I have built a number of dinghys over the last few years starting with a Bolger Nymph. The Nymph was a 5 plank pram with a flat bottom and only two side planks. It was very unstable for both sailing and rowing as it tended to fall over on the chines. My most recent effort was the Portuguese Dinghy with free plans from the internet http://koti.kapsi.fi/hvartial/dinghy1/simboii.htm It is as simple as boatbuilding gets and takes only 1 1/2 sheets of plywood. In fact I built two one 8 foot model and one 6 footer. I gave the 8 foot boat to my brother and have discovered that the 6 foot would be a great boat for double amputees because there is simply not enough space for the legs of a six-foot person. With a flat bottom it also suffered from a high degree of tippiness. It was, however, a tough little boat as I discovered when it blew out of the back of my brothers truck and needed only cosmetic repairs.
With crisp new plans from Selway-Fisher for the 7’9″ Greenshank pram in hand I have scouted the lumber year for the necessaries. My earlier dinghys were all built from 1/4 inch exterior plywood with the exception of the 12′ Highlander which was constructed of very fancy 1/4 inch Okume marine plywood, a gift of my father-in-law. Since the new dinghy will probably be sheathed in fibreglass and painted, the fancy wood would be wasted. The chief difference between exterior and marine grades appears to be that in exterior there may still be voids in the plys where knots have been removed. As the dinghy will mainly be “dry-sailed” this may not matter. Any edge voids can be filled in the construction process. The “good-one-side” plywood is considerably cheaper and still gives a nice outside finish. The interior of the craft may be less finished but a multitude of sins are covered up with a matt finish paint.
With my earlier boats I always had a (probably unreasonable) fear that if I hopped into the boat without due care and attention I could put a foot through the bottom. Quarter inch is easy to work with but this time I wanted something stronger and so have decided to go with 3/8 inch. It adds a little weight to the boat but that is not a bad thing. With a bit of a keel and some stringers or rub strips it should give a nice form and strength to the bottom. I had considered using 3/8 on the bottom three planks and 1/4 inch above but the layout got a little more complicated and the potential for waste a little greater. The plans show a tight cutting pattern and I will need more wood for the bow and stern. With these I am considering a better grade of wood and I may leave them varnished.