Here are the final figures on my 1978 246 aquasport cummins tunnel drive.
hurth hsw630V drive 1.6 - 1 ratio and running a 19 X 24 four blade prop, four people on board and 1/2 tank of fuel in main tank
test was done with a separate fuel tank calibrated with sight glass marked every quart and fuel supply and return lines.
water temp 85 degrees and 100 degrees ambiant.
4.8 gph at 24 mph at 2200 rpm, 850 degrees exhaust temp at 9psi boost.
with that said, the prop is a used prop, has been repitched, ideally i will end up with a 20X25, would be happy with a 20 x 24..efficiency to be gained there....
also, the propeller has a full stainless cage around it built out of 1/2" stainless flat bar stock 6" wide.
with the right prop, i should cruise at 22 at 2000 rpm at 4gph. boat handles great, the last mechanical addition would be a small heat exchanger for the fuel as its a constant loop and heats up at every pass thru the injection pump.
considering the drag from the prop cage and the not so perfect prop, i think ending up at 4.5 gph at 24 is a great improvement and a good end result out of a " free" hull i got...
i started out with a stock 160 hp dodge cummins engine, minor mods to injection pump, custom grind on fuel pin, 3200 governor spring, fuel screw turned in, i guess im at about 220 - 240 hp and close to 500 ft lbs of torque.
i should gain another 15% fuel economy when i bump the timing up a bit... so, its close but not done yet but i am happy so far and can live with it as it is right now...
