Prop Size?

[philmc]

I have to replace the prop due to de-zincification. The existing prop is 14" dia, 3 blade fitted to a BUKH 10, on a Cobra 750 yacht. The supplier of the new prop says it should be a 15" for the BUKH but the increased dia. will bring it very close at the tips, within 3/8" to the hull. Anybody know of the usual size for the Cobra with a BUKH 10? Thanks for any response.

Phil

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[supermalc]

I don't know the correct prop size for you engine/boat you have, but I do know it can be difficult to find the right one.

My boat came with a right hand prop on a left hand engine !!! and it was under propped.

I've now been able to get to mine, from an inflatable, to investigate, I had a substantial plastic bag around it. I've found it is very close to the hull, and is only approx 10 inches in diameter. It is on a YSE12 which needs an anticlockwise prop. Unless the prop has been moved towards the engine when it was installed, resulting in the small clearance (the thickness of my thumb knuckle) it means I must put up with a major compromise, with an inefficient small prop with relatively large thrust.

However, after all my investigations, I would suggest getting one the same size. It did the job before, and now doubt will do again.

<hr width=100% size=1>Malcolm.
<A target="_blank" HREF=http://www.angelfire.com/rnb/malcsworld/>http://www.angelfire.com/rnb/malcsworld/</A>
Take a look at my boating pages.

 

[mickp]

try asking the Cobra-Seawolf owners club search the net for cobra yachts they have a website with a forum.

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[Benbow]

A knowledgeable supplier will be able to do the calculations for you (they also need to know gear box ratio (if they gave you answer without asking that then I would run away from them)) and give you the options if, for example, clearance is limited.

I have used French Marine Motors (Googleable) and have been very pleased with them.

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[alec]

From what I understand, tip clearance should be a MINIMUM of 10%, so that means 1.5 inches at each end.

The ends are where we get the most power from our props ( think of a wheel of soldiers, the poor buggers at the ends are belting round while the inner ones are taking a stroll). This is why normal props are as wide as they can at the ends. It is very tempting I know to go for a wider prop but there just isn’t enough room to get a decent day’s work. Also, a prop too close to the hull will produce banging and thumping noises.

To make up for lack of width, you could go for bigger blade area. As suggested, I’m sure the pro’s will help you.

Regards,



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[snowleopard]

for a given application there will be a range of suitable props, trading off diameter against pitch, for example my setup could use 17x12 or 16x13 (dia x pitch) i'm told that the smaller coarser one will give a bit more ultimate top speed but larger & finer will give more drive in choppy conditions. taking the tips too close to the hull will increase turbulence, decrease efficiency and probably strip off your antifoul in short order.

any prop supplier will be able to give you the options.

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[JackFrobisher]

Try CJR. I used them to check whether I had the correct sized prop and they were good. They faxed me a form, which I filled in and faxed back. They used the details to calculate the correct prop for my boat. They are:
C J R Propulsion
72 Quayside Rd
Bitterne Manor
Southampton
Hants
SO18 1AD

Tel: (023) 8063 9366
Fax: (023) 8021 1832

By the way, I have a three-bladed 14 inch prop (14x8) which you can have for the cost of the postage if you'd like it (or you can collect - Southampton). It's a CJR11738 manufactured in Nov. '88 (this is stamped on the Boss).

The taper goes from 7/8 to 11/16 and is 2 inches long.

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[sailorman]

y not have a 2 bladed prop as youre a sailer ?.

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[philmc]

Thanks for your offer, it is appreciated but the tapper is not the same as the existing shaft. Sorry about the late reply - have been away sailing on a friends boat.
Phil McLaughlin



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[supermalc]

I have been busy as well, but an update on my prop situation.

I found a 12 x 8 LH on ebay, which is what someone who repairs and works with these engines said I would most probably need. Imagine my surprise and delight, when it was only 30 miles away, in the small market town I grew up. I fetched it straight away.

2 days ago I put the boat in the lock with a strap around the stern and dropped the water a little. To my dismay, after some vigorous wire brushing, I've found the prop already fitted is a 14 x 8, and the boat could do with a slightly larger one, or with a greater pitch.

Someone has found a 13 x 9 which he sent yesterday, and arrived this morning. However I'm fairly sure the taper is much too much, and the small hole won't fit over the shaft.

All I want to do is get out of reverse....

<hr width=100% size=1>Malcolm.
<A target="_blank" HREF=http://www.angelfire.com/rnb/malcsworld/>http://www.angelfire.com/rnb/malcsworld/</A>
Take a look at my boating pages.