These are good points, but already taken into account. I did not mention surface prep for the backup plate since I expect it will fall off shortly after its usefulness has expired, thereby obviating the worry about a stress concentration. In a saltwater environment, the aluminum will not exist for long as a piece of sheet metal anyway; it will turn to mush. This is especially true for the structural alloys like 2024. This is one reason they like architectural 5052 or 5056 aluminum for 'tuna towers' on fishing boats; it is itself FAR more corrosion resistant than 2024, and we haven't even touched on anodizing. 2024 is NOT anodized. Weight does not matter that much to power boat builders; if it did, they would not build those boats with a chopper gun.

I don't think any one will notice if a little piece of aluminum weighing perhaps a few grams is down in the bottom of the boat for a short while, assuming it is complelely unretrievable. There's no engine to make it rattle, after all

Jimbo