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Hydrodynamic Design Aspects forConventional Fast Ships
Manfred FritschVolker Bertram
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Focus on “conventional” fast vessels
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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
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Catamarans
• 70% more deck area• 20-80% more resistance• high transverse stability• similar roll periods
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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
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Simple design estimates possible
Froude number
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Foil-assisted cats at high speeds
• improved resistance• improved seakeeping• efficient ride-control system• controllable flaps forward and aft recommended
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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
The