By Jeff Kerr - S20 #338, Fleet 15, Dallas
Amended for work by Greg Smith
There are 2 things that can cause the stringer to compress. The first is that first couple of hundred boats that were made had an air gap under the stringer down to the hull. When enough pressure was applied via the mast post, the stringer would flex. If the backstay were left on for a period of time, this flex could become permanent. The symptoms of this problem are typically poor performance in heavy air. The reason is that as more backstay is applied, the more the stringer flexes, and this effectively reduces rig tension. So the rig gets loose just when you want it to be tighter. This problem was realized by W.D. Schock and boats since then have been made with a fiberglass filler between the stringer and the hull.
The second cause of mast post compression is the way the stringer is cut to mount the keel on the boat. In order to get a socket on the forward keel bolts, part of the stringer is cut away with a hole saw. This is fine if you live in a dry climate or dry sail, but if the boat is left with water in the keel pan, the stringer will eventually soak up the water and get soft. So again when backstay pressure is applied to the mast, the compression is transferred through the mast post and the step compresses. If not treated this can cause some minor cosmetic damage, but severe structural damage would occur only after the problem is very visually obvious. Here is a photo of a mast step that has gone soft and suffered from the large mast post shear force.
Photo 1 134K
A better solution was to keep the mast load as a closed system. Since the mast was connected to the shrouds and the shrouds connected to the bulkheads via the chainplates, I needed to get the mast post compression forces into the bulkheads. Since the stringer for the mast post is glassed into the bulkheads, the best solution seemed to be to spread the load of the mast post along the stringer. This would keep all the loads of the mast in one mechanical system. It would also keep my hull from deforming at the leading edge of the keel.
I had a 3/16" stainless steel plate made from this template at a cost of about $50. To install the plate, I sanded a bit where the outside edges of the plate would come in contact with the stringers, then filled the middle part with epoxy filler before using #12 wood screws to permanently attach the plate to the stringer. I also reinstalled the mast post before the epoxy could cure to prevent is from cracking once it became loaded. As with other methods, I had to use a hydraulic jack on the inside of the boat to get enough room to reinstall the mast post. I also cut about 1/4" off the post to allow for the width of the newly installed plate.
This solution has been in my boat for several years now. From looking at the repaired step it may be noticed that the plate has a slight bow from the load. This is probably because I weakened the step when I cut the big hole in it (although I cut the 4x4 down so it wouldn't touch the hull and put it in place with epoxy filler). In any case, my speed and heavy performance have increased. Most of the other boats in Fleet 15 have implemented this same fix and our boats were 2nd and 4th in the 1995 Nationals. In fact the boat from the above "soft" photos was 2nd!
Greg Smith has come up with an alternate mast plate template. It can be downloaded here (209K). It is a scanned image so it will look big in the browser. Save it to your local hard drive and open it with an application like MS Photo Editor or Adobe. Then you can print it out in a landscape format and force it to fit on one page.