7_Attachment 25 - NSWCCD-23-TM-2009-65 RevC Towing Fitting Design and Certification Process Guidance.pdf

Naval Surface Warfare Center Carderock Division West Bethesda, MD 20817-5700

NSWCCD-23-TM-2009/65 Rev C November 2009

Ship Systems Integration and Design Department

Technical Memorandum

TOWING FITTING DESIGN AND CERTIFICATION

PROCESS GUIDANCE

by Combatant Craft Division (Code 231)

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited

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F it ti n g D e s ig n a n d

C e rt if ic a ti o n P ro c e s s G u id a n c e

TOWING FITTING DESIGN AND CERTIFICATION

PROCESS GUIDANCE

NSWCCD-23-TM-2009/65 Rev C

Post-Release Revisions

Revision Reviewer/ Approver

Date Change Description

Page, Section

B M. Russell 231 11/3/2010 SF, text Numerous

B M. Riley 23 11/8/2010 Editorial Numerous

C M. Riley 23 8/22/2012 F593F stainless 11, 14 i

REPORT DOCUMENTATION PAGE

Form Approved

OMB No. 0704-0188 Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

05-11-2009

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Final

3. DATES COVERED (From - To)

Oct 2009 to Nov 2010

4. TITLE AND SUBTITLE

Towing Fitting Design and Certification Process

Guidance

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT

NUMBER

6. AUTHOR(S)

NSWCCD Code 231

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) AND

ADDRESS(ES)

8. PERFORMING ORGANIZATION

REPORT NUMBER

NSWCCD-23-TM-2009/65

Naval Surface Warfare Center

Carderock Division

Detachment Norfolk

2600 Tarawa Ct Suite 303

Virginia Beach, VA 23459-3239

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S

ACRONYM(S)

Attn PEO SHIPS, PMS 325G

Commander

Naval Sea Systems Command

1333 Isaac Hull Avenue, SE Building 197

Washington Navy Yard, DC 20376

11. SPONSOR/MONITOR’S

REPORT NUMBER(S)

12. DISTRIBUTION / AVAILABILITY STATEMENT

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

13. SUPPLEMENTARY NOTES

PAO approval received from NSWC Carderock, Congressional and Public Affairs

Branch, Code 3830 on 11/24/09

14. ABSTRACT

This document was developed to guide designers in industry and the U.S. Navy in the design, installation, testing and certification of safe and effective towing fittings for installation on boats and craft.

15. SUBJECT TERMS

Towing Fittings Design Guidance

16. SECURITY CLASSIFICATION OF: 17.

LIMITATION

OF

ABSTRACT

See 12.

18.

NUMBER

OF

PAGES

19a. NAME OF RESPONSIBLE

PERSON

Malcolm W. Whitford

a. REPORT

UNCLASSIFIED

b. ABSTRACT

UNCLASSIFIED

c. THIS PAGE

UNCLASSIFIED

19b. TELEPHONE NUMBER (include area code)

757-462-3274

Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. Z39.18 ii

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Contents

Page

Figures ........................................................................................................................................... iv

Administrative Information ......................................................................................................... v

Acknowledgements ....................................................................................................................... v

Summary

Introduction

Background Objective Scope

Towing Requirements

Towing Forces and Design Loads Towing System Design

Arrangements Fitting Types Strength and Factors of Safety Material Selection Material Properties

Design Details Load Path into Craft Structure Design Calculations

Fabrication and Installation Welding Surface Finish Mechanical Fasteners FRP Craft Metal Craft

Testing Towing Fittings

Documentation and Labeling

Documentation Towing Point Labeling Towing Bridle and Pendant Labeling Boat Information Book (and other craft documentation)

References

Symbols, Abbreviations, and Acronyms

Distribution iv

Figures

Page

Figure 1. Stern Tow: Towing Craft and Towed Craft Figure 2. Examples of Side Tows Figure 3. Integrated Tow Bitt and Weapon Foundation Figure 4. Beach Extraction: Towing Craft Using Bow Towing Fitting Figure 5. Bow Padeye with Webbing Pendant on FRP Hull Figure 6. Multi-purpose Bow Eye on Trailered Aluminum Craft Figure 7. Towing Fitting and Line Safety Factors Figure 8. Polyester Web Core with Protective Sheath Figure 9. Failure of Deck Mounted Bitt Figure 10. Permanent Pendant Label v

Administrative Information

This work was performed by the Combatant Craft Division (Code 231) of the Ship Systems Integration and Design Department at the Naval Surface Warfare Center, Carderock Division (NSWCCD). This effort was sponsored by the US Navy Small Boats Program Office, PEO SHIPS, PMS 325G under funding document N0002409WX31038 during fiscal year 2009.

This document (NSWCCD-23-TM-2009/65 Rev B November 2009) replaces Revision A, which is obsolete and should be destroyed.

