7_Attachment 28 - NSWCCD-80-TR-2014-38 Weight Management Process Guidance.pdf

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

NSWCCD-80-TR-2014/038 December 2014

Naval Architecture and Engineering Department Technical Report

WEIGHT MANAGEMENT PROCESS GUIDANCE FOR

BOATS AND CRAFT

by Combatant Craft Division (831)

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

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W ei gh t M an ag em en t P ro ce ss G ui da nc e fo r B oa ts a nd

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NSWCCD-80-TR-2014/038

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Weight Management Process Guidance for Boats and Craft

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NSWCCD Code 831

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NSWCCD-80-TR-2014/038

Naval Surface Warfare Center Carderock Division Detachment Norfolk 2600 Tarawa Ct Suite 303 Virginia Beach, VA 23459-3239

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Attn PEO SHIPS, PMS 325G Commander Naval Sea Systems Command 1333 Isaac Hull Avenue, SE Building 197 Washington Navy Yard, DC 20376

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PEO SHIPS, PMS 325G

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

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PAO approval received from NSWC Carderock, Congressional and Public Affairs Branch, Code 1031 on 1 December 2014.

14. ABSTRACT

This document was developed to guide designers in industry and the U.S. Navy in the effective application of weight management techniques for boats and craft.

15. SUBJECT TERMS

Weight Estimating, Weight Reporting, Weight Classification, Weight Tracking, Allowances, Margins, Boats and Craft

16. SECURITY CLASSIFICATION OF: 17.

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ABSTRACT

See 12.

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Willard Sokol

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

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Contents Page

Figures ................................................................................................................................................. vi Tables .................................................................................................................................................. vii Administrative Information ............................................................................................................ viii Acknowledgements ........................................................................................................................... viii Summary

Introduction

Background

Objective

Scope

Understanding the Design Requirements

Weight and Center of Gravity Limits

Load Conditions

Project Specific Weights

Project Specific Allowances and Margins

Identifying the Design/Construction Phase

Concept/Feasibility Design

Preliminary Design

Contract Design

Detailed Design

Construction

Developing the Weight Estimate

Establish Tools and Processes

Establish the Coordinate System

Definitions

General Coordinate Systems

Preferred Coordinate System for Craft

Establish a Work Breakdown Structure iv

Identify Appropriate Weight Estimating Methods

Databases

Unit Weights

Parent Craft

Parametric Methods

Ratiocination or Scaling

Class Society Rules and Design Standards

Top Down versus Bottom Up Method

Synthesis Programs

Identify and Apply Appropriate Margins and Allowances

Margins versus Allowances

Weight and Vertical Center of Gravity (KG) Margins

Longitudinal Center of Gravity (LCG) Margin

Service Life and Other Allowances

Use Margin Best Practices

Appropriate Margins

Avoid Excessive Margins

Apply Margins at the Appropriate Level

Avoid Compounding of Margins

Separate Margins and Allowances

Clearly Communicate Margins Used

Identify the Basis of the Estimate

Tracking the Weight

Weight Budgeting

Margin Tracking

Margin Consumption

Design Requirements Tracking

Early Performance Indicators

Quality Control

Weight Reporting

Weight Control Plan

Allocated Baseline Weight Estimate (ABWE)

Accepted Weight Estimate (AWE) v

Periodic Weight Reports

Inclining Experiment Report

Scale Weighing Report

Final Weight Report

Problem Areas and Corrective Actions

Symbols, Abbreviations, and Acronyms

References

Background Resources

Appendix A: Weight Management Process Summary................................................................ A-1

Appendix B: Standard Allowances for Common Items and Typical Materials ...................... B-1

Appendix C: Work Breakdown Structure for Boats and Craft ................................................ C-1

Appendix D: Sample Weight Tracking Tool ................................................................................ D-1

Appendix E: Why Use Margins? ................................................................................................... E-1

Appendix F: Margins other than Weight or KG ......................................................................... F-1

Distribution vi

Figures

Figure 1. Preferred Coordinate System for Craft

Figure 2. Illustration of Group Weights with Error Bands

Figure 3. Wrong Approaches to Cumulative Errors

Figure 4. Typical Tracking Tool Metrics

Figure 5. Weight Budgets vs Margins

Figure 6. Typical Line Items at the Element Level

Figure 7. Typical Maturity Metrics

Figure 8. Planing Hull Dimensional Definitions

Figure 9. Typical Flow of Weight Reporting - Contractor Developed Design

Figure 10. Typical Flow of Weight Reporting - Government Developed Contract Design vii

Tables

Table 1. Typical Load Conditions

Table 2. Weight Estimating Method Applicability

Table 3. Recommended Weight Margins

Table 4. Recommended Service Life Allowances

Table 5. Planing Hull Design Parameters of Interest

Table 6. Recommended Limit on Variation in Weight of Subassemblies viii

Administrative Information This work was performed by the Combatant Craft Division (Code 831) of the Naval

Architecture and Engineering Department at the Naval Surface Warfare Center, Carderock Division (NSWCCD). This effort was sponsored by the US Navy Small Boats Program Office, PMS 325G, in support of all boat and craft acquisitions.

