Modified Centerboard
Without a specific design it is hard to be terribly precise about the weight of your hypothetical boat. If I had to roughly estimate the weight of your boat, I would start by comparing it to a known design. For example, I would suggest that we use the 26 foot Thunderbird for a staring point. The Thunderbid is a moderately robust and comparatively heavy plywood boat, especially if compared to more modern designs, but pretty light compared to the more traditional designs of its day.
The Thunderbird has a 1/2" plywood hull and deck. The design weight of a Thunderbird weighs roughly 3,700lbs with 1,550 lbs of ballast. In other words the hull and interior weighs roughly 2200lbs. Based on past design calculations that I have done on similar boats, the hull and deck structure would be roughly 50% of that weight, in other words roughly 1100lbs.
If we compare the Thunderbird to the boat that you are proposing, if your boat had a 3/4" outer hull, plus a 1/2" inner hull that is roughly 2.5 times thickness of the planking on the thunderbird and so your planking and support structure would weigh roughly 2.5 times the weight of the hull of the Thunderbird. If you add the extra beam, the glass sheathing, and the foam core, then the hull/deck structure of your hypothetical boat is probably closer to 3 times the weight of the Thunderbird.
So as a rough guess the you are looking at something on the order of 3,300 lbs for the hull and something like 4,400 lbs for the boat without the keel. The Thunderbird has a 41% ballast ratio. Carried over to your boat that would mean 3,100 lbs of ballast and an overall weight of roughly 7,600 lbs. Even with a less conservative ratio of 30%, you are looking at 1,900 lbs of ballast an overall displacement of roughly 6,300 lbs. Either displacement is at the way overweight end of the D/L scale somewhere in the range of 260 to 330.
You can see that this is in the wildly heavy range of things, if, for example, you compare it to the moderately heavy Dudley Dix designed Cape Henry 21 with a D/L of around 180. I think that your hull scantlings and the resultant of weight of your double skin concept is way beyond excessive. It may be time to rethink your concept for yoru intended purposes.
Sorry Dude!
Jeff
