Expert propeller advice needed.

[kindredspirit]

My propeller disintegrated due to electrolosis and I ordered a new one according to the book specifications. 18" diameter & 20" pitch.

However when the old one was taken off it was found to be 18" diameter but 22" pitch. If I fit the new one as is, (It was ordered from the UK and has been paid for.) will I get a significent change in performance or will it be neglible? I presume a reduction in top speed would be involved by using a 20" instead of a 22" pitch.

Boat is an Astinor Flybridge with a fair bit of weight and equipment aboard. Engine is 250hp (single.) 3,800 WOT gives 21 knots. 3,300 revs gives 15/16 knots. 2,900 revs gives 12 knots.

Returning and re-ordering propellers involves €135 carriage each way. (€270).

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[MadMat]

What is the rated maximum revs for the engine?

<hr width=100% size=1>http://www.boatmad.co.uk

 

[PaulF]

If you want a real expert I will PM you details of one , meanwhile IMHO you can consider this or not as you wish.

You say at WOT the rpm is 3800. Is that what the manufacturer states is the correct maximum rpm?. If it is, AND the propellor you took off was at one time, when complete, reaching that figure, then with the propellor you ordered there will be a reduction in speed. That reduction would likely coincide with the 10% less pitch, somewhere near 10%.

It may be worth referring to the manufacturer to see if they tried different pitches, and trying to find somebody with an identical boat/engine set up and see what they use. If you try the new propellor and find the performance is down you will have 3 choices. 1/ accept the change. 2/. Buy another propellor. 3/ compromise and have the new one modified to give a small increase in pitch. This latter option is controversial in some quarters because for a propellor designer it is incorrect practice. They would say the correct pitch only should be fitted. Those of us lesser mortals are sometimes faced with economic choices that may exceed the purists angle by necessity. If you want to go down this 3rd route pm me as I know some people who may be able to help you.

BTW make sure your new prop does not suffer in the same way, get some measurements done on your cathodic protection anodes. The resistance between your propellor shaft and the anodes should be very low, less than half an ohm
Good luck.

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[kindredspirit]

Thanks for a very considered reply.

The manufacturer's rec. max. rpm is 3,800 so the previous propeller must have been correct.

The 3 options you give, are what I thought I would have. If I take Option 1, will there be only a reduction in top speed? or will it reflect right throughout the range including acceleration (although acceleration is not important as she is a cruising boat.)

Kevin.

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[Dave_Snelson]

Hi Kevin, being as you seemed to have "propped-up" correctly in the first place you should see a corresponding reduction in speed at WOT but also a corresponding increase in acceleration, which could get you planing more easily (assuming you have a planing boat that is!). Also there could be an effect on fuel consumption with one school of thought saying that higher revs per speed means more fuel and the second school of thought saying a lighter engine load means less fuel. If it is going to cost you so much to change the props, then why not suck it and see?

<hr width=100% size=1>Madoc Yacht Club
<A target="_blank" HREF=http://www.madocyachtclub.co.uk>http://www.madocyachtclub.co.uk</A>

 

[PaulF]

Hi Kevin,

Re the change in performance you should find an increase in rate of accelleration. The speed at any given rpm will be proportionately lower throughout the range. A good analogy would be to compare it with using 4th gear instead of 5th on a car. Not quite as fast, but gets there quicker. A plus side may be the engine will not labour as much with the boat being fully loaded. One of the difficulties when faced with this situation is not only the direct costs of props but to try the boat out it has to be launched again. Then to effect any change it has to be slipped again, prop modified relaunched etc. Not funny and it adds up.

I hope it performance is acceptable to you.

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[Its_Only_Money]

As well as the rated max rpm you should find out the governed max rpm (may be the same), if you are already achieving this then you >will< go slower with a 20" rather than the 22" as the engine won't allow itself to rev any higher.

You are sure that the 22" hadn't been re-pitched down to 20"ish????

<hr width=100% size=1>Rgds

Simon
Its Only Money
Fairline Sprint
Solent-based

 

[kindredspirit]

Thanks Dave, Paul, Simon for the help.

I'll probably bite the bullet, exchange the propeller (hopefully) and pay for the extra carriage involved.

I've faxed the factory in English (as I've no "technical" Spanish), they're getting it translated and I should hear back from them tomorrow or Monday about trials they would have done on propellers with various pitches.

Kevin.

<hr width=100% size=1><A target="_blank" HREF=http://www.begleys.com/weather.htm>Useful links for Irish waters.</A>