7_Attachment 20 - NSWCCD-23-TM-2009-33 RevB Hoisting System Design and Certification Process Guidance.pdf

Naval Surface Warfare Center Carderock Division Bethesda, MD 20084-5000

NSWCCD-23-TM-2009/33 Rev B Oct 2009

Ship Systems Integration and Design Department

Technical Memorandum

HOISTING SYSTEM DESIGN AND CERTIFICATION

PROCESS GUIDANCE

by

Jason T. Marshall Combatant Craft Division (23)

DISTRIBUTION STATEMENT A. Approved for public release;

distribution is unlimited.

N S

W C

C D

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-T M

-2

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R e v B

H o is ti n g S y s te m 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

HOISTING SYSTEM DESIGN AND CERTIFICATION

PROCESS GUIDANCE

NSWCCD-23-TM-2009/33 Rev B

Prepared by:

Jason T. Marshall, PE

Senior Craft Design Manager, Naval Architecture

Branch

NSWCCD Code #231

Reviewed by:

Jen Burrell

Naval Architect, In Service Engineering Branch

NSWCCD Code 234#

Michael J. Russell, PE

Naval Architect, Naval Architecture Branch

Christopher A. Shields

Naval Architect, Naval Architecture Branch

Malcolm W. Whitford

Technical Warrant Holder, Combatant Craft and

Boats

NSWCCD Code #2303

Approved by:

Willard Sokol, III

Branch Head, Naval Architecture Branch

NSWCCD Code #231

Post-Release Revisions

Revision Reviewer/ Approver

Date Change Description Page, Section

A JTM/ Code 3830 Oct. 14, Approved for Public Release cover, i

B JTM Aug 16, 2012 Sample spreadsheet, sling leg number, material properties, label plate details, sling safety factor, removed professional engineer certification

1, 3, 12, 14, 15, 27, 32, C- 1:16

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1. REPORT DATE (DD-MM-YYYY)

October 2009

2. REPORT TYPE

Final

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Hoisting System Design and Certification Process Guidance

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Marshall, Jason T

5d. PROJECT NUMBER

5e. TASK NUMBER

N0002409WX31038

5f. WORK UNIT NUMBER

09-1-2300-252-41

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

NAVSEA Carderock

Surface Warfare Center Division

Combatant Craft Division

2600 Tarawa Court, Suite 303

Norfolk, Virginia 23521-3239

8. PERFORMING ORGANIZATION REPORT

NUMBER

NSWCCD-23-TM-2009/33

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES)

Attn: PEO SHIPS, PMS 325G

Commander

Naval Sea Systems Command

1333 Isaac Hull Avenue, SE Building 197

Washington Navy Yard, DC 20376

10. SPONSOR/MONITOR’S ACRONYM(S)

PMS 325G

11. SPONSOR/MONITOR’S REPORT

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 10/14/2009.

14. ABSTRACT

This guidance was developed to help designers in industry and the US Navy to fulfill all requisite steps in the design of hoisting fittings installed on boats and craft. A process outline is presented as well as design tips and worked examples and illustration of proper and improper installations.

15. SUBJECT TERMS

Hoisting Lifting Guide Guidance Certification NSTM 583 Fittings Slings

16. SECURITY CLASSIFICATION OF:

17. LIMITATION

OF ABSTRACT

18. NUMBER

OF PAGES

19a. NAME OF RESPONSIBLE PERSON

Jason T Marshall

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified

See 12.

19b. TELEPHONE NUMBER (include area code)

757-462-3503

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

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Contents

Page

Figures .................................................................................................................................................. v

Tables .................................................................................................................................................... v

Administrative Information .............................................................................................................. vi

Acknowledgements ............................................................................................................................. vi

Summary

Background

Scope

General

Process Guide

Determine Requirements

Type and modes of lift

Growth Margin

Lifting Arrangement Constraints

Determine the Loads

Weight and CG estimating and margins

Determine the arrangements

Calculate sling loads (force diagram)

Sling assembly hardware selection

Sling type, material / size

Rings, links and shackle selection

Spreader bars

Straps for Trailer Lifting

Fitting design – structural details

Load Path Alignment

Fabrication / Design Details

Weld requirements

Material selection/properties iv

Strength Analysis

Design Example

System Verification

Validation

Weight and CG determination

Sling arrangement check

Re-check versus design calculations

Overload tests

Slings

Fittings

Trailer tiedown / combined lift fittings

Labeling and Documentation

Documentation

Boat Label Plate

Equipment Labels

Symbols, Abbreviations, and Acronyms

References

Distribution

Appendix A – Hoisting Design and Certification Checklist........................................................ A-1

Appendix B – Sample Drawings .................................................................................................... B-1

Appendix C – Sample Calculations ............................................................................................... C-1

Appendix D - Sample Scale Weighing Report.............................................................................. D-1

Appendix E - Sample Sling and Fitting Overload Test Report .................................................. E-1 v

Figures

Figure 1. Example of Impact of LCG Shift

Figure 2. Padeye design shown aligned to the sling load and located over craft frame (note – location of lifeline stanchion in close proximity)