Acknowledgements

The Combatant Craft Division wishes to thank Mr. J.M. Coughlin, Program Manager at the US Navy Small Boats Program Office (PEO Ships, PMS 325G), for sponsoring the original development of this guide, and Mr. Denis Bushey, PMS 325G for his overall management direction for revision B. Mr. Michael Russell and Mr. Malcolm Whitford provided technical guidance and Mr. Michael Riley served as technical editor. The collective support of these individuals is sincerely appreciated.

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Summary

This document provides design guidance to assist boat builders, designers and engineers in the design, fabrication, installation, inspection, and verification testing of safe and effective towing fittings on all Navy boats and craft. References for both Department of Defense and civilian design standards are provided, typical contract requirements are discussed, and best practices for tow fitting design, material selection, and installation are presented. Acceptance testing methods and boat builder information packages are discussed, and design safety factors established to mitigate the risk of unsafe conditions are presented. This guide provides a useful tool set, but it should not preclude the use of other detailed engineering practices or processes that can also deliver safe and effective capabilities. While not contractually binding, it presents a collection of Navy preferences and expectations that will be updated as new lessons are learned, and as new materials or techniques are developed. The following list is an outline of the towing system design process presented in this guide.

1. Determine towing requirements

Stern towing Bow towing Side towing Being towed

2. Determine loads Propulsor thrust (for towing, including astern and alongside as required) Craft resistance for the craft being towed, including effects of wind and sea state

3. Design towing fittings Arrange towing fittings to provide safe access, fair lead and chafe protection Select fitting types and materials Identify material properties Determine load path into the craft structure Determine towing forces Determine design loads and moments, including force vector resolution Apply factors of safety Select rope size for towline (and webbing or rope size for bridle and pendant, if applicable) Design or select towing fittings Design towing fitting supporting structure Refine design details

4. Fabrication and Installation Welding Surface finish

Mechanical fasteners Inspect fittings, welds and fasteners Repair and re-inspect as required

5. Testing Fitting pull tests Inspection Repair or replace and retest as required

6. Documentation and Labeling Drawings and Calculations Towing gear specification data Test reports P.E. design review and certification Tow point labels Towing bridle and pendant labels Boat Information Book / Owner’s Manual

Introduction

Background

Towing involving Navy craft, including towing and being towed, is a critical safety evolution. Flaws in towing fitting design or fabrication can result in catastrophic failures, loss of mission capability, and personnel injuries or death. Historically, these critical fittings had been engineered by the Government and a detailed design provided to the boat builder. In the last decade or so, the design of towing fittings has been entrusted to boat builders for craft purchased from the GSA schedule. However, government technical evaluation of towing fittings and design methods provided by boat builders has identified problems in the basic assumptions and engineering approaches used. There was a need for a single reference that describes a tow fitting design and integration process from a blank sheet of paper to final safety testing that could be applied across the various types of boats and craft. This design guide identifies acceptable design, fabrication, inspection and certification practices and materials to ensure that all fittings used in towing, and their integration into the craft, are safe and suitable for Navy use.

Objective

This design guide provides general guidance to assist boat builders, designers and engineers in the design, fabrication, installation, inspection and testing of safe and suitable towing systems for Navy craft.

The layout of this guide includes technical information as well as “TIPS” that highlight lessons learned and cautions for avoiding common issues. Typical contract requirements are presented, best practices are summarized, and lessons learned related to tow system designs are presented.

TIP: Successful acquisition and delivery of a new craft relies on collaborative teams of knowledgeable individuals. Ask the TPOC immediately if there are questions related to design requirements, applicable standards, or acceptance criteria.

Scope

This design guide is intended to provide general design guidance to assist boat builders, designers and engineers in the design, fabrication, installation, inspection and testing of safe and suitable craft towing fittings for craft that perform towing incidental to their primary function. It does not address requirements for craft specifically designed as towing vessels, which are typically designed to meet unique requirements.

This guide is applicable to conventional aluminum, FRP, and steel construction, and includes minimum best practice approaches to arrangement, material assumptions, and engineering methods to ensure adequate towing fittings and arrangements.

This design guide is not intended to provide detailed procedures for the design of towing fittings and their supporting structure. Loads from specialized operations such as beach extraction, recovery of grounded craft, “hasty tow” or other high-load operations are not covered herein, and will be addressed by specifications or guidance provided in unique solicitations or contracts. For example, the design and installation of towing fittings in specialized craft using highly stressed carbon fiber laminates would be specially addressed when applicable.

The breadth of technical issues addressed in this design guide is as complete as possible;

however this guidance is not exhaustive and does not cover all aspects of design or all situations.

Design of the towing fittings and their integration into the craft requires the technical skills and judgment of an experienced engineer, and no guide can capture all the factors that must be considered.

