The following is a little geeky, maybe a lot geeky, but I am a geeky kind of guy:
Larry's boat used 12 gph per engine to go 15 kts. This was the result of a fill to fill measurement.
Looking at the Cummins data sheet for this engine, 12 gph is 225 hp. If it were propped correctly I believe 225 hp would be fine for long term cruising use. Tony Athens advised me personally to that effect when I was considering buying a boat with a Cummins 6BTA. That is 38 hp per liter and is just under Tony's magic 40 hp per liter max for continuous duty.
In its over propped state 225 hp is certainly high, because it is making that 225 hp at a lower rpm than Cummins specifies in its data sheet. Making the same hp at a lower rpm means the torque is higher so the engine stress is higher.
But recreational boat engines are made for fun and if going 15 kts for a few dozen hours over the summer makes it more fun, well ok. It will result in a little bit less engine life, but as pointed out by a PP it is probably no worse than the previous 10 years since its repower.
How about fuel economy. Well 24 gph total at 15 kts is .625 NM/gallon, pretty high.
Running at 10 kts and below should be even better for this boat. Fuel economy will be better at 10, much better at 8 and the engine will be happy at either speed. My SWAG is about 1 NM/gal at 10 kts and maybe 1.5 at 8 kts. That is where Larry reported he plans to run the boat until he corrects the prop pitch.
David
"In its over propped state 225 hp is certainly high, because it is making that 225 hp at a lower rpm than Cummins specifies in its data sheet. Making the same hp at a lower rpm means the torque is higher so the engine stress is higher."
And the EGT is over the limit with unknown pyro accuracy, boost is high - lower rpm HP extraction is always a tight edge to walk.
"But recreational boat engines are made for fun and if going 15 kts for a few dozen hours over the summer makes it more fun, well ok."
My experience with the 6B has led me to conclude that utilizing these engines over the edge will lead to failures in the hundreds of hours and not thousands. There are at least 3 persons on this site that have replaced there 6B's due to these reasons and many more I have been in touch with over the years at other sites - they have sent me similar rpm, speed , boost and EGT data with some just above these readings having dire consequences.
"It will result in a little bit less engine life, but as pointed out by a PP it is probably no worse than the previous 10 years since its repower."
Please advise these details about the previous 10 yrs / 650 hours:
- were they run with the same props?
- are the tachs know accurate?
- did they achieve the posted 26 knts WOT?
- did the engines cruise at 20 kts?
- was the boat ever run on plane?
- what # hrs was the boat run on plane?
I know almost nothing about this boat and these engines at this time so it is very hard to agree or disagree with anything.