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Old 06-04-2021, 00:32   #1
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how to size a prop

I have a warrior 38 sailboat and my Brunton auto prop has failed, due to Brexit there appear to be complications in getting it repaired, I am in Portugal.
So, I need to get a new fixed prop, I don't have a fixed one to get data from, (it fell off!!! a few years ago, still don't know why)
The engine is a Volvo D2 40 with a 130S saildrive, I have used the data from Vicprop, and come up with a size tha falls between Volve sizes. should I go up to the nearest pitch/diameter or diameter/pitch.
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Old 06-04-2021, 02:55   #2
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Re: how to size a prop

When we repowered the professional we were working with did a bunch of calculations on repitching our prop to match the new engine. When we splashed it wasn't quite right and we had it 're repitched for correct match. My point is it is part science part art and part luck. Sure if it's a boat with lots of sister boats and your engine/transmission is just like theirs you stand an excellent chance of getting it right the first try. Add a few variables and it is less of a certainty
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Old 06-04-2021, 03:11   #3
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Re: how to size a prop

I would go to the nearest diameter as the pitch is easier to change.
You have to be careful with the room you have though. i.e. make sure you have the minimum tip clearance from the hull.
I dont know the market in Portugal but you can get some cheap used prop bargains if you are looking at a temp replacement for your autoprop.
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Old 08-04-2021, 06:36   #4
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Re: how to size a prop

Gotta get the diameter right, as Compass suggests. If you are between pitches at that diameter, go with the finer - the result if you err will be the engine not being fully loaded as you approach your red line. It will not produce full power and the boat will not go quite as fast. If the prop is too coarse, you will be overloading the engine. Be nice to your engine.
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Old 08-04-2021, 07:01   #5
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Re: how to size a prop

FWIW, when choosing between a few props I’d recommend going with the largest diameter that will fit properly (tip clearance, etc.) and then pitch from there. Sailboat props are nearly always smaller in diameter than the ideal in order to make them fit where they need to go. Of course, that’s a generalization and there are always exceptions.... YMMV.

But, if you’ve had two prop failures within a few years I’d recommend looking for root cause first. That failure rate is unusual/high (to me anyway) and I’d want to know why before investing in my third prop.
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Old 08-04-2021, 08:41   #6
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Re: how to size a prop

Quote:
Originally Posted by noblewarrior View Post
I have a warrior 38 sailboat and my Brunton auto prop has failed...
Let me guess- it threw a blade?
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