Figure 3. Load Aligned Fitting

Figure 4. Aligned Fittings in Profile

Figure 5. Misaligned Fitting

Figure 6. Designed Fitting Installation

Figure 7. As Built Fitting Installation

Figure 8. Location of Lifting Provisions

Figure 9. Single point lift with hoisting sling assembly definitions

Figure 10. Force Diagram

Figure 11. Proper Sling Marking

Figure 12. Proper Sling Marking

Figure 13. Model of hoisting fitting with proper load alignment

Figure 14. Model showing deflection from unequal loading

Figure 15. Allowable clearances between shackle and hoisting fitting

Figure 16. Excessive clearance between shackle and hoisting fitting

Figure 17. Hoist/Mooring Fitting

Figure 18. Sample Saddle Bracket, Bracket and Backing Plate

Figure 19. Sample FEA to check fitting integration into global structure

Tables

Table 1. Steel Properties

Table 2. Stainless Steel Properties – reference unknown

Table 3. Aluminum 5086 Minimum Properties for Design

Table 4. Composite Properties

Table 5. Stainless Steel Fastener Properties vi

Administrative Information

This work was sponsored by the US Navy Boats and Craft program office, PEO SHIPS, PMS 325G under funding document N0002409WX31038 during fiscal year 2009.

Acknowledgements

The author wishes to thank Mr. Jack Overman of CDI Marine Services who initiated the first draft of this report which, although much different in final presentation, is inclusive of most all the content he originally assembled. Mr. Mack Whitford, Mr. Ted

Sadowski, Ms. Jen Burrell also contributed greatly to the content of that first draft. Special thanks are extended to Mr.’s Rich Wilson and Mike Russell for early reviews of the proposed outline and flow of this guide. In addition, Mr. JM Coughlin and Mr. Denis Bushey should be acknowledged for their persistent support in the drafting of this guide.

vii

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Summary

Hoisting ship’s boats and other craft is a critical safety evolution. Flaws in hoisting system design or manufacture can result in catastrophic failures, loss of mission capability, personnel injuries or death. The best design practices discussed are important to safety and aid in achieving a safe and suitable hoisting system. Hoisting design criteria has been provided to builders and most recently a requirement for certification has been imposed. However, evaluation of hoisting systems provided by boatbuilders has identified problems in the basic assumptions and engineering approach used. This design guidance is intended to assist boat builders, designers and engineers in the design, fabrication, installation, inspection and testing of safe and suitable hoisting fittings and slings. This guide is primarily focused on the hoisting slings, fittings, and the attachment points to the boat. The guide addresses, steel, aluminum, and composite construction and includes minimum best practice approaches to arrangement, material assumptions, and engineering methodologies to ensure adequate hoisting arrangements, hardware and fittings. This guide encourages applying a simple hoisting system to allow for a straightforward analysis of the system. The breadth of technical issues is as complete as possible but some areas are probably lacking in the depth required in some situations. As always, there is no substitute for the judgment of an experienced designer. This guide should be considered a

‘first’ issue to be updated as lessons learned, new materials and techniques are derived.

An outline of the hoisting system design process presented in this guidance is provided on the next page.

Hoisting System Design Process Outline

Determine Requirements o Type and modes of lift and requisite safety factor o Growth Margin o Lifting Arrangement Constraints o Combined lift requirements (boat only, trailer only, boat and trailer lifted together)

Determine the Loads o Weight and CG estimating and margins o Determine the arrangements

Fitting placement and support

Sling assembly arrangement

Sling assembly lengths for proper bow attitude o Calculate sling loads (force diagram)

Sling assembly sling and hardware selection o Sling type, material and size o Rings, links and shackle selection o Spreader bars (if applicable) o Straps for Trailer Lifting

Fitting design – structural details o Load Path Alignment o Fabrication and Design Details o Weld requirements o Material selection and properties o Strength Analysis o Design Example

System Verification o Physical Validation o Scale Weighing and CG determination o Sling arrangement check o Re-check versus design calculations

Overload tests o Slings o Fittings o Trailer tiedown and combined boat-trailer lift fittings o Trailer straps

Labeling and Documentation o Documentation o Boat Label Plate o Equipment labels

Background

Hoisting ship’s boats and other craft is a critical safety evolution. Flaws in hoisting system design or manufacture can result in catastrophic failures, loss of mission capability, personnel injuries or death. Historically these critical fittings had been engineered by the

Government and a detailed design provided to the boatbuilder. In the last decade or so, the design of hoisting fittings has been entrusted to boatbuilders for boats purchased from the GSA schedule. The design criteria for the fittings have been provided to builders and most recently a requirement for certification of the design and installation has been added. However, evaluation of hoisting systems provided by boatbuilders has identified problems in the basic assumptions and engineering approach used. This design guidance identifies acceptable design practices, material assumptions and engineering methodology is provided to ensure that all hoisting fittings are safe and suitable for Navy use.

