Hi Bob

While waiting for epoxy to set, I measured the old GP firewall incidence, mine has about 3 degrees up thrust with the wing at 0 degrees, funny thing if you have a good look at full size photos especially where the spinner backplate meets the cowl, you would swear that they have downthrust check out the photos by ladislav.
I think the other factor we have to deal with is we also need to take nautical facts into account being the hull, i remember years ago having a dumas deep vee that porpoised badly, to stop it you had to get the balance just right at 33% from the back. The seawind has a big hull and this may be a factor as well, the flight balance may be upsetting the nautical balance.
At one of our float flys a guy had a seamaster that he had built downthrust into it (the seamaster is reverse of the seawind being the thrust is in front of the cg) it poiposed as bad as the seawinds untill we took out the downthrust.
I am going for 5 degrees down as a start point and will let you know the result after the weekend, will let you know cg point, weight, prop size, rpm and any other data, we have a big float fly coming up next month which attracts 50 plus entrants, it would be good to go there with a succesfull seawind, but I will have 3 other planes, Toledo Special on Thunder Tiger floats, Eflite Ultra Stick 25e ( an absolute blast to fly) and a plane called the Sea Dart, its a Japanes design float plane made for Pattern flying (photo attached), these things are quick and fly on rails.

Mark