03 Design Impacts

8/12/2019 03 Design Impacts

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TS 4001: Lecture Summary 3

Design Impacts


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20 February 2002 Design Impacts 2

Weight and Volume Impacts

Added weight and volume of installing a component or system can be much more than

just that component or system weight and volume.

Support systems.

Added electric and HVAC loads.

Weight of increased volume.

Larger displacement may mean larger engines to make speed.

Second and third-order impacts. Impact on total ship system may vary greatly depending on the ship.

Excess volume and length tend to hide impacts of new systems.

Intact stability.

Topside length.

Impacts tend to depend on the order changes are applied.

Addition of heavy component may drive up volume .

This additional volume is then free to next change.


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Example: Design Tradeoffs

A Notional Case

FEATUREWeight Impact

Add 1 knot 200 LT

Add 500nm range 130 LT

Add LAMPS Mk III facilities 400 LT

SPS 48 to SPY 1 radar ` 700 LT

3psi to 7 psi blast overpressure 100 LT

Add level I fragmentation protection 160 LT

Add accommodations for 10 crew members 40 LT

Add a 32 cell VLS 250 LTReduce standard margins by 50% - 550 LT


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Weight Impacts

Increased resistance

Decreased endurance

Decreased maximum speed

Change in ship motions

Change in maneuvering characteristics

Degraded damaged stability

Lowered structural reserve

Lowered freeboard



Weight and Volume Impacts (cont.)

Machinery Step Functions:

Engines and generators come in discrete sizes.

Impact depends on need to increase or decrease capacity.

Impacts are not always linear:

Depend on what other system changes are tripped.

Fuel tankage.

Manning.

Regulatory requirements. System impacts may cancel when combined.



Machinery Weight Impacts

Propulsion Engines:

Weight of engines (with fluids).

Reduction gear weight.

Shafting and bearing weight.

Lube oil, seawater, and compressed air systems.

Weight of larger intakes and uptakes.

Weight of larger foundations. Fuel weight.

Manning weight for operation and maintenance.

Control and other support system weights.

Generators:

Weight of generator and prime mover (with fluids).

Switchgear and cable weight.

Fuel, foundations, support systems, manning weights.



Machinery Volume Impacts

Propulsion Engines:

Volume for engines with enclosures.

Space for reduction gears.

Shaft alleys.

Intake and uptake volumes.

Fuel tank volume.

Support system volumes.

Manning volume for operation and maintenance.

Generators:

Volume for generators and prime movers.

Intake and uptake volumes for prime movers.

Switchgear volume.

Volume for cable runs.

Fuel, support system, and manning volumes.



Limits on Displacement

Strength limit: Displacement at which load carrying ability of the

hull will be exceeded. To exceed risks structural failure of thehull.

Damaged stability limit: Maximum displacement at which ship

can sustain designed level of damage and remain afloat. To

exceed risks loss of ship with lesser level of damage than it was

designed to withstand.

Speed/Endurance limit: Maximum displacement at which ship

can reach designed speed/endurance. To exceed means ship

will go slower or less far than designed.


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Machinery Arrangements

Engine room number and locations.

Start with two main engine rooms, with at least two compartment separation, and

one auxiliary machinery room. Required volume and trim considerations usually drive engine room location to the

center of the ship vs. near the stern.

Electric drive allows more flexibility in locating engine rooms, although taking

engines out of their normal low hull location increase KG. Shaft lines and gear locations.

With mechanical drive, shaft lines drive gear and thus engine locations.

Not more than 5 of rake and 2-3 of splay in shafts.

Shaft alley volume.

Electric drive decouples shaft lines from the engine locations.

Engines need to be located with minimum bends in intakes and uptakes to reduce

losses.


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Machinery Arrangements (cont.)

Intakes should be located high enough to preclude water ingestion, and uptakes should

be located so the exhaust plume does not constantly blow into vital electronics or

manned weatherdeck areas.

Survivability considerations:

Separation of machinery components increases survivability.

Each shaft should have its own prime mover(s) in a separate space.

Redundant auxiliary systems should be located in separate spaces. Important to lay out machinery spaces early in concept design stage.

Soon after powerplant is selected and initial hull form is developed.

Just a block arrangement, not detailed.

This validates propeller locations, shaft lines, and gear and engine placement.

Laying out individual auxiliary components and fluid systems usually not necessary

unless you have specific concerns.