Towing Requirements

Craft may be required to have the ability to tow from astern and to be towed from the bow as illustrated in Figure 1. There may also be a requirement to have the ability to perform side tows as shown in Figure 2 (i.e. the towed, or disabled, craft alongside the towing craft), or other unique towing evolutions. Note in Figure 2 that the stern of the towing craft is aft of the stern of the towed craft to improve maneuverability.

Towing Forces and Design Loads

The “towing force” is the applied force resulting from a towing operation. The “design load” is the actual load on any given component of a towing system that results from the towing force and the geometry of the towing system. Depending on the design of the towing system and its geometry, different components of the towing system may have different design loads.

The towing force to be used in the design of the towing system will normally be specified in contract documents. Unless otherwise specified, the towing force should be the maximum propulsor thrust, including the combined thrust of all propulsors in multi-engine craft. Note that the maximum propulsor thrust could be achieved at bollard pull condition or with the craft underway, depending on the propulsor characteristics. If the use of maximum propulsor thrust is determined by the TPOC to result in excessively large design loads, other means of determining the towing force and design loads will be specified. Such specification could be a defined towing force, or a requirement to tow a specified craft at a given speed in a given sea state, which would require calculation of the towing force based on the resistance characteristics of the towed craft.

Towing Craft Towed Craft

Towing Bridle

P/S (if used)

Towline

Pendant

(if used)

Figure 1. Stern Tow: Towing Craft and Towed Craft

Figure 2. Examples of Side Tows

The towing force used to determine design loads may be different for different towing fittings, depending on their use. For example, stern fittings are generally designed for maximum propulsor thrust for towing from the stern, while bow fittings might be designed for the resistance of the craft while being towed for the astern thrust of the propulsors, or for side towing forces. In some cases, a craft may be designed for multiple towing arrangements. All fittings on the craft used for towing or being towed should be designed to accept the towing forces and design loads that result from the towing requirements specified in the contract.

Towing forces for towing and being towed generally apply to forces generated with the craft waterborne. Loads on towing fittings resulting from beach extraction or other special cases may also be specified in the design, including the load magnitudes and direction of load application on the fittings.

Where towing fittings serve multiple purposes (e.g., when a bow eye is used for craft recovery onto a trailer and for towing the craft, or a bow bitt is used for anchoring, mooring and towing), the fitting should be designed for the worst-case design load and load direction. Figure 3 shows a stern towing bit that also serves as a weapons foundation, so its design had to take into account specified tow loads as well as weapon recoil loads.

TIP: Design loads for all towing points must take into account the vector resolution of loads resulting from use of bridles, alongside towing geometry, or from off-axis loads that could be imposed by events such as yawing, waves, maximum steering effect, and other forces. In some cases, resultant loads can exceed the propulsor thrust.

Figure 3. Integrated Tow Bitt and Weapon Foundation

For the towing craft, unless otherwise specified, the towing force used to design loads on the towing fittings should be no less than the maximum total thrust of all propulsors on the craft.

Depending on the type and design of the propulsion system, this value may range from 8 to 30 pounds of thrust per installed horsepower, so the actual propulsor thrust characteristics must be calculated or provided by the propulsor manufacturer, and verified by testing. As a minimum, testing should include a bollard pull test, but note that bollard pull is not necessarily the maximum thrust condition. Where a towing fitting is used only with the propulsor operating in reverse (such as a bow fitting used for beach extraction), the maximum reverse thrust of the propulsor may be used as the towing force, or another towing force may be specified in the contract depending upon mission requirements.

Unless otherwise specified, the towing force used to determine the design load for the bow towing point should be no less than the maximum towed resistance of the craft across its operational speed range (note that the maximum resistance is not necessarily at the maximum design speed), or at a towing speed specified in the contract.

TIP: The calculation of craft resistance should take into account the weight of service life margins, pre-planned product improvements (P3I), the resistance of inoperative propulsors, and the effects of wind and sea state.

Where used for other purposes (such as beach extraction, as shown in Figure 4), the bow towing point should also be designed for the design load resulting from such use.

Figure 4. Beach Extraction: Towing Craft Using Bow Towing Fitting

Towing System Design

Design of a safe and effective towing system requires consideration of the arrangements of the towing components on the craft necessary to accomplish the range of missions, selection of the craft fittings, selection of towline and hardware, and analysis of the strength of the towing system components and the craft structure.

High-strength fittings for Navy craft are designed for zero failures under foreseeable (not just normal) service conditions throughout the craft’s service life. This is because high strength fittings present the potential for a single-point failure with severe consequences, including injury or death resulting from, for example, a loaded line pulling a towing fitting out of a deck. This is feasible because the cost and weight penalty of designing high strength fittings for zero failure is low, compared to the cost and weight penalty of zero failure design for hull structure or other systems that typically do not present the potential for catastrophic single-point failures.

