Page 37Connecting the Engine and Propeller Shafts

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A lot of effort has been spent in making an engine and also in measuring the peak power.

It is very important that when the boat is out there, cruising, that the engine revs are at the
correct revs and deliver the peak power. That the engine and propeller are matched.

If the engine is not very powerful then it will only be able to turn the propeller slowly.

If it is very powerful, then the engine will spin the propeller faster than peak power revs.

Either way, peak power is not being delivered.

There needs to be the correct gearing between the engine shaft and the propeller shaft.

What ratio?

The only way to find out is to launch the boat and measure the speed.
How? See the next page.

Hopefully, for efficiency, you have fitted the largest propeller that can fit under the hull.

The likelihood is that you will need to reduce the engine revs for the propeller shaft.

Let’s say peak power is delivered when the engine runs at 500 rpm.

If, when the engine has reached thermal equilibrium, the revs when cruising are 400,
then change the gearing and re-measure the speed.

Keep changing the gearing until you are definitely over revving.
i.e. faster than the revs for peak power

Then step the gear ratio back one step.

The procedure I describe has various advantages.

It can all be carried out in the boat. Whilst afloat.

Another suggestion is that a propeller of same diameter and lesser pitch be fitted.

Ok. Number one. Source the appropriate propeller. Not easy.
Number two. Either get in the water and fit it under water.
Or retrieve the boat, fit it and relaunch.

Other advantages.
It is most likely that the engine shaft is above the propeller shaft.
Using a toothed drive belt between the two accommodates alignment issues.

For a small boat, passenger position and weight can flex the hull shape.
Precise alignment of engine and propeller shafts is not easy.
Misalignment causes friction.

Direct, straight line connection between engine and propeller shaft requires the
propeller shaft to be at a steeper angle.
The engine likely has a flywheel and this needs to clear the bottom of the boat.
The propeller shaft has to be tilted to rise up and connect with the engine shaft.

Ideally, the propeller shaft should be horizontal.
Vertical gearing makes this more possible.

So, to start, equip yourself with three alternative toothed belt pulley arrangements.
Then find out how much the revs change between the three settings.

You can then work out what number of pulley teeth are likely to enable peak revs.
And you can try a new set and hopefully select the correct ratio.