Tohatsu 18hp - engine 'racing' at high revs but not converting to speed on water...

[sailorinoz]

I have a Tohatsu 18HP (model M18E2S) on an inflatable. All professionally put together about 2.5 years ago).
Boat has been on the water about 4 times in our 30 months of ownership - and so have done about 6-7 hours of engine use.
Problem is the engine has a tendency to "race" - ie I go to put it into high throttle and the engine feels as it high pitch screaming and definitely does not convert to speed on the water.
So we drop the revs back so we ensure we get home.
I recall this happened the last time we were out about 18 months ago (and probably put me off a bit which is why we havent used it) - but would now like to have the problem fixed. I think when it happened last time we perservered with it and the engine seemed to clear itself and we were able to give it higher revs and this converted to also good boat speed.
I am not engine friendly - and also a sailor - so any help appreciated. David

PS - when we took it out yesterday we put in new fuel and oil - and still doing the 25:1 ratio as we have not done ten hours yet. And the engine fired up on about the second pull as I pre-tested it before putting it into the water.

 

[MattFL]

Re: Tohatsu 18hp - engine 'racing' at high revs but not converting to speed on water.

We need more info. If your prop gets air around it, then it will loose bite on the water and the engine will rev freely. Backing off the throttle usually resolves this. Also a prop with a broken hub will allow the motor to rev freely without spinning the prop, but as far as I am aware that is typically a permanent problem and doesn't clear up until you have the prop re-hubbed. The third thing I can think of is an out of adjustment shifter might allow the gearbox to jump into neutral. We really need more info, like what happens just before this problem occurs? Is this a new problem?

 

[sailorinoz]

Re: Tohatsu 18hp - engine 'racing' at high revs but not converting to speed on water.

Thanks for replying.
Okay - more info. I am not a boy racer or anything with the boat - as said, am still running it in so pretty careful with the use of it.
The motor is linked up to a steering wheel on a console - not sure if that is important or not. But when I increase the throttle to go faster it was during pretty much calm waters - the boat is definitely not jumping over waves or anything.
Have tried dropping the revs, or putting it into neutral, and then trying it again at the higher rev range - but there was no change other than a bit of an increase in boat speed but really not worth the higher pitch whining noise of the higher revs. When something just doesnt sound right you know - even to a sailor like me!

Have no idea what a broken hub is - I wouldnt have run over any rocks or anything.

I have taken the boat out and used the motor maybe four times in its 30 month life. I recall it happening the last time about 18 months ago. Same situation - doing easy motoring for say the first 30 minutes and then just wanting to increase the revs just to see what happens (again - in calm waters) and it may go for about 1-2 minutes and then with the throttle still in the same position the engine starts to get that high pitched whining sound but boat speed no where near what I know it can do.

I did phone up a local Tomatsu dealer and he said he could work out what the problem was in 3 seconds once he saw the motor - and mentioned it may be the brushes of the prop? I think I heard that right from him - but have no idea what that is, or how costly to fix. Thanks for the reply again.

We need more info. If your prop gets air around it, then it will loose bite on the water and the engine will rev freely. Backing off the throttle usually resolves this. Also a prop with a broken hub will allow the motor to rev freely without spinning the prop, but as far as I am aware that is typically a permanent problem and doesn't clear up until you have the prop re-hubbed. The third thing I can think of is an out of adjustment shifter might allow the gearbox to jump into neutral. We really need more info, like what happens just before this problem occurs? Is this a new problem?

 

[pvanv]

Re: Tohatsu 18hp - engine 'racing' at high revs but not converting to speed on water.

You may have a spun hub in the prop. Try a new prop.

 

[MattFL]

Re: Tohatsu 18hp - engine 'racing' at high revs but not converting to speed on water.

Agreed, without more info or taking it to the mechanic, then your next easiest task is replacing the prop. Or taking your prop to a prop shop or the mechanic to be inspected. Sometimes the hub in the prop gives out and the hub can spin without transferring the power to the prop, resulting in the engine revving but the boat not going anywhere.

 

[Sea Rider]

Re: Tohatsu 18hp - engine 'racing' at high revs but not converting to speed on water.

If no spare prop to test is available and before ordering a new one, perform the hub slip test. Remove prop from lower tail, mark 2 dots or a straight line with white marker as seen in picture, place prop back, go for a wot short spin, remove prop again, check pre aligned dots or line marks, if they have moved away from each other, you've got a spun hub. Buy same number stamped on prop body, should have a 9 which is a medium pitched prop.

Happy Boating