Propellers

Propeller Type

The propeller is where “the rubber meets the road” so to speak. More accurately for marine applications, it is where the “metal meets the water”.

There are many factors that determine the best choice of propeller for a given application. For auxiliary sailboats, it is often a compromise between sailing drag and performance when motoring. In particular, is effectiveness when maneuvering.

In this regard, a fixed three blade propeller will provide the best overall motoring performance. It will also provide the best regeneration under sail. On the downside, of course, is additional sailing drag.

There are a number of types of propellers in common usage. Each has its advantages and disadvantages.

In general, a two blade fixed is a good compromise with respect to overall performance and reduced sailing drag. Typically, their motoring performance is good, but they suffer reduced effectiveness when maneuvering.

Of course, feathering and folding propellers provide the greatest reduction in drag. This must be weighed against their effectiveness when motoring or maneuvering. All brands are not equal.

Propeller Sizing

Anyone familiar with a diesel repower is aware of the importance of matching the propeller to the engine and transmission ratio. The most important factors are diameter and pitch. This will determine how much power a propeller will absorb at a given shaft speed.

If the propeller absorbs too much power, the engine will be over loaded. It will not get to its designed maximum speed because it will reach its maximum torque capability before reaching the designed maximum speed (RPM). This is a serious issue and will cause engine damage.

If the propeller does not absorb enough power, the engine will over speed at full throttle. This is also a serious issue and will result in engine damage unless the operator restricts engine speed to the designed maximum.

The situation is different for an EP motor. As long as the system is properly designed, it will not be damaged by a mismatch. However, a proper match is still necessary to obtain full power from your EP motor.

Unlike other EP suppliers, Electric Yacht can supply a range of speed reduction ratios. When properly matched, the electric motor is allowed to run at its optimum torque/speed while the propeller shaft turns at the best speed for the boat/propeller combination. In most cases, we can match your existing boat and propeller combination such that you will get the most of your EP motor.

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