Propeller size comparison?

Gixer

Gixer

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I’ve just got some quotes for a propeller from SBS and want some advice from the collective mind. I’m going to go back to the people who quoted but want to improve my understanding before I do.
The two sizes of propeller I’ve been offered are 14x9 and 13x10. First of all, is there any real world differences to these?
I’ve read up on the principals of propellers and understand the 13x10 could in theory give the same speed for lower revs. Buts it’s smaller so won’t have the same ‘traction’?
The calculators I’ve used myself give me answers closer to the 14x9.
Any thoughts?
 
I don't think you can look at the prop in isolation. For our engine, it's recommended to motor at 80% of the fuel stop limit, ie 2,600rpm. The Operations Manual for the engine on our boat (AWB sailing vessel) says:

prop.jpg
 
Thanks guys, sorry I didn’t mention that the new prop I want to get will be a folding.
My current prop is a 14x9 fixed.
 
I changed from a 12 x 9 to a 13 x 8 and found the 13 x 8 was more powerful than the 12 x 9, leading to a drop in full throttle revs by ?200+rpm. Are you changing more or less like for like - eg. 2 blade for 2 blade? Total blade surface area difference probably worth a look.

If the 14 x 9 still allows full rpm (or near) then as a folder it will probably have greater surface area and more 'grip' than 13 x 10 equivalent. If there are any benefits to smaller diameter / higher pitch in flat sea motoring, in my view outweighed by marina manoeuvring and motoring in more demanding seas.
 
Pye_End said:
For motoring, a big slow turning prop is best. For sailing, the smallest (unless folder/featherer) is best.
Click to expand...
Generally speaking larger diameter is preferred. As you say 1" diameter is roughly the same as 1" pitch in terms of the engine revs. So 14*9 = 126 and 13*10 = 130 and the RPM for any given boat speed will be virtually the same.. You presumably have enough tip clearance to fit the 14" and there is no advantage in going for a smaller diameter. The greater blade area of the 14" may give some advantage in heaver conditions but doubt in reality you would notice any difference. Your prop runs in nice clear water, unlike, say my long keel boat with relatively wide keel where greater diameter puts more of the blades in clear water.

Which make props did you get quotes for?
 
Flexofold from Darglow is number 1 for folders. Not worth spending the extra on a 3 blade feathering prop for your boat/engine. I had 2 blade Flexofolds on both my saildrive Bavarias and they were better than the standard 2 blade fixed. Featherers start to make sense in heavier displacement boats or where the prop is in an aperture so no space for a folder.
 
Thanks Tranona, I’m pretty much set on the Flexofold. I haven’t spoken to Darglow yet, will give them a shout next week.

Any thoughts on the difference in performance between 14x9 and 13x10?
 
As I suggested in post#6 the 14" might be a bit better in heavier conditions because of the greater blade area. Much the same as your fixed blade of the same size.
 
You are unlikely to notice the difference between the two, in normal conditions, assuming they are both similar profile, so going for the bigger diameter is probably the best option. Other factors such as shaft speed and blade area will make a difference too. A higher shaft speed can result in increased cavitation, while a narrow blade will not grip so well as a broad one. OTOH a broad blade will cause more drag under sail, but give more thrust when the going gets rough
 
Thank you oldharry, that’s exactly the answer I was looking for.
This photo I’ve found online gives you the idea of blade difference between the Radice I have now and what a flexofold looks like. The difference in the blade area looks huge.

IMG_0317.jpeg
 
A folding propellor will not cause drag - thats the whole point. So the bigger blade area of the flexfold will give better grip in difficult conditions without losing sailing performance. The shallower pitch of the bigger diameter will enable the engine the engine keep its revs up when it is having to work hard.
 
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