Hydrodynamic Design Aspects forConventional Fast Ships

Manfred FritschVolker Bertram

Focus on “conventional” fast vessels

Monohulls

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There is considerable overlap in speed ranges

Speed ranges tested at HSVA for MONOHULLS

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Displacement ships

• frigates, corvettes, ...• 0.3 < Fn < 0.6

+ good seakeeping+ good course-keeping+ low dynamic trim– steep power increase

• V-shaped section in forebody• slender waterlines• round bilge with decreasing R going aft

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Planing hulls

• patrol boats, S&R boats, racing yachts,...• 0.8 < Fn < 1.7

+ low resistance at high Fn – dynamic instability– poor seakeeping

• straight sections and knuckle lines• slender waterlines• deadrise angle decreasing aft to L/2 then nearly constant > 10º• trim wedges with adjustable tabs frequent

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Semi-Displacement ships

• patrol boats, pilot boats, pleasure craft• 0.6 < Fn < 1.2

+ good seakeeping+ good course-keeping– dynamic instability

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Simple HSVA power prediction availableplaning hullsPB = f(,B,V)

semi-displacement hulls:PB=RT·V/(D·M)RT = CT ·½ ·V2·2/3

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(All) designs can be improved

• spray rails

• trim wedges

• appendages

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Spray rails effective and cheap

PE

Speed V [kn]

w/o with

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Sometimes considerable improvements

50 knot patrol boat with original trim wedge

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Sometimes considerable improvements

50 knot patrol boat with modified trim wedge

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Sometimes considerable improvements

50 knot patrol boat modified wedge + spray rails

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Trim influences resistance

Influence of LCG

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Recommendations for trim given

Recent designshavelower trim

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Trim wedges frequently employed

• most effective for Fn = 0.4...0.5 (10% savings possible)• almost no effect for Fn>1.2

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Interceptors allow speed-dependent trim

Height of interceptor

PE

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Appendages influence resistance strongly• RAPP = 6%-15%RT

• avoid oversizing shaft brackets, bossings, rudder profiles• V-brackets may have 7% higher resistance than I-brackets• Align brackets with flow (CFD or experiment)• power changes by 3%-5% depending on sense of propeller rotation for twin-screws• shaft inclination reduces efficiency• inward inclination of rudders for twin-rudder designs can increase propulsive efficiency by 3%• keep strut barrels small; nose rounded or parabolic• Align bilge keels with flow• determine angle of attack of least resistance for non-retractable stabilizers

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(All) Designs can be improved...

History of fast vessel project at HSVA

Catamarans

• 70% more deck area• 20-80% more resistance• high transverse stability• similar roll periods

Catamarans cover wide speed range

Displacement • Fn 0.5• large platform

Semi-displacement• Fn 1• round-bilge or hard-chine

Planing• 50+ knots• hard-chine• waterjets

Simple design estimates possible

Froude number

Foil-assisted cats at high speeds

• improved resistance• improved seakeeping• efficient ride-control system• controllable flaps forward and aft recommended

Foil-assisted cats often with aft immersed

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Seakeeping tests are sometimes performed

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Deep-V addition serves as anti-slam device

faired

knuckled

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Longitudinal rails alternative ASD

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Thank youall

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