Happy Valentines Day everyone!
What I get from the discussion.
Air tight spaces are equal to foam filled spaces, actually better because you don't have the weight of the foam. (I know its not alot)
Given, you don't worry about hull rupture or need to additional structural support afforeded by pour in foam.
When I retrofitted my skiff, I removed the offending wet stuff and added a jack plate (12 lbs) that set my motor back 5.5 inches. Also added a poling platform, weight less than 30#. I figured the reduction of 100# of wet form would offset the set back and the weight of the platform, boy howdy I was wrong.
Small skiff owners beware, set back equals alot of weight. I lost about 3 inches of free board at the transom, only had about six to start with. Reverse gear results in wash over even at the lowest of idle and slowing down from WOT if abrupt would swamp me. This creates a safety issue, can't stop fast....not a good situation for everyone while running the Econ.
Any mathmaticians out there? What does each inch of set back equal, in #'s, or is that continent upon the displacment of the hull?
BTW, the speed gain by adding the jack plate is almost unbeleavable, 7 mph by my handheld GPS. Now I am left to concider removing the jplate and building the transome 'up' a bit. (with the JP my motor is set 5 inches higher than before)
Thanks for all the great information.
Tim