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04-03-2014, 11:47 AM
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#1
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Member
City: Greeneville
Join Date: Dec 2012
Posts: 12
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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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04-03-2014, 12:43 PM
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#2
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Guru
City: Cape Cod, MA
Vessel Name: Island Seeker
Vessel Model: Willard 36 Sedan
Join Date: Oct 2007
Posts: 1,306
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Quote:
Originally Posted by Trichamp
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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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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04-03-2014, 02:52 PM
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#3
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Valued Technical Contributor
City: Litchfield, Ct
Join Date: Aug 2012
Posts: 6,785
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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.
David
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04-03-2014, 08:37 PM
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#4
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Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
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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.
__________________
Eric
North Western Washington State USA
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04-04-2014, 07:12 AM
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#5
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Guru
Join Date: Jun 2008
Posts: 5,037
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Quote:
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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And Cummins 6BTAs.
__________________
Jay Leonard
Ex boats: 1983 40 Albin trunk cabin, 1978 Mainship 34 Model 1
New Port Richey, Fl
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04-04-2014, 07:39 AM
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#6
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Guru
City: North Charleston, SC
Join Date: Sep 2010
Posts: 4,870
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Quote:
Originally Posted by djmarchand
........
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..............
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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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04-04-2014, 08:40 PM
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#7
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Guru
Join Date: Apr 2014
Posts: 1,288
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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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