It is important to integrate the design and development of towing fittings into the initial design process of a boat. Early integration of towing fittings into the boat design is required to avoid structural and arrangements problems requiring redesign and rework. The general design practices discussed herein are important to achieving safe and functional towing fittings and arrangements. Calculations and design of towing fittings and their integration into the craft structure should follow these guidelines and sound engineering practice.

Arrangements The arrangements for towing and the design of the towing fittings and their integration into the craft design should minimize risks to persons carrying out towing procedures. To the extent practical, towing fittings and their attachment to the hull and the supporting structure should be designed and arranged so that failure of the fitting or towline does not result in injury to personnel aboard the craft, or catastrophic failure of the craft (e.g., flooding).

Towing fittings should be arranged to provide fair lead of the towline during all operations to prevent chafe of the towline, damage to the craft or its equipment, or injury to personnel. Towlines should not bear on rigid inflatable boat (RIB) tubes or sponsons, and towing fittings should be readily accessible from aboard the craft for use with the craft afloat.

Where a bow eye not safely accessible from onboard the craft is used as the forward towing point, a pendant secured to the bow eye with a shackle should be provided, and include a means to secure it for safe and ready access from on board the craft.

Where a stern eye or eyes not safely accessible from onboard the craft are used as the stern towing point, a pendant or towing bridle secured to the stern eye(s) with shackle(s) should be provided, and should include a means to secure it for safe and ready access from on board the craft.

Towing fittings should be arranged so that the towline load does not result in yaw, heel or trim that cause difficult steering or hazard to the craft or its occupants. Stability of the craft when towing should meet the requirements of reference [1], or other requirement specified in the contract.

Fitting Types Selection of towing fitting types and size should consider the service of the craft, the line size, the craft arrangements and the ability of crew to safely make up and cast off a towline.

Fittings for belaying towlines with a hitch or spliced eye may be bitts or cleats. Chocks may be of the open or closed type. Where bow or stern eyes are used for towing, they should be fabricated plate pad eyes. An example is shown in Figure 5.

TIP: Bow or stern eyes with U-bolts should be avoided for towing operations, but for small craft with low towing forces, ask the TPOC for consideration and approval of selective use of U-bolt fittings.

Figure 5. Bow Pad eye with Webbing Pendant on FRP Hull

TIP: On trailered craft, where a bow eye is used for towing and a towing pendant is installed, a second eye, as shown in Figure 6, should be provided in the bow eye fitting to avoid the need for the pendant to be removed when the craft is on the trailer

Figure 6. Multi-purpose Bow Eye on Trailered Aluminum Craft

Strength and Factors of Safety A factor of safety (the ratio of the failure load to the design load) is applied to the design load (DL) on each component of the towing system. The purpose of the factor of safety is to address unknowns in the load and material properties of the towing system components to assure that the component does not fail when subjected to the design load. Different factors of safety are specified for different components and types of material. The factor of safety on the towline is selected so that it will not break in normal use, even when the effects of load and materials uncertainty addressed below are considered. The factor of safety on the towing fittings (bitts, cleats, towing eyes and other towing points) and supporting structure is based on the breaking strength of the towline so that the towline will always fail before the fitting. This avoids the great danger of a failed fitting, propelled by the strain energy of a towline, striking people or objects.

Factors of safety are based on experience and specific engineering analyses, and address a range of issues, including:

Loads:

Dynamic loading Wave encounter Towing geometry variations (bridle length, direction of pull, etc.)

Materials properties:

Weld quality Corrosion Fatigue Environmental aging of rope or webbing (UV light, ozone, chemicals) Wear in rope or webbing (internal and external) Wet strength of rope or webbing Variation of splice or sewn eye quality

The factor of safety is not intended to compensate for damaged fittings, line or webbing. Fittings, line and webbing should be inspected before load testing or use in accordance with the contract (or Boat Information Book for craft in service), and repaired or replaced before testing or use if damage exceeds specified tolerances.

Towlines, pendants and bridles must be selected with a factor of safety of not less than

5.0 on the design load, applied to the manufacturer’s minimum breaking strength (MBS) of the rope or webbing used. Components of towlines, pendants and bridles such as shackles and links must also be selected with a factor of safety of 5.0 on the design load, applied to the minimum breaking strength of the component.

Towing fittings and supporting structure fabricated of metal must have a factor of safety not less than 1.5 on the MBS of the towline, against the ultimate strength of the metal (use as-welded strength, if of welded construction).

FRP structure must have a factor of safety of not less than 2.0 on the MBS of the towline against the ultimate strength of the FRP material. Note that the factor of safety for FRP structure is greater than that for metal structure to account for the greater effects of fatigue and environmental degradation on the strength of FRP components during the life of the craft.

The resulting safety factors are illustrated in Figure 7.

