Aft Seat

The seats are one area of the boat where I took some creative liberties with the aim of reducing weight.
As designed, the aft seat is to be made of 1" x 10" mahogany, almost 3 feet long. Most builders also add foam flotation to the dinghy, as there is no flotation in the dinghy.
I figured that if I was going to add foam, I might as well make the seats out of foam. Then it would provide flotation as well as strength to the boat.
The foam I used was 2" pink polystyrene foam from Home Depot. This particular foam is rated to a pressure of 30 psi. This is important mainly in terms of durability. Brand names to look for are "Celfort 300" or "Foamular 300".

Foam and Balsa

I started off by cutting the foam to roughly the size I needed. I built the aft seat slightly wider than designed. The upper surface is three pieces of model aircraft balsa, 1/4" x 4" x 4' long. So the seat is 12" wide, versus 9-1/2" in the plans.
The balsa is then test fitted on the foam. Labels are used to mark the various parts, saving confusion once the parts are wet with epoxy.
I did a test layup on a piece of scrap foam, to decide how many layers of fiberglass would be needed on the seats. In one layup, I applied one, two, and three layers of 6oz fiberglass cloth to the pink foam, and compressed it using vacuum bagging techniques. The number of layers corresponds to the numbers visible in the photo.
The result was that one layer could be easily dented with a finger, two layers was more durable (but could be damaged by extreme force), and three layers was bombproof.
As a compromise between strength, amount of cloth, and difficulty of layup, I decided to apply two layers of cloth to the seats.
The sides of the seat were cut to a complicated 3D bevel to match the shape of the hull. The aft seat also attaches to the stern transom, which it does not in the plans.
The front lower edge of the foam was rounded prior to applying the fiberglass cloth. This gives a nice radius for the cloth, as well as making the seat appear to be thinner than it actually is.
I marked cutting lines at 1/2" away from the corner, and cut the 45 degree bevel using the Fein Multimaster. Next, a double bevel was cut to bring the shape closer to a radius.
Final shaping was done using a wood file and a sheet of drywall screening. It is very important to use VERY little pressure when sanding foam, to prevent tear-out of chunks of foam.
Weight of the foam + balsa seat: 700 grams.


I used a two-step process to apply fiberglass to the seat. First, the fiberglass was tacked in place using 3M 77 spray adhesive. Once both layers of fiberglass were in place, I could apply epoxy without worrying about both layers sliding around.
If you're shopping for 3M 77 adhesive, do note that there are two formulations. One uses cyclohexane, which is OK for foam. The new type uses acetone, which will dissolve polystyrene foam (e.g. the pink foam I'm using here).
On the aft seat, I used two large pieces of fiberglass which wrapped around the front edge of the seat. I masked off the fiberglass with newspaper, sprayed adhesive on the seat, and then carefully rolled the fiberglass into position.
Weight of the aft seat, with fiberglass: 1000 grams.

Vacuum Bagging

Due to the time constraint of working with 12 fl oz of epoxy and closing up the vacuum bag, I don't have any photos of the process itself. In their absence, here are the steps I followed:
Photo 3 shows my recently constructed vacuum pump system. I was pulling 22" Hg of vacuum on the aft seat. That doesn't sound like much, out of context. Here's what it really amounts to:
So in essence, I parked a large truck on my aft seat to press the fiberglass against the foam.
This is the seat just after I removed the bagging materials.
At this point, I applied a fill coat of S1 Sealer to the aft seat. It's a very thin epoxy, functionally equivalent to CPES which is available in the USA.
It is also at this point that I should have read the MSDS (Material Safety Data Sheet) for S1 Sealer. The ingredient that makes it so thin is xylene. To my dismay, xylene is also very good at dissolving polystyrene foam!
I ended up having to fill a number of voids using thickened epoxy using a small syringe. In some locations, the epoxy was coming through the weave of the fiberglass. The circled highlight in the first photo caused me quite some angst while injecting the epoxy. The epoxy was thickened with brown microballoons. However, a clear fluid oozed out of the fiberglass. It had been many hours since the S1 incident, so it wasn't S1 Sealer. I can only conclude that the fiberglass weave was filtering the microballoons, and clear epoxy was coming through the weave.
As installed, with two layers of 6oz cloth, the aft seat weighs 1100 grams.

