Propellor Advice please

[Blue_Blazes]

I am getting ready to re-engine my boat and have been advised that, taking into account hull type, engine hp, etc, it should be fitted with a 17" prop. problem is that, without major surgery to the cabin bulkhead and total re-arrangement of internal layout, I don't have sufficient room to shift the engine far enough forward to allow for a 17" prop, even with the engine tilted to it's maximum permitted angle of 15 degrees. I have enough room for a 16" prop with about 2" clearance between the hull and the blade tips. I f I use a four bladed 16" prop of the same pitch as recommended for the 17", would the extra disc area ratio make up for the smaller diameter? The boat is a 23foot Christina, planing hull, very similar to a Fairey Huntress. The engine is a Sabre 180, gearbox is Borg Warner 72c 1:1 ratio.
Any advice much appreciated.

Bill.

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[dignity]

I had the same problem when installing my new engine, end up using an equipoise propellor they have 15% more blade area than a standard prop, thus enabling you to use a slightly smaller diameter. I purchased from Lancing marine, found them very helpful.

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[oldsaltoz]

G'day Bill,

I suspect this will provide you with some answers

<A target="_blank" HREF=http://www.deltaprop.com/linksi.htm>http://www.deltaprop.com/linksi.htm</A>

Avagoodweekend Old Salt Oz /forums/images/icons/cool.gif

 

[snowleopard]

you can make up for lack of diameter by going for a coarser pitch. i was offered a choice of 17x13 or 16x14 for mine. prop manufacturers said the smaller one would give me better speed in flat water but the larger would give a bit more push in a chop.

a good prop supplier should do your calculations for you and guarantee the results (i.e. chance it f.o.c. if it's wrong).

if you were a raggie, increasing blade area would slow you down when sailing so 4 blades would be bad news but in your case would give better efficiency.

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[oldharry]

Theoretically if you reduce the diameter by an inch, and increase the pitch by the same amount, you will get approximately the same result, and vice versa. The size you have been quoted is optimum, and there is usually room for a little leeway before other problems swing in.

The main effect of reducing the pitch and increasing the diameter will be noticeable at low speeds, when the engine will have to work harder to produce a given speed, and the further you move away from the optimum, the more pronounced this effect will become.

However, before committing yoursewlf to an (expensive!) propellor, check out your particular engine/ hull configuration with someone like Lancing Marine, and get their advice. They will be able to tell you for certain whether you can get away with a smaller steeper prop without too much loss of performance.

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[Blue_Blazes]

Thanks All, You've given me some food for thought. I will contact Lancing Marine for their advice, they've been helpful in the past. Thanks Oz for the web link, the FAQ's on there will save me asking a lot more questions on this forum.

Cheers, Bill.

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[AndyL]

There ya go! The definitive answer to a thorny Q.

Q. On a sailboat under sail, should prop be rotating or locked?

A. There is less drag when prop rotates.

He he

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[oldsaltoz]

Re: There ya go! The definitive answer to a thorny Q.

G'day Andy,

A rotating prop on a sailing vessel may produce less drag, but think about the wear on cutlass bearings, couplings, gearbox and stuffing box.

Locking the prop may slow you but save a bundle.

Avagoodweekend Old Salt Oz /forums/images/icons/cool.gif

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[AndyL]

Oh I agree - both the drag and the wear,

but a LOT of folks insist that a fixed prop will cause less drag than a rotating one. In my case, I have proved by trials on my boat, there is less drag while rotating.

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