Re: trim tabs on 22' Islander
I agree with B2. Metal tapping screws into a single layer of aluminum is a little risky. I used rubber "Well-nuts" rather than hacking up the floor (and removing my fuel tank) for access. I would have preferred to have used nuts and bolts with large washers inside. The Wellnuts I used have a 1/4"-20 brass insert that is isolated from the aluminum hull by the rubber body. I had to drill 1/2" clearance holes to insert the Wellnuts which caused me some apprehension, but in the end, I rationalized that a good portion of the stress on the bolts is a vertical push as well as the pull astern. Are you planning stainless tabs or plastic? Although the mounting plate is on the outside of the transom, I think that stainless mounts will at least spread the stresses on all of the fasteners equally as a unit, since they're not likely to bend like plastic might.
I'm not sure why there's a need to have the tabs 3/8" above the hull. My logic says that the fewer imperfections in the planing surface, the better. Obviously they can't be below the hull, but 1/8" above should be adequate. Maybe someone more knowledgeable in tab design can chime in. - Grandad