This document (NSWCCD-80-TR-2014/038 November 2014) replaces the following documents:

• Margin Management and Procedures For U.S. Navy Small Craft, NAVSEACOMBATSYSENGSTA Report No. 60-196 May 1988

• NAVSEA S9009-AB-GTP-010, 1978, Weight Classification for U.S. Navy Small Craft.

Acknowledgements The author wishes to thank Mr. Chris Rozicer, U.S. Navy Program Executive Office (PEO)

Ships, PMS325G, for sponsoring the original development of this guide. The experience and knowledge of the following individuals contributed greatly to this guide: Will Sokol, Jason Marshall, Allan Demmelmaier, Michael Russell, John Almeter, Nathan Leightner and Rich Wilson. Ms. Elena Murphy provided editorial and formatting assistance. The collective support of all these individuals is sincerely appreciated.

Although weight management and margins are not new subjects, recent challenges related to operational needs, shipboard integration, and contractual compliance sparked an initiative to consolidate and update existing guidance for boats and craft. Mr. Robert Kaidy's presentation on weight estimating at the International Boatbuilders’ Exhibition and Conference (IBEX) 2012 and Mr. Ben Fisher's diligent work and consideration of weights, margins, and allowances on the 78 foot Patrol Boat (PB) MK VI provided industry perspectives that have been considered in the development of this guide along with many other lessons learned since the publication of previous guidance documents.

Summary This document provides guidance for the effective application of weight management techniques for boats and craft of the U.S. Navy. The information is intended to provide naval architects, engineers, designers and boatbuilders a useful, consistent, and applicable set of guidelines for estimating, tracking, and reporting weights and centers of gravity and appropriately using margins and allowances. It should not preclude the use of other good engineering practices or established processes that also support delivery of a craft that is fit for the intended purpose. It presents a collection of available information to be updated as new lessons are learned and as new techniques are developed. The following is a top level outline of this guide:

1. Understanding the Design Requirements

2. Identifying the Current Design/Construction Phase

3. Developing the Weight Estimate

4. Tracking the Weight

5. Weight Reporting

6. Problem Areas and Corrective Actions

A complete outline of the weight management process is included as Appendix A.

Introduction

Background Accurately predicting and controlling the weight of a new craft is the most effective means of ensuring that required performance parameters are met. Discovery of unplanned and uncontrolled weight growth during the latter stages of design or construction requires costly remedial actions and often results in undesirable compromises in needed mission capabilities.

Conversely, overestimating weight can lead to craft that are larger and more expensive than necessary or the cancellation of planned procurements that are erroneously thought to be infeasible.

Objective This document provides guidance to assist boat builders, designers and engineers in weight management during design and construction of U.S. Navy boats and craft. Weight management is a critical contributor to the ultimate objective of successful delivery of a craft that is fit for its intended purpose.

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

TIP: Weight is important!

For Proposals - inaccurate and poorly documented weight data, or weight considerations that aren’t fully integrated in all aspects of the design (performance, stability, hoisting, trailering, etc…) can keep a proposal from being found technically acceptable.

During detailed design and construction – ineffective weight management is a leading cause of not meeting requirements on a first-of-class vessel.

Uncontrolled weight growth can have a cascading effect on performance, stability, trailer sizing and other transportability and hoisting provisions.

Recovering from these secondary effects can be very expensive.

Scope This guide is applicable to all U.S. Navy boats and craft and presents a disciplined and integrated technical approach to weight management for design and construction. It conveys typical government preferences and expectations as a tool for designers and builders offering boats and craft to the Navy. The information contained herein is not contractually binding and does not supersede contract requirements; however, these are considered Navy best practices.

Builders departing from these best practices should do so knowingly and with careful consideration as to what unique circumstances justify such departures.

Accurately managing weight is one of the most important contributors to successfully completing a new boat or craft that meets all contractual requirements. The weight management process assists the designer and builder in assessing progress and monitoring processes. At completion of the boat or craft, it provides some of the necessary evidence that required capabilities have been met.

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.

Effective weight management requires the technical skills and prudent judgment of an experienced engineer, and no guide can capture all the factors that must be considered.

