Newlayout for 12' Dinghy

I recently purchased a 12ft sailboat with daggerboard. It has a 5 1/2 ft beam with a wooden thwart across the beam. Picture attached.

The seat across the back and the thwart were badly rotted and I basically had to remove everything except the outer shell due all the damage.

My plan is this:
- install a solid 1x8 across the beam as a seat/thwart
- install/epoxy/fiberglass 1/2" marine plywood inside the transom to give me a nice mount point for the rudder
- Replace the bench across the back, with 2 seats up the sides.
- make 2 seats from the transom to the thwart leaving leg room in the center.
- make a wood frame (1x1 or 2x2?)
- epoxy to bottom/sides of boat, fasten to thwart and transom
- cover with 1/2" marine plywood
- fiberglass everything
- 45 degree all the edges so the fiberglass cloth has smooth edges.

Questions:
- Is this approach sound?
- Will my transom idea work or do I need to use some hardware fasteners to hold this?
- How many layers of 6oz fiberglass fabric should I use to cover the seats?
- Any other worldly guidance is greatly appreciated!
- Am I completely nuts?? I know it's cheaper to buy something else, but hey what the heck!

Thanks,

 

you appear to have been doing your homework.
Seat framing might be 1 1/2" on average, which allows fitting to the hull, and will accept screws. If the rtansom never had plywood, a 6" strip of plywood bonded vertically in the center will be fine for gudgions and pintles. No hardware but through-bolt (never wood-screw) the gudgions/pintles.
Be sure to generously epoxy-coat the wood (all sides) contacting the hull where standing water might occur. Do this before adding additional framing to ensure separation so that moisture has no chance to migrate from unsealed wood parts. Then screw other frames (verical and top longitudinal)
to the pieces bonded to the hull.
The treatment of the plywood pieces can go two different ways. First, you could be creating sealed air spaces. Secondly, you could be creating lockers or foam floatation cavities.
If sealed air spaces, seal everything with epoxy and tape the seams, and possibly also glass the exposed surfaces.
If (and this is far cheaper) it's only a foam flotation cavity or locker or empty space, I'd allow for easy access from above and just epoxy-seal the ply edges and paint because you can easily add another coat down the line and inspection is no problem each season. I'm big on this type of construction. The hull-attached parts are totally encased in epoxy but the balance of the structure is screwed on and accessible. On most small boats, this approach is far chasper to do. If the innerds of the boat aren't necessarily contributing to the boats overall shape and stiffness, the extreme inside-outside/bonded joint/w/coved corners, glassing over etc., isn't needed.
Allow a lot of ventilation if not coating it all with epoxy. It will last a long, long time.
45 degree corners: Better to round them off. Though a simple angle is better than a square one, the rounding off will hold paint better (and glass cloth will more easily run over it). A router helps a lot but lacking one some 1/2" quarter-round from the lumber yard will work just as well.

 

Thanks for the reply!!

So you're saying that I can just epoxy several 1 1/2" strips of wood coated in epoxy and use those as mounting points for a seat/locker area. Do I need to attach these with anything but epoxy? Do I need to bolt through the hull, or something else?

I was thinking of just putting some kind of flotation foam inside. Do you have any recommendations of who makes this?

I like the paint/wood idea since it's lots cheaper. I like the additional ventilation as well. I'll just paint everything with several coats and make sure there's several places for water to drain out.

 

The pieces bonded to the hull are a bit challenging to make. You might level the boat and make a simple grid across the gunwales to drop a plumb bob from, something like a plan view made out of 3/4" x 3/4" sticks. This locates your hull pieces, which may need some bevelling and carving to fit the hull nicely.
You need not screw them, just set them in (e.g., Cabosil) thickened epoxy (now a 2 x 4 clamped across the boat makes a good surface from which to spring those pieces in until the epoxy semi-cures) and use the squeeze-out to cove the edges and save extra to adhere the next piece.
Before the epoxy cures, within a few hours normally, drag some 40 grit sand paper across the coving to clean any crumbs/ridges of epoxy off and tape the frame in (if the piece was rounded on top you could run tape across and over, but it's also adaquate to run tape up the sides of the frame pieces. You'll be totally encapsulating these pieces in any case). Tape widtyh can be 2", 3", 4" and so on, but try to buy a whole roll and stick to one size (4" x 50 yds is about $35.00 and that size and quantity is about right for your application. Two layers is fine for your job, and in larger coves you can overlap the tape to taper the edges, making it easier to fair into the hull later.
Try to keep working the epoxy "hot"--- not letting it cure but just tack up a bit before adding the next layer. This will eliminate any serious sanding, which becomes necessary for a good bond once the epoxy fully cures.
It allows a chemical bond, which is ideal because sanding is such a chore. It also makes you work clean because the next layer goes on better if the surface is smooth and free of crap.

 

Your flotation question: I'm no foam expert. I have used blue building insulation styrene foam with success. I've seen this type of foam waterlogged too, but you can avoid problems by keeping the foam out of the bilge and sealing the foam with epoxy. In a small boat, you have the seats and the cuddy or bow area. Each cubic foot of foam weighs a little itself (which gets subtracted) and displaces 62 lbs of fresh water (64 of salt water).
It costs about $4.00 a cubic foot in 2" lifts. So twenty bucks should neutrally float about 300 lbs of boat and swimming crew, which accounts for a boat weight of 200 and 100 lbs of flotation to support the crew hanging on.
That's a bare minimum, and twice that (my recommendation) will be cheap insurance.
Here's the thing... you will likely not use up a whole $130.00 5 quarts of epoxy and hardener, but buying quarts is too expensive so you ought to buy a gallon. You could save out some to coat your flotation with a pint-quart or so. First glue the edges of the pieces to build a bigger block (just a bead of epoxy to seal the joint). Then roll some over the whole shaped block at least three coats. Now it will never draw in moisture and you will never sink.

 

Thanks for all the guidance! I'll give it a shot and take some pictures along the way so someone else can learn from the mistakes I still have yet to make!!