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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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