Hydrodynamic Design Aspects for Conventional Fast Ships Manfred Fritsch Volker Bertram

Focus on “conventional” fast vessels

Monohulls

Speed ranges tested at HSVA for MONOHULLS There is considerable overlap in speed ranges Speed ranges tested at HSVA for MONOHULLS

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

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

Semi-Displacement ships patrol boats, pilot boats, pleasure craft 0.6 < Fn < 1.2 good seakeeping good course-keeping dynamic instability

Simple HSVA power prediction available planing hulls PB = f(,B,V) semi-displacement hulls: PB=RT·V/(D·M) RT = CT ·½ ·V2·2/3

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

Spray rails effective and cheap PE w/o with Speed V [kn]

Sometimes considerable improvements 50 knot patrol boat with original trim wedge

Sometimes considerable improvements 50 knot patrol boat with modified trim wedge

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

Trim influences resistance Influence of LCG

Recommendations for trim given Recent designs have lower trim

Trim wedges frequently employed most effective for Fn = 0.4...0.5 (10% savings possible) almost no effect for Fn>1.2

Interceptors allow speed-dependent trim Height of interceptor

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

(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

Seakeeping tests are sometimes performed

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

Longitudinal rails alternative ASD

Thank you all The