Why change it? The vent we had leaked gently at times, into both our head compartment and our head. It wasn’t easily accessible with the setup we had, nor could we close it when the weather turned nasty. And a close look suggested that the leaks might be because the dorade box the vent was sitting on wasn’t all that effective - so we decided that we would change to a closable vent without a dorade.
We had already changed the original cowl vent, and had a round vent (plastic, and unhappily destroyed when a heavy object hit it) sitting on our rectangular dorade box. So our first step was to remove the vent - and then the dorade box. We had already decided on a vent with a stainless steel cover that promised to move a lot of air, so the next part of our work was based on that purchase.
The vent would work best if it was sitting level (as level as anything ever sits on a sailboat), so creating a level surface for the vent to sit on was the next part of the project. Once we had cut the dorade off we were left with a hole around which to build the new base, and an area around it that needed to be repaired.
Since we had decided a round base would look nice and work well, Richard started by cutting circles of fiberglass mat. There were two considerations when cutting the mat:
* The base would be larger at the bottom than the top, and
* We had to build up one side (the out side) more than the other (the in side), due to the curve of the cabin roof.
- First, the size of the top circle was matched to the base of the vent we were installing.
- Then the bottom circles were cut larger than those for the top. To allow for the slope of the roof he cut progressively more off one side of the circle as he cut the lower circles. In other words, the bottom circle had the largest area removed and the longest straight line to be laid along the cabin, and the top circles were complete circles.
Since there was already an existing vent hole, Richard cut matching holes in the center of the circle for each piece of mat, and used the center hole to help center the layers of mat and resin as he worked. He also used the inside of the old vent to keep the hole open and even while he was working. Inside the head compartment he put tape over the hole to prevent resin dripping through.
Then he laid the circles up in a number of layers, centering them by matching the holes in the pieces of mat to the original vent hole. Each layer consisted of eight circular pieces and was laid up like this:
* First, polyester resin was rolled out on the surface to be covered.
* Then a layer of mat was put in place.
* The layer was saturated with resin, and the resin rolled out (pressed down into the mat) with a cheap 3 inch roller.
* Another 7 pieces of mat were put on the same way.
* After the eighth layer of mat, Richard used a steel roller on top to get rid of any air that might have been trapped during the lay up.
(When using the rollers, it’s important to press hard enough to get rid of any trapped air - you can see the air bubbles moving - but not so hard that the layers of mat move. This takes some practice.)
Eight pieces of mat create about a 1/4 in thick layer; Richard laid up 8 or 9 layers altogether, allowing each layer to cure before putting the next one on.
The first part of the process left us with a shape which was roughly the one we wanted. Refining the completed shape began when Richard put on the last layer of mat and resin. He mixed up ‘gunk’ (resin thickened with silica and talc) when he was preparing the resin for the last lay up, and spread the gunk on top of that layup to make a smooth, level top. On top of the gunk he put a piece of waxed plexiglass we happened to have, weighed down with a handy bit of former outboard bracket, and this left the hardened top surface nice and flat and smooth when we removed it.
( A note: we often work with things we happen to have at the time. You should know that waxed MDF board would have worked much better on the top of the vent base than plexi.)
Once everything had set, Richard mixed up and used more gunk to smooth off the sides of our new base. He applied the gunk with a flexible plastic spreader; by the time he was finished we had the basic shape.
After that I took over. I did a lot of sanding, to make the shape as smooth and symmetrical as I could. Then Richard gelcoated the whole base, and I went back to sanding - this coat of gelcoat helped me to see imperfections I would have missed otherwise and filled in some small areas. Another coat of gelcoat, brushed on thickly, was followed by more sanding. Then Richard used polyfair to fill in the remaining irregularities, and I sanded yet again.
Once the vent base was just about ready Richard used a rotozip to open up the hole in the middle to the size we needed for the new vent, and then cut away the thin layer of resin that had accumulated on top of the tape inside. With the hole was the right size and everything looked ready, another coat of gelcoat was applied, using a roller and covering the whole area that we had been working on. He also brushed gelcoat on the inside of the vent hole- that will be sanded when we start work on the inside of the head compartment.
A little sanding made the surface a little more even, and there will be more sanding before we put paint on the cabin and non-skid on the areas that need it, but for now we had gone far enough. So then we put the vent on!
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