There probably isn't a P-19 owner alive who hasn't cursed the mast support post at one time or another. Those who have made the centerboard cable removable all remark on how much roomier the cabin becomes. From there, it's not a huge step to start imagining the cabin without the post.
I actually started thinking about this project before I bought the boat. While doing research on various trailerable boats, I checked out Precision, a line of boats in the same market space as the Potters. For a variety of reasons, a Precision wasn't the right boat for us, but I was impressed with their use of an arch instead of a post for mast support.
I got involved with many other boat projects, but the arch idea kept rattling around in my head. I even went so far as to work up a design, just to satisfy myself that it could be done using common materials and fabrication techniques. I put the plans in a folder and forgot about them for over a year. In the fall of 2003 I pulled out the folder for another look. I improved the design further and decided to go for it.
This project is more about design than fabrication. There are many possible approaches to the problem. For me, the design needed to be one that I could be sure of and one I could actually build. That meant using common materials and fabrication techniques. For example, I lack the equipment and skill to do high quality aluminum welding. I settled on a structural arch built from 6061-T6 aluminum. Some of the key design goals were:
Since the arch is being added to a finished boat, it must be assembled in place. This is not how you would do it if you were the builder. In that case, you could drop in a finished arch before attaching the deck.
Here is a picture of the assembled arch. The weight came in at 28 pounds.
I won't go into all the analysis and calculations, but a few points are worth noting.
The top beam gets all of its strength from the top and bottom plates, which are 0.25" x 3" bar stock. The wood is just there to maintain the spacing. The sloping top reduces cabin intrusion without lessening the strength, since the stress of a center loaded beam gets lower as you move toward the edges. The bolts that fasten the top and bottom plates at their ends are critical. The holes are match drilled with zero clearance for the bolts. Even a small amount of creep will allow the beam to sag.
Each change of direction is built as a triangle.
The bolted design can be assembled inside the cabin.
Aluminum is great stuff to work with. All of the cutting can be done with standard woodworking tools. Plan on sacrificing a blade to the project. There is a lot of cutting and drilling, but the work is not difficult. Getting the four bends in the top piece just right is a little tricky. It's partly a trial and error process.
The side pieces and those that form the upper triangles are 0.25" x 2" flat bar stock. They connect to the top beam through 0.125" x 1" angle. The lower gusset plates are 0.125". Each vertical piece is formed from two 1.25" square tubes with 0.125" wall thickness. When the arch is installed, there is a wood block between the hull/deck joint and the top of the square tubes.
The stainless steel bolts are size 10-24.
It is important to load the hull at a point of strength and to distribute the load as much as possible. Here is a photo of the hull attachment.
The load is right at the chine, one of the strongest points on the hull. The nearly vertical side of the boat is loaded in shear, the same as it is for a chainplate. Just to be safe, I added three layers of 6 ounce fiberglass cloth, with each layer being slightly smaller than the one below it. Given that my boat has the bluewater layup, this may be overkill. On top of that is a solid teak block that is rounded to fit the chine. It is bedded in a thick layer of 3M 5200. A final layer of glass covers the top.
The vertical square tubes are cut at an angle to match the teak block. They rest on a plate and are held in place with two quarter inch bolts. The arch was built to be slightly wider than the distance between the supports. The bolts pull it in to pre-stress the arch upward.
The arch is assembled in place. Parts of it, such as the top beam, can be pre-built. In the process of getting everything to fit, I assembled and disassembled the arch several times. Before final assembly, all pieces were cleaned and painted. Once it's in place, blocks of wood were added between the side pieces to maintain spacing (i.e. to prevent buckling) and to provide a surface for the covering material. The outside surfaces are covered with quarter inch plywood. The plywood is drilled for the round head bolts. All edges get a quarter round before the cover is added. Four large screws connect the arch to the cabin top to keep it from shifting. They are screwed in from the outside.
Before adding the cover, I raised the mast and tested the arch under a static load. There was almost no deflection. There was no practical way to test it under dynamic loading except to go sailing.The final cover is made from the same headliner material that came with the boat. IM was kind enough to supply this at a reasonable price. (this stuff isn't cheap) The nifty two axis stretch of the headliner material isn't necessary, but I wanted it to match. Here is a photo of the finished installation. The wires at the top center are for a future cabin light.
The arch provides an ideal place to mount reading lights. The lights are protected by the arch from accidental damage. I experimented with several locations and finally decided that a lower position was best. That way, you don't get blinded if you happen to look up. The wiring is concealed inside the arch.
With the post gone, the V berth filler board can extend to the centerboard without a notch. Here is the new filler board. The old board used a cleat on the mast post for support. The new board has extra material along the front edge. The V berth is much easier to get in and out of.
We spent 21 nights on our boat this summer (2004), 19 of them with the board down. The difference from last year is hard to describe. The project was a fair amount of work, but for us, it was worth it.
I removed the covers and inspected everything at the end of the summer. The arch seems to be holding up. There is no creeping or shifting anywhere and no cracks in the glassed in supports.