Yesterday, on my way in I put my 300hp 3116 on the pin to check that I was getting max RPM fully loaded. 31 Duffy, 200 gals fuel, 3 men gear and 75 gallons of water. She went just a hair over 2800, which I feel was perfect for the load. Now the question--On the pins, on the GPS, I hit 21.8 knots steady. Does this mean that basically, if I repower with an engine that is producing 300hp at rated cruising RPM I can expect 21 knots? It makes sense, but things that seem to make sense hardly ever work out in the real world. I am thinking ahead for when I sell my 3116 to lobstercatcher

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I'll take a stab @ this
Simple math says yes. burning 15 gals per hr. +- a few % equals 300 hp in any marine diesel engine.
Now you have to get the "rubber to the road".
21.8 knots with 300 hp is @ 2800 rpm with a particular gear and prop pitch.
1st thing is you'll need is an engine that is designed to produce 300 "cruise" hp.
The Cummins QSB 5.9 480 comes to mine. The engine is designed to produce 300 hp (15 gals/hr) over many thousands of hours.
There are many others engines that will do the same but plan on buying an engine designed for 300 hp cruise rpm.
You may have to change the gear to get the same shaft speed @ a lower cruise rpm or increase the pitch of the prop or some combination of the two.
Making the change could effect efficiency few percentage points either way??
How about some "more" info?
What is the boat geared and propped to now.