Outboard - Slipping and no power - Yamaha 4hp

[MS_Adventure]

Hello. I have a Yamaha F4A - 4 stroke - that’s about 15 years old. It has a problem: when I accelerate (not much beyond idling revs) there seems to be some slippage - to the point that no more power comes to the prop and the dingy will not accelerate. I can hear the engine revving higher, but the revs do not appear to translate to prop revs. Any advice as to where to start the investigation would be much appreciated!

 

[AllanG]

Have you checked the propeller drive shear pin, as this may have broken?

 

[MS_Adventure]

I have not. What is the best way to access and assess it? Assume simply remove prop and see if it is broken? If it was though, would the prop turn at all? Because in my case it does turn at low revs.

 

[Graham376]

Many small outboards don't have a shear pin, they rely on the bonding of the prop to the bush failing and the symptoms you have would indicate that's what's happening. Usually means a new prop is needed as not many small outboard props can be re-bushed.

 

[RichardS]

I have not. What is the best way to access and assess it? Assume simply remove prop and see if it is broken? If it was though, would the prop turn at all? Because in my case it does turn at low revs.

The prop will still turn due to friction but will not drive the dinghy. Once the prop is off, usually just a nut and a split pin or similar, you will quickly see what kind of impact protection mechanism was in place and what you need to do to fix it.

Richard

 

[VicS]

Hello. I have a Yamaha F4A - 4 stroke - that’s about 15 years old. It has a problem: when I accelerate (not much beyond idling revs) there seems to be some slippage - to the point that no more power comes to the prop and the dingy will not accelerate. I can hear the engine revving higher, but the revs do not appear to translate to prop revs. Any advice as to where to start the investigation would be much appreciated!

Have you checked the propeller drive shear pin, as this may have broken?

I have not. What is the best way to access and assess it? Assume simply remove prop and see if it is broken? If it was though, would the prop turn at all? Because in my case it does turn at low revs.

These are the classic symptoms of a "spun hub".. ........ The failure of the rubber bonding in the hub. ....................It does not have a shear pin. The prop fits onto a splined prop shaft

To prove put a mark on the prop in line with the head of the split pin through the retaining nut using a waterproof marker pen. Take it for a short run and open the throttle until it apparently slips.

Examine the mark. If it no longer lines up with the split pin head it confirms the spun hub.

 

[seumask]

As a true PBO bodge / quick fix if it does not have a split pin, easy to replace , it is possible to drill holes and put several screws through from the metal of the propeller through to the metal of the Hub that slides off the axle when you take the prop nut off. I've done this on the 8 hp Yam prop that I suspect is similar. Clearly you don't want to drill through the axel!

 

[duncan99210]

As a true PBO bodge / quick fix if it does not have a split pin, easy to replace , it is possible to drill holes and put several screws through from the metal of the propeller through to the metal of the Hub that slides off the axle when you take the prop nut off. I've done this on the 8 hp Yam prop that I suspect is similar. Clearly you don't want to drill through the axel!

This dodge does work: done it my myself on a couple of occasions. But I regard it as more of a “get you to a chandlery” measure than a permanent fix. I found that all too quickly the screws either sheered or worked loose. Probably new prop time.

 

[MS_Adventure]

Thanks for all for the info. Looks like I need to buy a new prop. as the longer term fix.

 

[thinwater]

Thanks for all for the info. Looks like I need to buy a new prop. as the longer term fix.

Look around. In the US there are businesses that re-bush for ~ $25-50.

 

[Boathook]

As per others it sounds like the rubber bush has failed because the prop has hit something. Steel developements ( https://www.steeldevelopments.net/propeller-repair-service/rebush-service.html ) replace the bush but the prob might need repairing. Quite often this makes it cheaper to buy a new prop.
I have found marine tech ( https://www.marine-tech.co.uk/parts/ ) good for Yamaha spares.