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Old 23-03-2006, 15:59   #1
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Matching prop to engine

Demonstrating my considerable ignorance here:

I am toying with the idea of re-propping. Or, perhaps, keeping 2 props. Insatiable currently has a rather anaemic 2-blade feathering prop - showing her race-orientated design. I wouldn't mind having a decent 3-blade fixed prop for which I can change in for extended cruises.

The question is, how does ne select the appropriate size / type / pitch, etc to suit the boat and engine (Volvo Penta 28hp)?
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Old 23-03-2006, 17:39   #2
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Here is a beginning

http://www.miwheel.com/MIWheel/PropA...InboOne.asp#fn
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Old 23-03-2006, 17:51   #3
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Chuck is correct, in fact just today I emailed them the specs of my boat and within an hour I had the information I needed, and of course with a quote for a new one.

Mike & Paula
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Old 23-03-2006, 18:01   #4
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I personally would not put a fixed prop on an ex race boat.. performance impact will be noticeable. Instead look at a 3 blade feathering prop. Number of options out there and delta between the folder and a good feathering prop like a Maxprop is fairly small...
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Old 23-03-2006, 18:08   #5
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Thanks for the info guys. Here is the current arrangement:


As you can see - pretty skinny!

I intend to keep the 2 blade folding prop for when I am racing, but if possible I would like to get hold of a 3 blade prop for when I do some extended cruising...whether that be fixed, folding or feathering....I would just swap them in and out as necessary...
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Old 24-03-2006, 13:44   #6
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I did mine last fall. It's all numbers.

Sail Boat / power boat
Engine
Transmission

Mine was correct in diameter and pitch BUT it was a power boat 3 blade prop instead of a sail prop.

It lugged at low RPM and did not reach full rpms. The new one does both and it sails and motors faster too. Any reputable prop sho has the references to compute it and if you pitch is off they can generally be adjusted by up to three inches.

Mine couldn't be adjusted so I had to buy a new one. I wouldn't assume because the boat has the original prop that it is the correct one either. For about $75 most shops will rebalance and shine it all up pretty. Not a bad time to als ask them to run the numbers for you. Just get the engine numbers and and the transmission numbers and they should do it.
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Old 24-03-2006, 20:17   #7
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We had considered changing from a 2 blade to a 3 as well ... primarily to lessen prop walk. We came to find out (from forum members) that going to a 3 blade wouldn't lessen the prop walk ... but could induce other problems. The true test came when bringing the boat home under power dead into a 35+ knot wind ... and she still made hull speed. I've learned to deal with the prop walk ... we will NOT go to anything different!! This 2 bladed prop does anything we could hope for!
Bob & Lynn
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Old 25-03-2006, 06:05   #8
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Weylan

One other thought -- if you're going to keep racing her why not look at a good two blade feathering prop. When I hade the Frers we went from a anemic two blade folder to a 2 blade Maxprop and it made all the difference when motoring both in forward and reverse. From a drag standpoint my speed against the polars was no different. Also some rating systems give you credit for a feathering vs folding prop.
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Old 25-03-2006, 09:10   #9
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I have to endorse the use of a feathering prop over a folding prop.
Folders after a few years need to be rebuilt. THe pin the blades pivot on wears and the blades themselves will wear at the stops from opening. If you can spend the bucks go for a Maxprop.
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Old 02-04-2006, 15:59   #10
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Try this site as well.

http://www.osbornepropellers.com/Design_data.htm

Phil
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Old 15-03-2008, 15:41   #11
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The best and only source I have found for selecting a propeller is the book by Dave Gerr, Propeller Handbook. A condensed treatment is by Dave Gerr in Sail, Feb 2001, pp81-84.
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Old 15-03-2008, 16:13   #12
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j.g. You had to go back a ways to resurrect this thread
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Old 15-03-2008, 19:55   #13
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You can call any good prop shop and give them all the information. If I remember correctly that information would include: waterline length, beam, weight of boat, horsepower, maximum RPMs, shaft size, transmission ratio and right or left hand. In additon, give them any special problems (such as blade tips being close to the hull - a problem I had which causes cavitation) . They will plug it all into their super duper computer and tell you almost instantly what size, pitch etc. you need. Not being that trusting, I would check with two or three prop dealers to compare both the specs and prices given by the various dealers.

Good luck!
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Old 15-03-2008, 23:08   #14
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I had michigan size mine for me too and ended up buying from one of their dealers although it wasn't a michigan wheel as it was a special order. Anyhow, I prefer to stick with fixed props after having my feathering prop eat itself (luckily i was hauling out).
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Old 18-03-2008, 09:42   #15
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boatdiesel.com has a website where you can find all kinds of answers to tech questions about prop size. for example, Yanmar engines has a link on that site to allow you to compute the prop size for any of their engines. ( boat diesel is one of the bargains in the family budget at $25 a year for unlimited access)
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