12-10-2024, Rev. 8-17-2024
This spring I spent several weeks replacing a bunch of rotted deck caused by the use of flimsy fender washers. That encouraged me to dredge up this old post. There are longer versions, with more data, in Practical Sailor and Good Old Boat.
As ramp-up for some Practical Sailor testing, I thought I would share a preview.
First, unable to secure scraps of deck material for which I could be sure of the pedigree, I laid up some of my own. The testing will based upon 1/2-inch balsa core with (1) 6-ounce cloth and (1) 17-ounce biaxial layers on the deck side and (1) 17-ounce biaxial layers on the under side.
I drill a 1/4-inch hole (no epoxy plug, block of wood on the back side) and tightened down a fender washer against it. At 10 in-pounds (about 675# load) the washer had distorted and the laminate was failing. for comparison, the bolt working load of a Lewmar 40 winch (1/4-inch bolts) with a strong grinder is about 500 pounds each. In other words, without an epoxy plug the bolt will fail under working load and standard ASME bolting load, with no safety factor for aging and fatigue. It is about 5x weaker than good design suggests. It also explains why I had a PO installed winch rip out.
By 18-in-pounds the fender washer was buckled and the nut was well into the core. For comparison, this is about 50% better than a plain pine board in each case.
I repeated the test with only lock washer. The same result! The fender washer resulted in no increase in strength. The point being, that the bolting washer provided better support in close, the end result being the same.
Testing for the actual project will involve proper epoxy plugs. However, since under the load the bolt will NOT be supported on the other side (the winch or cleat will be lifting) in the real world, the top side support will be supplied by a 4-inch diameter ring spacer, allowing the washer to pull through, if that is what it wants to do. I've tested this without the epoxy plug; not surprisingly, it lowers the failure load and creates top side damage much like I saw on my failed winches.
We'll see. But for now, the moral of the story is that fender washers are basically useless; they fail as soon as they are actually needed.
You can buy extra thick fender washers that are double the thickness, 4 times the strength, and 8 times the stiffness (that's the way increased thickness works). Bolt Depot has , them, as well as many others. This is what the chandleries, including West Marine and Defender should carry. But they would be higher in price and we would buy the cheap ones. Which is why chandleries don't sell crappy rope. They shouldn't sell crappy washers.
[From Bolt Depot. Buy the extra thick ones.]
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