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Old 04-03-2014, 12:47 PM   #1
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Proper prop size

Mainship 390: currently looking at different age Mainship 390's. Question, how do you determine if the prop is of proper size for the respective boat? Thanks
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Old 04-03-2014, 01:43 PM   #2
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Mainship 390: currently looking at different age Mainship 390's. Question, how do you determine if the prop is of proper size for the respective boat? Thanks
Normally, the engine should be able to turn up to it's max. rated RPM with the correct prop and conventional wisdom is that 4 blades work best behind a keel & are smoother and 3 blades are ever so slightly more efficient.
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Old 04-03-2014, 03:52 PM   #3
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First, make sure that your engine will rev up to its maximum rated rpm + 200 or so, in NEUTRAL. If it won't do that, then you have throttle stop or other more serious problems.

Then take it out, warm it up and run it for a few minutes at wide open throttle in gear. It should be able to hit maximum rated rpm plus maybe another 100 rpm. If it hits max wot rpm + 200 then it is underpropped. If it hits less than maximum rated rpm then it is overpropped and you are a candidate for a prop repitching job.

The Mainship 390s were delivered with Cats, Yanmar 6LPs, Yanmar 6LYs, twin Yanmar 4LHs. All have different maximum rated hp rpms.

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Old 04-03-2014, 09:37 PM   #4
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Call Michigan. Gather up all your numbers and call. They have given me great information and if you want to ask about # of blades they will be able to tell you if you need enough blade area to warrant a 4 blade. Faster boats w lots of power and little prop clearance are good candidates for 4 blade props but most trawlers are better off w 3. When you get Michigan on the phone ask them about it.

The OP asked about size. Diameter and blade area is basically it. The maximum size of your prop may be limited to the space on your boat for the propeller. Once you've got size scoped out one should strive to get the best dia pitch relationship. Michigan again can help you w that. Most of our trawlers have a relatively low prop loading and many have an excess of blade area and too little pitch. Too much power is wasted turning blades through the water so there's less power available to develop thrust. The above is IMO only.
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Old 04-04-2014, 08:12 AM   #5
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The Mainship 390s were delivered with Cats, Yanmar 6LPs, Yanmar 6LYs, twin Yanmar 4LHs. All have different maximum rated hp rpms.
And Cummins 6BTAs.
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Old 04-04-2014, 08:39 AM   #6
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........

Then take it out, warm it up and run it for a few minutes at wide open throttle in gear. It should be able to hit maximum rated rpm plus maybe another 100 rpm. If it hits max wot rpm + 200 then it is underpropped. If it hits less than maximum rated rpm then it is overpropped and you are a candidate for a prop repitching job..............
That only works with a clean bottom. Any growth will slow the boat down and it won't be able to hit the rated RPM.
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Old 04-04-2014, 09:40 PM   #7
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If you want education about this go to Seaboard Marine site and read Tony Athens articles on the subject also Dave Gerr"s book The propeller handbbook. Otherwise go to a good prop shop and ask for help.
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