I have a 2 bladed sailing propellor fitted to my Seal 28. She has the Bukh DV20 engine. I would like to fit a three bladed prop to get a bit more 'push' at the expense of a little more drag when sailing. I would like to know if anyone has a three blade prop fitted, and if so, what size and pitch is it?
Neil Sinclair

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have a look at this site, I downloaded the excell file and it seems to work fine

http://www.surfbaud.org/...r/surfprop/Propkingx.xls 
Mike Edwards

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Thanks very much for the propking link, this looks very promising. My Seal 28 has been raced in her past life and I guess the prop has been selected accordingly. As a family cruiser, though, she feels very weak under engine power, only managing about 4.5 kts in calm conditions, despite the 20 BHP.
Neil Sinclair

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Just a bit more info for you, I replaced my 2 blade 14 x 10 prop with a 3 blade 13 x 9 prop.
The result was the same speed 6kn aprox., at 2600 rpm(1300 shaft rpm)but the cavitation stopped and the drive felt much smother and quieter. Stopping was also improved. The drag must be greater as the prop shaft wants to turn when sailing, (I lock the shaft in reverse gear to minimise wear on the bearing) But I have not noticed an appreciable drop in speed.
My engine is a Petter 12hp
The shaft was 25mm, with a 1:10 taper, taper length 70mm, minimum shaft diameter 15mm, Keyway 8mm.

(Have you checked the accuracy of your log 4.5 kn is a bit slow a badly fouled bottom would account for this loss of speed.)

Mike
Mike Edwards

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Thanks for the extra info. My speed was off the GPS. It's a little better after a scrub, I get about 5 kts now. I can't see the size of the prop-not so easy on the beach with water up to your neck! Its about 13 inch dia two bladed, though. My first run through Propking advises a 16X12 for the Bukh 20. I'm not sure that would fit in the aperture! When you fitted your 3 blade prop, did you find you got more 'punch' into head winds and sea? I don't seem to have any. Cheers for now!

Neil
Neil Sinclair

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I would check with an "expert" before ordering any new prop as 16 x 12 seems large to me.
I believed that my original 2 blade prop was just thrashing water as much as it was producing propulsion.
The 3 blade grips the water better, this is aparent when you put the engine in gear the revs drop and when you push the throttle forward the increase in engine revs builds up more slowly than it did before. This must be better when punching into a sea and the prop is bouncing up and down in the sea. However the engine is reving to the same as it was before so I am happy that the size is correct for my setup. (My engine is a bit on the small side for the 28)
The most notisable change is the lack of noise (2 blade props produce a pulsed propulsion, as both blades line up with the deadwood)
Mike Edwards

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Many thanks, Mike. I'm pleased to hear you got better 'push' from your three blade prop. I take your point about not leaping in, there is a lot more info to collect, yet. I only have approximate data about the dimensions of the hull to enter into Propking and I also know of at least two other Seal 28's in the Chichester area which I can go and look at.

Cheers for now!

Neil


Neil Sinclair

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i have a seal 28 with fixed keel ,20 hp bukh wit a 3 blade prop.
i will take it out of the water in november or the beginning of december.I would be glad to measure my prop because she makes good speed with force 7 head wind at the waddenzee and in calm conditions she does about 6.5 knts full throttle.So be patient and as soon as she's dry ill give you the measurements of my prop. maybe you have the info i requested in my other message that i posted on this bulletinboard greetings Sjerp from holland
sjerp noorda

Thankyou! I look forward to hearing from you a bit later. I have to measure to get the correct dimensions for the prop calculator, so I am waiting until the boat is out for the winter.

Neil
Neil Sinclair

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hello again ,
i've taken the boat out of the water and what do i see that i have a 2 blade prop as well but i'll look in to a 3 blade prop here at a friends yard who might be able to get the information.
sorry i've kept you waiting for nothing
sjerp noorda
sjerp noorda

I'd be very interested to know what size your 2 blade prop is, since it seems to drive your boat well. Mine's 16 X 12 inches, which is just the size Surfprop says the three bladed prop should be.

Neil
Neil Sinclair

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i'll look in to it and measure the prop next week
greetings sjerp
sjerp noorda

Many thanks! Incidentally, I saw your request for sail number information on your Seal 28 - did you find the maker's mark under the starboard settee? I checked on mine (No. 27) and found nothing!

Cheers! Neil
Neil Sinclair

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i'll check it this saturday and i'll look at the prop
greetings sjerp
sjerp noorda

I have just found a book which goes fully into props.

"Propellor Handbook"
Author Dave Gerr
ISBN 0-07-138176-7
Publisher International Marine

It is technical, but very thorough.

Frank Marsden

Frank Marsden