2008 R25 Cummins 150 Hp will only do 12kts

[jjbussell]

I have a 2008 R25 with a cummins/Merc 150Hp. I can get her up to 3500 RPM, but she is only doing 12 Kts. Also I can not sustain that RPM as the temp starts to rise to 195 or higher. The cooling system has been checked without any problems being found. The prop is 17" diameter by 13.5 Pitch. Would a smaller diameter prop help solve this problem?

 

[scross]

Several things could be going on with your boat.

First, Is the bottom clean? A fouled bottom could easily cut top speed by 2-5 knots depending on how dirty the hull was.

Second, what is the WOT RPM for your Cummins suppose to be? My Yanmar 150 hp 4BY2 engine has a published WOT of 4,000 RPM. The best I can get is about 3,950 RPM at WOT. You should be able to get close to your published WOT RPM, whatever that is for your engine. Not getting close to that means you are not seeing the top speed you expect. In a test I did a year ago, I was getting 14.6 knots at 3,950 RPM.

Third, are you properly trimmed? You can’t get to speed if the stern is low and the bow high. I added fins to my trim tabs to make them more effective in getting to the proper trim angle.

I don’t think 195 degrees is that hot. At least it isn’t for my Yanmar. My WOT temp is 196 degrees. Several factory techs at Yanmar told me the engine is ok with temps up to 205 degrees without doing any damage. Your Cummins may be different.

For any sustained cruising, the experts, recommend keeping at or under 80% of WOT RPM which is 3,200 RPM for us. At that RPM I get just over 10 knots. Fuel consumption at that RPM is about 4.6 to 4.8 GPH. The R25 Classics are just not fast boats.

PS: The weight of your boat may also make a big difference in top speed. The published weight of the 2009 R-25 Classic was an absurdly low 5,700 pounds. With options, minimal fluids in the tanks and our cruising gear we weigh in at the certified scales at about 7,700 pounds. That’s with no one on board! As others have noted, lightening up the boat will improve speeds at all cruising RPMs.

We have a Acme 1390 Prop 17X14 with .105 cup.

 

[PanaSeaAh]

On our 2009 R25 with Mercruiser Cummins QSD 2.0 150 HP and stock prop (don't have exact specs) I see the following as of 2 days ago: Clean bottom (trailered), 3/4 fuel, 1/2 tank fresh water, < 10 gallons in black tank, 2 people on board, lake water temp 72, lots of "stuff" on board, oversized trim tabs by previous owner:
WOT 3840, 16.3 MPH
80 % throttle 3390 RPM, 14.2 MPH, Temp 186, Boost 15.6, Oil press. 90 - 94
68 to 69% throttle, 2850 RPM 12.4 MPH, 4.2 GPH, Temp 180, Oil press 90 - 94
2240 RPM 7.6 MPH, 2.7 GPH, Temp 176 - 180
Boat is sensitive to trim tabs for sure. Would guess trim planes are 3 to 4" wider and 2" deeper than original. Usually run at 2850 or 2240 RPM. Above that, just using more fuel. I can't find the WOT number right now, but I am sure it is between 3850 and 4000. Don't think it has ever gone over 3860. Assuming you have a very clean bottom and prop and without knowing the water temp, I would be sure that the water pump is OK. Also, note that specs say thermostat is fully open at 203F, and you are not there yet. You did not mention if you have extra weight on the stern (propane, dinghy, kicker, etc.)
Higher speed may also be caused by the Michigan State banner I fly :lol: