dinghy and outboard issues .....

[kashurst]

Upgraded very tired 4hp mariner to much new and better 5hp mariner. put this on 2.3m v floor inflatable and goes mostly very well. However when planning after a few hundred yards the boat drops of the plane all of a sudden and the engine is screaming. First time I thought the prop had come off. However prop seems fine, the inner rubber bush seems OK ( I can't turn it around by hand when in gear). As soon as I drop the revs back off we go again. It's as if the prop is sitting in an air bubble.

Am I doing something wrong or do I need a different pitch prop???

 

[Nick_H]

It's as if the prop is sitting in an air bubble.

I think that's exactly what it's doing, it's called "prop ventilation", and is caused by the prop sucking air from the surface. The outboard may be mounted a bit high.

 

[[2068]]

Have you tried trimming in a notch or two?

 

[kashurst]

I tried different trim angles and it did improve but not cured. The outboard is fitted as low as it will go on this transom design. I will try and find a way to lower it a bit more
thanks for the help

 

[Searush]

I tried different trim angles and it did improve but not cured. The outboard is fitted as low as it will go on this transom design. I will try and find a way to lower it a bit more
thanks for the help

Cut an inch off the top of the transom? Sit further astern so that it is "only just" on the plane?

 

[VicS]

The outboard should be mounted with its AV plate roughly level with the bottom of the transom.
See http://www.smalloutboards.com/shaft.htm

I would feel inclined to go along with the idea that it is a spun hub.

You do not give the model details to look up anything about it but the normal way to check is to mark the end of the shaft and the prop, take it out and run up to the speed where the trouble occurs, then inspect the marks. If they no longer line up then its a spun hub.

Check the drive/shear pin as well.

 

[kashurst]

fixed problem

checked out the prop all OK. measured high and attitude of cavitation plate and decided to cut 30mm out of the transom. it now goes like a stabbed rat. thanks all for the advice