Understanding the Design Requirements The weight management process begins with a knowledge of the design requirements that comes from a careful review of requirements documents. There are often specific requirements related to weight which will have to be identified, estimated, tracked, reported, and verified as part of the weight management process.

Weight and Center of Gravity Limits Requirements documents may explicitly set weight and Center of Gravity (CG) limits or they may indirectly set overall limits by specifying that the craft be capable of being lifted by certain equipment or transported by certain modes of transportation. Weight and CG limits may also be inferred by the designer in the early stages of design as critical to meeting explicit performance requirements. Weight and CG limits can be based on one or more of the following:

• Hoisting – Working Load Limits of shipboard davits or other weight handling equipment may place a limit on the weight of the craft in the hoisting condition.

See Reference [1] for hoisting guidance.

• Transportability – Various transport modes each have their own limits on total shipping weight and weight distribution. See Reference [2] for transportability guidance. If it is not clear what specific transportability standards apply, contact your government Technical Point of Contact (TPOC) for clarification.

• Powering – Overall weight and CG is a significant factor in meeting speed requirements and selecting a propulsion system appropriate for the craft’s operating profile.

• Intact and Damaged Stability – Once the hull form parameters are determined, weight and CG will become important factors in determining compliance with stability standards.

• Dynamic Stability – Weight and CG information can provide early indications of dynamic stability concerns. For planing craft, basic hull dimensional information may dictate limits on weight and CG. Weight and CG limits may also be available for other hull types based on dimensional and speed parameters. These should be investigated early in the design to ensure the final design is fit for its intended purpose.

• Other Interface Requirements – Boats and craft are often subject to constraints based on requirements to interface with unusual or unique systems. Look for key phrases such as ‘interface with’, ‘operate with, from, in, or on’, and ‘compatible with’. Understand any interface requirements as early in the design process as possible.

Load Conditions Requirements documents may refer to specific load conditions. Load conditions can be defined to identify any condition of interest and are typically used to represent operating conditions, transportation and handling situations, test or demonstration requirements, limiting stability conditions, or simply to provide a common design baseline. The weight management process must identify, estimate, track, and report on the status of the craft relative to any contractually defined load conditions. Other load conditions of interest may be included at the discretion of the designers. In all cases the conditions must be clearly defined to be useful. The definitions shown in Table 1 are typical conditions used in boat and craft design for the Navy and should be adhered to whenever possible for maximum consistency. However, these definitions can and should be modified to suit any specific craft features or requirements so the definition is complete and unambiguous.

TIP: Contact your government TPOC for clarification if there is any question about what is included in a contractually required load condition.

Other conditions which are commonly used, but whose definitions are very specific to a given craft include transport conditions (see Reference [2]), beaching conditions, and ballasted conditions. Full Load Condition is sometimes split into Full Load and Full Load - End of Service Life. In this case, the Service Life Allowance is included in the second condition, but not the first.

Table 1. Typical Load Conditions

Load Condition Definition Purpose/Remarks

Light Ship Craft complete in all respects and ready for service with liquids in machinery at operating levels but no personnel, fuel or other variable loads.

Provides a clearly defined design baseline. Forms the basis of other load conditions. Can be useful for weight validation at completion of construction. Not an operating condition.

Full Load Light Ship plus full fuel, crew, passengers, any items of variable load (full fresh water, payload or cargo, mission equipment, etc.), and Service Life Allowance (if applicable)

Often defines the condition at which other requirements must be met. Should include hoisting slings for boats that are frequently lifted as part of their normal operation. If applicable, levels for other tankage should be specified.

An additional “Full Load, no cargo” condition may be needed for trim and stability analysis of craft for which cargo is a substantial fraction of Full Load condition.

Hoisting Condition

Light Ship plus full fuel, crew, hoisting slings, and other items as specified.

Basis for determining compliance with hoisting weight and CG limits. Basis for hoisting sling and boat fitting design.

Number of crew should be specified, as in the case of a ready service rescue boat crew, which may be more than the minimum crew necessary for operation. Other weights to be included should be specified. Typical items include mission equipment not readily off loaded before hoisting, a rescued person on a stokes litter, etc. An additional allowance is often specified to allow for weight growth after delivery. See detailed hoisting guidance provided in Reference [1].

Minimum Operating (MinOp) Condition

Light Ship plus crew; fuel and all other tankage at low suction level, all stores and other consumables fully depleted.

Used to define lightest operating condition for craft susceptible to stability limitations when fuel, stores, and ammo are at their lowest possible levels. Details of loading can be modified to represent the worst operating case for stability.

Arrival Condition

Light Ship plus crew, passengers and payload or cargo. Fuel and all other tankage at low suction level, all stores and other consumables fully depleted.

Waste tanks (gray water, black water, oily water, etc.) full.

