Centerboard and Case

The centerboard shape evolved a little as I drew it out at 1:12 scale and then made one out of an old cedar shingle offcut that happened to taper about  the right amount from top to bottom anyway. As a bonus, the aromatic red cedar smelled nice as I cut and sanded the model centerboard to a rough NACA 0010 profile.

I also spent a bit of time making a spreadsheet to help calculate how much lead I would need to make sure my centerboard goes down properly. Cedar with fiberglass over it weighs a lot less than the equivalent volume of water, so needs ballast added to sink it. I worked out the volume of the centerboard by approximating the cross sectional area at top and bottom using the same method (since they have the same proportions of width to thickness) and then used the average to work out the volume. It was then a matter of comparing the weight of fiberglassed cedar to water and adding a suitable volume of lead. It worked out that if I replace the bottom 3.5 inches of the board with a lead casting that the board should sink. Alternatively I can build a cavity of a suitable size into the board and fill it with lead.

Centerboard Details - There are 2 more even more detailed pages of sketches showing how the centerboard case goes together and how the pin the board swings on is made

The next details to work out regarding the board and case were the overall structure. How I will take the board our from inside the boat, and how to make the pin, which the board will pivot on. The latter will be below the waterline so must be carefully made and water tight. It took me a couple of days of part time research and design to sort all this out. There is a wealth of material out there in particular on BoatDesign forums at boatdesign.net.

I made the case from basswood with ripped strips of spruce to simulate the douglas fir structure holding it together. When I went to fit it into the boat today, the slot I had cut a few days ago was in the wrong place because I have had to shorten the boat by about 3 inches to ensure the longest planks will come out of scarfed 1/2 inch plywood. I also shortened the board slot a bit by putting a notch at the forward end of the board.

Bottom with centerboard case in place. White stuff is patch repair to fill in gaps

Centerboard shown fully lowered

I have been reading up on stitch and glue construction, which is the method that I intend to use to make the hull. This uses wire and small holes drilled in pairs of sheets of plywood that will be joined together to make the hull. It differs form the method that Welsford suggests to build Penguin and I have realized that I will need to use thicker plywood than if I had built Penguin. So the hull panels and bulkheads will end up being 1/2" instead of 3/8" as I had initially thought. This means that I will be better off using proper 1:8 scarfs them rather than using fiberglass butt scarfs as I had intended. Hence I need more plywood for this as each scarf takes up 4 inches, so I was forced to shorten the boat.

I have begun work on the bulkheads that will support the planking, deck, and cabin. They are defined by offsets that can be output from the boat design software (FreeShip) I am using. You specify where along the length of your boat you want to make each 'slice' through the hull and the program will figure out the shape at this point. This is defined as a set of XYZ coordinates. I used a simple text processing script to remove un-needed points and to define both halves of the boat in one go so that I can have Google sheets graph these numbers. I also had to adjust these offsets for the planking thickness I will use because the coordinates are to the outside of the hull and I need the shape of the inside of the hull. I did this by resizing the boat by the appropriate amounts and regenerating the coordinates of each bulkhead. Once I have  graph, I will print this out at 1:12 scale.

Next steps will be to add detail to each bulkhead so that in addition to being the right overall shape it also supports the various seats, floors, and other furniture we need as well as allowing access into and within the boat. I must also determine each bulkhead and stringer's thickness and material. Once this is done, I will model all the bulkheads in basswood and spruce and set them up on top of the bottom making ready for planking the boat,