rudder bushing material, loads

[Burnsy]

If you want the short version:

What sort of loads can I expect (or a source for equations) for a 33'' 7500 lb. displacement racing sloop with a 4.5 ft deep x 2 ft long spade rudder. What tensile strength bushing material should be accepted as a minimum?

The long version:

I dropped my rudder and removed the old bushings, which I believe to be some sort of nylon after a quick burn test. Too much play in the rudder stock (which is aluminum) and such, the whole post was banging around in waves. I am planning on doing the epoxy/graphite solution on dan pfeiffer''s P26 page (http://dan.pfeiffer.net/p26/howto.htm) to create new smooth bearing surfaces on the rudder shaft.

So, I need to come up with a material I can have machined into new bushings. Lower bushing was made up of two 1" high pieces and 3 1.5" high pieces, all of about 2.7" ID and 3.3" OD. Upper bushing looks like a mushroom cap on which the aluminum stock-to-tiller set block rides. The hole through the mushroom cap is about 1.5" ID, 2.5" OD, and the cap diameter is about 4".

Some criteria I would like from this material:
Low cost - under $100/ft; Low coefficient of friction, high tensile strength, no water absorption, weather resistant, UV resistant, dimensionally stable, low thermal coefficient of expansion, decent impact strength, easily machined, scratch resistant, decently chemical resistant, must come in a 4" size rod.

Of course, this exact material doesn''t appear to exist. Time for tradeoffs.

I have been poking around www.mcmaster.com for some plastics ideas and specifications. Materials I have narrowed it down to include Delrin (either black or white), Oil-Filled Cast nylon, MDS Filled Cast nylon 6, and UHWM polyethelene.

Some I have eliminated include Cast Nylon 6 (too much water absorption), Delrin AF (not weather resistant, becomes chalky and brittle, and costs around 300/ft), and Teflon/PTFE products (also too costly at 200''s/ft, low scratch resistances).

UHWM would be perfect if it was UV stabilized, but still might be fine for the lower bushings. Coeff of friction is only .11 and price is dirt cheap at 19/ft. Has a higher thermal coeff of expansion than some others but since it probably won''t ever see temps over 100 F and we''re talking about expansions of 11x10 exp(-5) /in/in/deg F I don''t consider this a very high-weight variable. The biggest problem with this material may be the tensile strength. It is listed at 3050 - 4600 psi, and the nylons (which I believe my old bushings are made of) are generally in the 9000 - 12000 psi range, so I am not sure that UHMW will be able to hold up to the rudder loads without deforming... hence my earlier question.

Oil Filled Cast nylon would also be perfect if it were more scratch resistant, which leads me to be concerned about durability. It says it is very stable dimensionally, but i''m still concerned about the water absorption properties of nylons in general. Price is ok at 67/ft.

Delrin is slightly higher friction .2 - .25 and only moderately UV resistant but seems otherwise good, and I''ve heard of many people using this.

MDS-filled cast nylon 6 is a better overall nylon than the oil filled, but coeff of friction is up at .22 and I''m very concerned about water absorption of this one since it is not noted as dimensionally stable and the other regular Cast nylon 6 is very water absorbing.

Ok, there you have it, so if anyone has made it this far and has any recommendations or comments, please share!

Thanks in advance!

[RichH]

Rudder bearing typically fail due to abrasion and not compressional loads. I wont venture a guess about what rudder load you can expect; but, do offer the following:
Nylon will hydrolyse (the breakup of the long chain molecules)) in water; should NOT be used in ''wetted'' applications. Delrin is subject to "face transfer" - meaning the delrin will abrade quickly and attempt to transfer/attach to the rudder stock. UHMWPE is too soft to bear loads unless you have comparitively large bearing surface areas. PTFE or allied fluropolymers if purchased in cylinder or barstock will be prohibitively expensive.

I assume that you have worn rudder shaft bearings. An adequate strength and frictional repair is possible by using your present bearings and filling the voids with carbon filled epoxy and Technora (PTFE thread) or "PTFE dental floss". Method: polish the present rudder shaft to a "mirror" polish using "Tripoli" buffing compound (available in most old-fashioned hardware stores). Coat the shaft with wax; rub on then heat lightly with a torch to apply and smooth-out a ''few mil'' thickness of wax. Wind the PTFE thread in an very open ''diamond pattern'' on the shaft but not so tight as to not deform the wax. Mix epoxy resin and fill with carbon powder to the consistency of thin mayonaise and apply to shaft/bearing surface. Assemble the bearing and mix, allow to cure. When fully cured, rotate the shaft to break the wax free. This ''fix'' is quite popular on many Pearsons that use delrin rudder shaft bearings. Do a websearch for ("Pearson + bearings + rudder") for other info.

BTW - If you''re machining base polymer from scratch, your bearing clearance should be approx. 0.002-0.003" per inch of shaft diameter (less any press fit/interference fit that you use to press the bearing into the GRP to mechanically hold it in place.

[DelmarRey]

I have one word: Acetal

Its water and UV resistant. It comes in differt blends the Tucite* A (blue) being the strongest.