If you have used stitch and glue before that's the way to make a light ply dinghy.
Mine was sized to be light enough to carry and load on a roof rack single handed and to all come out of 2 8x4 sheets of ply. It is only just large enough for two adults and some gear though.
With hindsight I'd have used two different thicknesses of ply so that the bottom was stronger without increasing the weight more than necessary.
If you might want to add sails at a later date during construction is the time to think about how a cb case will be added, what support might be required for the mast and a pads for screwing on rudder fittings etc.
Make a cardboard model so you know what it will look like and that it will fit together.
Go to the Classic Marine site and spend £50-60 on a set of plans - it will save a whole lot of trouble during the build. In the grand scheme of construction and time costs, a decent set of plans really isn't that much money
Suggest you get a copy of INSTANT BOATS by H."Dynamite"Payson there are several dinghy types with plans,brilliant book as are all his others. Simple step by step instructions and really light reading on board. Search ing on his name will bring up his website(he's an American) His Model boat book is really good with full size dimensions. I made several small sailing dinghy models from the plans Xerox copied X%, as recommended ,and cut out from 1mm and 2mm ply they were constructed exactly as the full size jobs. Incidentally, he also sells the full size plans as well.
I built a couple of these about sixteen years ago - sold one and kept the other. Best dinghy I have ever used, fantastic fun to sail and rows and sculls perfectly. Like Vic's dinghy it is built from 8' sheets. It takes just as much effort and money to build something that looks like a bathtub as it does to build something that looks right, so I agree with the other posts who suggest getting a decent set of plans to work from.
This is the one I kept - by chance I re-discovered the other one hanging off a set of davits in Island Harbour last summer.
I have built a couple of "Sabot" dinghies. They are a sailing class 8ft long transom bow. The plans were free from a Canadian ply wood supplier.
It was fairly simple to build on a strong back. It used a keel 2 chines and 2 gunwhales to the 2 transoms. The biggest difficulty was getting the chines and gunwhales to bend into place. Plywood was nailed and glued onto the chines and gunwhales.
With no sailing gear it was light and rows beautifully and would take 4 adults in very calm water.
I must say that recently I got an aluminium dinghy for a tender. It is left chained up on the beach. It is lighter than the previous f/g dinghy though another foot longer son is OK with 5 pob. I love the Al dinghy.
But certainly light plywood is the way to go for light weight. I might be able to find and photograph the plans for Sabot if you PM me with email address. good luck olewill
I did a "Tippy the Tender" a few years ago. Not as pretty but goes v. well. Not a load carrier though and the name is apt. Basic plans £ 15.00 from Classic Boat.