Stern towing fittings and supporting structure:

ultimate strength = 1.5 x MBS metal ultimate strength = 2.0 x MBS FRP

Bow towing fitting and supporting structure:

ultimate strength = 1.5 x MBS metal ultimate strength = 2.0 x MBS FRP

Towing Craft Towed Craft

Towline pendant, bridle, shackles, and links:

MBS = 5 X DL

Figure 7. Towing Fitting and Line Safety Factors

The bending moment used for design of bitts, cleats and chocks is the product of the design load on the line multiplied by the height above the base of the top of the bitt crossbar, or the height of the line path at the cleat or chock. Components of the towing fittings such as bitt crossbars or cleat horns must be designed for the maximum bending moment produced by a belayed towline, to the same factor of safety. The effect of nonstandard belays that may be used by inexperienced crew should be considered where this might increase the bending moment.

Bending moments on towing eyes should account for side loads that could reasonably be expected in service. In addition, specific bending load directions, moments or forces may be specified by the Government.

TIP: Many commercial off-the-shelf cleats and bitts have fastener holes that will not accommodate bolts large enough to achieve the same strength as the fitting. These holes must not be enlarged since this may result in failure of the fitting in way of the enlarged hole. Towing fittings, whether fabricated or off-the-shelf, must be designed so that all parts provide the required strength.

In the structural analysis of towing fittings and supporting structure, the material ultimate strength used in each part of the analysis is the value appropriate to the stresses, including tensile, compressive, and shear, as appropriate. Buckling and other structural stability effects must also be considered.

Where towing is conducted using a bridle to two towing fittings (e.g., when two bitts, cleats, or pad eyes on the transom are used for towing) each bridle leg must be designed using the full towing force, to account for asymmetric loading due to turns or yaw of the tow that could result in the full towing force being taken by one bridle leg. The legs of the towing bridle must be long enough to avoid acute angles that result in loads in the bridle legs greater than the towing force.

Material Selection

Towing fittings on aluminum or FRP craft may be of stainless steel or aluminum, unless otherwise specified. Towing fittings on steel craft should be fabricated of stainless steel to prevent corrosion that would occur on steel surfaces due to wear of the protective paint.

Stainless steel used in welded fittings should be AISI 316L (UNS S31603). Cast stainless steel fittings should be 316 alloy (UNS S31600). Aluminum used in welded towing fittings should be a suitable marine alloy of the 5000 series. Aluminum plate should conform to ASTM B928 (reference [2]).

TIP: 5086 is the preferred alloy for fabrication of welded aluminum towing fittings, since it offers good post-weld strength retention and is more resistant to corrosion in a marine environment than other readily available 5000 series alloys. If extrusion availability in this alloy is limited, contact the contract TPOC for alternate material consideration.

Cast aluminum fittings should use alloys suitable for use in a marine environment that will prevent corrosion in service, with particular attention given to intergranular corrosion and stress corrosion cracking. Effective quality controls and inspection methods should be used to avoid deleterious defects in cast fittings, such as porosity, cracks, or other material defects.

Molded plastic towing fittings should not be used. FRP towing fittings should not be used without specific TPOC approval.

Stainless Steel Alloy 316 ASTM F593G, Group 2, Condition A, is the best practice for fasteners. ASTM F593 specifies that the fasteners be heat treated to reduce susceptibility to inter-granular corrosion, and ensures that the alloy is resistant to stress corrosion cracking.

Material Properties

As-welded material properties should be used for welded components of towing fittings and their supporting structure.

Properties of FRP laminates used for design of towing fitting supporting structure should be verified by testing. In the design of bolted fittings, core materials should not be considered to support bolt bearing loads.

Line and webbing design strength should be based on the minimum breaking strength (MBS) of new, dry rope or webbing. Note that minimum breaking strength of line or webbing is less than the average or typical strength. If only average or typical breaking strength data are available, reduce the value by 15% (i.e., MBS = 0.85 * average BS) to approximate the minimum breaking strength.

Design Details Towing bitts and cleats should be sized to accommodate typical belays using the towline size required by the design calculations, specified by the contract, or provided by the builder, whichever is largest. Chocks should be designed so that the opening will pass an eye splice or bight of line in addition to two parts of the size line to be used with the chock. Where it is expected or specified that chafe protection will be used, chocks should in addition be sized to accommodate such protection.

Access should be provided to both sides of bolted towing fittings for installation and inspection. For FRP applications where only one side of the fitting is accessible, standard nuts should be tack welded to the backing plate and bolts with prevailing torque self locking inserts should be used. Inaccessible bolted connections should not be used in metal boats.

TIP: To the extent practical, towing fittings should be designed so that bolts are loaded in shear, not tension. This reduces the potential for failure resulting from, e.g., a cracked bolt, since the remaining length of the bolt may still provide some load-bearing capability by acting as a pinned connection. This is not always achievable due to load direction and practical limitations on the geometry of the towing fitting.

