Mast compression post

Is your mast compression post sliding aft?

[2019 edit by DS : The premise of this issue as described below is not entirely correct, and best I explain this up front. In the mk1 there are two possible mast and compression post positions. The fwd mast position was originally intended for use with the mk1 ketch, and the aft mast position was originally intended for use in the mk1 cutter/sloop. Subsequently the mk2 design revision adopted the fwd mast position for the sloop/cutter, in combination with the bowsprit, so as to resolve the weather helm that affected some of the mk1’s. Therefore the issue of the mast compression post sliding aft – as described below – can in principle affect the mk1 ketches as well as the mk2 cutter/sloops. Remember also that some of the mk1’s were completed after this weather helm issue was understood and so in fact adopted the fwd mast position.]


I’ve been hard at work below decks on hull #145, Luff Shack. Since I purchased the boat this past Spring, I was kicking around shimming the compression post, once the rig was lowered, to address a slight depression in the deck around the mast step. I knew that the deck was solid fiberglass in the region of the compression post, so it wasn’t a wet deck core issue. I just didn’t like the little puddle that appeared right behind the mast step after a rain or heavy morning dew. The depression would only get worse once the boat is fitted with a hydraulic backstay adjuster. The 2-1/2″ diameter schedule 40 stainless steel compression post has a welded deck flange that has the same dimensions as the cast aluminum mast step/halyard organizer. At the bottom of the post, a single 1/2″ diameter stainless steel bolt passes thru both the compression post and an 8″ tall piece of 3″ diameter stainless steel schedule 40 pipe that acts as the post socket. The socket itself is welded to the 1/4″ thick stainless steel baseplate weldment.

So, to fix the deck depression, I pulled the compression post to install some shims in [under?] the post socket. Upon closer examination of the compression post mounting socket, I realized that it was going to take a bit more than a few shims. As I cleaned up the bilge sump directly behind the compression post mounting area I realized that what I thought was sloppy fiberglass tabbing was actually distortion of the thin fiberglass tabbing due to the compression post mounting area shifting backwards about 3/4″. What happened???

When the [original, mk 1] boat was originally designed, the original location of the mast/compression post was on top of the fiberglass laminates that capped the lead ballast. This presented a nice flat surface for the compression post to sit on. Once the mast and chain plates were moved forward on later Corbins, the mast step now intersected the leading edge of the fiberglass keel laminates. The leading edge of the keel has a pronounce swept back angle, so the downward force of the mast resulted in a secondary force aft and down (down the hill so-to-speak). This arrangement is not unique with the Corbin 39. My previous boat also had the keel sited forward of the lead ballast in the region of the fiberglass laminates that defined the leading edge of the keel sump.

The reason why I found that the compression post keel sump reinforcement shifted aft was due to poor surface preparation of the primary hull laminates, poor material selection used in the keel sump reinforcement, and inadequate surface area/total number of plys of the fiberglass tabbing to handle the thrust loads. Luckily the bond of the fiberglass tabbing was so poor that I was able to literally pry the entire structure out of the leading edge of the keel cavity. I was left looking at shiny (read un-sanded) fiberglass hull laminate and a total of 12 layers of 3/4″ thick mahogany plywood packing that was cut to fit the bottom of keel sump prior to capping off with several layers of fiberglass for the compression post to land on. It is also worth noting that once the compression post keel mount shifted aft, water that found its way aft from the chain locker to the keel sump ended up completely saturating the mahogany packing material making a bad situation even worse. The upcoming repair will find things better than new due to proper surface preparation, epoxy resin, a proper laminate schedule and the elimination of any wood in the compression post mounting area. Here is a sketch that will help you, Compression Post Support .

Here is an outside view of the keel in the compression post step area, Outside View . Note the 2 holes. The upper one is just above the keel sump floor behind the step prior to the tear out of the wet plywood. The lower hole is about 2″ above the polyester resin fill that was found at the bottom of the plywood stack. Once the glasswork is done in the spring, this will be the new elevation of the keel sump. The lower hole will be finished with a 1/2″ bronze flush thru hull fitted with a threaded cap. The cap is removed during winter storage so that any water that comes aboard can drain away rather than accumulating and freezing. This is a common feature on wooden boats, called a garboard (plank) drain plug. The upper hole will plugged with new laminates.
So if anyone finds trouble with a sagging deck or maintaining rig tension, a good look in the forward bilge at the compression post step might be in order. I hope that the chainplate knee laminates continue to hold! Jeff Shutic (#145, Luff Shack).

a. My Corbin 39 is hull #101, an original Corbin with the mast compression post sitting flat on top of the encapsulated lead ballast. Sounds like moving the mast forward on the Mk 2 was not well thought out at the factory. Jeremy P. (#101, Two Pelicans).

b. A2b: Great post – I don’t have the problem but I will watch for it happening. John Gleadle (#181, Spinnaker).

c. I too had the same problem when I first bought the boat. I ended up extending my Compression Post almost 2.5 ” and by making a much larger and a beefier flange ( 1/2 ” SS ) at the top of the post, to spread the deck load more. In my case, there was a large SS saddle c/w short sleeve that held the bottom of the post in place. Also, I ended up making longer all 8 of the deckside turnbuckle bolts in order to gain that 2 1/2 ” without redoing the rigging. Lastly, I had a close look, in the bilge, at the fiberglass around the post and it looked fine to me.
Thanks again. Damn!! I miss that boat ……. Frank Bryant s/v #186, Visitant