Scope

This document is intended to provide design guidance to assist boat builders, designers and engineers in the design, fabrication, installation, inspection and testing of safe and suitable hoisting fittings and slings. This guide is primarily focused on the hoisting slings, fittings, and the attachment points to the boat. Other considerations related to structural adequacy of surrounding structure are outside the scope of this document.

The guide addresses, steel, aluminum, and composite construction and includes minimum best practice approaches to arrangement, material assumptions, and engineering methodologies to ensure adequate hoisting arrangements, hardware and fittings.

The guidelines in this technical memorandum should be followed in the development of the hoisting system. The requirements of Chapter 7 of Naval Ships Technical Manual Chapter 583, Volume 1, reference [1], must be met for approval and acceptance of hoisting sling assemblies.

This guide encourages applying a simple hoisting system to allow for a straightforward analysis of the system. The breadth of technical issues is as complete as possible; however some areas are probably lacking the technical depth required for some situations. As always, there is no substitute for the judgment of an experienced designer. No guide could ever come close to capturing all the intricacies an experienced designer may consider in the design. This guide should be considered a ‘first’ issue to be updated as lessons learned, new materials and techniques are developed. The Combatant Craft Division should be contacted for any desired clarifications.

General

It is important to integrate the design and development of hoisting fittings into the initial design process of a boat. Hoisting fittings not integrated into the boat design results in a poor design and often requires re-work during the test phase of the build cycle. The design practices discussed are important to safety and aid in achieving a safe and suitable hoisting system.

Calculations and design of hoisting slings and fittings are to be in accordance with the guidelines of this document. Appendices are provided with various examples of documentation required to certify a hoisting system.

An outline of this process is presented on page 2 as part of the summary.

Process Guide

Determine Requirements

At the outset of the design process it is imperative to understand the base objective of the hoisting system. US Navy personnel should specify one or more of the following types of lift based on the requirements provided by the Fleet. The stated objective and subsequently derived requirements should be included in the contract or delivery order.

Type and modes of lift

Ship lift at sea - launch of a ships boat or force protection boat by a ships crane or davit at sea, whether dead in the water or underway. Boats and craft lifted by a ship-borne davit or crane have the potential for large accelerations and the dynamics of lifting at sea can significantly increase the loads. Also, the seawater environment promotes more corrosion and wear of hoisting slings than most land stations. Further more, these lifts generally involve personnel and mission gear onboard during the lift that can not be loaded after the boat is launched. Therefore a safety factor of at least six (6), based on the ultimate strength of the material, is necessary in the design of hoisting sling assemblies. The safety factor provides a longer life. As a boat or craft mission may evolve from shore based to ship based during its lifecycle, it is recommended the safety factor of six (6), be applied for all boat and craft hoisting arrangements. At the time of initial acquisition, the cost and weight impact of requiring a safety factor of six is relatively low when compared to potential retrofit costs. NSTM 583, ref [1] governs these lifts and includes detailed requirements.

Shore lift or Ship lift at shore – loading or offloading of a boat onto a ship for tactical or logistical transport. The standard hoisting safety factor of five (5), based on the ultimate strength of the material, applies to these lifts. NSTM 583, ref [1], governs these lifts and includes some detailed requirements. These lifts may require a cradle or the craft trailer to set the craft down onto the shore or the deck. If that is the case, the cradle may be strapped to the boat. Recently, we have found that fleet personnel were loading boats and trailers onboard ship by lifting with the boat lifting system with the trailer strapped under the boat. In some cases, the craft lifting system was not designed for the weight and additional LCG shift this imparted. Additionally, the trailer tiedowns straps and fittings become trailer lifting gear in this scenario and should be subjected to a more rigorous safety factor, pull tests and certification as part of the hoisting system. It should be noted that a combined lift of boat and trailer will not be conducted underway at sea and is therefore subject to a safety factor of five, even if the boat only lift is required to meet a safety factor of six.

External Air Transport (EAT) under Helicopter – MIL-STD-209K, ref [2], and MIL-

STD-913A, ref [3], govern these lifts, and certification by US Army Natick command is required. It should be noted that strength and safety factor criteria from MIL STDs 913 and 209 differ from NSTM 583. Neither standard is fully inclusive of the other and if invoked, requirements from both must be examined to ensure compliance.

Growth Margin

A hoisting weight growth margin for potential service life increases of craft weight should be specified by US Navy personnel in the contract or delivery order. This is typically

10% on the Hoisting Condition (see below), however the contract may specify a differing value for weight critical craft or other special considerations. Note, NSCSES 60-196, ref [4], recommends a 5% service life margin for a boat in service. For hoisting fittings, the high cost of future rework and low weight impact and cost of providing added margin, indicates that a 10% growth margin should be used in the hoisting system design, in addition to any Pre-Planned

Product Improvements (P3I) and the margins appropriate to the stage of design. However, discrete rigging sizes, weight constraints or other considerations may necessitate lowering of the margin. If the 10% hoisting margin can not be achieved, consult the US Navy Technical Point of

Contact (TPOC).

Lifting Arrangement Constraints

Various constraints may be placed on a given lifting arrangement, such as lift point height and cradle clearance issues from a davit. Crane direction of travel may also limit the path of a lift over a gunnel. It is important to understand these offboard physical constraints and account for them in the design.

