To make sure the keel will be placed at the correct location on the hull there is a set that should be done. Even though the hull's manufacture marks the keel's trunk slot or you find it yourself, it is a good idea to do this test. The main reason for this is each hull from the same mold can be different. Because there may be a difference, each hull will sit differently in the water. This test will check this for you.
Again fill the bathtub with five to six inches of water. Do not remove the brown paper take on the hull in which you have marked on, you need that later. To do this test we are going to place the finished lead bulb, radio board, batteries, and other weights to act for keel fin and sail rig inside the boat hull. By putting these items inside the hull will weigh the boat down into the water as if the boat was completed. Doing so will give how the boat will sit in the water to where the weight is.
Where the weight is in the hull is important. If the weight is too far forward the boat will sail bow down. That is not good because by being bow down causes more drag at the bow, and when there is a gust of wind the bow will plow into the water causing more drag. If the weight is too far back the stern will be under water causing extra drag. The trick is to find the right area.
Before we continue be very careful about putting the radio board (with electronics) in to the hull. If you are not careful the hull can roll over, and dump it in the water. You can put the radio board in a zip-lock bag, weigh it and put the same weight in the hull, or test the hull to see it rolls. If it does not then put it in the hull; you have been warned.
The goal is to get the stern down to almost (within a sixteenth of an inch (1/6)) from the waterline, see photo 1. To start, first place the lead bulb in the hull some where in the center of the hull. Move it back and forth until the stern is real close to the waterline. Next, the keel I am using weighs two (2) ounces so I put two ounces of lead weights right on top of the bulb. I then put radio board in right behind the bulb (the exact position at this point is not known so just put it in the general area).
Right behind the radio board put in the batteries. How does the hull look? If the stern is below the waterline move the bulb forward a little, and re-adjust the other components. If not down far enough move the bulb back some. The idea is just get the stern close to the waterline. Final adjustment will come later.
At this point comes the sail rig. It is obvious you can't put the real sail rig into the boat hull. So to accommodate for the sail rig's weight, weigh the sail rig on a small scale. My sail rig weighs six (6) ounces so I found some objects that weigh the same weight. I put the same weight objects right above the forward part of the bulb, see photo 2.
Now again check where the stern sits to the waterline. If too low or too high adjust the objects inside the hull starting with the lead bulb first. We do this because the lead bulb is the main weight for the hull. The other object's final placement can't be known at this point in time, but their general weight is still important. The idea is to move the bulb and other objects until the stern is right at the waterline. If the bow is up off the waterline, see photo 3 that is ok. Once again when a wind moves the boat along it will bring the bow down, and that is what we want.
When you have the bulb at the right spot and everything looks good now comes the measurement. To do this just lay a yardstick on the hull with the tip of the yardstick right at the bow's tip. Now look down at the yardstick, and find the point where the forward tip of the bulb is at on the yardstick, see photo 4. If you have a good eyeball you can do it that way or use a plumb bob. Write down the measurement and remove the hull from the water.
Now it is time to get this new information down on paper so we can make sense of it. On a 36 inch long piece of paper draw out a line around 30 inches long. Put a mark at the 18 ¼ inch mark, this is where there the leading edge of the fin will be at. To get an idea of what I am describing look at drawing 1. To make it easier to understand, I drew an outline of the keel on the drawing.
When I put the needed weight into my hull, the forward tip of the bulb is at the 15 ½ inch mark. On the drawing I measured the 15 ½ inch point and marked it. At this point I drew an outline of the bulb to show this in detail, again see drawing 1. Now the question is how does one figure out if the bulb is at the right spot that agrees with the placement of the keel trunk slot?
First, the designer of the hull has already tested the hull and found the spot, that is the measurement for the leading edge of the fin. Our goal is to make sure that measurement is correct for the hull being worked on. To do that there are a couple more tasks that needs to be done first. Once done the calculation is rather easy.
The first task is to find the exact center of the finished lead bulb. This is easy to do, all that is needed is some strong string and a place to hang from. Just tie the string into a loop and put the bulb in to the loop, and tighten it and hang the bulb in the air, see photo 5. Move the bulb back and forth in the loop until the bulb hangs level. When done, mark the spot. If the bulb is designed right the center f the bulb should be very close to the thickest part of the bulb.
Next the look at the keel fin, it should have a foil shape to it. To find the center of the foil, look at the end. There you will see the thickest part of the fin, see photo 6. Next look at the upper part of the fin, there too you will see the thickest part, that is the center part of the fin. This center part run through the full length of the fin. Most fin makers place the bolt or hole to keep the keel in the hull at the center line at the top of the trunk.
The center lines of the bulb and keel should line up with other. If these two center lines are off from each other, that is ok so long it is not by much. When I checked mine the two points are off by 1/16 of an inch. So when I attach the bulb to the fin, I will do so by bringing the two center lines together. By being off by a 1/16 is fine, it just means the bulb will be back a hair more, and won't be a problem. So the way it seams the 18 ¼ inches for this hull is right on, and will work well.
This may seem like a lot of work. Well I guess it is, but if you want a model yacht that sails well it is worth the effort. It is better to put in the work now than realize that you should have done it when it is too late. The next job is cutting the keel trunk slot in the hull. So get busy, check your work and then recheck your work. See you next time.