Used to define end of mission operating condition when fuel and stores are at their lowest possible levels and the weight of passengers, payload or cargo has a significant effect on stability. Details of loading can be modified to represent the worst case for stability.

Trial Condition Option 1: Same as Full Load.

Option 2: Same as Full Load minus one half of full fuel load.

Option 3: Light Ship plus specified items and specified quantities of fuel and other variable loads.

Used to define the load condition of the craft to be established for trials and operational demonstrations.

This should match the design requirements relative to the conditions under which performance parameters must be met. Option 1 is the simplest case where all requirements must be met at full load. Option 2 is often used for patrol craft with both high speed and long range requirements.

Option 3 is to be tailored to the specific design requirements. More than one Trial Condition can be specified if necessary to cover all design requirements.

Project Specific Weights If a craft has unique mission requirements or is being developed for a specific set of user requirements, there are often specific mission related details that have weight and CG implications. Project specific weights may include the following:

• Mission Equipment – Specific equipment to be carried or incorporated into the craft may be itemized with weight and CG information. Identify the loading conditions that include this equipment and ensure it is accounted for accurately in the weight estimate.

• User Specific Personnel Weight – The intended user of the craft may specify their own standard allowance for the weight of personnel with their required personal or mission equipment.

The possibilities for project specific weights are nearly limitless. While reviewing the design requirements, make note of these items and ensure they are incorporated in the weight estimate appropriately. This guide provides default values for many of these weights if they are not defined by higher level requirements (Appendix B).

TIP: If there is any doubt what weight to use for mission equipment or personnel, contact the government TPOC for clarification.

Project Specific Allowances and Margins The design requirements may also include specific allowances and margins that are to be incorporated into the design process through the weight estimate. These may be driven by the desire to avoid problem areas based on past experience, or may be specific to the intended user of the craft. Typical project specific allowances include:

• Hoisting Weight Allowance – An allowance added to the hoisting weight to account for weight growth after delivery.

• Trailer Weight Allowance - An allowance added to the weight of the craft on the trailer to account for weight growth after delivery.

• User Specific Fuel Allowances - The intended user of the craft may have their own allowance for reserve fuel for the purpose of range calculations and tank sizing based on their standard operating procedures.

• Other User Specific Allowances – The intended user of the craft may have their own benchmark for other allowances such as freshwater usage or general stores consumption that must be incorporated in the weight estimate.

Identifying the Design/Construction Phase The weight management process is closely tied to the phase of design/construction. As the design moves from the concept phase through detailed design and construction the extent to which the details of the design are known or can be determined changes significantly. The weight estimating techniques and margins used must take this into account. Distinct lines between design phases are not always apparent. Phases may blend together or overlap, but thinking of the design process in phases provides a useful framework. The concept of design phases becomes more concrete if contractual requirements call for specific determination of end dates for defined phases and associated formal reviews.

Concept/Feasibility Design This is the least detailed level of design. It describes the general configuration of the craft with enough definition to illustrate how it will function to meet the design requirements.

Hull type and major dimensions are identified as a basis for feasibility level estimates. Design estimates indicate rough sizing of major systems. Risk areas (the most significant challenges and most difficult requirements, and how they may affect the design) are identified for further development. This level of design allows assessment of the likelihood that the design requirements can be achieved.

Preliminary Design This is an intermediate level of design that is sufficiently developed to allow significant design characteristics such as powering, stability, structure, range, and payload to be quantified with a moderate level of analytical rigor. This level of design should be sufficient to identify conflicting requirements and trade-offs necessary to achieve a converged design. Typically, this would include preliminary identification of commercially available major components such as main propulsion engines, marine gear, and propulsor. This level of design should provide confirmation that all design requirements can be met.

Contract Design This is a more advanced stage of design which determines the configuration of major structure and systems with a high degree of certainty, and conveys construction requirements to a potential builder in the form of drawings and specifications. Auxiliary systems, their capacities, relative locations, controlling dimensions, interface requirements, and technical specifications are defined. Significant design characteristics can be determined with a high degree of analytical rigor providing high confidence that all design requirements will be met. This phase of design is typically associated with award of a contract to complete Detailed Design and construct the craft.

If completed by the Government, it becomes part of the technical requirements of the contract. If completed by the boatbuilder, it becomes the contractual representation of the craft he intends to provide to meet the stated requirements. The technical data package (TDP) generated during this phase is commensurate with a Developmental Level TDP as defined in MIL-STD-31000A, Reference [13].

Detailed Design This is the most advanced stage of design where the details necessary for production are added to the contract-level design. All components and materials are identified with sufficient detail to support production planning activities such as ordering parts and materials and developing work packages for the production shop. The technical data package generated during this phase is commensurate with a Production Level TDP as defined in MIL-STD-31000A, Reference [13].