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HM&E Impact on Signatures

Acoustic:

Machinery noise radiates into the water through the structure.

If engines are located above the waterline, their acoustic signature is reduced,

which can be an advantage of electric drive.

Acoustic enclosures reduce noise level in engine rooms as well as reducing

acoustic signature (high volume and arrangement penalty).

Propellers can be noisy, especially if cavitating.

Cavitation in struts, rudders, and sonar domes produces noise.

Infrared:

Hot stack gasses and hot metal of stack shine brightly in IR spectrum.

Eductors mix ambient air with exhaust gases to cool the plume.

Use of air to cool rings at the top of stacks, reducing visibility of hot metal.

Insulation around uptakes and over engine rooms reduces IR signature.

Wake:

Hull and particularly bow shape.


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Impacts of Increased Length

Length is a very expensive dimension (building).

Length helps powering.

Less power required to make speed requirement could reduce cost if it meansa smaller engine or fewer engines.

Less power required at cruise speed means less fuel, which translates to

reduced volume, weight, acquisition cost, and annual O&S cost.

Length tends to improve seakeeping.

Greater hull bending moment means larger scantlings and higher structural

weight and increased cost.

Longer runs for cables, pipes, ducting, fiberoptics, and shafting mean higherweight and increased cost.

Volume (especially at the ends) becomes less arrangeable because of

narrower beam and finer entrance angle, driving up total volume and cost.


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Impact Example

Basic performance

BHP at specified conditions.

Noise level at engine bedplates

without isolation mounts.

MTBF/MTTR in factory

environment.

Ship performance impact

EHP delivered into the water.

Ships radiated noise signature.

Propulsion plant availability (Ao).


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Sonar Impact Example

Weight Percentage

Direct 60 tons 10%

Indirect (1st Order)

Structure 173 29%

Support Systems 23 4%

Manning 32 6%

Space 90 15%

Indirect (2nd Order)

Fuel 150 25%

Other 66 11%TOTAL 594 tons 100%


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Signature Considerations

Hull

Radar Cross Section Wake

Acoustic

Magnetic

Combat Systems

Radar Cross Section

RF Emissions

Active Sonar

Propulsion

Infrared (IR) Wake

Acoustic

Magnetic

Auxiliaries

IR

Acoustic


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HM&E Impact on Signatures

Acoustic:

Machinery noise radiates into the water through thestructure.

If engines are located above the waterline, their acoustic

signature is reduced, which can be an advantage of electric

drive.

Acoustic enclosures reduce noise level in engine rooms as

well as reducing acoustic signature (high volume and

arrangement penalty).

Cavitation in propellers, struts, rudders, and sonar domes

produces noise.


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HM&E Impact on Signatures (cont.)

Infrared:

Hot stack gasses and hot metal of stack shine brightly in IR

spectrum.

Eductors mix ambient air with exhaust gases to cool the plume.

BLISS caps use air to cool rings at the top of stacks, reducing

visibility of hot metal.

Insulation around uptakes and over engine rooms reduces IR

signature.

Wake.


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Ship Synthesis Models

Design tools which model iterative nature of ship design

Spreadsheet models

Warship-21 and other preliminary design tools

Parametric weight, volume, electric load, manning and cost estimates

Some calculations (powering, endurance fuel, etc.)

Minimum inputs, quick response, but low fidelity

Advanced Surface Ship Evaluation Tool (ASSET)

Flagship of NAVSEA preliminary design

Originally based in DD07 destroyer design code

Surface combatants, amphibs and auxiliary ships, and now carriers

More input to run, but can achieve higher fidelity

More first principle calculations vs. parametric estimates


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Propulsion Plant Selection

Payload-limited and Power-limited approaches

Payload-limited fixes the payload and selects smallest plant to carry payload at

required speed

Power-limited fixes the plant size and adds as much payload as possible while still

making required speed

Transmission type and number of shafts

Based on operational profile, survivability, and operator desires Engine selection approach

Make a guess at required power

Choose type of engine (GT or diesel)

Based on required power, type of transmission, and number of shafts, choosenumber of engines and engine size

Estimate weight and volume of ship with selected plant

Check required power and resize plant as needed


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Generator Selection

Estimated electric loads

Parametric scaling from similar ships

Known system loads

HVAC and lighting calculations

Electric power margins

20% design and construction margin

20% future growth margin

90% generator loading

Always one generator in reserve

Size of generators and type of prime movers

Discrete generator sizes (500kW, 1000kW, 1500kW, 2000kW, 2500kW)

Based on generator size, choose prime mover type

Manufacturers offer specific prime movers matched to generating capability


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Definition of Hull Coefficients

Displacement-Length Ratio.

