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**David M** · 22-04-2012, 08:18

I don't see a problem then since you can reach full RPM. A larger diameter prop as they are suggesting is only going to increase drag when it is locked while you are sailing.

Irish rambler · 22-04-2012, 08:41

There's lots of help here for you my friend and if I may also add, why not contact one or two propeller makers direct ? they all offer a prop matching service free, as has been pointed out you will need to furnish them with the following details :-boat maker and model, engine make and hp, gearbox make and ratio's. If the boat is out of the water it can help if you measure the distance between the bottom of the hull and the centre of the propshaft, and the distance between the centre of the propshaft to the skeg. Basically the diameter of the prop gives power and the angle(pitch of the blade gives the speed. The hull shape determines the speed of your boat and is calculated as the square root of the waterline length multiplied by 1.4 = hull speed.
Time spent on reconnaisance is never wasted.

**hpeer** · 23-04-2012, 17:55

My new prop (20x12) just came in. The old prop (19x13) looks very different.

The prop shop said I had a planning power boat prop. The difference in shape is pretty evident looking at this web site. My old prop looks like what they call a power boat prop.

Michigan Propellers // Michigan Wheel Marine

Anyone have a source of info on why the difference in shape?

Avayak · 23-04-2012, 18:06

I recently went through the same exercise and found that prop people were concerned with my provided figures and requested that I check gear ratios directly by manually turning engine and counting rotations on the prop shaft. This was requested as they have had instances of gear ratios being changed.

Irish rambler · 23-04-2012, 23:46

hpeer, in answer to your question as to the shape of the blades. To design a prop takes in many variables as other have posted. The speed of the prop turning, the engine torque characteristics and the underwater profile of the hull which determines the flow of water over the prop and consequentially the blade shape. Many props are a compromise, for example a large slow turning variable pitch prop is very efficient on commercial vessels but for a sail boat you also have to take into account of the drag effect when sailing which is why there are folding propellers marketed by the likes of Gori. As I suggested earlier I would have been inclined to get several prop manufacturers designers to give their opinion to choose the best suited to your boat. Remember that a salesman is just that,a salesman. Good luck with your new prop and happy sailing.