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Calculation information… and why we need it (Part III)

This posting is related to engines and gearboxes, and how they impact vessel speed and performance.

It is a common misconception that if you simply add more power to a vessel, that speed will increase. Though, on one level this supposition is true, on a different level it is misleading. Oftentimes, a higher power engine weighs more, sometimes significantly, and any gains in speed are largely offset by the higher displacement.

France Helices is very specific about engine data we require. We need to know the number of engines, the make, model, and rating. On some engines, such as Cummins, a single model can have as many as ten different ratings, with significant differences in power curves between ratings.

engine gb

After we get the engine details, we build an engine power curve, supplied by the manufacturer, giving us the power and torque at different RPM. We build this curve and, based on the propeller we design, we match the power curve, propeller demand curve, the hull resistance curve, and the torque curve. Where these curves meet is the final projected speed of the vessel.

We are, occasionally, asked to make engine recommendations for a particular vessel. While we sometimes will recommend a particular engine for a specific vessel due to our experience in past projects, we work with any and all appropriate engine makers and espouse no “favorites”. For example, we frequently work with MTU’s 2000 series engines, as they are very popular on several sizes and types of vessels. Similar with Caterpillar, Volvo, and so on.

As to gearbox, we are usually given a make, but not model, of gearbox and asked to provide a recommended gear ratio. There is no single ratio. We look at the hull and performance characteristics of each vessel and recommend an ideal or efficient ratio, unless we are specifically told to use a specific gearbox and ratio (Usually on retrofits). Sometimes maintenance, reliability, or bearing wear becomes critical (For instance, on heavier vessels that are borderline on power, sometimes bearings will show excessive wear).

With power, fuel consumption and vessel range are critical. In addition to the extra displacement of the fuel, we need to determine how much fuel a specific engine consumes in order to determine and acurate range, top speed, cruising speed, and efficient speed.

This leads into the first question we ask customer: “How will the vessel be used?”



Landing craft.







Long Range.

Rough weather.

Passenger ferry.

Each type of use requires different types of performance. Sometimes a different propeller or gearbox will be required. A longer drive arm. The difference between two engines.  Maintenance comes into play, too. Continuous use requires slower, heavier engines with larger propellers. Intermittent use, as on pleasure craft, requires different propellers and performance characteristics.

engine charteng curvesfuel range cons

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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.