2014 D3-150 WOT question

[Bob B.]

When cruising with our R25SC with a full load ; at least half fuel, 1/2 - 3/4 water, 1/2 black water tank, dinghy on davits, 2 large adults, 1 70 pound dog, 2 loaded 45 quart coolers, tools food etc., we top out at about 2850 to 2920 RPMs depending on conditions.

We travel between 160O to 2000 rpm 90 percent of the time ( mostly 16-1700 ) kicking it up to 26,2700 rpm at the end of a trip for a few minutes then idle down before we anchor or tie off at the end of a day.

By Not hitting 3000 RPM, am I working the engine a bit too hard, slightly over propped, or is this ok?

Bob.

 

[BB marine]

If this is your fully loaded state and the engine can turn 2900 rpm fully loaded, full fuel, full water, two adults, dog and gear. If this is the standard way you use the boat and plan to cruise. This is what you prop the boat for. Fluid Motion doesn't know how folks are going to use their boat. They prop it for the best overall performance as it leaves the factory. The engine should turn 3100 rpm as it leaves the factory. This maintains the Volvo Penta recommended WOT specification of 3000 rpm +/- 100. At this rpm the engine is loaded properly. Anything above 3150 rpm is wasting energy and anything below 2900 rpm is loading the engine outside the manufactures recommendations. I always have propped my boats based on the normal way I use the boat. I want it to turn full recommend rpm. By doing this if I do add extra passengers, gear, get caught in disturbed seas and wind the engine is still properly loaded. When I owned my Cutwater C26 I found the boat to be weight sensitive. My meaning of this, when I added a few hundred extra pounds of gear, food, dinghy and dinghy motor the boats performance suffered and the engine could not make full rpm. I removed .035 of cup from the blade tips and the rpm was now in the proper range. Perfect right? Not really my engine was not over propped but the prop efficiency was no longer there. ( prop slip increased and overall efficiency dropped) I lost about 1kt in speed and about .3 of a mpg of fuel economy. Bottom line what was worse? Before I made the prop adjustment 3880rpm WOT after 4020 rpm WOT. If I had it to do over I probably would have left it alone. I was 20 rpm off of recommendation. It would have been a different story if I was seeing 3800 rpm or 3780rpm. That is significantly loading the engine out of range. I am an advocate of operating the engine within the recommended rpm range for proper loading. In order for me to do that with my C26 a completely different prop would have been needed. I had Acme spec out a prop. It was suggested that a 4 blade replace the 3 blade with a pitch change. I was not willing to spend the extra money. I removed the cup and lost efficiency to properly load the engine.

2850 rpm is borderline if this is the way you plan on cruising.

 

[Bob B.]

Thanks Brian, sort of what I thought … not sure what I am prepared to do. A buddy of mine, as well as myself have wondered about a 4 blade for our boats (25SC). I think I would have to step down the pitch and likely cup as well. Traveling around 8 mph do not think mpg would be affected dramatically. Hoping to not overburden the engine.