Shear Pin

rothwell_neil 11:36am, 2 July 2008
Put motor in reverse, motor off pontoon, put into forward and no drive, then float around trying to get back on any pontoon. OK should have checked out the shear pin before now but you don't do you? Takes 5 minutes to check and replace if necessary.

Photo uploaded and as a materials engineer I can tell you this is classic fatigue with the dark areas the years of crack growth and the shiny bits the final failure. Been driving around with about 25% of the cross section for a while, glad it didn't fail when a big handful of reverse applied on entering the pontoon as that would be embarrassing.
Skykomish E29 10 years ago
mmmm lesson to us all unless you have a Mercury 6 then you don't have a shear pin, which I found out when I tried to get a spare. Apparently these work on a bush system, which strikes me as an expensive part to reploace if you need to.

I remember owning a Mariner 4 h.p. two stroke and I was forever having to replace the shear pins on that.

Should be part of an annual service to replace the pin as they are soooooo cheap and can really spoil your day if they fail.
santalars 10 years ago
I would have a non Achilles related question for you Neil.
I have a Land Rover Defender and as all of them the aluminium body is corroding especially at points where connected to the steel (partly galvanised) chassis. That makes sence to me as connections should be isolated. However, I will never manage to isolate all connections.
Would it help to connect a magnesium anode to the aluminium body? What do you think?
rothwell_neil 10 years ago
Galvanic corrosion is a problem on all wet things and particularly on wet muddy things near the sea. The problem is that despite all the magazine adds for cathodic protection of cars that you see every couple of years nothing works. In theory zinc and magnesium are higher up the galvanic series and would thus protect aluminium sacrificially. Trouble is that you need a complete electrolytic path between the metals (to be protected) and the anode. Thus would have to have fully wetted conditions with high conductivity such as seawater for any chance of this working. Tag with anodes and driving into the dock would work. This is unlikely to be practical, otherwise unless you tagged each point of contact with an anode (even a very small one) would be tricky to get it to work.

As it happens a mate of mine did a PhD on landrovers and corrosion and the best thing to do is pressure wash, leave to dry and then use the spray on wax wherever you can see the body to chassis joins to seal them. The underside benefits from liberal doses of this Waxoyl type product. If you happen to remove any fasteners then paint joint with the wax and then re-join as a barrier joint, can't avoid the fastener but this minimises the area of attack, no water ingress no corrosion. This stuff is actually very good and if warmed up can be applied with paint brush to the external side of the joint without looking horrendous. If you refit any fasteners then a splog of this under the head will again work as this keeps the water out of the joint area. Most exposed areas are fine as rain water is not that corrosive. Joints that let water in then concentrate all the salts from roads and air through dry/wet cycles and then you have crevice corrosion from salt concentration. Seawater and near the sea is much worse but again keep it clean and free of mud and grime and the body dries out and then no corrosion. Only corrodes when wet.

Very similar approach to stainless and aluminium, I have just fitted slab reefing and Barton make a big point out of using a stable grease between the boom and track, can't avoid the contact but can avoid the effect by keeping water out and by electrically isolating. They have a special grease for purchase but recommend in the absence of this any barrier grease or vaseline/petroleum jelly products just to fill the crevice and keep water out. Used to be called wet jointing.

As a point of proof I used to have an Opel Manta and they were renowned for corrosion, used plenty of the spare wax that my mate had for his PhD and no corrosion years later. Now have an Audi and they have fixed the problem on Aluminium bodies by the same process of wet jointing. Aeroplanes stay in the air for years by wet jointing and the use of cadmium treatments, we can't use Cadmium as it is not good for us, like an earlier thread we can't use TBT for anti fouling as it turns all female dog whelks and other Molluscs male and thus has a pronounced effect on their population. The military still can apparently.
santalars 10 years ago
Very helpfull, many thanks for this!
busy home [deleted] 10 years ago
1986/7 ? On board a Seamaster 23 (I later bought it) Albin 10 petrol inboard, Owner on bow to push off the pontoon we were approaching too fast with F5 astern. (Finger berth) Tried to engage reverse but cable broke , Owner jumped over pulpit but slipped and one leg went between pointy bit and pontoon ,crushing upper leg.
Stern of boat lurched away from pontoon that I was about to jump onto and wind took charge, No one about! Sh..... t
Just goes to show , Things can go badly wrong to the point of serious injury because of small mechanical faults when not even at sea. (sea master 23 great boat to bring up 4 kids on)had her 5 yrs
Groups Beta