powers up but no push

[gateway]

I have a 12 aluminum jon boat with a 79 johnson 4hp on the back... It runs good till about half half throttle, when I go to full thottle the motor rev's up high but, the boat doesnt seem to pick up any more speed. Im not expecting to fly across the water with only 4hp but, I think there should be more there? Could a prop change help?<br />thanks for any help.

 

[gateway]

Re: powers up but no push

The model # on the motor is 4w79d if that helps?

 

[naughtybynature]

Re: powers up but no push

check your engine height. you might be too high and the prop is ventilating (or is it cavitating jb?) rule of thumb is that the cavitation plate on your motor should be atleast even if not below the bottom of your boat. if your engine sounds like it is gaining rpm, then your prop must be slipping. i would assume that if you throttled up and the engine stayed the same note or sounded like it was bogging down that you have no juice left and need to go to more horsepower.

 

[gateway]

Re: powers up but no push

What is a cavitation plate??

 

[naughtybynature]

Re: powers up but no push

on your motor, its the horizontal plane just above your prop.

 

[gateway]

Re: powers up but no push

The plate rest just below the water level, The boat pulls out good but, if im tooling along at say half thottle and I gas it to WOT the motor rev's very high but seems to gain speed very slowly if any at all?

 

[JB]

Re: powers up but no push

Howdy No4njnk.<br /><br />I think NBN is right. Sure sounds like ventilation to me, though you could have a "spun" prop or you could simply be reaching displacement speed.<br /><br />The "cavitation plate" should be level with the bottom of the boat so that the prop grabs only water passing under the bottom. If the prop is too high it grabs air and tries to push the boat with foam. That is ventilation.<br /><br />There are two possible causes of ventilation. The engine could be mounted too high. Though I don't think I ever saw a little tinny with a 20" transom, if you had a short (15") shaft engine on a long (20") transom that would happen.<br /><br />If you have the engine trim set too far out the prop will ventilate. Engine trim on your 4 is set by placing the pin that the engine rests on when down. Set it so that the "cavitation" plate is parallel with the boat bottom.<br /><br />Next possibility is a spun prop. Either the rubber hub has broken and allows the prop to slip on the shaft, or there may be a broken shear pin in the hub.<br /><br />Final possibility is that the engine is getting your boat to it's "displacement speed" which is the highest speed you can get without planing. It would take at least twice as much power to lift the boat onto plane, so applying more power after reaching displacement speed will not increase the speed unless there is enough thrust to plane it. Your 4 hp would not be enough to plane even a 12 footer.<br /><br />Hope that helps.

 

[gateway]

Re: powers up but no push

Thanks JB for the info, I understand now! Im looking for a 15 hp now.<br />no4njnk

 

[naughtybynature]

Re: powers up but no push

jb you are totally right. it sounds like that hull is at displacement speed. there is no way that 4 is going to make a 12ft tinny plane. a 15 on the other hand will put you near bare footin speeds. if you want to learn just give me a shout. good call on the trim too, missed that in my first post. that would make the prop ventilate as well at higher rpms.