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Fully agree, most washers used by the industry are fender variety, and they are under-rated for the application.
Hi Steve,
Good to hear from you. I bet you see a distressing number of fender washers?
I’ve been writing about that for years. I did a lot of similar tests. We agree. Bottom line: if the load needs distribution, a fender washer will do no better than a common bolting washer. Most of them end up bent. If you are using a backing plate, the plate should be (better be) stiff enough that all you need is a bolting washer.
The Bolt Depot is a good source of extra thick fender washers. You have to buy a small box, but for a few common sizes, that’s nothing. Lots cheaper than core repair.
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Want to match the stiffness of an extra thick fender washer (about twice as thick) by using several standard fender washers? Remember what John said about stiffness increasing with the cube? You will need 8 standard washers, which will be more expensive and stand about 1/4- to 1/2-inch tall. Using two will probably just result in two bent washers.
Hi Drew,
Thanks for the confirmation.
Hi Drew,
Thanks for the fill. Interesting about how many fender washers you have to add to equal one thicker washer. Gets me thinking about another tip.
Fender washers are not backing plates!
They are intended for soft or thin materials, where you need to spread the force of a fastener out over a large area to prevent the material from tearing or crushing. They’re a great choice for something like a thin piece of sheet metal serving as an exhaust heat shield, or hanging an electrical enclosure on drywall / gypsum board, or securing sheets of thermal insulation.
If you use them in a situation where the bolt is torqued to a non-negligible fraction of its yield strength, they will deform into shallow cones, concentrating the load under the bolt head and defeating the entire point of having a large washer.
Fender washers are, by definition, thin. They have to be thin because they are *supposed* to deform in a way that minimizes the tearing forces and stress concentrations in the thin material where it touches the outer edge of the washer. A thick oversized washer is not a fender washer and, if properly specified, may make a fine backing plate.
Hi Matt,
Thanks for the fill on what fender washers are actually for. Drew had some stuff on that too. Just another case where the marine industry adopts something because it’s easy without doing any basic engineering. Reminds me of saildrives: what engineer in their right mind would design a drive that requires the whole engine to be removed (in most cases) every six years to change a gasket!
On structural washers, I guess I can see a few cases where they would work but I think in most real world cases on a boat they too are a bit of cop out since a bonded backer plate will be a lot better.
Oops. Matt beat me to it, and of course with better clarity. Agree 100%.
Fender washer will protect the FRP from abrasion at the surface. FRP is not great in abrasion. If there is cyclic loading at the bolt, especially if the load is off axis, the fender can protect the surface of the frp from getting worn away.
Hi Robert,
I mentioned abrasion as a secondary use of washers in a footnote. That said, if the bolt is moving due to cyclic loading the joint was not done right or the bolt was not preloaded properly: https://www.morganscloud.com/jhhtips/under-torquing-is-dangerous-too/
John,
Unless the material is infinitely stiff, the structure moves under load, ie the structure ALWAYS moves under load.
Hi Robert,
That’s true, but in this case not if the bolt is preloaded at a higher load than the maximum design load, which every joint always should be. I explain this here: https://www.morganscloud.com/jhhtips/under-torquing-is-dangerous-too/
One point of contention: I would argue that a bent fender washer is not a useless fender washer. When a washer is bent (yielding plastically), it is still loaded on the elastic stress-strain curve and applying near yield stress level loads to the surface underneath it. There may be another discussion about long-term creep of that washer, and it clearly is not functioning the same as a thick, stiff washer, but the thick stiff washer also may not distribute load if it is installed with a couple of high points and or a dished surface, where it will put all the load on the high points or edges. Acknowledging that the elastic load (in washer bending mode) on the outer edges of the washer are not huge or optimal, it does seem to me that the standard fender washer would act somewhat like a thick hard washer with 100 tiny springs under it, distributing load under it’s surface.
I’m open to correction here and would love to hear some discussion on this.
Link to example of elastic response of a yielded specimen: https://mechanicalc.com/static/img/Materials/Theory/Original/unloading-reloading-01.png
Oh, and backing plate is clearly the MUCH better solution, no argument at all there.
Fender washers are *supposed* to bend, at least a little bit. The pressure on the thin material being fastened and protected by the fender washer will be low at the washer’s outer edge, and higher at the bolt head. This is how they are supposed to work: they spread out the stress concentration that would otherwise tear the soft / thin material where it touches the edge of the washer.
By contrast, general-purpose washers for use on hard / thick materials are not much larger than the bolt head, and produce a roughly uniform pressure on the material being fastened.
Horses for courses.
The error here is that in many cases, once the fender washer deforms, the core is damaged. Additionally, the boat loses its pretension ans is now loose–very important. The bolt then moves under load, perhaps imperceptibly at first. Then the core delaminates or rots, and is further damaged by fatigue. The cycle can repeat until there is nothing left of the core but mush and the washer tears through the skin.
Except for cases such as insulation or thin sheet metal, where the material is not damaged in the process of distortion and where resulting fatigue or water damamge are not factors, a bent washer is a failed washer.
If the area is not cored and was of sufficient thickness, the fender washer will not deform. In fact, a fender washer was not needed and a thick (won’t bend) bolting washer is actually structurally better. Some builders are good about reinforcing under winches and such, and as a result, fender washers and backing plates are not needed. After all, additional lay-up is a backing plate. In fact, the very best backing plates is to layup ~ 1/4-inch (amount varies) additional glass in the area, which will spread the load and function as one with the hull, providing many times better stiffness and strength to weight than a separate non-bonded backing plate.