Maximum Lifting Point Height - For interface to the lifting equipment, the maximum allowed height for the intended load should be specified in the contract or delivery order.

Obviously, the longer the slings the lower the sling loads due to lower longitudinal and athwartship force vectors. If no detailed requirement exists, as in the case of a davit lift where the pick point is predefined, it is recommended the overall height of the attached sling apex above the lowest extremity of the equipment (typically running gear or keel of boat or wheels on the trailer) when suspended should not exceed 24 feet for boats up to ~ 50 feet in length and 32 feet for craft ~50 to 100 feet in length. [Note: The 24 foot recommended value is similar to that in

MIL-STD-209 and the 32 foot value was derived from a typical crane used on the Mark V

Special Operations Craft program.] Care should be given if there is a requirement to lift a boat off a trailer or perform a combined craft and cradle/trailer lift as the ground clearance of the boat on the cradle/trailer must be considered.

Within the Navy, the existence of NAVFAC P-307 "Management of Weight Handling

Equipment" ref [5] should be recognized. In general, P-307 should not be referenced for any boat hoisting requirements. P-307 contains requirements that are not compatible with boat hoisting operations and boat lifting slings and fittings often must meet requirements that are not addressed by P-307. Additionally, Navy Crane Center letter, ref [6], exempts slings procured in compliance with NSTM 583 from NAVFAC P-307.

Determine the Loads

Weight and CG estimating and margins

This subject is covered by a number of other guides, NAVSEA S9009-AB-GTP-010, ref

[7], and SAWE Recommended Practice Number 12, ref [8], however this discussion is limited to establishing a weight estimate for design of the hoisting system.

Design Hoisting Weight definition - The design hoisting weight of a boat or a craft generally includes the hull, machinery (wet), full fuel, full outfit such as boat hook and fire extinguishers, a 10% growth margin, and may include crew on shipboard craft. Specific crew weights and a Hoisting condition definition should be specified in the contract or delivery order and general guidance is supplied in ref [7]. The design hoisting weight may include personnel that must be in the boat for operational reasons such as in the case of a ready service rescue boat.

Outfit items required for operation that cannot be readily removed for hoisting are included in the design hoisting weight. Special cases, such as: 1) the need to load the craft and its trailer onboard a ship for overseas transit; 2) External air transport; or 3) cases where large payload loads/packages may not be onboard during lifts, will create additional hoisting scenarios than just a base boat lift (e.g. typically for davit or EAT lifts masts and antennas have to be lowered, this may impact LCG of vessel). This will lead to two or more design hoisting conditions that must be analyzed. If multiple conditions exist, they should be identified in the contract, however, design parameters (e.g. load placement during lift) within the builders discretion may also create conditions requiring discrete analysis.

DESIGN TIP- Rarely does the weight estimator in the office accurately predict the as-built weight, that’s why build margins exist. Make sure the design weight includes a build margin.

Often times hoisting weight estimates used in a calculation do not include any build margin. Hoisting weight estimates must include the margins appropriate to the status of the design. The 10% growth margin is intended only as in-service margin after the craft is fielded.

If the as-built hoisting weight exceeds the estimates used for design, then the hoisting system may not be contractually compliant at time of delivery. This is especially true for first of class vessels or repeat buys with weight changes in systems or structure that may increase hoisting weight over craft from a previous contract. Refer to ref [5] for a further treatment of margins.

Trailer weight should also include a 10% growth margin.

LCG When hoisting using a single point lift the LCG of the craft will align vertically under the lift point. Sling leg lengths must be determined accordingly to achieve the desired lifting attitude. This allows for the boat to be easily positioned on stowage arrangements such as a cradle, a trailer, or blocks.

The boat should be hoisted in a slight bow up attitude in all conditions. U.S. Navy ship boats are often recovered dynamically while the mother ship is underway. A bow down attitude while hoisting under such dynamic conditions is dangerous because the craft can become directionally unstable. With a bow up attitude, drag forces imposed at the stern of the craft will promote directional stability during the hoisting evolution. It is common practice to ensure a minimum 2-degree bow up attitude in hoisting arrangements for ships boats. For craft with anticipated variability in longitudinal center of gravity, this angle may be as high as 5-degrees with the worst-case aft LCG, but no less than 2 degrees with the worst-case forward LCG.

In some cases, lifting a boat at end of mission with no fuel vice full fuel or with trailer creates large LCG shifts. This LCG shift impacts attitude of the lift as well as the loads on the fittings.

Other ‘extremes’ of LCG that should be examined include: people vice no people for ship boat’s, trailer vice no trailer and with or without large payload packages.

DESIGN TIP – Be complete! A range of possible loadings will result in varying LCG’s and may change the loadings on the fittings even though the weights are lower. Also a difference in the estimated LCG from the as-built LCG could result in purchasing of unnecessary slings or costly re-work.

The 10% growth margin weight should typically be located at an LCG the same as the hoist weight estimate LCG. That is, the LCG of the margin should be the same as the combined weight LCG. However, as mentioned above, some designs may have a fairly large range of LCG that should be accounted for in design to ensure a fitting is not overloaded at one of the LCG extremes.

