This is the second version of the design. The first was made of plywood, which broke, and used brass screws to secure the treads, which corroded the aluminium through galvanic action. This one is made of solid mahogany with nylon bolts to secure the treads.
The ladder has a U-shaped notch at the top to fit over the boat rail. The top is reinforced with a horizontal member, glued and bolted with stainless-steel bolts.
The treads are aluminium tubing fitted into holes drilled in the uprights, secured with epoxy and with nylon bolts which prevent the tube sliding out of the hole. Pure aluminium is chemically active but has a passivating oxide layer which renders it resistant to corrosion. In contact with less active metals such as steel or brass in salt water an electrochemical potential is created which etches away the aluminium. So I chose non-conducting nylon.
There are pieces of nylon set into the inside of the upright, secured with recessed stainless steel screws. These prevent the ladder from rubbing on the GRP boat hull and scratching it.
The ladder has been in service many years, works well and is still in excellent condition. The only drawback is that when climbing with bare feet, the narrow treads are a bit uncomfortable.