Prop refurb

robertj

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Having fitted a new three blade fixed blade prop last year, I lost about 3/4knt of speed and feel it needs altering.
Has anyone had their prop repitched and diameter increased etc. and by whom?


Bob
 

Simes

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First Marine Propulsion

At Shamrock Quay, Talk to Paul.
These guys are terrific, knowledgeable, good engineers and understand hull dynamics.

As ever no connection, just work next door and they work on our / my boats.

Satisfied customer.

Simes
 

Ex-SolentBoy

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The first thing to do is get some proper calculations done to see exactly what prop size and pitch you need. You can do these yourself. All you need is in Nigel Calders book.

Once you have established that, you can see if it is within the realms of prop alteration. You may find that it is simpler to buy another one and sell yours on eBay.

The other advantage of getting a new one is that you can make sure your calculations are correct. It would be a shame to repitch yours and then find it made no difference.
 

oldharry

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If you have lost speed the prop size or pitch needs reducing, it too big or too coarse pitch for the engine. Did you notice whether the engine had lost revs at full throttle? Otherwise you may have noticed the engine reaching full revs very easily but not full speed - then the prop is too small, and usually the only solution is to fit a larger prop. There are various prop size calculatior programmes around for download, but matching props is a bit of a black art. Manufacturers are sometimes helpful. Not always!
 

robertj

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The one I replaced was a two bladed 16"x11" and fitted a three bladed 15"x11". No difference in engine speed no black smoke etc.
These were calculate by prop supplier.
 

Robin

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The important factor is making sure that the engine can JUST reach it's design maximum revs at full throttle in gear. If it can't the prop is too big (dia or pitch) if it can do it too easily it is too small. The old prop isn't necessarily a good starting point unless it was perfectly suited. Three bladed props will usually be smaller dia and/or pitch than a two bladed in the same application.

It is relatively easy to repitch a prop up or down by an inch or two, reducing diameter can be done but increasing it would probably mean a new prop.

When I did the exercise on re-engining our then W33 I did the calculations myself from Nigel Calders book on marine diesels, compared that with other sizes from Perkins, Volvo (it was a Perkins Prima engine sold via Volvo as a MD22L) and a marine surveyor's computer program and all gave different results. I went with my own figures and luckily got it spot on first time. BTW the original prop fitted by Westerly was oversized and the then Mercedes engine couldn't reach full revs so we had the dia reduced by an inch and the blades reset to the stated pitch as all three were actually different!
 

Tranona

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The one I replaced was a two bladed 16"x11" and fitted a three bladed 15"x11". No difference in engine speed no black smoke etc.
These were calculate by prop supplier.

If nothing else has changed then one would expect similar performance with those new props. Have you taken it up with the supplier?
 

robertj

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Yes taken it up but they say it's good their calculations, I cannot blame them as this seems a bit of dark ark.
I have noticed that the cord length is larger on the 2 blader compared to the new 3 blade.
I will contact mike Lucas for a chat see what he says.
Thanks guys any more thoughts please keep them coming

bob
 

LONG_KEELER

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Yes taken it up but they say it's good their calculations, I cannot blame them as this seems a bit of dark ark.
I have noticed that the cord length is larger on the 2 blader compared to the new 3 blade.
I will contact mike Lucas for a chat see what he says.
Thanks guys any more thoughts please keep them coming

bob

As a matter of interest, did you find the three blade produced less vibration and noise ?

Regards,
 

Tranona

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Yes taken it up but they say it's good their calculations, I cannot blame them as this seems a bit of dark ark.
I have noticed that the cord length is larger on the 2 blader compared to the new 3 blade.
I will contact mike Lucas for a chat see what he says.
Thanks guys any more thoughts please keep them coming

bob
Can you get maximimum revs? You say that the revs have not changed but it could be that you can get more speed by flattening the pitch to allow the engine to get up to full revs, or just below.

Put all your boat details into the Propcalc programme on www.castlemarine.co.uk this usually gives a good indication of size so you can see if you are way out and in which direction.
 

Sailfree

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At Shamrock Quay, Talk to Paul.
These guys are terrific, knowledgeable, good engineers and understand hull dynamics.

As ever no connection, just work next door and they work on our / my boats.

Satisfied customer.

Simes

+1

They supplied both my last 2 boats with Variprop feathering propellors. Paul Bell is really helpful.
 
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