Construction In this phase, actual production proceeds in accordance with detailed design documents.

Small adjustments may occur to correct for dimensional errors, incomplete design data, or changes in component availability. Production processes may affect actual versus ‘as designed’ characteristics. Quality Control processes should confirm adherence to design specifications.

Developing the Weight Estimate Weight estimating is essentially an accounting process that is sufficiently complex to require planning, organization, and documentation to ensure consistency, thoroughness, and traceability. The process continues throughout the design and construction of the craft. The weight estimate is continually refined as the level of design definition increases and more is known about specific components and materials.

Establish Tools and Processes A common computer spreadsheet application is more than adequate for weight estimating and tracking. Organizing the inputs and relationships between sections of the spreadsheet are the keys to an effective tool. This tool needs to have the capacity and flexibility to accommodate the entire design and construction process without changes to its basic structure and organization.

The best spreadsheets minimize the number of inputs and the number of manual changes required when updating those inputs. All inputs need to be tracked by revision and many organizations include the name or initials of the person making changes so they can be consulted, if questions arise.

Always document the construction and function of any complex spreadsheet. Basic documentation allows a new user to understand how to update inputs and how the various outputs are linked for automatic update. On a larger project with multiple users making changes to the spreadsheet, the documentation should include business rules regarding who can make what changes and how the changes will be documented. Version controls should be used to clearly indicate who will decide when a version change is required and how to ensure the right version is in use.

Early in the design process the weight engineer will use this tool to collect and compile weight information he generates or which is generated by others. At first the inputs will be rough estimates, possibly broken down only in large high level groupings, but the tool must be flexible so these large groupings can be broken down to a finer level as the design progresses.

While organizing the totals and summaries, keep in mind the items that need to be seen individually, as well as in their cumulative effect. An example of a workable tool is provided in Appendix D.

Establish the Coordinate System Coordinate systems, reference planes, and associated sign conventions are used to ensure that numerical data related to weights and centers of gravity can be clearly understood and used by others throughout the design, construction, and life cycle of the craft.

Definitions The following terms are used in this guide:

Molded Line – a line generated from the hull lines either directly or by a process of full scale lofting. Molded lines have no thickness. Physical parts of a craft located by a molded line must specify where the molded line lies relative to thickness or other geometry of the part. For example, is the molded line on the inside or outside of shell plating, or is a bulkhead on the forward or aft side of the molded line.

Origin – starting point of the x, y, z coordinate system, often written (0,0,0) indicating it is the zero point for all three axes.

Design Waterline (DWL) – the waterline at which the craft is expected to float when unconstrained and at rest under the as-designed condition of loading, usually Full Load.

Baseline (BL) – typically parallel to the keel line for planing craft, but may be parallel to the design waterline or a deck particularly for craft with keel drag, rocker, or other underwater hull features.

Base Plane - the base plane is the horizontal plane which includes the baseline and is used as the reference plane for vertical dimensions.

Centerline (CL) –longitudinal or vertical reference line typically defining the axis of symmetry of the hull in views that lie on a horizontal plane or transverse vertical plane.

Centerline Plane - the centerline plane is a vertical plane used as a reference for transverse dimensions; all hull centerlines lie on the centerline plane.

Forward Perpendicular (FP) – vertical reference line typically located at the intersection of the design waterline and bow of the craft.

After Perpendicular (AP) – vertical reference line at the after end of the craft. The AP is often most conveniently located at an easily identified physical feature near the aftermost part of the craft. The transom-keel intersection is desirable, especially for planing hulls. Using the intersection of the design waterline and the hull as the AP is also acceptable for non-planing craft.

Mid Perpendicular, Midships, - the mid perpendicular is a vertical reference line mid-way between the FP and AP. Midships is defined by a vertical reference plane perpendicular to the centerline located at the mid perpendicular and is often designated by the symbol

Vertical Lever Arm – the perpendicular distance from a horizontal plane through the baseline of the craft to the center of gravity of an item.

Longitudinal Lever Arm – the perpendicular distance from a transverse plane through the craft's longitudinal reference to the center of gravity of an item. Typical longitudinal references are the transom-keel intersection or midships.

Transverse Lever Arm - the perpendicular distance from the vertical centerline plane of the craft to the center of gravity of an item.

Moment - the product of a weight and its lever arm. For example, the longitudinal moment of an item is the product of the weight of the item multiplied by its longitudinal lever arm.

