Engine speed increases but boat does not

[canoeboy1]

I have an early 60's Bundy. Italian made 30Hp outboard. similar to a Johnson/Evinrude. Two cylinder.

I got both cylinders firing. Then fixed the fule pump diaphram. Got the boat in the water and it ran great.

Next day it started fine, got up to about 3/4 speed when something went clunk. The motor would move the boat at slow speed, but when I would increase the throttle, the engine would speed up but the boat would not. I checked the prop and shear pin all ok. The prop would rotate in neutral and would not in gear. I would rotate the motor by hand and the prop would turn.

I'd start it up and move slowly. When I accellerate, the motor would spead up but not the boat. It was almost like running an automatic transmission with little or no fluid.

What did I do?

I have not had a chance to pull plugs or anything yet.

Thanks,

Paul

The motor is on a 15' 1954 Feathercraft.

 

[jtexas]

Re: Engine speed increases but boat does not

except for the "clunk", sounds like the spun hub...but you mentioned a shear pin...so I take it your prop doesn't have a rubber grommet surrounding an inner hub...

Drain the gearcase oil...a small amount of metal filings (small like sand) is normal but look for chunks or splinters. I don't know of a gearcase problem that could cause those symptoms but somebody else might.

other possibilities:
Ventilation...it's pulling exhaust gas or surface air onto the prop blades so they're trying to push air stead of water.

Cavitation:...something is interupting the flow of water over the prop causing extreme low pressure on the leading edge which boils the water, creating bubbles. Can be caused by damaged prop blades...bent out of shape or broken off tips or sharp corners on the leading edges...or a sharp edge on the gearcase...

doesn't explain the "clunk" though...you'll probably end up rebuilding the lower unit...hope the shop manual is in English not Italian...

 

[F_R]

Re: Engine speed increases but boat does not

Not a Bundy expert, but chances are 99.9% that you have a spun prop hub. All gearshift motors since 1950-something have rubber cushioned prop hubs. The fact that you can't slip it by hand is meaningless. Here's how to tell: Make a mark on the prop nut and another on the propeller, aligned with the first one. Mark the propeller, not the hub. Then run the motor till you experience the problem. Stop the motor and look at the marks. Are they still aligned? No? Then the hub slipped. If they are still aligned, repeat the test, there's a slim chance it stopped spinning in the same position. The prop can be be rehubbed by a prop shop....If they have one for a Bundy that is. Otherwise, replace the prop....If you can find one for a Bundy that is.

 

[ezeke]

Re: Engine speed increases but boat does not

It could be like the OMC props - most of those that had shear ["drive"] pins had the rubber grommet as well.

If the shear pin is not broken, check by making a mark across the inner and outer portion of the prop from the propeller shaft and see if the marks are not alligned after running.

If they are not, you will need to find a prop shop to repair the prop.

 

[canoeboy1]

Re: Engine speed increases but boat does not

It could be like the OMC props - most of those that had shear ["drive"] pins had the rubber grommet as well.

If the shear pin is not broken, check by making a mark across the inner and outer portion of the prop from the propeller shaft and see if the marks are not alligned after running.

If they are not, you will need to find a prop shop to repair the prop.

 

[canoeboy1]

Re: Engine speed increases but boat does not

This all makes sense. I did not know about the prop have the possibility of slipping. Great news is that I have a second parts motor that does have a prop on it. Looks like I'll put that one on first and see what happens.

I guess you can damage the rubber part without shearing the pin. I was in a shallow lake.

Thanks much for the help.

I hope the fix it that simple.

Paul