Pendants and bridles, if used, should be of synthetic webbing protected against chafe and ultraviolet light exposure with a nylon webbing sheath or guard. Alternatively, double-braid nylon rope with eye splices and stainless steel thimbles may be used. Figure 8 shows a pendant fabricated using polyester webbing with an integral stitched nylon protective sheath.

Figure 8. Polyester Web Core with Protective Sheath

Shackles should be RR-C-271D, type IVA, class 3, grade B; or SlingSaverTM shackles may be used with webbing. The design of towing fittings, shackles and pendants or bridles should preclude asymmetric loading of the shackle or fitting. Holes for shackle pins should be no more than 1/16” larger in diameter than the pin. The width of shackle openings should be no more than 1/8” greater than the pad eye thickness. Where the shackles used in the hoisting fittings are of the same or greater strength as required for towing and are suitable for use in towing, consideration should be given to using the same size shackles for towing as in the hoisting fittings (see reference [3]), even if oversized for the towing requirements. This reduces the potential for a spare shackle being used in the wrong application and simplifies logistics support.

Load Path into Craft Structure The design of the load path from towing fittings to the supporting craft structure should be direct and linear, avoiding stress concentrations and other unsound design practices. Insert plates, doublers, reinforcing framing and other structure should be designed and fabricated so as to avoid hard points and stress risers.

FRP structure should have brackets, doublers, inserts, and/or special framing installed as necessary to distribute towing fitting loads. Areas of cored structure in way of towing fittings should have the core replaced by a high-density foam insert and the laminate suitably increased in thickness to prevent compressive or other failures due to bolting forces or loads seen in service. Alternatively, the core may be deleted and the laminate thickness suitably increased in way of the fitting.

Design Calculations Design calculations should account for the loads and stresses on all towing components throughout the load path, including the towing fittings, towline, pendants, bridles and fittings, and supporting craft structure. Use of Finite Element Analysis (FEA) is encouraged, particularly for highly-loaded towing fittings and weight-critical craft.

Fabrication and Installation

Welding Only qualified weld procedures, materials and personnel should be used in the fabrication of welded components for towing fittings. Welding and qualifications should be to NAVSEA TO300-AU-SPN-010A (reference [4]) and NAVSEA 0900-LP-060-4010 (reference [5]), or other military standards, American Bureau of Shipping (ABS) or American Welding Society (AWS) standards as specified for the craft. The builder is responsible for maintaining appropriate procedures, materials and personnel qualifications and record keeping to show compliance, or if welding is subcontracted, for imposing the same requirements on the supplier.

Surface Finish The edges of all metal components should be rounded and smoothed to prevent chafe or other damage to the towline. The radius of any line bearing surface should be suitable for the diameter of the towline, i.e., large enough to prevent chafe damage, or damage to the towline from internal friction due to running over an excessively tight radius. The belaying surfaces of cleats and riding surfaces of bitts, cleats and chocks should have a surface roughness average value, Ra, measured in accordance with ANSI/ASME B46.1, not greater than 3.2 µm (125 micro-inches).

Mechanical Fasteners

Where mechanical fasteners are used for towing fittings, they should be 316 stainless steel conforming to ASTM-F593G (bolts) and F594F (nuts), Group 2, Condition A (see references [6] and [7]). Note that Condition A is a solution annealed condition that reduces susceptibility to intergranular corrosion. Washers should be of the same material as the nuts.

While stainless steel fasteners using less common alloys with higher strength than 316 are available (e.g., duplex stainless steels), for logistics support and safety reasons, it is generally preferred that 316 fasteners be used, since this alloy is commonly specified for all Navy boat fasteners. This assures that replacement fasteners are readily available, and reduces the chance that 316 fasteners would be inadvertently substituted for high strength fasteners during maintenance. Use of stainless steel fastener materials with higher strength than 316 alloy must have at least equal corrosion resistance, and such use must be specially requested by the builder and approved by the TPOC. Such materials should be clearly documented in the Boat Information Book (BIB) and integrated logistics support (ILS) documentation to avoid inadvertent substitution of lower-strength replacement fasteners during the service life of the craft.

TIP: Note that stainless steel type 304 or 18/8 fasteners are commonly available and are sometimes improperly substituted by suppliers when type 316 is specified. Type 304 or 18/8 stainless steel fasteners do not provide adequate resistance to inter-granular corrosion and stress corrosion cracking in a marine environment and their use is prohibited.

Bolts should be used with one flat washer under the nut and one under the head of the bolt, except for flat head bolts. The length of bolts should be selected so that two to five threads of the bolt protrude from the nut. Additional washers should not be used to reduce thread protrusion. Prevailing torque type self locking nuts should be used. Acorn nuts should not be used.

Bolts should be selected with sufficient unthreaded length that shear and bearing loads are applied through the unthreaded portion of the bolt.

TIP: If loading bolts through the threads is unavoidable, fastener strength calculations should account for the reduction in cross-section area in way of the threads, and bearing load calculations should account for the reduced bearing area in way of the threads.

