Well, we just need to know how far the stabilising fin (and/or rudder?) sticks down below the bottom of the planing shoe, and that could very well be the missing inches; it certainly looks to be the case here-
Even at the most rudimentary of glances, the bottom of the stabilising fin to the top of the bodywork and the height of the tail fin appear to be the same- and as the tail fin is about four feet, that makes it all about 8 feet. Sorted!
It doesn't help the calculations of position carried out with the wrong figure that have gone into print, but at least we might be able to bottom where '8 feet' ever came from in the first place.
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"You never had the things you thought you should have had,
and you'll not get them now..."
is mos redintegro