Unless the washer can sustain the full load without bending, it has failed. Obviously, it has been pushed past its yield strength and will continue to fatigue and bend. Surface irregularities can be bridged with sealant, epoxy, or by grinding the surface level before installing the washer. Not a flexi-washer.
Sorry, rant generated by a pet peeve. Yes, chandleries and fender washers are that bad, in most applications, IMO.
I feel like I need to repeat this point. A bent fender washer is nearly always associated with a bolt that has lost tension. It can only be thus. When you retighten the bolt the washer immediately bends more. If you do not retighten the boat, the bolt will be loose in its hole, will move, leak, and further damamge the hole. The bolt itself may either bend or fatigue. It’s all bad.
Matt is correct (below), but those things don’t seem common on a boat.
Hi Drew,
Thanks for reinforcing that. I should have used “a bent washer is a failed washer”. Just makes it simple.
Excellent and thought provoking article. Quick question. At what point does stacking x-number of fender washers equal a space limited backing plate?
I am re-doing the “coffee can” hull-to-deck joint on my Havsfidra 20. This joint is an odd one. It was originally screwed together from the outside to locate everything, (with metal clips on the hull lip which the screws passed through,) and then heavily fiberglassed on the inside.
I was going to use 1/4” stainless elevator bolts fed from the outside and thru-bolted. Because the elevator bolts, without a bolt-head, give a flat profile, I planned to glass over the outside of the joint, adding a glassed in 60mm Coosa board rubbing strake, to which a commercial sized PVC rub-rail will be added. All this hull work is getting re-gel coated…..(I know….I know…LOL),
Lastly, now I’m wondering; since it is already glasses together inside, is re-bolting even necessary?
You have me re-thinking my bolting method. Any thoughts on this greatly appreciated. Thanks you.
Regards,
Martin
s/v Pippi,
1973 Fisksatra Havsfidra 20
Hi Martin,
I guess the first question is why are you doing this? My guess would be leaks? If so, and given that your are glassing from the outside, I would not bother with bolts, and particularly not elevator bolts since my guess is you would not be able to torque them high enough to do much good. So my approach would probably to just glass over from outside but using epoxy resin, not polyester, to make sure I got a good bond. Do note that you can’t use normal fibreglass that’s intended for polyester with epoxy, but need to source cloth for use with that resin. If it were me I would get all supplies from West System to insure compatibility. Follow their instructions to the letter and you will get a strong and water tight covering for the existing joint.
Hi John,
Thanks for your reply. I think you’ve hit it spot on, and your suggestion will also save me a lot of extra work. I’ll contact West Systems to make sure about compatibility and carry on.
Best regards,
Martin
Hi Martin,
Glad it was useful.
Washers serve another important purpose in a bolted joint, as a sort of sacrificial “slip plane” between the rotating bolt head or nut and the clamped material. Even when the average contact stress under the bolt head wouldn’t yield out the base material, the local contact stresses often do, and you get galling. With galling, you increase friction, reducing your effective joint preload. This represent much of what the “preload uncertainty” factor used in your bolted joint calculator is meant to address.
Washers, especially when plated or lubricated, reduce the chance that the “torqued” side of the joint galls during final torque. Even if one side of the washer galls against the fastener or clamped material, the other side will slip instead.
I am a trained ship electrician who completed a three-year apprenticeship at a fishing-trawler building shipyard in Germany and spent three years as a ship electrician in the Navy.
We learned that when using a nut on a vessel, one should always use a washer and a spring washer! Mind you, this was between 1979 and 1985. Perhaps nowadays, Loctite has largely eliminated the need for spring washers to secure nuts against loosening due to vibration.
Hi Robert,
What is best is highly application dependent and you may well have been taught an appropriate method for the application. In electrical applications, conductivity is very important so putting something like Loctite 243 on is undesirable. Instead, dielectric grease, a regular washer and a split lock washer is probably the most common solution these days but some applications call for things like a star/cut washer if a part is likely to be oxidized. Some applications use things like wedge-locks or disc springs but those are pretty specialized. The number of solutions out there are enormous and can get quite application specific, one example is how taperline nuts actually mechanically lock threads to retain bearings.
I use a fair amount of Loctite 243. On the boat with all the stainless fittings, I don’t like to do anything dry as sooner or later something galls on you. 243 is enough to prevent galling in all but extreme circumstances, it gives a reasonably consistent k factor and provides extra retention. Living in the rust belt, I make sure to take out all regular service fasteners relatively early in a car’s life and get never seize on them so that I stand a chance of servicing it later in life.
All of this thread-locking should be a bit of a bandaid or belt and suspenders as a well designed bolted connection won’t suffer from the bolt backing off. The 2 things that I see violated the most often are: 1. The joint must never move meaning that the preload (and friction if a shear joint) are high enough that the loads never exceed them 2. The joint design has enough length of bolt that is being stretched that the torque to tension relationship is predictable. This second one can be hard to follow in certain circumstances such as sheet metal but is usually not too hard and it is laziness or cost that causes it to be forgotten. If you look at large industrial machinery that has a lot of vibration, they put their effort into having good bolted joint design and rarely use any form of locking features or thread locking compound (in fact, they typically grease all threads).
Eric
Hi Eric,
Thanks for fielding that. Thanks to you and Matt I was pretty sure that a properly done joint does not need lock washers, but good to have that confirmed.
I must have picked up this “fender washers aren’t the best” attitude somewhere, because years ago, when I was driving my 1973 Viking 33 close-hauled in lively conditions, the lee jib sheet track tore up. I recall doing the standard “fill and drill” on the core and ignoring the fender washers in favour of 1/4″ aluminum backing plates, with a bead of sealant around the perimeter, and just regular washers and nuts. I did the same with circular backing plates on a couple of suspect stanchion bases.