Masthead revisited

[Sea Change]

The refurb of my masthead continues.
I've sourced a replacement divider and cut it to size. Just need to drill the holes in it for the sheave axles and the two transverse bolts which secure it to the mast.

I've had new sheaves made from Delrin.

Next problem is that the inner surfaces of the box are in really poor condition where the sheaves have been rubbing on them. The guy who made my new sheaves has persuaded me that I ought to do something about it before I install them, otherwise they'll be chewed to pieces pretty quickly.

The aftermost sheaves can, with difficulty, be removed and replaced with the mast up. Three forward ones cannot, without removing the mast box, which requires dropping the fire and back stays.

His suggestion was to cut out a complete circle to remove all the damaged aluminium, and weld in new pieces. So all the work is done from the outside. Sounds pretty good, but he doesn't do aluminium welding.

Any other thoughts? I wondered if I could just make up thin sheets of aluminium to line the sides, and accept that the sheaves would have to be a little thinner. I'm replacing some of the running rigging anyway so I could drop down a size.

The bodge approach would, I guess, be to just fair it all off with thickened epoxy, and sand it smooth. But I know that getting anything to adhere to aluminium is difficult, so it might risk chunks breaking off and jamming the sheaves.

Any other suggestions?

 

[ash2020]

Could you not just put a washer either side of the (thinner) sheave so it doesn't touch the sides.

 

[Sea Change]

Could you not just put a washer either side of the (thinner) sheave so it doesn't touch the sides.

Yes that occurred to me too.
The other thing is, it's really only the main halyard that needs to be good. The others just have to be good enough.

 

[RunAgroundHard]

There are epoxy methods that include graphite in the mix for bearings such as rudder stock heals. West System has a product.

423 Graphite Powder | WEST SYSTEM Epoxy

West also have instructions for Epoxy to aluminium bonding https://www.westsystem.com/app/uploads/2022/12/650-K-Aluminum-Kit-Instruction.pdf

If it was me, I would try and repair with epoxy, inspect later and then take it from there. Cutting out and welding in is a big deal, easy for a skilled worker, but at a higher risk to you for a person who thinks they are skilled and trashes your truck.

 

[chris-s]

Would a ‘finger belt sander’ help clean it up?

 

[William_H]

The sides would have worn when the sheave hole wears allowing the sheave to flop sideways. New sheaves with neat fit on axle should mean no contact with sides. If you can fit in a washer each side so much the better but if not (sheave too wide) then epoxy will smooth out the surface. ol'will

 

[PetiteFleur]

Could you fit a thin piece of plastic - say PTFE which is slippery each side of the sheave?

 

[vyv_cox]

My aluminium gooseneck wore badly against the stainless steel pivots. I drilled out the aluminium and used lengths of PVC water pipe as bushes, expecting them to have a short life. I made several spares, which are still waiting to be used after about 10 years. A gooseneck is a considerably more arduous duty than your sheaves. I would make up thin PVC washers.

 

[vyv_cox]

Could you fit a thin piece of plastic - say PTFE which is slippery each side of the sheave?

Surprisingly, in a research experiment carried out by a colleague looking for seals in large ball valves, he found that the stainless steel wore whereas the PTFE did not. Apparently white PTFE is thickened with powdered glass. PVC or MDPE would be better.

 

[justanothersailboat]

When I had my mast down and fixed up the sheaves in the head box, I think (been a while and I didn't take pictures... I should have taken pictures!) I found thin tufnol sheet between the sheaves and the aluminium head. Something like that. That seems like not a bad option.

 

[Sea Change]

Good ideas, thanks everyone.

 

[Minerva]

How about a finger belt sander to help clean up the faces, and then if you have space between the shiv and the alloy face, a / a pair of 1.2mm thick polycarbonate large washers (ie old CD's) cut to fit the void?

 

[Sea Change]

How about a finger belt sander to help clean up the faces, and then if you have space between the shiv and the alloy face, a / a pair of 1.2mm thick polycarbonate large washers (ie old CD's) cut to fit the void?

Or, similarly, I'm thinking of making up a sanding disc that can fit within the box.

I could buy a finger belt sander but it would either be a cheap 110v device, or an expensive cordless one.

 

[geem]

Surprisingly, in a research experiment carried out by a colleague looking for seals in large ball valves, he found that the stainless steel wore whereas the PTFE did not. Apparently white PTFE is thickened with powdered glass. PVC or MDPE would be better.

If you go this route, make sure you consider the UV resistance. Some PVC and MDPE is not UV resistant.

 

[Sea Change]

If you go this route, make sure you consider the UV resistance. Some PVC and MDPE is not UV resistant.

I'm not sure how much UV it's going to see, with the sheaves on top of it?

 

[geem]

I'm not sure how much UV it's going to see, with the sheaves on top of it?

I had pvc washers on my gooseneck fitting. They suffered UV damage and fell apart. They were mainly under the boot on the stack pack but UV still destroyed them.
I replaced my masthead sheaves last winter. They had suffered UV damage. They were cracked in several places. I am not sure what the plastic was but it wasn't delrin.

 

[Sea Change]

Would Delrin be a suitable washer material? I'm not sure how it behaves at this sort of thickness.
The guy who made my new sheaves could probably knock me up something suitable.

 

[vyv_cox]

Acetal should be good for this, tensile strength 60 70 MPa, at least as good as PVC and polyethylene, good wear resistance, fairly rigid.

 

[Graham_Wright]

How often do the sheaves rotate? I have in mast furling and a roller Genoa so the only usage my sheaves have is for the spinnaker (but I don't have one!).

 

[Sea Change]

How often do the sheaves rotate? I have in mast furling and a roller Genoa so the only usage my sheaves have is for the spinnaker (but I don't have one!).

The main is the only one I'm really concerned about. Slab reefing. I usually manage to get things set up so that I don't have to adjust the topping lift. The genoa will go months without turning. And the fourth one doesn't currently do anything.