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Old 05-11-2016, 06:13 AM   #21
FF's Avatar
Join Date: Oct 2007
Posts: 14,908
I would go for a ride and with the engine warm and a GPS that reads speed create a graph from 100RPM on up .

Take at least 2 min at each RPM for the vessel to stabilize.

At some point you will add RPM and hardly any boat speed.

This would be your go home RPM , as anything above that simply is heating the lake.

To actually go faster may require a larger prop or a different transmission.

If course if the boat is a pure displacement boat you basically will NEVER go much faster.

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Old 05-11-2016, 09:09 AM   #22
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City: Port Credit
Country: Ontario
Vessel Name: DIRT FREE
Vessel Model: Benford Fantail 38
Join Date: Sep 2012
Posts: 1,817
37' full displacement, 165hp Perkins turbo, 1.92:1, 8knots at 1600rpm, 11knots at 2400rpm (ugly and noisy with major squat) ..... 24X20 prop.

Your prop is seriously wrong if your numbers are correct. Suggest you double check prop size and run one of the on-line prop calculators.
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Old 05-11-2016, 09:25 AM   #23
City: Carefree, Arizona
Country: usa
Vessel Name: sunchaser V
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 5,366
Originally Posted by tedted View Post
Sorry I haven't replied in a while. things have been a bit busy. I suspect that more load on the engine wouldn't hurt anything. I'm more concerned about running at 2400rpm forever and doing only 6.5 knots. That seems to make no sense to me. With almost double the hp as the original engine I should be able to run along at 9 knots wide open. The hull is a semi planing. Very little deadrise. I still haven't gotten the boat back home. Winter just isn't giving up this year and I don't want to cross an angry Lake Erie with a boat that probably needs more attention than I am even aware of yet. Thanks for your input, Ted

IMHO, you are indeed correct, running at 2400 RPM is something to be concerned about. The sweet spot for your engines is 1600 to 1800 RPM, this is the design range for gensets, farm equipment and pumps. Harmonics are balanced best in this range by design as well as noise being less. Bottom line, reprop to get where most of the rest of us are.
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Old 05-11-2016, 10:06 AM   #24
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City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 11,937
If your mechanicals are dependable I'd run it home at 1800rpm .... whatever speed that is.

Why your boat is going so slow can be sorted out later. Sounds like the boat is designed for higher speeds and has a hull somewhere between an Island Gypsy and a Californian. If the boat is excessively heavy or the gear ratio is Way different .. or both it will eventially be obvious what's going on. Sounds like it's got a 3-1 gear to me.


North Western Washington State USA
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