Yamaha engine question

I had just bought a used 31t contender w/ twin 300 hpdi yahamas on it and the first time we took it out to the Edisto Banks we only burned about 100 gallons the whole day. This past sunday we took it out and and was could tell something wasnt right, we had to run about 5000 more rpm’s to reach the same speed as we did the fist time. I also burned twice as much gas. I did fill up the boat upon arrival after the fist trip with gas at the Edisto Marina. After we got back to the dock, i checked both water/fuel seperators and they were pretty clean, I also checked the spark plugs, fuel filter in the engines and everything seemed to look clean. Any Suggestions on what i should check or do. Also on the port motor the throttle control lever on the console would creep back slower and had to keep pushing it forward to keep the port engine running the same rpm as the starboard engine. I did tighten the tensioner and that seemed to work. Could the two be related?

you keep it on the trailer or in the water

We keep it in the water


There could be a lot of factors playing in. Did you fish the same length of time both trips? How many miles did you travel total? What was your load like on trip 2? I assume the 5000 extra RPMs really should be 500. If it is 500 then that could be because of load. 500 RPMs could mean 0.5mpg loss in some cases.

There are days that I burn 80 gallons then there are days I burn 120. I have a 26 with four strokes and I can get 2+MPG (I usually don’t)… 100 gallons for an all day trip in that boat seems a little low to me.

Check the hull for crud.


Stonoman is correct. If you have to run the engines at a higher speed to reach the same boat speed then your hull is fouled. Nothing is wrong with the engines. During the summer you can have enough fouling to slow you down in as little as a week.

Iain Pelto
Sea Hunt Triton 160
Native Manta Ray 14

you will need to have the bottom cleaned at least once a month if you run the boat often,once the water temp gets up, figure twice a month

My father told me once “If a dirty bottom slows you down, imagine what it does to your boat”

shut up and listen…you might learn something

Agreed. Check the hull first

Captain Steve Little
TowBoat US Charleston
Psalm 107:23-32

i keep my boat in water at ripley light once a week bottom scrub or baby barnacles appear and they will slow u down…100 gallons for 80plus miles of running and trolling with two 300hpdis musta been creeping along

2660 Sailfish
150 Yamahas

Just got done servicing the hull of our boat… not fun, but that’s where I would start. Tight lines -

Fish to live.

those motors have several internal fuel filters also. 1 low pressure and another high pressure filter I believe. I always get mine serviced so not 100% sure, But I have the same motors on triton 35 (much heavier boat) &we get 1-1.1mpg running between 4,000-4,400 rpm. Replace external fuel filters often & use ringfree or the merc premium plus oil which has it in it. 1,200 hrs on them so far.

I am going to pull the boat out, clean the bottom and get the engines serviced to check out all the internal filters. Thanks Satisfied and everybody.