This keel box mold is made totaly out of cardboard.
It has been coated with the release agent
polyvinyl alcohol, and is ready to be fiberglassed.
For this issue in the series on building a 36/600 I will write about making a keel box. The reason for building on a keel box now will become obvious in the next issue. Now days if you are building a 36/600, the boat should be designed in a modular form. This means the keel and hull are separate. They are separate to allow easy transportation and changeable keels. A keel box allows the skipper to be able to change keels depending on the wind strength at the pond before the regatta. In the event a keel is damaged during a regatta, a spare keel can replace the damaged one within minutes.
The three factors in building a keel box are strength, weight, and watertight. The keel box must be able to support the downward weight of a keel's lead bulb. The keel box must also be able to withstand the stresses of heeling and pitching of the sailboat. If you would think you need a keel box that is built like a steel box, not so! A keel box can be built that is both light and strong. Weight is a premium factor, every ounce that can be thrown overboard is less weight to sail with. So how do you build a keel box that is both light and strong?
The three materials that can be used to make a keel box are wood, fiberglass, and carbon fiber. Wood can be used if it is protected by water proofing the wood. Not doing so will cause the box's wood to warp and crack. Wood keel boxes are also heavy and add more weight to the boat. Carbon fiber offers strength, is lightweight, and is waterproof. The only problems with carbon fiber are its high cost, and availability. Fiberglass is much less in cost, easy to obtain, and is waterproof, so fiberglass is a popular choice.
The keel I will be using has a straight rectangular trunk. Straight rectangular trunks are difficult to build a box for. The reason is not the box itself, but rather trying to get the box off of a straight form. Straight rectangular trunk forms don't release well from the box (even with mold release). If the keel's trunk is tapered then building a form that will release the keel box is easy. I chose to make the mold out of dense cardboard glued together with white glue. Once the fiberglass keel box has cured, I can drop it in a bucket of water, and let the white glue and cardboard soften up, and release the fiberglass keel box.
The problem with using a single piece of fiberglass cloth is it is hard to keep medium weight cloth from lifting off the mold when applying resin. This is because the forward and aft ends of the keel box are narrow, and medium weight fiberglass cloth does not lie well on the mold when you round corners. There had to be a better way to apply fiberglass on to a mold to make a keel box.
The solution was to spiral a yard long, one-inch wide strip of fiberglass cloth around the mold. Lay the cloth around the mold in a spiral pattern. This prevents the cloth from raising up off the mold, causing air pockets between the mold and fiberglass cloth. Start by taping one end of the strip to one end of the mold. This will keep the strip from slipping on the mold when the cloth is spiraled on. Once you have taped the start end of the fiberglass cloth, apply a good covering coat of resin that has had hardener mixed in on to the entire mold. Start spiraling the one-inch cloth on the mold with some tension being applied to the cloth. This tension is important because it makes the fiberglass cloth lay down on the mold without lifting off the mold forming bubbles. There are always small, little bubbles, what you don't want is the large bubbles. Large bubbles weaken the keel box and that is not what you want. Brush the resin on to the cloth as you spiral, this will saturate the cloth with resin for a good finished product. Don't loosen up on the tension when you spiral on the cloth. If tension is not applied all the time, the cloth won't round the corners of the mold well.
As you spiral on the fiberglass cloth, over lap the strip of cloth at least one quarter of an inch up to one half inch. This will insure good coverage of cloth on the keel box. Once the strip of fiberglass cloth has been spiraled and resin applied, there is one more item. For added strength, put another strip of lighter weight cloth on both the forward and aft ends of the keel box. You can do this during the first glassing, or after the resin has hardened and has been sanded. This will ensure all four corners of the box will be strong with very little weight gain. To finish, scrape the resin to make it smooth before it starts to jell-up.
After the fiberglass has hardened it can be difficult to get a straight rectangular keel box off a mold. By using polyvinyl alcohol, cardboard, and by dropping it in water, the mold will come apart and release the keel box. To help the release, I now use 1.5 mm thick sheet styrene for the two large sides of the box. By using sheet styrene with polyvinyl alcohol, the release of the keel box is much easier. In addition, the two styrene sides of the mold allows the reuse of part of the mold which saves time to make another keel box.
I'm sure there are better ways to make keel boxes, but I hope the way I have described may be of some help to you. It is important to make a good, strong keel box that you can depend on. The next article in the series will be on a important topic, how to install the keel box in the boat.