Phase 4 - Interior Fiberglass Work - Rear Flotation Tanks

Ok, I know we all love grinding, sanding, scraping, and stripping paint. But let's face it - there are tasks in this project that are supposed to be more fun. One of the primary reasons I bought this boat was to learn how to work with fiberglass. So it's time to learn dang it!

During the week of 6/9/2008 we began to look at how to construct two new rear flotation tanks for the boat. These would replace the previous installed flotation; a block of Styrofoam wrapped with part of an old sail. While this previous method was certainly functional and met the requirements of the class rules, it lacked the style, grace, and elegance that I'm looking for. *Grin*

So we spent some time measuring the rear flotation tanks on my wife's boat and then brought those dimensions back to see how it laid out on my boat. We then constructed a paper template of the tank to test the layout and fit. Once we were satisfied with that, we transferred the template to cardboard and checked that fit as well. As part of this process we realized a slight difference in how the D&M and Clark manufacturers constructed the stern of the thistle. On my wife's boat (D&M), the wood piece the traveler attaches to does not come down below the level of the rear grate. On my boat (Clark), that piece of wood comes down further, two to three inches below the level of the rear grate. So my air tanks will be flatter as a result since I don't want to attach the fiberglass air tank to the wood. Even with that slight difference I will still have the required volume in each tank.

Once we were happy with the fit of the cardboard mock-up, we then transferred those shapes to some 1/2" MDF that I had. I used a jigsaw to cut those shapes out and then cleaned them up on the band saw. We then used wood glue and a combination of screws and finish nails to put them together. We then added support pieces to the edge of the molds to form the flange that will be needed to provide the attachment point to the boat. So now we have a mold that when fiberglass is laid in on the inside will produce a perfectly sized flotation tank, complete with attachment flange. However, we're still not there yet. Although the MDF is very smooth, it is still not smooth enough so as to not show the MDF surface pattern when the gel coat is applied over it. To create a perfectly smooth surface on the inside of the mold for the gel coat to cover, we lined the mold with tile board. This product is available at your local home center and is used as an inexpensive shower surround finish (in place of ceramic tile). It's basically an 1/8th inch thick piece of fiberboard with a thin, hard, smooth finish. So, using our cardboard patterns, we marked and cut out the three pieces needed for the mold. We used 3M No. 77 spray adhesive to attach each piece to the inside of the mold. Now we have a smooth bearing surface for the gel coat. We're finished now, right? Wrong! Yes, we do have a very nice looking mold, but it has on 90 degree square corners. That's not very functional when trying to lay the cloth into the mold. We need nice rounded corners for the cloth to lie against, both for the inside corners as well as the outside corners where we transition to the flange.

Rear Air Tanks

On the first mold we made I was concerned that we wouldn't be able to use a router and round over bit to make a nice smooth curve because I was afraid that the tile board would fracture and splinter. So Bruce didn't argue with me (I wish he had!) and we attacked those edges with a rasp and file. What a mistake that was. I succeeded in making a nice rounded over edge but it just took too long. Then because it was so rough, we had to spend a lot of time applying wood filler and sanding to create the smooth transition we were looking for. On the second mold we used a router and 1/2" round over bit to route the outside corners and it worked very well. The tile board did not splinter and we ended up with a very smooth edge. We still had to do a little bit of hand shaping and filling since the router can't go all the way into the inside corners, but it was much simpler than on the other one.

So now our mold has nice rounded corners where it makes the transition from the vertical surface to the flange, but we still have the issue of how to deal with the inside corners. We discussed several different ways of doing this including using wood filler, drywall compound, or modeling clay. In the interest of time we decided to go with a fast drying, light weight, drywall crack filler. While in theory this was a good idea, the material was difficult to work with and was finicky about sticking to the very smooth tile board. We made it work but it was hard to get it as smooth as we wanted. We have since had a conversation with someone who does this sort of thing for a living and they use modeling clay for this purpose. The next time we do this that is what we'll use. It's soft enough to mold easily but hard enough to allow you to work it into the corners and smooth it out with either your fingers or simple tools.

After we got the corners and edges the way we wanted them, and everything was as smooth as we could get it, Bruce applied several coats of lacquer over all the exposed wood. We are now ready to wax the interior surfaces with Meguiars mold release wax. That's the final step before actually laying up the fiberglass and resin.