I am starting a series of articles on building a 36/600 model yacht. To start this building program, I'll start from the bottom and work up. By doing it this way will give you the chance to see how I do it and it may give you ideas on doing it better. This complete project will take some time, so don't get too excited. I will try to get one article done and online each month.
To start the project I am going to work on the keel bulb. There are several ways to start the bulb's construction, I choose to start with a bulb that has been made in a mold. For the most part it is not hard to obtain or purchase a bulb in a rough mold condition.
The torpedo shaped bulb is the classic design shape for keel bulbs for the 36/600 (as well as for the Marblehead, and other). It is easy to design and make (or buy) for most modelers for the needed results of keeping the boat upright. But before you attach your bulb onto the fin, there are a couple of important items you must consider. Not doing so will give your model yacht less than favorable performance, and you don't want that.
The lead bulb I am using is from a mold that I obtained from a fellow model yachter. The over all condition of this "bulb in the rough" is fairly good, all this considered. By no means is the bulb "keel ready", so there will be some work to get it in the desired condition. It is at this point where you the builder/skipper of the model yacht need to make some decisions before you start the work. Not doing so will only lead to less than desired results and needless disappointment.
This decision is what are the average wind conditions your new 36/600 is going to sail in? Is the average wind speed of the pond where you sail are light, medium, strong, or a combination? This is important, if you select a bulb weight that does not work well with the average wind speed of your pond, the boat's performance will reflect it. A bulb too heavy for light wind conditions will be too much weight and the boat will be slower. The same is true for a bulb what is too light in weight and the boat will heel too much loosing power.
The next important item you need to know before you begin work on your bulb is the recommended bulb weight for the design of boat hull you will be sailing. This information should be easy to get from the designer/builder of the boat hull. Knowing what is the recommended bulb weight will give you a good baseline to work from. Without it, it will cause you more effort to choose the bulb weight you are looking for.
The 36/600 boat hull I'll be working on is the Orco Millennium. The recommended bulb weight for this hull is around 3 lbs. With this baseline information I am not too far from the recommended weight for the average wind speed of the area where I sail. The wind speed at the pond I mostly sail at is light to medium, the average wind speed is light - 1 to 6 mph. With this the wind speed condition, I wanted a bulb that would be best for the yacht to sail with. I have to admit experience is also helpful in matters like this. The more experience you have and information from others can help give that inside tip in making the bulb weight decision. For my new boat I wanted a three pound bulb for my keel.
To start the project I have the basic carbon fiber and the bulb is lead. Carbon fiber has become a popular material of choice of skippers because of its lightweight, it allows the keel's weight to go down to the bulb. To remove the unwanted lead, the best way I have found is to use a wood rasp. It does the job fairly quickly and not too messy. Since lead is not kind to the environment or to humans, I capture the lead shavings into a plastic tub. When working with lead it is best to use a plastic glove since lead can rub off on to your hand, which is not good if you have a cut on the hand.
The bulb in its rough form weighed three pounds, eight ounces, eight ounces over the weight I wanted. To start rasping off lead with no plan is a mistake waiting to happen, it is easy to take too much lead off and very hard to put back on. Using the basic bulb shape as a guide, I started to rasp lead off the nose of the bulb to match the shape first. After reaching close to the desired shape in the nose, I turned to the bulb's tail end.
Again, using the basic bulb shape as a guide I worked on taking lead off the end. I would switch back and forth between the two ends working the bulb to the guide. All during this part of the process I would weigh the lead shavings on a small food scale to keep check the amount of lead in weight I was taking off. Rasping lead off is not hard, just rasp lead off by stroking down the length of the bulb and curving the stroke around the bulb. Rasping lead off this way prevents flat spots from occurring.
All during the process of getting the bulb to the shape desired while removing most of the weight, use the flat rough part of the wood rasp. When close to the required shape and desired weight, switch to the flat fine end of the rasp. Doing it this way will allow the final shaping without the risk of removing too much lead or a mistake in the shaping.
It is important to remember when working on shaping the bulb, take your time and refer to the bulb shape guide often. To check for shape and for flat spots, feel the bulb. You be surprise how well you can find the spots that needs more work. When getting close to the final product, check the bulb against the pattern, and hold the bulb up and look it over. It is better to go slow and do the job right the first time. Weight the lead shavings often, doing so is the only way to check the weight you want to take off. Close to the end (you pick the time), weigh the bulb on a mail scale to check the bulb's weight, it is another guide to check your work.
When you are finish, now is the time to wet sand the bulb to do the final finish. Sanding doesn't take off much lead (leave yourself a little extra weight for sanding). To fill in the remaining scratches and pits, use Bondo auto putty, it does a good job and is easy to sand.
In closing some skipper have two or three keels of different bulb weights. This is a good idea if you sail in a wide range of wind speed. This way you can go to the pond, check the wind strength, and choose the correct weighted keel. The only thing you have to remember, once a yacht is sailing in a regatta, you can't change keels short of a keel loss or breakage. Well, I hope this article is a help to get you started on how to shape a keel bulb and make it "keel ready".
