Shear pin

[Mhvoiceuk]

I took my five year old son out today for a mini motorsail round Chichester Harbour. I put up the main, started the engine (8hp yamaha longshaft in a well) and off we went. After about five minutes I realised that we were going very slowly and quickly worked out that it was the mainsail alone that was propelling us. The propeller was definitely turning but not pushing us at all. There was a (seemingly weak) vortex behind the boat which stopped when I put the engine into neutral. As I increased the revs the engine started to over rev. I can only assume that the shear pin had broken and that the outboard was still turning the propeller by a bit of weed or barnacle or something. Does my diagnosis sound right? Or could the problem be something else? I managed to sail slowly back to my mooring and pick up the buoy. Luckily the weather was fine, the sea state very calm and there was a gentle breeze. Not something I want to repeat with child in tow. And I will always remember to have the jib sheets attached on the furling gear in future as I might just need two sails! I did lean over the back as I was leaving in the dinghy and could spin the propeller freely with my hand so nothing was snagging it. I'm going to beach the boat this week with a spare outboard on a bracket and have a look.

 

[PeteCooper]

Yes, most likely the shear pin, and only a couple of minutes to check / replace. However it could be the drive shaft which has snapped. The drive shaft has a hole through it about half way up or down, and they have a tendency to snap across this.

 

[Talbot]

beaching in order to do the shear pin seems a bit extreme! should be able to do it afloat without any problems. lift the engine on the tackle attached to the boom if it is too heavy.

Sounds very much like the shear pin, and suggest you carry a few spare!

 

[Mhvoiceuk]

Thanks for the confirmation. I hope it is just the shear pin. Yes I could probably wrestle it up but it's a very snug fit in the well and took me about an hour last time when I took the engine for a service. I'll beach it to give it a scrub as well (bilge keels) - kill two birds with one stone.

 

[david_bagshaw]

If the shaft had sheered at the split pin holes, wouldnt the prop have fallen off?

 

[simonfraser]

the three outboards i have had, had spare pins under the cover

 

[sparkie]

Had all sorts of problems with shear pins in my Honda 5. Always went -with no provocation -just when I needed the engine most. Replaced mine with a stainless pin, best thing I ever did.

Safe sailing

Sparkie

 

[PeteCooper]

The shaft I was referring to is the vertical drive shaft running from top to bottom of the leg, not the short shaft to which the prop is fastened.

 

[Joe_Cole]

Does that engine have a shear pin? I thought that shear pins were generally only used on smaller engines. On bigger engines I think that the prop is usually on a splined shaft.

When I had a problem like that on my engine (8hp Mariner) it turned out that it was the gear selector that wasn't working, the wretched thing was permanently in neutral. Easy enough to fix (once I had worked out what was wrong!), just one screw needed tightening.

 

[Mhvoiceuk]

I am a little more worried now as i found this pn the internet:

outboard thread

this is kind of the same - prop turning but no forward motion

 

[VicS]

If the engine uses shear pins then it should be mentioned in the manual together with some indication on the method of replacement. Also spares should have been supplied with the engine. If on a rubber hub that should be mentioned.

Take the prop of first and confirm how it is driven once it is off turning the shaft by hand should enable you to accertain if the gear selector is engaging or if there is a mechanical failure else where.

The website may be useful
http://www.yamaha-motor.co.uk/products/marine/index.jsp

The i boats forum may also be useful
http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi

 

[Mhvoiceuk]

Spoke to Home Marine who service the engine. They say prop is on spline and that most likely rubber hub mounting is worn and slipping - particularly as engine worked ok in reverse.

 

[VicS]

I missed the bit about being OK in reverse. It could still be a problem with forward gear not engaging due 'linkage' somewhere needing attention but hopefully nothing serious. You'll be able to check that once the prop is off.

 

[Lakesailor]

If it is a pin on the prop, look at it really carefully. In fact remove it even if it looks OK.
I had just the same symptoms on my Honda 5 and it was a broken shear pin, but cunningly broken at an angle so that it sometimes caught up and drove and other times slipped and freewheeled.
Considering they're well regarded I must say I don't have a lot of confidence in mine.
A mechanic at the agents (and I won't say where) suggested Mariner are a better motor.