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Old 04-13-2015, 08:28 AM   #1
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Ford Lehman 120 - Are these good performance numbers?

Yesterday, we took our Defever 44 out for a run on the Sassafras River on the Upper Chesapeake. At full throttle both engines topped out at 2475 RPM. She was lightly loaded. A full load of fuel and water would add about 4,500 lbs. She is equipped with 25x20 props. Cruising speed was 1600 RPM, measured by phototac, and averaged 8.8 MPH up and down the river with negligible current and very little wind.
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Old 04-13-2015, 09:00 AM   #2
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The WOT numbers look to be spot on
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Old 04-13-2015, 11:34 AM   #3
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Originally Posted by catalinajack View Post
Yesterday, we took our Defever 44 out for a run on the Sassafras River on the Upper Chesapeake. At full throttle both engines topped out at 2475 RPM. She was lightly loaded. A full load of fuel and water would add about 4,500 lbs. She is equipped with 25x20 props. Cruising speed was 1600 RPM, measured by phototac, and averaged 8.8 MPH up and down the river with negligible current and very little wind.
I'm curious about the transmission gear ratio.
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Old 04-13-2015, 12:18 PM   #4
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Looks good to me. If one props for max rated rpm the engines should turn 2,500 rpm at WOT.

We cruise at 1650 (2 FL120s) in a 30,000 pound boat and that gives us 8 knots give or take.
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Old 04-13-2015, 03:40 PM   #5
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The transmissions are B-W Velvet Drives turning at 2.5:1, perhaps it's 2.57 but close enough.
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Old 04-13-2015, 05:06 PM   #6
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The transmissions are B-W Velvet Drives turning at 2.5:1, perhaps it's 2.57 but close enough.
Thanks. I was scratching my head as our similarly sized boat has twin 250s and is propped for max engine rpm (2800) with 24x20 wheels. But it's a semi planing hull with 2:1 gears. Now it adds up. By the way I checked your set up against the Dave Gerr Prop diameter/power/rpm chart and it's right in there. (I'm always surprised how close that fairly crude tool comes to real world).
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