2 for the price of 1 - Nylon bearings & Cold curing epoxy

[Stemar]

I need to replace Jissel's lower rudder bearing.

According to the Owner' Club, the right way to do this is to cut off part of the old skeg to free the rudder and the old bearing, then get a plastic through hull of suitable dimensions and mount it in a metal bracket, thoroughly glassed onto the skeg. Weight bearing surfaces are supplied by oversized SS washers and a little ingenuity (SS rudder stock)

Problem 1:
Since most plastic through hulls seem to be nylon, and nylon swells in water, I need to know how much clearance between the rudder stock and the inside of the through hull I need to allow for the swelling. Are we talking 1 or 2 mm, or more on a stock of, from memory, about 40mm?

Problem 2:
I'll have to work outside, during the winter. Keeping the job above 15 degrees for long periods of time would be a problem as I only have limited electrical power. Will epoxy cure, albeit slowly at lower temperatures (5 - 10 degrees), or will it go manky and never reach full strength?

If the latter, is there an additive to help things along?

 

[Talbot]

amazed the people are still talking about using plaastic for this purpose. Delrin is much more suitable.

 

[oldsaltoz]

G'day Steve,

You need heat AND low humidity, less that 74% is ok for most epoxy brands, perhaps some sort of temporary chimney built around the skeg with a kero heater or other heating system that does not require electricity will help. If the surface has cured before it gets too cold, the rest will cure given time and temperature, if the temperature drops too low before the surface cures it may never full cure. Sand off any white or light coloured areas indicating moisture has entered prior to full cure.

Avagoodweekend......

 

[TigaWave]

Delrin is a plastic...its an acetal plastic, but it is OK for a rudder bearing, and doesn't absorb much moisture. 0.22% I think is typical?
Check with the manufacturers for expansion and absorption they should be able to advise on different makes of Acetal or Nylon.
Thes people are usefull
http://www.midlandplastics.com/srtd_acetel.htm

 

[TiggerToo]

Go here for the Vesconite bearings: I have not done so myself, but will do when needed

 

[Poignard]

Looks like good stuff but do you know where we can buy the small quantities we need for our applications?

 

[srp]

Vesconite definitely - very little expansion in water, and it's made specifically for bearings in aggressive environments. There are contact details for the UK agent on the Vesconite web site. If you ring them they will quote you for an offcut.
I've used it for cutlass bearings - excellent stuff - no measurable wear after 6 years on my last boat. Vesconite also do a good design manual so you can calculate the exact size you need to machine it to.

 

[Avocet]

I can help on the epoxy one - having just asked the same question myself! I'm using SP 106 resin. I spoke to a very helpful chap at Gurit who said that it cures below 10 degrees but much slower. At 3 degrees, it (effectively) stops but it WILL carry on when the temperature rises again. It's not all good news though. The hardener is much more willing to combine with moisture than with resin so if there is any moisture, it will have a go at that first (hence the milkiness). Obviously, any hardener that has combined with water, won't be available for curing resin so you might end up, as OldSaltOZ says) with slightly under-cured resin when the temperature does rise again. If you can keep it BONE dry until the surface is at least dry to the touch and reasonable dry thereafter, I think temperature probably won't matter too much. If, howervr, it's cold AND damp, you might experience a loss of strength. if your application is strength-critical, this might be a problem. The guy I spoke to couldn't over-stress the importance of MIXING it at a higher temperature. The resin goes really viscous at low temperatures and he said it was extremely difficult to get the hardener to fully disperse by stirring it if the whole mixture was too thick. After mixing, he wasn't too bothered about low temperatures.