which would inevitably result in large amounts of air entering the waterjet inlet. To use waterjet propulsion, one needs to use special inlets that don’t draw water from the boundary layer close to the hull. Examples of such inlet types are pitot, ram and ‘shoe’- type inlets. A suitable alternative to waterjet is the use
of surface piercing propellers. This might be a good alternative for relatively small vessels, since the concept of using a stepped hull is favourable only for large Froude numbers, which means that it will mostly be applicable for smaller vessels; primarily below about 30m in length.
Seakeeping The model tests reported here did not include tests in waves. It is reported from full-scale that stepped hulls often have less pleasant motions than conventional V-shaped hulls without step. The magnitude of speed loss in waves of stepped hulls compared to conventional hull forms is not known, but it can be expected that the stepped hulls will have a slightly higher speed loss, since the waves would tend to disturb and temporarily destroy the ventilated cavity that provides the resistance reduction. One would expect the same to happen in
case of an interceptor placed underneath the hull. Thus, a natural next step in the investigations will be to test the model in head sea waves, both with a step and with interceptor. The interceptor could possibly be actively controlled as part of a ride control system.
Conclusion It has been found that the use of a step under the bottom can significantly improve the resistance characteristics of planing hulls at high Froude numbers. The reduction in resistance is mainly attributed to reduced wetted surface. The use of an interceptor at the step will increase the lift of the fore part of the hull, and will further increase the ventilation length and reduce the wetted surface of the aft part of the hull. In addition, it has also been established
that an interceptor can also be used under the middle of a smooth hull to obtain basically the same benefits as a stepped hull. A most remarkable possibility is to use a dynamically controlled interceptor below a smooth hull; then the interceptor depth can be optimised for each speed, so that there is practically no resistance penalty at low speed.
References 1. Clement, EP and Koelbel, JG, ‘Effects of Step Design on the Performance of Planing Motorboats’, Fourth Biennial Power Boat Symposium, SE Sect, SNAME, Feb 1991.
2. Hansvik, T, ‘Resistance of planing catamarans with step’ MSc thesis, Department of Marine Technology (2005), NTNU, Trondheim, Norway.
3. Savitsky, D, ‘Hydrodynamic design of planing hulls’, Marine Technology, Vol 1, 1964, p.71-95.
4. Tsai, JF, Hwang, SW, Chau, SW and Chou, SK, ‘Study of hydrofoil assistance
arrangement for catamaran with stern flap and interceptor’, FAST 2001, Southampton, UK, p 69-78.
Nomenclature AT Wetted surface of transom stern (m2
B Breadth of hull (m) CBD CF
DCF CR CT Dis
DSS DS
FBD Fn g
Base drag coefficient (-)
Frictional resistance coefficient (-) Roughness resistance coefficient (-) Residual resistance coefficient (-) Total resistance coefficient (-)
Depth of interceptor in full scale (m) Step height in full scale (m) Step height (m)
Drag force due to base drag (N) Froude number (-)
H Hull roughness in 10-6 LCB LCG LPP
R Resistance (N) RTS
Acceleration of gravity (m/s2 m (mm)
)
Longitudinal centre of buoyancy from AP (m) Centre of gravity (m)
Ls Length from transom to step (m) PBD
S Wetted surface (m2 SB SF
) )
Length between perpendiculars (m) Pressure at transom/base (pa) Total resistance in full scale (N) Area of base (m2
Wetted area ahead of base (m2
V Speed of craft (m/s) Vs Speed in full scale (m/s) T Draught (m) b
Deadrise angle (deg)
∇ Displacement volume (m3 Specific weight (kg/m3
r ) Reeds Marine Surveying
By Thomas Ask. Published by Adlard Coles Nautical, 38 Soho Square, London W1D 3HB, UK. 2007. Paperback. ISBN: 978-0-7136- 7714-0. Price: £30.00.
T
HIS book is an updated version of the author Thomas Ask’s Handbook of Marine
Surveying, originally published in 1997. Reeds Marine Surveying has expanded upon the original version with new sections on hull and deck loads, combustion and pollution, electrical systems, and noise and vibration, among others. Aimed at students of marine surveying,
professional marine surveyors, boat yard operators, and technically-minded boat owners, the latest surveying technology is covered. This includes: an analysis of the mechanical behaviour of materials, stress concentration, failure analysis, fatigue and fracture, corrosion, wood-damaging organisms, and the composition and characteristics of common plastics, metals, and composite materials. This publication is divided into three main
sections, primers, design and application guides, and techniques and checklist. Primers deals with understanding various elements of vessel wear and materials which should be covered in a survey, including buckling, fluid mechanics, and non-destructive testing.
SHIP & BOAT INTERNATIONAL MAY/JUNE 2007 Design and application guides covers the
survey analysis of hulls, (including varying shapes and drag) propellers, fasteners, engine systems, (such as heat exchanger cooling, exhaust and fuel systems, and ventilation) and fire-fighting. It contains clear directions
on how to evaluate each system, and which warning signs to look out for to gauge seaworthiness. Survey techniques covered in the final
section relate primarily to small craft. Diagrams to show the difference between a weak boat and a strong boat are included, demonstrating possible hazards. Following the advised techniques section are guidelines on how to format a survey, and a useful checklist provides practical hints on how to conduct a survey. The survey notes contain assor ted
photographs of various possible failures and issues to be considered. Usefully, the appendices detail IMO conventions and classification societies. The overall aim of this text is to provide
a solid foundation of marine surveying knowledge upon which experience and skill can be developed, and the author feels it is much more important to understand likely causes for troubles and common areas from which they originate, rather than listing frequent problems experienced on a given vessel design. A large amount of topic areas are covered
regarding different materials used in boatbuilding and their failure mechanisms, but the text remains concise and relatively easy to follow. It is a very useful book for practical, theoretical and regulatory information, with regards to marine surveying.
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