Whats harder nylon or metal?

Peppermint

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I was supposed to skipper a Bav last weekend. It was out of commission. I'm told it had rudder problems caused by the nylon bearings chewing up the stock. It also is reported to have a blade full of water, so repair was taking a long time.

Anyone else had a situation where metal came of second best to plastic?

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Heckler

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used to work on diesel electric locos yrs ago, the con rods on the wheels used to have brass bushes and they wore out on a regular basis but didnt do too much wear to the steel pins, however when we started using a "plastic" material to make the bushes they lasted a lot lot longer but the pins wore out!!
stu

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wrapped around prop shafts melts and welds together and quickly wears away the shaft ......


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briananddoreen

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It is a fact that in engineering terms soft wears hard! As nylon also absorbs water and swells this means that the bearing becomes tight so therefore the shaft wears quickly, add a little grit or sand and the wear and scoring on the shaft increases rapidly. PTFE [polytetraflouroethylene] would be a much better bet for the bush and can be obtained in tube to save waste.
If you must use Nylon, it pays dividends to make the bore of the bush anything up to 25 thousands of an inch larger than the shaft to allow for the decrease in diameter of the internal bore when pressing in and also to allow it to swell without becoming tight.
Stainless steel is not really a hard material in engineering terms and is easily bent and deformed, [ look at your pulpit for instance] try it with your pen knife and you will see how easy it is to mark.

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halcyon

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Once had a demo film of a rubber band, cutting through stainless steel due to electrolytic corrosion.

Brian

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vyv_cox

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Many engineering plastics are filled with glass for mechanical strength. This can cause wear to even highly hardened wear-resistant coatings and treatments. Wear of stainless steel trim by PTFE seals used to be a major problem in large ball valves in the oil industry.

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dick_james

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Agree, My father owned a garage and used to show me how the brass rivets on worn brake shoes would gauge deep trenches in the steel drum but remain relatively untouched. I was told it was bits of grit etc getting embedded in the soft material forming an effective abrasive for the harder material.

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Chris_Robb

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Nylon is the most stupid substance to use for a lower rudder bearing. It will swell and stop all movement.

Rudder full of water as well? This is probably very common, with many boats affected. I am not sure what the effect is on the stainless steel skeleton in terms of corrosion, but it is a major danger in freezing weather (if ashore) that the rudder will split when the water freezes.

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maxi

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The vast majority of rudder blades are full of water, or at least exhibit saturation readings when tested with a moisture meter, you wouldn't expect a Bav to be any better or any different to the rest, would you?

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charles_reed

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Tribology

is an interesting and abtruse subject, but generally the harder bearing material will be worn by the softer.

Whilst I hold no brief for Bavaria, most of their design engineering is fairly sohisticated and I doubt the lower bush fitted by them is standard nylon which swells considerably in water, it's more likely to be Delrin (which is as pricey as PTFE but has rather better structural characteristics).

It's quite interesting that the most common failure point with modern boats appears to be the rudder shaft. Wasn't the failure of a carbon-aramid stock shaft which resulted in the scuttling of a big Hunter in last year's ARC?

Still any competent mariner should know how to sail a rudderless boat - a far more likely challenge than riding to a para-anchor or having your in-mast furler fail fully out.
It's not, I believe, in any of the RYA Yachtmaster exams.

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boatmike

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Re: Tribology

I agree entirely Charles. Most are in fact Delryn which is not as hydroscopic as Nylon66. (don't swell up in water so much)
Wear has also little to do with hardness as you say. The reason Delryn is used is because it is a good bearing material and has low wear characteristics. There are two reasons I have heard put forward why rudder shafts in stainless steel wear (apart from the fact that it is a pretty rotten bearing material itself). One is if the vessel is dried out on soft mud and abrasive material gets in embedding itself in the soft bearing material making it like sandpaper and the other is oxygen depletion causing crevice corrosion in the shaft. Sometimes the latter is responsible for reducing the shaft to something like Aero chocolate which then wears rapidly or can even break. I have never known however why some boats have really bad crevice corrosion on rudder and prop shafts but others seem completely free of it. Anyone an expert on electrolysis?

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Evadne

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Re: Tribology

Not an expert but I've used S/S 316 underwater a few times. All I know is that the corrosion is between the skin and body of the material, which have differing electropotentials, and occurs in anoxic conditions - I assume any oxygen will skin over any exposed "body" material depriving the cell of its anode. If there are no other variables involved then the differences would be the oxygen content of the water and the quality of the steel. I'd guess that the oxygen is the deciding factor, enclosed, permanently submerged fittings are vulnerable, but so are items in still water - so the answer is to go sailing and get the water moving around the rudder! Failing that, get a swinging mooring that gets the tide flowing past it.

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ponapay

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Sailing without rudder

was done in my YM exam, and great fun it was too. Luckily it was in a nice old boat with a long keel (even if the forefoot was not too deep).

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