A sample of the impact an LCG shift can create is illustrated in Figure 1. In this case, weight was added near the aft fittings and the original sling system was used and resulted in an excessive bow up lift. This arrangement can overload components of a designed lift system, even if properly designed for the original craft weight and CG.

Figure 1. Example of Impact of LCG Shift

VCG - The vertical center of gravity (VCG) location of a boat is important for single point hoisting gear having only two sling legs. For a 2 point lift, to prevent overturning of the boat during a lift, ensure that the height of the hoisting fittings is far enough above the VCG. The margin should account for any foreseeable transverse shift of CG to one side (e.g. for ships boats, 2 crew moving to one side of the boat, or ~500 lbs of unsecured cargo shifting to the outboard side of the craft) to preclude any excessive heel angle (~5 degrees). If the VCG is above the hoisting fittings, the slings require tethers or side guides to both sides of the boat.

Hoisting slings with three or more sling legs are not as susceptible to boat overturning as two leg sling gear.

DESIGN TIP– The VCG must be estimated in order to calculate the extent of bow up attitude during a lift. As the craft rotates to place the CG under the lift point, the distance between the pick point and the VCG must be known to accurately calculate the trim. Often times designers mistakenly only examine the LCG along the craft baseline.

The 10% service life margin weight should typically be located at a VCG the same as the hoist weight estimate VCG. That is, the VCG of the margin should be the same as the combined weight VCG.

TCG - The location of the transverse center of gravity (TCG) of the boat is equally important. Maintain the boat parallel to its transverse waterline, allowing for greater stability and easier placement of the boat on a stowage cradle, trailer, or blocks.

Determine the arrangements

Once the requirements above are determined, the boat designer should examine craft structure and arrangements to determine the best hoisting system arrangement.

Fitting placement and support – Later on, there are sections concerning design of the hoisting fittings in more detail. For now, this discussion is about trying to locate fittings relative to the CG and the contributing major craft structure. This is one of the most important decisions in the hoisting system design. The two main factors that must be considered are 1) Support -ensuring the hoisting fittings are secured to major structure that can support the hoisting loads;

and 2) Placement -distance of the fittings relative to the CG.

Support - A hoisting fitting is a load transition interface member between one of the legs of a hoisting sling assembly and one (or more) structural members of the boat to be hoisted.

Major contributing structural members of a boat are longitudinal hull girders or stringers, bulkheads, transverse web frames, transom, or deck edge at sides and frames. Continuation of the load path established by the hoisting slings is critical to maintaining structural integrity and preventing failures. A direct line of force places sling legs, end fittings, and boat hoisting fittings in straight-line axial stress and precludes bending moments and shear loads. See Figures 2 through 4 below for an example of aligned fittings placed at major craft structure. Figure 5 illustrates a fitting that is not aligned to the sling path load. If the arrangement is not complicated, then the engineering to support it does not need to be complicated, although it does need to be thorough, and continue into the craft structure until the loads can be reasonably assumed to be transferred into main structure

DESIGN TIP – Be prepared for and take into account changes in craft structure. Below are two figures, Figures 6 and 7, illustrating how a design change can affect the hoisting fitting support structure. As seen in Figure 6, when the fittings were laid out solid structure surrounded an exhaust penetration, however in the as built picture, Figure 7, it can be seen that the structure around the exhaust will not be solid and the exhaust pipe will be laminated in as part of the structure, this change in design can invalidate the structural calculations performed during the initial arrangement.

If arrangements preclude placing fittings directly in way of major craft structure (e.g.

slings would have to pass through the pilothouse), then consideration can be given to adding structure in way of the fitting placement. ‘Sister’ or ‘half’ frames or longitudinal ‘headers’ that bridge between craft major structure can be added as a way to tie back into major structure.

Obviously this is more costly and weighs more than if the fitting were placed in way of major craft structure.

Figure 2. Padeye design shown aligned to the sling load and located over craft frame (note – location of lifeline stanchion in close proximity)

Figure 3. Load Aligned Fitting

Figure 4. Aligned Fittings in Profile

Figure 5. Misaligned Fitting

Figure 6. Designed Fitting Installation

Figure 7. As Built Fitting Installation

Placement - Other factors to consider when locating hoisting fittings are:

Select hoisting fitting arrangement to minimize interference with operations, maintenance requirements, and safety hazards to personnel such as tripping.

Properly located lifting provisions provide stability to the lifted item during a crane lifting. Lifting provisions should be located above the vertical center of gravity (VCG). If this is not possible, lifting provisions shall be located so that a line connecting adjacent lifting provisions is located outside a 120 cone having its apex at the CG and its axis of rotation about the vertical axis, see Figure 8, below. A reasonable goal is to spread out the lift points as far as possible to improve stability, consistent with the load and lifting height restrictions as applicable

Figure 8. Location of Lifting Provisions

DESIGN TIP - It is desired to keep the fittings equidistant from the LCG so that all sling legs are equal lengths. This simplifies use of the hoisting system and the parts logistics and makes the analysis easier! However, this is not always achievable due to structural or general arrangements.

