The washer is to protect the bearing from the back side of the nut and help keep even pressure on the bearing. I tighten the bearing to about 40 ft pounds (tight) spin the wheel then back it off tell it loose. Re tighten hand tight then then tighten it tell the cotter pin goes in. Most problems with wheel bearing is people over tighten them causing failure. I've made a living for the last thirty years servicing trailers trucks and heavy equipment. This applies to most single and tandem axle trailers.
This is how I've always done my boat and car trailers. A local trailer shop or boat dealer usually will be glad to give good info also. Nothing worse than braking down on the road, be safe out there.
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