Hydrodynamic%20Design%20Aspects%20for%20Conventional%20Fast%20Ships%20Manfred%20Fritsch%20Volker%20Bertram

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Hydrodynamic Design Aspects forConventional Fast
ShipsManfred 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 lt Fn lt 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, SR boats,
• racing yachts,...
• 0.8 lt Fn lt 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 gt 10º
• trim wedges with adjustable tabs frequent

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Semi-Displacement ships

• patrol boats, pilot boats, pleasure craft
• 0.6 lt Fn lt 1.2
• good seakeeping
• good course-keeping
• dynamic instability

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Simple HSVA power prediction available
planing hulls PB f(?,B,V) semi-displacement
hulls PBRTV/(?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
w/o with
Speed V kn
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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 designs have lower trim
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Trim wedges frequently employed

• most effective for Fn 0.4...0.5
• (10 savings possible)
• almost no effect for Fngt1.2

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Interceptors allow speed-dependent trim
PE
Height of interceptor
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Appendages influence resistance strongly

• RAPP 6-15RT
• 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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The