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API Journal Authors: John Wetherill, Liz McMillan, Carmen Gonzalez, Elizabeth White, AppDynamics Blog

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Stepped Hulls

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If you hang around naval architects, attend boat shows, or look on various design forums on the Net, you will often see quite the debate about stepped hulls and their relation to speed.

Though somewhat “trendy” in boat design currently, stepped hulls have been around since World War II, and they are intended to aerate the hull; introducing a thin layer of air between the hull and the water surface, thus reducing friction and drag, hence, increasing speed.

Indeed, a stepped hull design can somewhat drastically increase a vessel’s top speed, when properly designed. It can combine the benefits of a flat hull with a deep Vee. Sounds good, right?

Ah, but here’s the rub… Designing such a hull correctly is exceedingly difficult, and many, if not most, stepped designs are not done properly. Get the angles of the steps a tiny bit wrong, and you have just made a very expensive mistake. Steps, when incorrectly designed, negatively impact not only vessel speed, but can greatly decrease the maneuverability of the vessel, increase planning time, and reduce ride comfort. To use the American term, the boat will become “squirrely” when operated. In fact, improperly designed steps can weaken a hull to the point of catastrophic failure.

France Helices, as drive manufacturers and vessel designers, have been involved on projects with hundreds of shipyards and naval architects. Quite often, when there are performance or speed problems, the culprit can be steps that are carelessly or incorrectly added, often ruining a perfectly good hull design. When we are brought into a new project at the design stage, we can check the steps along with the rest of the design. Unfortunately, we are often brought into the picture in desperate “help us” scenarios when vessels did not perform as expected.

Make the steps too deep? Hydrodynamics issues and coriolis. Wrong angle? Air flow directed in the wrong direction. Too many steps? Inadequate hull surface for planning. The list goes on and on.

The math involved in designing a stepped hull is complex and far beyond the scope of this article, but properly designing steps requires sophisticated design software and nearly always requires extensive tank testing in order to get it right. Unfortunately, this is often expensive and beyond the means of small yards and shops.

Stepped hulls must also be designed with the propulsion method in mind. For instance, since surface drives are aerated, additional aeration from the hull must be taken into account. In part, this is why so many stepped hull designs fail, since very often an assumption is made about the drive and hull… Both must be designed together and in conjunction with each other. You cannot ignore the laws of physics due to cost or other reasons.

France Helices has gone above and beyond simply being drive manufacturers. We take the entire vessel into account when designing propulsion. We have the experience and expertise that ensures a vessel’s performance, stepped or not.


Read the original blog entry...

More Stories By John Miele

I am currently Sales and Marketing Manager, Asia / Pacific for France Helices. Created in 1977 by Paul BEZZI, FRANCE HELICES is today an international company. French marine propulsion leader, FRANCE HELICES is also one of the international leaders. Research and development are a very important part of FRANCE HELICES program, the CAD's department is equipped with the latest computer technology, using most up to date programs. Our engineers and technicians determine from the customers specifications and designs, the type of propulsion system to suit the boat's application. They can also advise on the ideal choice of power and optimum gearbox ratio to obtain the maximum thrust. There are many FRANCE HELICES' inovations and patents such as Surface Drive System (SDS) which enable boats to obtain very high speed with excellent handling capabilities and high quality Controllable Pitch Propeller systems (CPP) for both professional use and pleasure application. The constant research using cavitation tunnel testing, guarantees high efficient propeller blade shape for our customers. More than 20 000 propellers per year are manufactured by FRANCE HELICES workshops. In all sizes from small sailing boats to large fishing vessels to navy boats or mega yachts. The FRANCE HELICES workshops are equipped with modern foundries capable of casting propellers up to 3.5 tons in NiBrAl. They are also equipped with CNC milling machines and CNC lathe machines. FRANCE HELICES has four sites which cover the complete range of production. The development of FRANCE HELICES, supported by shareholders places our company as a leader in the international market and insures a constant development, worldwide, in order to be close to the end user.

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