A key objective is to not contact boat structure any where unless the structure is designed to contribute to the load and does not present a wear point. Not less than 1 inch of clearance should be maintained between any part of the equipment and the slings. If a sling must contact a part of the equipment, testing or computer aided engineering structural analysis must demonstrate that the affected part(s) has sufficient strength to withstand the force exerted by the sling leg to prevent permanent deformation of any part of the equipment, and that contact will not adversely affect the material of the sling.

Sling assembly arrangement - Most arrangements currently in use are a single point lift with either 3 or 4 leg slings attached to fittings on the craft. However several variations can exist and are illustrated in the accompanying design template worksheet for various arrangements:

Single point o 2, 3 or 4 legs

Dual point o 2 or 4 legs

Spreader bar with dual, triple or quad point lift with a number of sling leg variations.

Belly bands from a travel lift or spreader bar

Two-point hoisting arrangements are typically used with davits having two lifting points.

The davit may be a fixed or monorail overhead type, a pivoting arm, or a slewing type but always with two lifting points to the boat. Some ship’s boats are furnished with two hoisting fittings and with single lifting point slings. Section 9 of NSTM 583 Vol 2, ref [9], provides descriptions of the various boat davit types.

The most common hoisting arrangement is a single point lift with four sling legs. A typical arrangement is illustrated below in Figure 9. Once an arrangement is identified, the sling lengths should be determined to position the lift point slightly forward of the LCG. The LCG will naturally align itself under the pick point and this should cause the bow of the craft to come up to the desired attitude. The exact dimension of this offset varies for each craft but can be determined geometrically and is easily modeled.

When deciding which sling assembly arrangement to use, special consideration should be given to each arrangement type. Four legged slings have the potential, due to minor variations in sling lengths or other portions of the rigging assembly, to carry the entire hoisting load with either pair of diagonally opposing legs. Four legged slings also require more hardware, more sling testing and certification and therefore increase the weight and life cycle cost of the hoisting assembly. Three legged slings create potential interferences with arrangements and the bow hook or forward steadying line. The vessel will become unstable and potentially overturn when a variable or dynamic loading moves the transverse center of gravity outside of the triangle created by the three legs.

Figure 9. Single point lift with hoisting sling assembly definitions

Calculate sling loads (force diagram)

Once the 1) weight is estimated; 2) initial fitting placement is known and; 3) lift point is determined then the distance between the lift point(s) and the fittings will determine the sling lengths and the resultant forces. This can be easily calculated with trigonometry or vector math in a spreadsheet. An example of this calculation is included in Appendix C.

One of the common products of this calculation is a force diagram where the forces are related to the geometry of the lift. IT IS HIGHLY RECOMMENDED the force diagram accompany any calculations and be included on the hoist system drawing. A force diagram should be prepared for each hoisting load condition examined to evaluate the various sling loads.

Care must be taken in the development of the force diagram to avoid confusing the physical lengths of the hoisting sling legs with the lengths of the components of the force diagram. Figure

10 shows a sample force diagram.

Single lift point

Sling legs

Hoisting fittings on the boat

Hoisting fittings on the boat

End fittings

End fittings

End fittings

Figure 10. Force Diagram

Sling assembly hardware selection

A key consideration for the design of all rigging, slings, spreaders, and other hoisting hardware, is to take into account the need to be lifted, installed, and used by personnel within the range of 5th percentile female to 95th percentile male as defined in ASTM F1166, ref [10]. For lifts at sea, personnel on the boat usually attach the hoisting slings to boat hoisting fittings and the davit or crane hooks. In a seaway, hoisting slings or hooks can swing, cause safety hazards, and be difficult to attach to a boat. The abilities of personnel on the boat or ground should be considered as a design requirement.

Sling type, material / size

Typically each type of craft is assigned a unique sling set designed for the hoisting condition weight. Hoisting slings consist of flexible sling legs of chain, wire rope or fabric, end fittings, and a connection at the lift point(s). Hoisting slings may also include spreader bars to prevent interference of sling legs with boat structure such as pilot house, arch, or antennas. The boat hoisting fittings are attached to the boat by welding or bolting, and the sling leg lower end fittings are attached to the fittings using shackles or bolts. US Navy boat and craft slings are exempt from NAVFAC P-307, ref [5] per previously referenced Navy Crane Center waiver letter.

All slings should be rated to provide a safety factor on the material breaking strength of the sling material, with respect to the calculated sling load (which is the same as the safety factor for the hoist fittings). If the craft is intended to be hoisted at sea, the fittings will require a safety factor of six (6) on the ultimate strength of the material, with respect to the design load. In this case the sling legs will each require a material breaking strength with a safety factor of six (6) on the calculated sling load for that respective sling leg).

External Air Transport requires use of a specific adjustable sling set described in MIL-

STD- 913, ref [3]. Refer to that document if the requirements include EAT.

