I completed construction of the building jig this past week. In the case of the Simmons Sea-Skiff 20', the purpose of the building jig is to support the backbone which is simply a 2" x 10" cut to approximately 14' in length. It goes without saying that the the backbone must be straight, plumb and level. The backbone will support the floors that compose the bottom of the boat. Also, note that the boat bottom will be built upside down.
The plan set has no clear instructions on how to construct the building jig, and that really is no matter. But in reviewing some construction pictures that came with the plan set I built the building jig closely resembling what I saw in the pictures.
In reviewing Ellis Rowe's WoodenBoat magazine article on building the Simmons Sea-Skiff 18, I noticed that his building jig was fastened and secured to the wood floor of his shop. I was immediately reminded of Pete Culler's words: "To set up a new professional boat shop with a cement floor shows a total lack of understanding of wooden boat building, the trade in general, and the welfare of the working crew and equipment" which I read in his delightful book "Skiffs and Schooners". I gave this a lot of thought and consideration before building my shop and "none-the-less" elected to go with a cement floor despite his words of wisdom. The biggest factor in this decision was that I wanted to heat my shop (which Pete is also sternly states is unwise) with an economical radiant floor heat system and reasoned that I could build a wood platform of sorts when it came time to erect a building jig as I have done so with this jig. I figured Pete's words were from the perspective of an old-school crusty resident of Cape Cod, Massachusetts whom didn't have to deal with the 20 below 0 temperatures that are common here in Duluth, MN during the winter nights. In the future I'll consider installing a humidifier to help keep boat lumber at a constant moisture content if I find that it is getting too dry.
Anyway, if you look closely in the pictures you will see that I have carefully cut notches along the backbone. Into these notches will be inserted the various floors that makeup the shell constructed boat bottom. They are cut precisely to various depths reflecting the contour of the bottom of the boat.
I am somewhat concerned with how precarious the floors appear to be in being balanced and secured to the backbone in the center only. But this is the recommended method, and as such I will go with it.
Next up, cutting the floors.