Now that the "right spot" has been located for the keel trunk slot, it is now time to take the leap of faith. By now the location has been found and marked, the next task is to start cutting the slot into the hull. If all measurements were done correctly and checked (please say you did), this article will be on putting the keel trunk slot into the hull correctly.
The best way I have found to do this is by drilling small holes along the slot line. As you can see in the first photo there are nine holes drilled into the hull on the slot line. There are two important items to remember when you do this. First is to use a small drill, 1/16 inch works fine. The reason to use a small drill bit is to drill in pilot holes first. Small pilot holes won't hurt the hull on the inside when the bit comes through. The second important item is don't start the hole drilling at the two ends of the slot line. Doing so, will make the keel slot bigger than it should be. I drill the forward and end holes around 1/8 of an inch from the two ends. When I hand drill each hole, I space each hole 3/16 of an inch from the next.
Once the pilot holes have been drilled, I next use a larger drill bit to finish this part of the job. The important item to remember here is not to use a drill bit larger the width to the keel trunk. Doing so will mean the slot will be larger than needed. Since the keel trunk width is ¼ of an inch, I use a 1/8 inch bit to enlarge the holes. As you can see in the second photo, once I have used the larger drill bit, I still have a small amount to room at the two ends of the slot to file out, that is good.
To file, I use a metal nail file that women use to do their nails with. It is long, narrow, ridged, and cheep. The second file I use to do the 90-degree corners is a small square needle file. Both files do the job without any problems with fraying the fiberglass. To start the filling process, I take ether file, find a hole, and start filing. Since there is a lot of hull to file away, the main task here is to file out the drill holes first.
When filing make sure you are filing in a straight line. Take your time and check you work. Don't file for a long time, or too fast or hard with out checking. To check you work, use the keel trunk. This is the most important part, make sure you are filing an equal distance on ether side of the line. Too much on one side will cause the keel not to be on the exact centerline of the hull.
As you get close to finishing you will be using the square needle file to file the forward and back parts to the slot. Go slow and check your work often. File the square hole of the forward part of the slot first. This is done because you must place the forward edge of the fin at the correct point on the hull. Not doing so will cause a mistake, and you don't want that now. File on one side of the slot and then on the other equally and check often. Once the forward part of the slot is done start work on the end part.
When done the slot should be tight, (straight slot parallel with the keel trunk) and true in the corners to the trunk. Any gaps between the slot wall and keel trunk are not good when the keel box is being put in. To guard against this, check your work and file only enough to get the trunk to slide in, but you don't want a too tight fit. Once you sink the first pilot hole, you have just past the point of no return. So mistakes now will be costly, so check, check and re-check. It will pay off in the end.