FRP Craft

Where towing fittings are fastened to FRP, a close and accurate fit between the faying surfaces should be obtained. A backing plate should be used to distribute bolt tension loads and loads seen in service to obtain the required factors of safety. The backing plate should be aluminum or stainless steel of sufficient thickness to prevent distortion due to bolt tension or loads seen in service. Faying surfaces should be clean at the time of installation. On non molded surfaces, fittings should be set in high compressive strength resin-based adhesive putty. On molded surfaces, the fitting should conform to the shape of the surface, or bosses with contours matching the contours of the fitting should be molded into the FRP surface as necessary. On surfaces exposed to the weather, the fitting should be set in a bedding compound or a marine caulking compound such as 3M 4200 to exclude water. Silicone sealants or caulking compounds should not be used.

To prevent galvanic corrosion, metal towing fittings and bolts in carbon fiber reinforced plastic (CRP) laminates should be entirely electrically isolated from the carbon fiber by glass or Kevlar reinforced plastic (GRP or KRP), nonconductive plastic bushings (either prefabricated or cast in place) or other method(s) satisfactory to the Government. Caulks, bedding compound, putty or similar materials alone are not adequate for this purpose.

Metal Craft To the extent practical, towing fittings on metal craft, whether aluminum or steel, should be welded to the craft structure. Where mechanically fastened towing fittings on metal craft are used, a gasket, bedding compound or a marine caulking compound such as 3M 4200 should be used at the interface with the craft structure to exclude water, electrically isolate the materials where required, and prevent corrosion. Silicone sealants or caulking compounds should not be used.

Welded bitts that penetrate the deck and connect to structure below are stronger than towing fittings mounted to the surface of the deck and are therefore preferred for all craft other than small, low-power craft with low towing forces. Figure 9 illustrates a surface mounted bitt that failed due to improper surface mounting.

Figure 9. Failure of Deck Mounted Bitt

Where increased deck plating thickness is required in way of towing fittings, use of extra-thickness inserts is preferred to doublers to provide a stronger connection between towing fittings and the craft structure. Inserts or doublers should have their thickness tapered at the edges and have rounded corners to reduce stress concentration in accordance with references [4] and [5].

Testing

Towing Fittings The boat builder is typically required to proof test the towing fittings after installation on the boat. The tow fittings affixed to the boat (pads, pad eyes, and other fixed structure) are typically required to be tested with an overload of 50% of the design load (i.e., test load = 150% of the design load) in the design load direction(s). The duration of pull tests for composite craft is 10 minutes. This includes any craft made of composite, polymer or other non-metallic materials, adhesively bonded joints, or any material in the strength fitting load path that may creep. For metal craft with only welded or bolted connections throughout the load path, the test may be reduced to 90 seconds. After testing, the towing fitting and all supporting structure should be inspected.

Test failure is defined as any visible permanent deformation, yielding, cracking, or bending of the towing fitting, the craft structure or any other towing component, welds, fasteners or supporting structure, or any loss of watertight integrity. Any failure indication identified during the testing and inspection should be considered by the TPOC to justify the requirement to use more detailed analysis and testing methods including calibrated measurements, finite element analysis, magnetic particle inspection, X-ray, fatigue testing, ultimate strength testing or other means of assessing the soundness of the welds and affected parent metal.

All towing fitting and supporting structure welds should be visually inspected and tested with a dye penetrant test (PT) or Radiographic Testing (RT) after the load test. A test report and certification of results should be provided. A recognized Pass/Fail criterion should be specified, such as MIL-STD-2035A (SH), specifying the appropriate weld class. It is recommended a PT be conducted prior to the overload test to identify any pre-existing defects that may cause failure of the fitting during the load test or inspection failure after the load test.

If required by contract bridles and towing pendants should be tested by the manufacturer independently of the craft. Each pendant and each leg of a bridle should be tested individually, complete with fittings, with an overload of 100% of the design load (i.e., test load = 200% of the design load).

Documentation and Labeling

Documentation The following documentation is typical, and may be required by contract specifications.

Drawings defining towing fittings, craft structure, rigging and towing arrangements, including accessibility and towline clearances. The maximum towing speed, sea state, craft load condition and maximum force on the bow towing fitting, and the maximum towing force on each stern and side tow fitting, and other pertinent safety information should be provided.

Calculations for the loads and structural design of the fittings and craft structure.

Specification sheets for pendants, bridles and their fittings, and for purchased towing fittings.

Towing fitting, pendant and bridle load test reports.

Towing Point Labeling Where it is not obvious what fitting is to be used for towing, or where multiple fittings are provided and some are not designed to accept towing forces, the towing points should be labeled on or adjacent to the fitting in an obviously visible location. Labeling information should include the safe working load of the fitting in pounds, to the nearest 100 pounds for safe working loads over 1000 pounds, e.g.: “MAXIMUM LOAD 2600 POUNDS”, and to the nearest 10 pounds for safe working loads under 1000 pounds, e.g., “MAXIMUM LOAD 740

POUNDS”.

