The bolt holes in the mast and tabernacle are the reference from which a mast is measured, they should line up. Unfortunately the length of mast below the bolt hole varies. Also the holes in the tabernacle are not always in the intended location. The class rules allow some variation. Line the bolt holes up using thin filling pieces. Once the holes line up perfectly measure the thickness of the fillers used and make a new filler plate of that thickness. I made mine from some Delrin, cut from a Delrin cutting board. (Most people would call it nylon but it isn’t). The advantage of Delrin is it does not rot like a wood filler plate. These cutting boards are available in household shops or your local supermarket. (Or use the one from the kitchen and tell the misses……..).
Don’t screw the mast step to the filler plate and the filler plate to the boat, that construction is no good. Screw the mast step directly to the boat, that ensures all forces from the mast are directly transferred to the hull rather then via the filler plate. You don’t need to fix the filler plate at all. With some decent screws it will get stuck for ever between the mast step and the hull when you tighten the screws. There should be a solid piece of wood below the GRP that allows the use of long screws (5 x 60 or longer).
Before you fasten the mast step and your new filler plate you may want to fill the old holes with some thin epoxy. After it has thoroughly set you can be assured the new screws will hold like in a brand new boat. Make the holes in the filler plate slightly larger then the screw diameter, this is to avoid the screws biting in to the filler plate and preventing a tight fit between the mast step, plate and hull. While the screws are not yet tightened the plate should be able to move around ever so slightly. In the hull, drill the screw holes to the core diameter of the screws. 5mm screw – 2.5mm drill. Try 3mm if you have to use too much force tightening the screws. To avoid cracks in the gel coat you may want to drill the top of the screw holes with a 6 mm drill, but no more then 2 mm deep! And to prevent water from leaking in to the buoyancy tank, use some Sikaflex in the screw holes. Don’t be afraid to use liberal amounts of Sika as it will also glue the the filler plate to the boat and mast step, providing some extra strength.