This keelson is the result of several attempts to get it out of a 16 foot 2x10 plank. Please note that this is not exactly what the plans called for.

Making the keelson presented some special challenges that I did not really anticipate before hand. The plans call for a 3/4" thick plank with the plywood butt jointed down the centerline. Attempts to get that and the evolution of events are described below.

1. I tried to plane down a 16' 2x10 plank to half its thickness. I came to find out my thickness planer would not do the job. It smoked and the blades developed problems before getting half way through the first pass.
2. I set my tablesaw blade to as deep as it would go and started ripping the plank edgewise. The tablesaw struggled and soon began smoking. Not good.
3. I pulled out my cellphone and called a local lumber yard and asked if they could surface plane down a plank. They said they didn't have that ability and did not know of anyone locally who could. I recall having a plank planed down in this fashion at a lumber yard in Maryland.
4. I went to Home Depot and got a new plank. This time I ripped it into 3/4" wide strips. I was thinking that I could glue them up to make a 3/4" thick plank. But then I thought about all the epoxy and gluing and whether I had enough big C clamps to deal with any warpage and hold it straight. I set the strips aside and pondered the matter for a bit.
5. I went ot Lowes (I had picked over Home Depot's stock). I thought of cutting a rabbit 2" deep, then cutting a strip off the plywood bottom to compensate. I set my router to 3/4" deep and started cutting. First, I found that this was a bit much to try to route. It seemed it would take all day, and that I would go through more than one bit, maybe several. I tried a number of strategies. I also discovered a new, annoying quirk in the router - it tends to adjust itself to make a deeper cut than the original 3/4" setting. I was getting a bit annoyed by now as I had ruined the plank.
6. I got yet another plank from Lowes. This time I would set the tablesaw to vary carefully edge rip the board to two inches. I would make two passes on each side instead of trying to do it in one pass and taking a chance of destroying the tablesaw. Once this was done, I turned the board face down and ripped out the rabbit. I set the router back to 3/4" to finish the corner. At top you see the final result. At bottom is a stack of the failed earlier results.
7. In talking to the designer I realised some modifications are in order to make this work. The problems with the rabbitted keelson are that there could be shrinkage and checking. Also the plywood is not there to offer rigidity from the cross grain.

Here are various pieces set aside from all these attempts

The number 4a bulkhead cut out in two pieces, glued together, and framed up.

The centerboard slot is cut out. I used a circular saw and finished with the router in several passes. The quirk in the router didn't matter here. Over the next week I glued and screwed the bottom to the keel, one piece each night.