Where a fitting for mooring or other purpose that is not designed for towing is located so that it could reasonably be assumed by an uninformed craft operator to be a towing point, it should be labeled “NOT FOR TOWING”.

Label plates should be stainless steel, 0.8 mm nominal thickness, ASTM A167, UNS S31600. The stainless steel should be etched or engraved no less than 0.1 mm deep. Etching and engraving should be filled with black paint. Label plates should be secured in place by a means that will assure retention through the craft service life, and that will not impair the water tightness, structural integrity or functionality of the object to which they are attached.

TIP: Alternate means of labeling that provides the required durability and legibility will be considered by the TPOC.

Towing Bridle and Pendant Labeling Pendants and each leg of a bridle should be provided with a permanently attached label specifying the certifying organization, the date of the load test, the applied test load and the design load. Figure 10 shows a pendant with its label stitched to the material.

Figure 10. Permanent Pendant Label

Boat Information Book (and other craft documentation) The Boat Information Book (BIB), Owner’s Manual, or other documentation required by the contract to be provided with the craft should include information to assure safe towing. If required by contract, as a minimum, the following information should be provided:

Instructions for rigging the craft for towing or being towed, including proper use of the towing fittings, pendants, bridles and fittings as applicable, and other applicable operating and safety instructions.

Towing arrangements drawings.

The design line size and type for each towing fitting.

Line size limits for the towing fittings to assure line failure before fitting failure, in accordance with the factors of safety defined herein.

Design load limit on each fitting and component.

Directional limits on loading of the fittings, or a polar diagram of load limit vs. direction.

The maximum towed speed and sea state for the craft for each rigging configuration for being towed, including bow tows and side tows.

The maximum craft size, towing speed and sea state for towed craft for each towing configuration, including side tows and stern tows.

If applicable, engine RPM limits to provide design load limits on towing force, for each towing configuration.

If a bow eye is fitted for towing, its suitability or non-suitability for trailering or

Combatant Craft Retrieval System (CCRS) use should be noted, with the design loads and use limitations for those applications stated.

Provide rigging and operating instructions and limitations for “hasty tow”, beach extraction or other specialized towing evolutions where required.

References

1. NAVSEACOMBATSYSENGSTA Report No. 6660-99 Rev. A “Procedures Manual for Stability Analysis of U.S. Navy Small Craft”

2. ASTM B928 / B928M – 09, “Standard Specification for High Magnesium Aluminum-Alloy Sheet and Plate for Marine Service and Similar Environments”

3. Marshall, Jason T., “Hoisting System Design and Certification Process Guidance”, Naval Surface Warfare Center Carderock Division report NSWCCD-23-TM-2009/33, October 2009.

4. NAVSEA TO300-AU-SPN-010A “Fabrication, Welding and Inspection of Small Boats and Craft, Aluminum Hulls”

5. NAVSEA 0900-LP-060-4010 “Fabrication, Welding and Inspection of Metal Boat and Craft Hulls”

6. ASTM F593 “Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs”

7. ASTM F594 “Standard Specification for Stainless Steel Nuts”

Symbols, Abbreviations, and Acronyms

ABS ................................................................................................... American Bureau of Shipping

AISI .............................................................................................. American Iron and Steel Institute

AWS ....................................................................................................... American Welding Society

ASTM ......................................................................... American Society for Testing and Materials

BIB ............................................................................................................... Boat Information Book

BS ......................................................................................................................... Breaking Strength

CCD ......................................................................................................... Combatant Craft Division

CCRS ......................................................................................... Combatant Craft Retrieval System

CRF ................................................................................................. Carbon fiber Reinforced Plastic

DL ................................................................................................................................. Design Load

FEA ............................................................................................................. Finite Element Analysis

FRP ............................................................................................................. Fiber Reinforced Plastic

FS ............................................................................................................................. Factor of Safety

GRP .................................................................................................... Glass fiber Reinforced Plastic

GSA............................................................................................... General Services Administration

ILS........................................................................................................ Integrated Logistics Support

KRP .................................................................................................. Kevlar fiber Reinforced Plastic

MBS .................................................................................................... Minimum Breaking Strength

MIL STD ............................................................................................................... Military Standard

NAVSEA .......................................................................................... Naval Sea Systems Command

NSWCCD DN ................. Naval Surface Warfare Center Carderock Division Detachment Norfolk

NSTM ............................................................................................. Naval Ships’ Technical Manual

P .................................................................................................................................................. Port

P.E. ................................................................................................... licensed Professional Engineer

PEO .......................................................................................................... Program Executive Office

P3I ............................................................................................ Pre-Planned Product Improvements

RIB ............................................................................................................Rigid hull Inflatable Boat

S .....................................................................................................................