Materials and strengths of hoisting sling materials such as fabric and wire rope can be obtained from manufacturers. There are a wide variety of suitable materials: wire rope, polyester, Dacron, Kevlar, and others. When selecting materials, be sure that material fabrication and environment are considered. Slings are required to resist stretching, resist chemical attack and corrosion, be suitable for salt water, resistant to cuts and abrasion, and be capable of easy storage without kinks and damage to the fibers and fittings.

Certain sling materials will stretch a considerable amount under loading. This stretching should be accounted for in sling selection when an overall height for the hoisting sling assembly is specified. It is recommended to estimate sling stretch and initial loading prior to determining final length.

Consideration should be given to the adequacy of webbing and other synthetic sling materials. Very high strength materials may result in very small physical size to meet required load capacity leaving slings highly susceptible to physical damage. In other words, a small nick represents an unacceptable reduction in load carrying capacity. Stated another way, ensure a certain level of physical robustness in addition to adequate strength.

Sling protection (sleeves, chafe pads, coverings, wear pads in way of shackles and hooks, etc.) should be considered based on service, stowage and sling material

All slings must be marked for their proper use. Figures 11 and 12 illustrate sling labels.

Figure 11. Proper Sling Marking

Figure 12. Proper Sling Marking

Rings, links and shackle selection

Shackles – All terminating sling shackles shall be Type IVA, class 3 safety anchor shackles, in accordance with RR-C-271D, except for synthetic slings where the sling manufacturer requires a 'sling saver' style shackle. This requires, in part, that they be safety anchor shackles with a threaded pin, nut and cotter pin. The shackle manufacturer has to approve and stamp on the shackle.

Size, fabricate, and design shackles to the design load for the sling leg. The manufacturer working load limits (WLL), also referred to as safe working loads, typically apply a safety factor of 5, therefore lift requirements dictating a safety factor of 6, larger size shackles must be selected to achieve the proper safe working load.

For synthetic slings a sling saver style shackle is often required by the manufacturer although these shackles are not fully compliant with the RR-C-271D specification. The increased radius on the sling saver shackles provides a wider sling bearing surface. The bolt, nut and cotter pin configuration is required for these style shackles.

Note: If EAT is a requirement, it should be noted that MIL-STD-209 precludes use of shackles in lift fittings. This preclusion has typically been waived for combatant craft with EAT requirements but requires specific dialogue with Natick on each project by the

US Navy TPOC.

If the hoisting assembly will provide a single point ring, then the following requirements should be invoked: "The welded area of the ring shall be RT inspected IAW NAVSEA

Technical Publication T9074-AS-GIB-010'271 with acceptance criteria to MIL-STD-2035, ref

[11], class 3 for welds prior to initial use".

If the sling set is for general shore based use it is reasonable to assume the crane operator will have a type of single point attachment, such as a large shackle that will fit over the crane hook. For synthetic rounds slings or nylon web slings with a sewn eye, often times the sling ends can be placed directly around the crane hook.

Spreader bars

Spreader bars are sometimes employed when interferences occur between the sling legs and boat structure such as a pilot house. The spreader bars, make it possible to maintain a safe load path of the sling legs from the single lift point to the boat hoisting fittings. Spreader bars may also be required to lower the height of the single lift point above larger boats. If possible, avoid the need for spreader bars as these usually cannot be transported with the boat when it is in the water.

Straps for Trailer Lifting

Trailer tie-down straps are often used to secure the trailer under the boat for a combined hoist. Typically the safe working load of tie-down straps are rated at a safety factor of 3 to 1 on ultimate strength, so a 1000 lbs strap has an ultimate load of 3000 lbs. In a static shore based lift, a safety factor of 5 to 1 is required, so straps must be ‘overrated’ accordingly. For hoisting it would require an ultimate load of 5000 lbs for an expected safe working load of 1000 lbs. Use vector math to determine the actual loads carried by each trailer strap if it is being lifted under the boat. Take the resultant value and multiply by 5 and divide by 3 to determine the minimum standard strap rating that will work for a given application (e.g. - up rating from 3:1 to 5:1 safety factor, a typical 1000 lbs strap has an ultimate strength of 3000 lbs). Therefore, for an anticipated

1000 lbs load in order to achieve a safety factor of 5:1, the strap safe working load would need to be at least 1667 lbs (1000 lbs x 5 / 3). The arrangement shall provide that no single strap failure will result in the trailer being ‘dropped’ during the lift. Other factors pertaining to the arrangement, strap end connections, etc. will need to be addressed with the contract Technical

Point of Contact.

Fitting design – structural details

Load Path Alignment

As mentioned above, the hoisting fitting should be located to transition the load directly into the craft strength members without altering the load path (line of force). It is recommended to achieve this by lining up the attachment interface of the hoisting pad with the hoisting sling by bending it over and having the material of the pad in line with the load, and utilizing the existing boat structure to support the pad in the direction(s) in which it is most capable of carrying the load (e.g. don't use the deck to support vertical loads). Make the pad large enough and the connections (bolts or welds) numerous enough to spread the load into the major contributing structural members until the loads in the craft structure are well handled by the existing scantlings. The resulting design evenly transitions and spreads loads to the fittings, fasteners, welds, and boat structure. Designs that stray from the load path result in installations that are too bulky or heavy. Misaligned load paths invalidate assumptions of equally distributed loading of bolts, welds and other structural components and can result in progressive failure. Misalignment can also introduce off-axis loads that invalidate assumptions of tensile loading by introducing bending loads.