General Coordinate Systems Ideally the same coordinate system is used for all design and construction purposes to simplify engineering efforts and minimize the opportunities for misinterpretation. Clear definition of the origin, coordinate orientation, sign convention, and location relative to the physical craft is essential. As a minimum, the defined coordinate system and sign convention should be clearly identified, preferably through graphic as well as textual means, in each weight estimating and reporting document. In addition, drawings depicting the lines and offsets, inboard and outboard profile, general arrangement, and docking arrangement of the craft should clearly indicate the location of structural frames and bulkheads, with controlling dimensions and identifying numbers, relative to the defined coordinate system and stations used for the hull lines.

Large craft having multiple deck levels will typically involve a larger design team and the need to document basic information about the craft for coordination between team members.

Development of a document early in design that defines the coordinate system, reference lines and planes, and sign conventions being used to locate weight items will improve the efficiency and accuracy of the process. The document should also contain tables of longitudinal lever arms (distances from the reference plane), and heights above baseline for decks, platforms, and flats.

These are usually organized by frame number, bulkheads or other readily identifiable physical features. It’s also useful to include clarifying diagrams based on the lines drawing for ready reference.

TIP: To avoid simple errors, ensure all persons involved in the design effort have ready access to a description of the coordinate system, reference planes, sign conventions, lever arms, deck heights, and frame locations.

Origins and sign conventions for the purposes of weight estimating and tracking should not be confused with naval architectural conventions for station and frame numbering. The preferred convention for station numbering is to start with zero at the forward perpendicular and proceed with ascending positive numbers going aft. The preferred convention for frame numbering is to designate the most forward frame as Frame 1 and proceed with ascending frame numbers going aft. Frame locations are typically dimensioned to the molded line. The convention used for locating frames relative to the molded line must also be clearly defined.

Preferred Coordinate System for Craft The preferred coordinate system for craft is a right-handed orthogonal system which has its origin point, (0,0,0), on the craft centerline at the transom-keel intersection as shown in Figure

1. For hull forms where the keel and the baseline are not coincident the origin may be located at the intersection of the baseline and a vertical extension from the transom-bottom intersection at centerline. In this system the X-axis lies along the baseline and forward dimensions are positive, the Y-axis lies on the base plane and positive dimensions are to port. The Z-axis is vertical and positive dimensions are up. See Appendix D, page D-3, for an example.

Figure 1. Preferred Coordinate System for Craft

Establish a Work Breakdown Structure A Work Breakdown Structure (WBS) is an organized system of cataloging that allows all participants in the process to know where to find the specific information they need and where their contributions belong so that others can make use of them. It also serves as a checklist that promotes completeness by taking a large set of data and categorizing it into smaller groups of related items. Weight classification is the process of logically assigning weight items to a designation within a WBS. See Appendix D, pages D-7 and D-8, for examples.

TIP: Use the Work Breakdown Structure as a checklist to ensure the weight estimate is complete.

The specific scheme of the WBS is less important than the fact that one is established and provides complete coverage for all weight items on the craft. The level to which the WBS is broken down should be consistent with the phase of design and the weight estimating methods being used. This will change as the design progresses, so the WBS has to have the flexibility to expand as the level of design definition increases.

The recommended WBS for Navy craft is provided in Appendix C. It is a hierarchical structure on three levels of increasing detail; Groups, Subgroups, and Elements. Weight items are grouped under the appropriate Element. Related Elements are combined under Subgroups, and related Subgroups are combined into Groups. Navy ship designs are typically based on the Expanded Ship Work Breakdown Structure (ESWBS), Reference [3]. The WBS listing provided as Appendix C is patterned after, and is completely compatible with the ESWBS. For boats and craft, this listing can be substituted for references to ESWBS when interpreting requirements for contract level work breakdown structures in MIL-STD-881, Reference [4]. In adapting this structure for craft and boats, only those parts of the ESWBS pertaining to weights were retained.

Those pertaining to ship acquisition, maintenance, etc., have not been included. Within the parts retained those elements which pertain only to ships or submarines have also not been included.

The WBS must be sufficiently detailed for compatibility with the stage of design development, but excessive levels of detail in the WBS can hinder efforts to develop an accurate estimate.

These adaptations for use in craft design strike the right balance.

The total weight of the craft is made up of the seven major weight groups plus load items.

The major groups are sometimes called “Groups 1 through 7”, but more correctly they are “Groups 100 through 700.” The expressions are equivalent. These groups are:

Group 100 Hull Structure Group 200 Propulsion Plant Group 300 Electric Plant Group 400 Command and Surveillance Group 500 Auxiliary Systems Group 600 Outfit and Furnishings Group 700 Armament

The “Groups” are broken down into “Subgroups” each bearing a three digit number ending in a single zero, and these in turn are broken down into “Elements” each bearing a three digit number. Elements typically contain a number of individual weight items. An example of this structure is as follows:

(Group) 300 Electric Plant, General (Subgroup) 310 Electric Power Generation (Element) 312 Emergency Generators

The complete listing presented in Appendix C is too lengthy to comprehend at a glance.

