RE: SEAMASTER TWIN BUILD
I put a set of spray-rails on a Sig Sealane because it had a reputation of doing submarine duty on occasion. The rails are 5 inches long, 1 inch wide and set at a 20 degree angle (approx.) They sit on the waterline at the forward 1/3rd of the distance from the bow to the prop.
It was all a best guess but it kept the nose up, and the spray went distinctly off to the side.
Hope it helps. Your twin is a neat project!