Boats only have steel or alum plate , cause someone is trying to make it last a little bit longer.. the wood core ( plywood ) is the strength, the fiberglass adds strength but must be encased in glass, edges, top and bottom, once the wood core is gone the glass starts ripping and breaking.. and once rot starts they only way to stop is to remove the rotted wood, and get it clean and dry, before you lay in more glass.
To do this boat right you need to separate the top from the bottom and then plan on doing a lot of grinding, vacuuming, and wipe it all down with acetone. make sure you take lots of pics and measurements so that it will all fit back together properly.
I can send you a PDF book on the transom part.. it covers using polyester resins. epoxy is your best bet in the winter and will out last polyester resin .
To do stringers, floor, and transom on a 15-16 ft boat you can plan on a good 10 gallons of resin, you will need a variety of mats.. if you use poly resin you want at least enuff 1.5 oz mat to put two layers on the top side of the floor and one on the bottom.
The stringers and transom you are better off with epoxy resin, and any holes drilled to be filled and coated with epoxy resins.
For the stringers and transom with epoxy use 1208 or 1708 biaxal glass. if you use ployester resin , use 1.5 oz mat , and while its still wet uses the biaxal over that .. you want every bubble and air pocket out no matter what resin you use.
I get my epoxy resin at uscomposites.com .. no body sells it in the Chattanooga area cost for 10 gallons is 438 includes the hardener make sure you get fast hardener for 40-60 weather
for the best price on polyester resin get it at advanced plastics at 8928 Transport Lane Ooltewah, TN ph (423) 238-9303 ( cost is $108 per 5 gallon pale ) plus you will need the hardener you will need a good 10 gallons to do this project.
DO NOT use the crap they sell at auto body stores etc .. most of the time its out of date .. it has about a 8 month shelf life.. if you get from advanced plastics its fresh. and its a whole lot cheaper than the crapola they sell at auto stores, lowes etc .
Id be willing to show you what you need to do .. steps involved etc .. just do it right dont half **s it .. other wise why bother