Boat Thrust Problem - please help

Help. I’m having a problem with my boat. I normally make about 23-25kts at full throttle. It’s a 27’ crusing style Larson with a 300 hp I/O.

Anyway, last week when I went out, I could get into gear but the boat had significantly reduced thrust. I couldn’t even get up on plane. It sounded like the motor was straining and the boat was dragging. Also, I noticed that I can get over the max rated RPMs of 4600 now. I don’t know how high because I didn’t want to push it, but I know it was over 5K.

Anyway, I cleaned the outdrive of barnacles (minimal) and I checked the bottom for fouling (none).

My next step is to change the propeller (I have a spare) to see if it’s a prop problem, but I’m doubtful.

Could the prop be slipping on it’s shaft?

What else should I check?

Please help.

Thanks in advance.

AHHH a 27’ Larson with a 300 I/O huh?

You say you can hear it running over 5k rpm but you can’t get her to plane? Could be the prop, but most likely your trany is slipping and your losing gears. There is no bumping or clunking sounds right? I hope not cause if there is you may have blown the heads, did she have a yearly overhaul by a marine mechanic. I blew 2 out of the six cylinders on my 30’Grady and she wouldn’t plane running on 4 cylinders. Best bet, don’t try to take her out again with out having her checked by a mechanic. You could really damage the gears.
If you are not moored at a marina drive her over to a larger marina in your area and just ask the head tech, they should be nice enough to give you their two cents without charging you. And if they do give you good advice and you think you can do the work yourself, you will save a lot of money.
But definitley take it soon to a mechanic, its only the beginning of the season and there is too much fun to be had this summer on your boat to lose her now!!

Slipping tranny. Actually, it creates less drag on the engine, so the engine can spin faster.

Prop is not spinning as fast as it should for the given engine speed, so the final drivetrain is in need of repair.

Maybe clutch related. Does boat slide into gear very smoothly, or is it a bit rough?

What kind of tranny?

Sounds like the motor is ok, as you are turning plenty of RPM (more than enough).

You don’t mention which model, but I would guess it is a pretty nice boat.

Does this boat have more than one forward gear? I would think it wouldn’t as it originally did 25kts. If it does, I would look there first, make sure it shifting like it supposed to. Even if it is single speed, but uses a torque converter as an interface, if you get low on fluid, it could behave like this.

It sounds like you aren’t turning enough prop for some reason. That would account for the high RPM’s and the boat not going like it should.

To the more likely problems:
Does your outboard unit have counter rotating twin screws? Are both of them engaging?
Also, people steal the wierdest things. Maybe somebody stole your prop(s) during the offseason and a put smaller and/or less pitch prop(s) back on in their place?

I would say that it is extremely unlikely that the prop might slip on the shaft. That is a splined shaft. Most likely, any slippage of that would totally destroy the spline and/or prop hub. It should be pretty horrendous if that happened.

Whether it is the prop or the transmission, it certainly sounds like the engine is ok, you just aren’t getting the power to the water. If it was an engine problem, you would be turning low RPM and going slow.

Sound familiar:


I have a 1976 188 Mercruiser. It goes into gear, but under load and RPM’s it won’t push boat. I have trouble shot everything I can think of and still draw a blank as to what the cause of the problem is. The engine at 2500 RPM’s will barely push boat. When it is in yard, it seems to be functioning fine. The propeller turns fine, propeller locks when you turn engine off and put it in gear, can’t turn by hand. Any help you can give would be greatly appreciated.

Thank you

Ray Grant


Start with the easiest thing to check. My guess is that the rubber hub is slipping. You might confirm this by trying to turn the prop backward, not forward, when the engine is in gear. However, the hub may be gripping enough to resist hand pressure, but not the tug of 188 ponies. Borrow or buy a replacement prop and see if that doesn’t solve your problem.

Good advice. Thanks. I’m hoping it’s not the transmission…

A couple of other things…

The motor operates smoothly from idle up to full throttle. There are no strange sounds while in gear or going in/out of gear. Thrust seems normal at low RPM.

Another guy I talked to suggested it could be the rubber hub in the prop. I’m gonna change the prop tonight and see if that helps. (Apparently, there’s a rubber splined hub in the prop that protects the engine components if the prop abruptly stops. Like if you hit something…)

I SO want to believe that’s the problem. I don’t want it to be anything in the engine/trans.


I didn’t see the other replies. Thanks for the advice. I’ll post tomorrow with the results.

I have a nickel that says it’s the prop hub. LET US KNOW!