I have a 2017 Ranger 25SC Tug that has a Volvo D3 170 HP engine. Our cruising speed is normally 10 knots at 3000 RPM. When checked with a friends 25SC with the std D3 150 HP engine, he attains 10 knots at 1875 RPM. My bottom is clean and not really weighted down. Any suggestions on how I could lower my RPM for a 10 knot cruising speed?
Cruising RPM
- Thread starter Whyknot
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Automaticman865
Well-known member
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- Cutwater 28
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- Alana Kaye
You are in the correct ballpark as long as you can hit redline rpms. I had to change the prop on my Cutwater 28 as I could never hit the 3500 redline. 2 inches of pitch removed moved it from 3300 to 3500 rpms.
Stuart
Stuart
The D3-170 is rated for 4000 rpm where the D3-150 is rated at 3000 rpm. Your wide open throttle should be some where around 4000 rpm, however your friend with the 150 his wot will only be 3000. So rpm is not a good approach to compare performance. I have a D3-150 and at 10% below wot (2700 rpm) I get about 13.5 knots and at wot (3000rpm) I get about 16.5 knots with a clean bottom. I would expect you performance may be somewhat better than the 150, so you might try wot and wot minus 10% and see what you get. I would think you should do as good as my performance or somewhat better.
Thanks for your inputs. I forgot about the HP curve being different between the WOT 3000 and 4000. My WOT is close to 3950 which means I should have the correct prop. An interesting observation is I have to go to 3000 and my friend has to go to only to 2000 RPM with the same engine. He has a fuel savings advantage.
Red Raven
Coupeville, Wa
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Whyknot":2a4ntg9i said:
Not necessarily, to achieve the different WOT rpms your props must be different (or the transmission). The different prop on your boat changes the load on the engine and thus the fuel economy at a given rpm. That said, with 170 hp vs. 150 hp you will likely have a small increase in fuel consumption. Rather than comparing rpm you should be able to compare fuel consumption directly by going the same speed in the same water and looking at the mpg on each of your Garmin’s.
Curt