Fiberglass can be lightweight

To dispel the myth that fiberglass coating is heavy, here are the numbers for the aft seat:
area covered with fiberglass6.86 sq ft
weight of glass cloth (2 layers)300 g
weight of epoxy100 g
weight of one layer of glass cloth150 g
weight of epoxy for one layer of glass cloth50 g
weight of one layer of glass, with epoxy, per sq ft29.15 g
So, vacuum-bagged, 6oz fiberglass cloth will add 1 oz per sq ft.
For those who still believe that removing excess epoxy with a squeegee is as effective as vacuum bagging, I present these numbers:
ounces of epoxy mixed to fiberglass the aft seat12 oz
ounces of epoxy actually in the fiberglass3.5 oz
ounces of epoxy absorbed by the breather fabric and discarded8.5 oz
It is important to note that while there is a financial cost to the 8.5 oz of discarded epoxy, there would be NO benefit to that epoxy still being on the seat. The goal is to have the fiberglass as close to the surface as possible, and an open cure will never be able to accomplish that as well as vacuum bagging.

Aft Seat Installation

This is the aft seat just before installation. Polyethlyene sheeting has been placed in the boat to catch the inevitable epoxy drips.
The green triangles of tape mark the position of the seat. A single pencil line would give no information as to whether the seat was above or below its target position, especially if the line was obscured. Now, when the point of the triangle is at the surface of the seat, it's in the correct position.
Also note that all the interior of the dinghy has been coated with epoxy, with the exception of where the seats need to be attached. This ensures that the bond between the seat and the dinghy will be as strong as possible, as it will be a primary bond between wood and foam. Coating the wood ahead of time would result in a secondary bond, which is not as strong.
One of the advantages of a small boat is that it's possible to rotate the boat as needed, so the epoxy is always flowing into the joint. We needed to fill the gap at the back of the aft seat, where there was still a small void. The easiest way to do this was to set the boat on end and inject epoxy with a plastic syringe.
Photo 3 shows that this is indeed an amazing dinghy -- it has standing headroom!
Once the seat was epoxied in place, we removed the green tape and epoxied a fillet in the upper corner. This was preparation work for epoxying strips of fiberglass over the seat/hull joint.

Surface Vacuum Bagging

The first preparation step of vacuum bagging onto a surface: applying tape to the surface. Here, we used clear packing tape. The edge of the tape was marked with a dotted line using a Sharpie marker. This made aligning the butyl bagging tape much easier.
Next, the butyl bagging tape is applied around the perimeter, on top of the clear packing tape. The paper backing is left in place for now, as the butyl is VERY tacky.
I opted to use a hose for the vacuum connection. A small piece of butyl tape is formed around the vinyl hose, pressed into the corners with a popsicle stick, and then the paper is placed on top again.
Bagging materials are test-fitted before starting with epoxy. The white cloth is felt breather fabric. The black cloth is the nylon release fabric. Also visible is the polyethylene bagging film under the seat. The difficulty of working under the seat made it prudent to attach the lower part of the bag ahead of time, without the time pressure of wet epoxy.
Here you see the completed vacuum bag with vacuum being pulled. The most difficult part of this operation was making sure there was enough slack in the vacuum bag, so that it could apply pressure right into the corner. If the bag was too small, then it would "bridge" the corner, and would not be touching the surface.
Two detail photos of the vacuum bag pickup. There is a small piece of nylon and felt over the end of the hose, to keep epoxy out of the hose, and to keep the bag from being sucked into the hose.
On this operation, I was able to pull 15" Hg, with about a 30% duty cycle. This was less vacuum than when bagging the seat inside a vacuum bag.
The dominant source of leakage was probably under the packing tape. Given that this bagging operation had 14 linear feet of bag edge, I'm still quite happy at being able to achieve this level of vacuum.

Aft Seat Complete

With the bagging materials removed, this is how the fillets and fiberglass looked on the aft seat.

© 2018 Melissa Goudeseune