Propeller Guidance

Whilst Beta Marine has had many years of experience learning which propeller normally goes with which engine, we can only give you general guidance, propellers are a complex subject with many variables that can affect a vessels performance and if optimum theoretical hull speed is required a propeller specialist should be consulted.

Propeller Dimensions

Propellers are measured in two dimensions:

• The actual diameter (X) and
• The pitch – the blade is set at an angle, if you followed this angle through one revolution and measured the distance travelled between the starting and finishing point you will have moved forward through the measurement called the pitch.

There are often physical limitations/restrictions imposed on the propeller. This would frequently be the diameter due to the space limitations between the position of the propeller shaft and the underside of the hull (Y). This can be compensated for by changing the pitch but there are limitations to what can be achieved.

Y = the minimum clearance between the tip of the propeller and the underside of the hull and should be 10% (or greater) than the propellers overall diameter (X).

Propeller Concept

The purpose of a propeller is to convert the power and torque provided by the engine into thrust and the design should deliver the most efficient forward or ahead movement of the vessel. It will also provide backward or astern movement of the vessel but depending on propeller type this can be compromised in favour of forward or ahead efficiency.

The propeller then needs matching to the power, speed of the engine, water line length and displacement of the vessel to ensure both engine size and fuel consumption are kept to a minimum but are still capable of achieving the maximum theoretical hull speed. Generally a large diameter propeller rotating slowly is more efficient in converting available power and torque into thrust than a smaller diameter, faster rotating one.

In some vessels a propeller may be “sized” to absorb and transmit the full engine power and torque at a specific engine speed rather than the maximum rated engine speed (RPM) it is capable of. Vessels with this criteria are known as being ‘over-propped, engine fuel consumption and noise are reduced but the full maximum rated speed (RPM) will not be able to be achieved whilst in gear.

Propeller Types

Displacement Narrowboats, Wide Beams & Dutch barges are generally specified with either a 3 or 4 bladed propeller.

Propeller Verses Engine Guide

The following chart is an approximation of the required propellers and does not represent a bespoke Beta Marine recommendation.

Calculated On A 3 Blade Right Hand Rotation Propeller and 2:1 Gearbox Reduction Ratio.

Engine	Beta 14	Beta 16	Beta 20	Beta 25	Beta 30
Propeller	12" x 8"	12" x 8"	13" x 8"	14" x 8"	15" x 9"
Engine	Beta 38	Beta 43	Beta 50	Beta 60	Beta 75
Propeller	17" x 10"	18" x 12"	19" x 13"	20" x 14"	Please Ask
Engine	Beta 90T	Beta 105T	Beta 115T
Propeller	Please Ask	Please Ask	Please Ask

A dedicated propeller calculation that is unique to your vessels configuration is highly recommended.

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