Best cruising speed for fuel economy?

I'm by far an expert on this sort of thing, but I've read that cruising 'hull speed' is the most efficient for miles per gallon. It's calculated by square root of length of hull at waterline x 1.34 . If your boat at waterline is 35 ft, then 5.92 x 1.34 = 7.93 knots. Of course getting anywhere would take way long, so it's a trade off between fuel efficiency and time. On my 32 aft cabin, we normally cruise at about 18 -19 knots, and tabs are kept down most of the time (except for list correction) otherwise the bow rides too high and we push too much water.

indeed the best ecomony on any boat is under hull speed if you have time, on long trip, it can really add up.

otherwise, on most boats, the best planning speed will be just over when the boat is fully on plane... probably around 19kts in your case.

the worst speed it half on plane, pushing a big wake... once you go beyond hull speed your fuel flow goes up a lot faster than speed. it depends on the boat, but 2 kts above hull speed can cost you as much as 40 to 50% more fuel

All good responses. I have a 42 carver w/ 454's. I once found a graph on the internet that showed best economy at hull speed. What was interesting was the same economy at 12.5 knots (plane) as at 8 knots. So you either have to go 6-7 knots to realize some economy or just get on plane. In my experience, it's about the same.

Also, I had someone ramp up the RPM's in 100 increments while I was in the engine room to note when the 4 barrels started opening up (I pre-marked the shaft with a white line).

You also have to account over the season for different loads (I had 12 people on board 1 trip), full or empty tanks, etc.

Keep ypur load under the 4 barrels (~3400 rpm) and still get on plane.

Hope this helps. Putting gas in is still cheaper as a rule than deisel equipment, parts and mechanics if your run time is moderate. All of which cost more than gas.

I've never met a gas engine that liked life at idle speed (fouled plugs & carbs). Generally 3400 to 3600 (just on plane) seems to be the spot.