The following list of Groups and Subgroups is provided to give the user a convenient overview:

100 HULL STRUCTURE

110 Shell and Supporting Structure 120 Hull Structural Bulkheads 130 Hull Decks 140 Hull Platforms and Flats 150 Deck House Structure 160 Special Structures 170 Masts, Kingposts, and Service Platforms 180 Foundations 190 Special Purpose Systems

200 PROPULSION PLANT

230 Propulsion Units 240 Transmission and Propulsor Systems

250 Propulsion Support Systems (except fuel and lube oil) 260 Fuel and Lube Oil 290 Special Purpose Systems

300 ELECTRIC PLANT

310 Electric Power Generation 320 Power Distribution Systems 330 Lighting System 340 Power Generation Support Systems 350 Grounding and Bonding 390 Special Purpose Systems

400 COMMAND AND SURVEILLANCE

410 Command and Control Systems 420 Navigation Systems 430 Interior Communications 440 Exterior Communications 450 Surveillance Systems (Surface) 460 Surveillance Systems (Underwater) 470 Countermeasures 480 Fire Control Systems 490 Special Purpose Systems

500 AUXILIARY SYSTEMS

510 Climate Control 520 Seawater Systems 530 Fresh Water Systems 540 Fuels and Lubricants, Handling and Storage 550 Air, Gas, and Miscellaneous Fluid Systems 560 Ship Control Systems 570 Underway Replenishment Systems 580 Mechanical Handling Systems 590 Special Purpose Systems

600 OUTFIT AND FURNISHINGS

610 Ship Fittings 620 Hull Compartmentation 630 Preservatives and Coverings 640 Living Spaces 650 Service Spaces 660 Working Spaces 670 Stowage Spaces 690 Special Purpose Systems

700 ARMAMENT

710 Guns and Ammunition 720 Missiles and Rockets 730 Mines 740 Depth Charges 750 Torpedoes

760 Small Arms and Pyrotechnics 770 Cargo Munitions 790 Special Purpose Systems

Except for concept and preliminary design estimates, weights are seldom estimated at the Group or Subgroup level. They are more typically estimated as a collection of weight items organized by WBS Element. For example, Group 100 is the sum of Subgroups 110 through 190.

Subgroup 110 is the sum of Elements 111 through 119. The three digit Elements themselves are made up of many components, or even several complete systems.

The abbreviation (WS) after an item in the listing in Appendix C means “with system”; that is, the weight of that item is included with the system it serves and does not have an Element number of its own.

Identify Appropriate Weight Estimating Methods Methods for estimating craft weights have been developed to address the needs of the designer or project engineer at various stages of design and construction. Methods used for early stage design must provide reasonably accurate results at the total craft level without detailed knowledge of systems and components. However, as the design develops, a more refined breakdown of weights is needed even while system details are still being developed. Successful weight control ultimately requires a high degree of detail in the weight estimate in order to assess the cumulative effect of many small changes. A detailed weight estimate, combined with a means of evaluating the maturity of the estimate, also allows margins to be successfully managed as the design develops.

Some of the broad categories of weight estimating methods are described here. More information is available in the Society of Allied Weight Engineers (SAWE) Recommended Practices Number 14 (RP-14), Reference [5]. It is common to combine more than one method of weight estimating during design development. Which method will be most useful depends on what information is available during the design development phases. It is also common to use one method to check the validity of the results of another method. A less detailed estimating approach is sometimes useful to check the reasonableness of a more detailed estimate; however, it is best to use the most detailed approach that can be applied using the information available during each phase of design. Table 2 provides a summary of the applicability of the various estimating methods.

Databases Several typical weight estimating methods below start with a database of similar craft or systems. The applicability of the data to the current project should always be carefully considered. Depending on the estimate being developed, the inclusion of widely varying craft types, mission profiles, or types of construction may lead to inaccurate results. Whenever possible, the validity of estimates based on a database should be checked by other methods.

Where separate validation is not possible larger margins are indicated.

Unit Weights The concept of an estimate based on unit weights is simply that of constructing a list of units that make up the whole item and multiplying each by the number of units and its unit weight. Summing the products gives the total weight of the item. The specific breakdown of items and units can be devised to suit the weight estimating needs and will vary depending on the system involved. In the area of habitability, the units may be items of furniture or square feet of floor covering. For mechanical systems, the units may be discreet components such as motors or pumps, but also may be linear units for piping or duct work.

