You go to a Sheet metal or engineering works to have one made.
- The centre-board can be made from mild steel or stainless steel.
- The thickness can be either 8mm (5/16) or 10mm (3/8)
- A mild steel one would be best galvanised if you can get a very good finish.
In mild steel, a 8mm one would weigh about 45 kilos, and a 10mm one around 55 kilos.
However, weight is not the essential consideration unless the hull is very light. It is the wetted area of the centre-board that does the work.
The leading edge of the board (1308mm) is best shaped round
The trailing edge (1702mm) may be faired to a maximum of 38mm, and the edge is best sharp – it may have a 2mm (1/16) flat, but this must be square and sharp. Breaking or a radius on this edge causes drag (Tune & Trim)
The following is from “Tune & Trim” part 3.
Grind, mill, or file the trailing edge in a smooth taper to a sharp edge. Alternatively, a 1/16" flat can be left, but the edges of the flat must be square and sharp. Breaking or a radius on the edge causes drag.
The leading edge of the plate should simply have a full radius ground or filed onto it. Do not chamfer or provide a fine entry on the leading edge.
Well, we now have a flat, streamlined centreplate, but what about the finish? Galvanising stops rust, but the finish is lousy! Paint it, commencing with a quality 2 pack etch primer, fill any minor blemishes then follow with a good, hard, two pack epoxy paint. Dulux put out an excellent marine paint guide, so check it, or one of their competitors' guides first. Sand the surface back very well before the final coat. After the final coat, allow to dry for several days, and apply a liberal film of silicone car polish.
Now you have the perfect centreplate, but what are you going to put it into? Is your centre-case smooth, or is it like mine was, with great knobs of epoxy resin sticking out of it? If you don't want to spoil that beautiful job, smooth the inside of the case as well.
Another trick is to apply some 1/32" thick insertion rubber to the bottom, and on each side of the plate case slot where the centreplate comes out. These strips can be placed between the plate case and the brass rubbing strips on the skeg of most boats. The idea is to reduce the turbulence of water by preventing it from surging into and out of the centreplate case. If mounted properly it can also protect the paint on the centreplate.