Hydrodynamic Design Aspects for Conventional Fast Ships

Manfred Fritsch Volker Bertram

Focus on “conventional” fast vessels

Monohulls

There is considerable overlap in speed ranges

Speed ranges tested at HSVA for MONOHULLS

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Displacement ships • frigates, corvettes, ...

• 0.3 < F n < 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 5

Planing hulls • patrol boats, S&R boats, racing yachts,...

• 0.8 < F n < 1.7

+ low resistance at high F n – 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 6

Semi-Displacement ships • patrol boats, pilot boats, pleasure craft • 0.6 < F n < 1.2

+ good seakeeping + good course-keeping – dynamic instability 7

Simple HSVA power prediction available planing hulls P B = f(  ,B,V) semi-displacement hulls: P B =R T ·V/(  D ·  M ) R T = C T  ·½  ·V 2 ·  2/3 8

(All) designs can be improved • spray rails • trim wedges • appendages 9

Spray rails effective and cheap P E w/o with Speed V [kn] 10

Sometimes considerable improvements 50 knot patrol boat with original trim wedge 11

Sometimes considerable improvements 50 knot patrol boat with modified trim wedge 12

Sometimes considerable improvements 50 knot patrol boat modified wedge + spray rails 13

Trim influences resistance Influence of LCG 14

Recommendations for trim given Recent designs have lower trim 15

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 P E Height of interceptor 17

Appendages influence resistance strongly • R APP = 6%-15%R T • 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 18

(All) Designs can be improved...

History of fast vessel project at HSVA 19

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

Catamarans cover wide speed range Displacement • F n  0.5

• large platform Semi-displacement • F n  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

Seakeeping tests are sometimes performed 25

Deep-V addition serves as anti-slam device faired knuckled 26

Longitudinal rails alternative ASD 27

The Thank you all

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