Estimating based on unit weights can be done during all phases of design, but is especially appropriate for contract and detailed design. This approach may require more detailed design information than is available for most systems during feasibility and preliminary design phases.

Parent Craft Where a parent craft exists and the differences between it and a new craft are limited, it may be useful to estimate the weight of the new craft based on differences between the new craft and the parent craft baseline. Baseline data may be available from a final weight report from a previous construction contract, an inclining report, or a design history for the baseline craft.

Weight items can be added and subtracted from the baseline to represent the configuration of the new craft.

Secondary effects of changes in major components must not be overlooked. A larger propulsion engine may also require upgraded auxiliary systems that will also involve weight changes. If range requirements are unchanged, fuel consumption differences may require a larger or smaller fuel capacity. Speed increases can lead to a requirement for heavier scantlings, heavier foundations, or a strengthened transom.

If the list of changes from the parent craft is complex, containing potentially offsetting factors, the magnitudes of which are not immediately evident, it may be necessary to use a method that is capable of accounting for these differences in a more integrated manner.

Predicting the Impact of Design and Requirement Changes by Almeter, Reference [6], presents a method based on the Normand Number concept which accounts for the interactions between various aspects of the design and their impact on overall weight using minimal early stage design data.

Estimating weights based on a parent craft is most appropriate for the feasibility design phase. It can also be applied on a system level for preliminary design work. Parent craft methods do not provide the level of detail needed during contract and detail design.

Parametric Methods Parametric methods are used to estimate weights based on certain craft characteristics, or parameters. These include statistical methods using regression analyses which are based on a database of existing craft. There are a wide variety of parametric methods in the craft technical literature. Parametric estimates of total craft weight or major weight groups as opposed to

TIP: When adapting a proven parent craft designed for commercial use to fulfill military missions it is a common mistake to under estimate the impacts of the military requirements and the rigors of service for items typically assumed to see only nominal loading (weather decks, hull sides, etc.). Pay careful attention to the specified capabilities and load condition definitions and fully account for all the changes.

individual elements are more common for this approach. Methods are typically tailored to a set of parameters suited to the stage of design. Early stage designs are characterized by a lack of precise values for many craft parameters, so the goal is to achieve accuracy with as few inputs as possible. Even so, parametric estimates can be quite accurate. Accuracy is improved by using databases that are as similar as possible to the new craft and by staying within the limits of the range of parameters the database contains. Parametric methods should include an honest technical assessment of their accuracy and a description of efforts made to validate the results.

Estimating Weight and CG in the Early Design Phase, by Aasen, and Reliability of Weight Prediction in the Small Craft Concept Design, by Grubisic, References [7] and [8], discuss typical approaches of this type.

Parametric methods of weight estimating are well suited to feasibility design and in some cases can be applied on a system level during preliminary design. Parametric methods do not provide the level of detail needed during contract and detail design.

Ratiocination or Scaling Ratiocination is an estimating approach which assumes craft weights vary in a manner similar to other naval architectural characteristics; and that weights can be scaled from one craft to another based on basic hull parameters such as length, beam, and draft or combinations thereof. Ratiocination is particularly useful for early stage design and synthesis models and can be easily incorporated in spreadsheets for estimates based on basic hull parameters. A typical ratiocination relationship might be of the following general form:

𝑊 = 𝑘 ∗ 𝐿 ∗ 𝐵 ∗ 𝑇 Where:

W is the weight of an item or group of items k is a scaling coefficient that depends on categorization of the craft by size, type, etc.

L is craft length

B is craft beam

T is craft draft

As in parametric models, an appropriate database of craft can be used to calibrate the scaling coefficient.

The use of cubic numbers (a general term for parameters containing a Length x Beam x Draft or Depth component) and equipment numbers (a general term for parameters usually using Length, Beam, and Draft or Depth combined with other dimensional or quantitative parameters) is a common theme in naval architectural estimates. Weight parameters from previous designs are often plotted against one of these terms to provide a basis for scaling to a new design.

More in-depth treatment of ratiocination approaches can be found in Society of Allied Weight Engineers publications, References [5] and [9].

Ratiocination and scaling methods of weight estimating are well suited to feasibility design and in some cases can be applied on a system level during preliminary design. Ratiocination and scaling methods do not provide the level of detail needed during contract and detail design.

Class Society Rules and Design Standards Classification society rules and other design standards are sometimes of sufficiently simple form that they can be readily integrated into a spreadsheet or other mathematical software. Rules and standards for structural design are particularly useful in this respect. If the input parameters needed are compatible with the level of craft definition in the phase of design, these standards can be used to develop an estimate of structural weight prior to development of a complete structural design. A sufficient rough cut may even be achieved by using class society rules or design standards to estimate the scantlings based on a representative section of the hull.

The single section…