The preferable fabrication approach is to ensure the primary load path is routed through straight or bent metal vice welded metal and that bolts, if used, are not placed in tension. Metal to be used in hoisting fittings should be cold formed to align with the design load path. For bolted fittings, arrange the fitting geometry to route the load path through the centroid of the fitting and the fitting fasteners. For fittings attached directly to metallic structure, ensure that the load path passes through the centroid of the base weld. This approach aligns axial loads to the fitting and equally distributes the load to the fasteners. If the load is not equally distributed among the fasteners, bolting pattern analysis is required to calculate the load on each bolt. Direct tensile loading of bolts and welds should be avoided. The rationale for avoiding tensile loading of bolts is that a deformation in the bolt attachment when used in shear is preferable to a tensile failure. This is especially of concern in stainless steel bolts due to the potential for crevice corrosion below the bolt head. The best arrangement does not rely on any single bolt or weld joint as the determining factor in the ultimate strength.

Figure 13 shows a mockup hoisting fitting being loaded with proper alignment to the fitting. Note, in the mockup the fasteners are incorrectly loaded in tension for the convenience of the simple model. The fitting is mounted on a sheet of synthetic rubber to allow observation of the deflection shape. The hoisting fitting needs to be backed-up with connections to a major structural member of the boat to prevent the deflection.

Figure 13. Model of hoisting fitting with proper load alignment

Figure 14 illustrates improper load alignment. In the illustration, the load is not in the plane of the fitting. The load path does not pass through the centroid of the fastener pattern. The figure shows the unequal loading and deflection resulting from the force couple generated when the load path does not pass through the centroid of the fastener pattern.

Figure 14. Model showing deflection from unequal loading

Be aware of the dynamic nature of hoisting boats aboard ships at sea. Consideration must be given to loads that will be introduced into the system due to pendulation of the single point crane or davit lift hook that may result loads in directions other than the primary load direction.

Fabrication / Design Details

General - The hoisting fitting thickness required for the applied loads and required safety factor is usually less than the throat (jaw) opening of the connected socket or shackle. Therefore, it is necessary to add bosses or cheek plates to the sides of the fitting. The increased thickness in way of the hole prevents bending of the shackle bolt and eliminates hole elongation. Figure 16 depicts excessive clearance between the shackle and the hoisting fitting. If fitting strength permits a narrower thickness than the required shackle throat dimension then boss or cheek plates should be installed to achieve the desired clearance of no more than ¼ th the pin diameter

(e.g. a 1” pin would have a total ¼” clearance or 1/8” on either side). This helps prevent the introduction of out of plane bending into the fitting.

The padeye hole diameter should be no larger than 1/8 th the diameter of the shackle pin

(e.g. a ½” shackle would have a hole 1/16” larger than the intended shackle pin diameter and a 1 inch pin would be 1/8” larger.).

The recommended clearances are shown in Figure 15.

Figure 15. Allowable clearances between shackle and hoisting fitting

Gap between padeye hole edge and shackle pin should be no larger than

1/8th of the shackle pin diameter.

Total gap (sum of clearance on both sides) between shackle ears and padeye should be no larger than 1/4th of the shackle pin diameter.

Shackle pin

Figure 16. Excessive clearance between shackle and hoisting fitting

Access - Access is required to both sides of the bolted hoisted fitting for installation and inspection. If the arrangement does not allow access to both sides, retain the nuts by tack welding or other means to allow installation and removal from one side.

Nut type - Bolted connections shall utilize prevailing torque type self locking nuts. For applications where only one side of the fitting is accessible, standard nuts shall be tack welded to the backing plate and bolts with prevailing torque self locking inserts shall be used. No acorn nuts should be used. In all cases, 2-5 threads of the bolt should be exposed on the back side of the nut.

Surface finish - The material edges of all metallic components of a lift system shall be rounded, 1/8” radius to round cross-section, or chamfered, 1/16” x 1/16” to 3/8” x 3/8”, and smoothed to prevent cutting or damage to the sling or tiedown devices.

It is recommended a stainless steel press fit bushing should be installed in aluminum pads for bearing wear and improved bearing area.

Multipurpose fittings - If the hoist fitting is to be used as an integral tow or mooring point, any unique features or loads associated with the supplemental use must be specifically analyzed to ensure safe and suitable design and fabrication. Figure 17, is an example of a hoisting fitting also used as an integral mooring fitting.

Excessive

Clearance

Figure 17. Hoist/Mooring Fitting

Ancillary Structure – It is important that structure not integral to the fitting not be welded to the fitting unless specifically accounted for in the design. Examples include lifeline/rail stanchion bases and side plating overlaps above the deck. Failure in the welded connection would necessitate repair and recertification of the craft hoist fittings, even if the failed weld was not required to meet strength requirements.

Metal…