J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Engine Specs - speed and fuel

I have a 79 J30 with the 2QM15 engine. What is a good crusing engine rpm and what is the max rpm one should run at. How much fuel for each is used per hour and what is your boat speed. Also what the the shaft diameter. What is a good way to determine how much fuel is in the tank (no gauge) or do you keep track via engine hrs. If so how much fuel is used as per the engine hr gauges.

I normally use 2200-2400 rpm as my cruising speed. For fuel consumption, I assume that it uses about 1/3rd gallon per hour. That level is higher than actual, so it gives a cushion. Unfortunately, I don't have a good way to measure the fuel tank either. Mine has an approx. 6 inch inspection port on the top that's attached with about 8 screws. To check fuel, I usually just undo the screw and look in.

Sorry, forgot to answer the question about the prop shaft. On the early boats it was a 3/4" shaft. Due to shaft vibration, it was upped to a 1" shaft in later boats.

On hull #359, I have a 3/4 inch shaft and a fuel gauge on the top of the tank. It reads accurately enough that I dont have to open the tank to check fuel quantity. My tack has not been functioning since I bought the boat so I dont know the RPMs at cruise. But I do know that I typically I cruise at 5.5 - 5.7 knots over the water. I can push the speed into the low 6's if I really want the additional noise. I dont really have a problem with shaft vibration. Its mainly idle vibration thats the problem. Fuel usage has been around 1/4 gallon per hour. Its so little that I havent filled the tank in a year or more.

I have a 1980 J/30 (hull 201) with a 3/4-in. shaft, 2QM15 diesel, and a 14-gal fuel tank.
Today I found my 2QM15 Service Manual. It says the engine has a continuous duty rating of 14 horsepower at 3000 rpm. The most fuel efficient speed for diesels is supposed to be 80% of the max horsepower rpm and sure enough, the manual shows that the best fuel consumption is 200 g/Hph between 2250 and 2450 rpm. I'm guessing the units are grams of fuel per horsepower hour. At 2400 rpm the engine produces 11 horsepower.

I have a 1985 hull #523. We cruise at about 2,200 to 2,400 rpm. This usually equates to between 5 and 5.5 knots and have found that I also conservatively figure about 1/3rd gallon per hour at this rate. My tank has a fairly accurate fuel guage.

I have #92 it has a top deck fill in the anchor bay. I added a West Marine external guage and mounted the guage near the electrical panel. I set the sending unit for 6 inch tank depth so when it reads below 1/2 a tank it takes a 4gallon jug of fuel without backing into the filler pipe. Care must be taken when fashioning a new cover plate and sealing the sending unit to the new plate. Use Teflon tape on all of the screw threads and it is best if you can punch in pim nuts for fastening the sending unit to the cleaning plate. All other info is as the other responses.