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Old 28-07-2019, 12:25   #1
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Yanmar prop size

Any suggestions welcome.Just installed new 3ym30 to replace 3gm30 .current prop fixed 3 blade 14”.Should I go to 14.5” or 15”.Cheers Geoff.
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Old 28-07-2019, 18:42   #2
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Re: Yanmar prop size

My suggestions:

Was the previous prop suitably sized, i.e. could you reach hull speed and max RPM at WOT with a clean bottom and prop?

Is your new transmission the same ratio as the old one?

There isn't much difference between the 3YM30 and the 3GM30 in terms of max power and max rpm however it really depends on transmission ratios and both engines had several ratios available.
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Old 28-07-2019, 20:57   #3
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Re: Yanmar prop size

The 30 in the model number indicates that both engines are 30 hp so if the gearbox ratio is the same then you probably should not change the prop pitch or diameter. Those little high reving Yanmars tend to bog down quickly with any fouling of the prop.
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Old 28-07-2019, 22:12   #4
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Re: Yanmar prop size

No different horsepowers 3gm30 24hp @3400 27hp@3600 3ym 26hp@3480 27hp@3600.14” prop revs out to easy need larger prop to increase load on engine though didn’t want to waste money buying 2 props 14 1/2” and 15”.
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Old 29-07-2019, 01:21   #5
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Re: Yanmar prop size

So at max RPM, both engines are producing the same power at the same RPM.
Are the transmission ratios the same?
Why not try repitching your existing 14" prop(s) before replacing them?
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Old 29-07-2019, 11:57   #6
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Re: Yanmar prop size

The 30 indicates the engine dyno flywheel horsepower but the 27 is the output shaft horsepower after the gearbox losses are taken into account.
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