Bronze or stainless for gudgeons and pintles? (etc.)

[LittleSister]

Our project boat (26ft) has a big 'barn door' rudder hung on gudgeons and pintles on the transom and back of the long keel (boat GRP, rudder wooden). The existing fittings are a bit worn and I'm wondering about replacements.

What are the relative merits of bronze and stainless steel (or any other material) for this purpose? I am particularly interested in which is likely to wear more slowly.

While on the subject -
1) Would it be advantageous to have delrin (or similar) bearings from the outset, so that dealing with future wear is a simple replacement, rather than having to machine out worn metal fittings first?
2) Would a solid GRP gudgeon (with a metal, delrin or whatever bearing) be a feasible and strong enough alternative to a metal one?

 

[TradewindSailor]

Silicon bronze castings or forgings with 316 stainless pins. The bronze is an excellent bearing material.

The problem with a solid grp gudgeon is dispersing the stress. To do this the bearing material would need to be of a large diameter, say 25mm, and then secured in place with the grp or carbon epoxy. I'd keep things simple and go for bronze.

 

[William_H]

I would always go for stainless steel pins for gudgeons etc. The larger the better so perhaps 10mm diameter for your boat. Much however depends on the other material of the bearing. I would think that GRP with a suitable bearing material set in would be OK for the part attached to the rudder in that you can locally reinforce with carbon fibre back into the rudder blade. It is all in the design of the shape for strength. Your attachment to the transom needs to be robust. I have a swing rudder on a huge cantilever to the bottom pintle. ie rudder tip is a full metre below the bottom pintle. This has caused some concern at the strength of the transom skin in my case a SS bracket just bolted on with backing washers. Because of difficult access to the inside of the transom I stuck a large square of carbon fiber on the outside which ahs stiffened up the transom at that point. If you have a long keel with rudder pintle at the bottom of the rudder you will have far less concern here. good luck olewill

 

[Neil_Y]

Stainless fittings and modern composite bushes will give you longest life. Bronze is not great where there are abrasives and the older plastics such as Delrin deform under load and soften with heat. All of the plastics swell with moisture (despite what some will tell you) but this is taken account of in the finished clearances. The modern materials are often bedded in epoxy either in a GRP tube or a stainless/Al or steel fitting.

One of the wear issues if the vessel sits for any time in tropical waters is the abrasive nature of marine growth.

Yes I work with and supply bearings, ours are at the top end of the performance spectrum, often Delrin will be good enough for yachts but the choice is yours there are a range of materials to choose from. Just make sure you get your clearances sorted when you decide on finished sizes of the bush.

 

[LittleSister]

Thanks for all the suggestions.

Silicon bronze castings or forgings with 316 stainless pins.

Doesn't the combination of bronze and stainless steel set you up for electrolytic erosion? (One gudgeon/pintle pair is out of the water, one on the waterline, and the third well underwater.)

 

[vyv_cox]

If you have the fittings already and only have to take up some wear, why not ream out the gudgeons and line with plastic bearings. I have done this with my gooseneck, aluminium casting with stainless steel pin. I reamed out the worn aluminium and lined it with plastic water pipe. Has worked perfectly, taking up the wear and reducing noise.