Measuring For Correct Transom Height

Measuring For Correct Transom Height

Boats are usually designed to take a short shaft outboard motor – or a long shaft outboard. A few boats may require an extra long shaft outboard but they are not common – typically yachts.

It is easiest to measure the outboard when it is actually on a transom or stand but small outboards can be measured while on their side.

Measuring For Correct Transom Height Mightyboy outboard motor

This outboard is too low in the water. It appears that the cavitation plate is lower, below the bottom of the hull, than it needs to be.

Measuring For Correct Transom Height

Measure down from the top of the transom bracket, where it will sit on top of the boats transom, to just immediately below the cavitation plate.

Some measurements are given from the top of the transom to the centre of the propeller and this will give you some idea of how far below the bottom of the keel you should have the propeller.

Typical Outboard Leg Lengths

A short shaft outboard is usually 15 inches (375mm) from top of transom bracket to bottom of cavitation plate.

A long shaft outboard is usually 20 inches (500mm) from top of transom bracket to bottom of cavitation plate.

An extra long shaft outboard is usually 25 inches (625mm) from top of transom bracket to bottom of cavitation plate.

 

MightyBoy Outboard Motors

The cavitation plate should be flush or just below the bottom on the ‘V’ of the boats hull so that the propeller runs in undisturbed water. Too deep and the motor has to work against extra drag caused by the leg and too shallow and propeller will be working in aerated water disturbed by the boat hull, loose thrust and kick up extra spray, wasting energy.

Different styles of boats will need different setups. A displacement hull will use a different propeller depth to a planing hull. Ask around experienced boaties for their advice. You’ll get some variable opinions but you will find they are all saying much the same thing.

Other information on mounting your outboard can be found here: CLICK here

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