Santana 20 Mast Step Compression

By Jeff Kerr - S20 #338, Fleet 15, Dallas

Amended for work by Greg Smith

The Problem

The classic Santana 20 has a wooden mast post inside the cabin that transfers the compression forces of the mast down to a marine plywood stringer that runs athwartship just in front of (or on top of) the forward keel bolts. This stringer is made of 2 layers of 3/4" plywood that are encased in fiberglass. The entire load of the mast is transferred into 2 square inches on the stringer. Along with the compression, a very large shear force is applied to the stringer.

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

The Solution

There are many half documented solutions in the Santana world. My first attempt to fix the compression problem started by cutting out the compressed area. As a temporary measure, I put in a piece of 4x4 lumber cut down to fit my cut-out area. This 4x4 was not attached to be stringer at all. The mast post rested on it and the 4x4 rested on the hull. I noticed very quickly that the the leading edge of the keel was starting to separate from the hull. When I removed the 4x4 and mast step (with the mast down), this separation went away. It could be that my hull is inordinately soft, but I didn't like the idea of transferring the mast post load to the hull.

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.