For displacement in long tons and length in feet, ratio equals

Combatants usually range from 40 to 100.

Prismatic Coefficient.

For Max Section Area (AX) in square feet and length in feet:

Combatants usually range from 0.60 to 0.68.

Length-to-Beam Ratio. Combatants usually range from 7.0 to 10.0.

Beam-to-Draft Ratio.

Ratio of waterline beam to hull draft (i.e. without appendages).

Combatants usually range from about 2.5 to 3.5

Maximum Section Coefficient.

For AX in square feet and beam and draft in feet:

Normal range is 0.60 (corvettes) to 0.99 (CVs).

( . )0 013

L

CA L

P

X

=

C A

BTX

X

=


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Optimum Coefficients

CAPT Saunders published design lanes for optimum displacement-length and

prismatic coefficient which still apply today. Combatants usually range from 40

to 100.

Optimum beam-to-draft is 2.0 (rarely achieved), which implies a semi-circular

midship section and the most efficient surface area for a given volume.

Combatants usually range from 2.5 to 3.5.

Higher length-to-beam tends to be better, since it implies lower resistance, but

could create stability and arrangement problems. Combatants usually range

from 7.0 to 10.0.

For powering, the optimum max section coefficient is 0.785 (or p/r4), which

again implies a semi-circular cross section.

For seakeeping, a max section coefficient close to 0.61 is better. Corvettes are

about 0.60, CVs are at 0.99.


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Typical Range

Length vs. Beam

TiconderogaSpruance

Belknap

AdamsPerry

Sherman

Gearing

Fletcher

Sumner

Benson

Burke

30

35

40

45

50

55

60

65

300 350 400 450 500 550 600

Length (feet)

Beam(

fee

t)

L/B = 8.0

L/B = 9.0

L/B = 10.0


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Prismatic Coefficient Design Lanes

These represent

normal and customary

values for

displacement type

hulls.

T i l L


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Typical Lanes

Captain Saunders Design Lanes

Burke

Benson

Sumner

Fletcher

Gearing

Perry

Spruance

Ticonderoga

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.12 0.16 0.20 0.24 0.28 0.32 0.36 0.40 0.44 0.48 0.52 0.56 0.60

Froude Number

PrismaticCo

efficient

Bl k C ffi i t


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Block Coefficient

Block Coefficient vs. Froude Number

Ticonderoga

Spruance

BelknapAdams

Perry

Gearing

Fletcher

Sumner

BensonBurke

0.40

0.45

0.50

0.55

0.60

0.40 0.42 0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60

Froude Number

BlockCoefficient

H ll P f I t


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Hull Performance Improvements

Bulbous Bows:

Reduce wave and viscous resistance by creating a canceling wave pattern and

altering the flow around the bow.

Generally tuned to a specific speed-length ratio, and can be designed for either high

or moderate speed regimes.

Increased wetted surface tends to increase resistance at low speed.

Bulbous Sterns:

Improve flow to the propellers and reduce energy lost in the wake.

Transom Sterns:

Reduce resistance by postponing separation at the stern.

Stern Wedges:

Direct flow downward as it exits the hull, which brings the stern up and reduces

squat at high speeds. This reduces resistance at those higher speeds, but causes

increased appendage drag at lower speeds.

Oth H ll F D i


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Other Hull Form Drivers

Intact Stability Criteria:

General practice is to maintain a GMT at least 8-12% of beam.

Depends on mission and type of ballasting. Will usually drive beam out higher than needed for area/volume.

Area and Volume Requirements:

Sufficient area and volume in hull and deckhouse to enclose what you need to carry.

Tends to preclude use of long and narrow hull forms which are optimum for

powering.

Volume-limited vs. weight-limited designs.

Topside Length and Beam:

Weapons, sensors, pilothouse, stacks, helo deck and hangar, and anchors.

Sufficient beam to enclose guns and magazines forward.

Maximum length and beam constraints.

Additi l R di


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Additional Reading

1.3.1 Ship Impact Studies (P. J. Sims)

1.3.2 Power Limited Design Approach for Combatant Ships (D.

A. Rains)

1.3.3 Stealth on the Water (M. Valenti)

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03 Design Impacts

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