All of the hull panels in First Mate, including the narrow flat bottom panel, are made from 6mm/1/4" marine plywood. Wayne Jorgensen, who helped build the first boat from the design to hit the water, expressed some concern about the stiffness of such a thin bottom panel. I wasnt particularly concerned, as the panel is only 856mm/34" wide at the widest point, and is very well supported by the centreboard case, 6 bulkheads and frames, and most importantly, by a 66mm x 19mm (2-9/16" x 3/4") external keel batten firmly glued to the bottom of the boat. In addition, I sheathed the boat Im building with 200gsm/6oz glass cloth in epoxy.
In a beach-cruiser, light weight is of critical importance for a solo sailor, and I also wanted to economise on plywood, getting the most possible out of standard sheets - it made sense to use a standard thickness for all panels. Having said that, there is nothing to prevent you from using 9mm/3/8" ply for the bottom panel if you wish.
As part of the design, I drew an adjustable set of foot braces to make rowing more efficient, and also to give something against which to lock your heel to prevent sliding across the boat when heeling under sail. These foot braces gave me yet another opportunity to stiffen the bottom panel. Following are a few images of the set Ive just installed in the First Mate which I am (slowly) building: -
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| An overview showing how the side rails of the braces are glued to the bottom of the boat, and extend forward and aft to teminate against the bulkheads. This is very important, because terminating a stiffner in the middle of a sheet of plywood risks the formation of a crack in the plywood eventually. Note how the side-deck knees in the background are tapered and lead all the way down to terminate on the chine between the bilge panel and the topside panel. |
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| Blocks to locate the foot brace. In this photo the longitudinal rails had been glued into position, but the blocks were just dry-fitted. They were subsequently glued to both the rails and the bottom panel |
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| Three photos showing the foot brace dropped into the three different locations. The longitudinal rails are rounded down at the forward and aft ends to allow drainage of water - sort of like an open limber. |
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