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Thorp 09-06-2012 12:36 PM

Mast Compression
 
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I have a 1964 Cal 28, hull 41.

The deck has deflected down somewhat as evidenced by a crack in a timber that rides along the upper edge of the main bulkhead, a gap that has opened up in the door, and a crack in the fiberglass liner.

The compression post and mast step look to be in good, solid condition. It is clear to me that the deck stepped mast has pushed down hard enough to deflect the inner liner.

I believe the boat has been like this for quite a while and I have been out sailing almost every weekend, without a problem. Though, I do not know if the deck is deflecting while I am sailing, hard on the wind. It is possible that the liner has settled in (read cracked/deformed) so that it is now better supported by the hull underneath and there is no deflection while sailing. It is also possible that it is.

In either case, I would like to sail blue water with this boat and need a good solution. It is odd to me that there is no beam under the liner. We hear about the 'dreaded steel beam'. This boat doesn't have one, but now I kinda wish it did.... I think I need to install some semblance of a beam to carry the load to the hull, or maybe the keel. Access is limited to the seat just adjacent to the compression post and from the galley. I do not want to cut the inner fiberglass liner to install a beam because in this boat, the inner fiberglass liner is the only thing supporting the compression post.

Do you guys have any suggestions?

SlowButSteady 09-06-2012 01:48 PM

Re: Mast Compression
 
My Cal 2-27 has a similar, although less extreme, problem. And I have heard of other 2-27s having problems with the compression post. In the case of my boat, the CP was obviously deflected down by about a quarter if an inch, but there was no apparent damage to any of the associated wood. I loosened the standing rigging a little and that seemed to take care of most (but not quite all) of the problem. It appears that there are two problems: first, the design wasn't meant to be tensioned nearly as much as I, the previous owner(s), and probably most folks, tension standing rigging; and second, the design of these boats really didn't take into account the effects of 20, 30, or 40 years of bending stress/strain on that fiberglass box/beam that is supposed to support the compression post. On my boat I can actually see that the top of the FG box/beam has a bit of a dimple in it under the butt of the CP.

For the moment, I'm just going to leave the standing rigging a little looser. But, in the long run, I was thinking of slipping a steel plate (maybe 1/8" or 3/16" thick) under the butt of the CP (between the butt of the CP and the top of the box/beam, to distribute the load along a larger section of the box/beam), and/or installing some sort of support under the FG box/beam. In another thread, someone poured concrete (less than a cubic foot, IIRC) under the support for his boat's CP. In that thread the poster figured that it only added a hundred or so pounds, nice and low in the boat, and it spread the load out nicely. That might be something to consider, if the geometry of the area in question on your boat would allow doing so (but from your pics, it doesn't look like such a fix would be easy in this case). On my boat, the "concrete fix" might be a bit easier. But, it does have the disadvantage (besides the added weight) of not letting the hull and the liner flex independently of one another (which I think was one of the original assumptions of the design).

Thorp 09-06-2012 02:17 PM

Re: Mast Compression
 
Interesting fix, concrete.

Why would the hull and liner flexing interdependently be an advantage? This is a highly loaded area, I would think the more stiffness in this area, the better. Are you worried about a stress concentration someplace else if this area is too stiff?

I need to decide a) how I am going to strengthen the step, ie athwartship beam, rib inline with the keel, concrete and b) if I am going to raise the deck/compression post/innerliner back up.

What would be a good a good material for the beam? Something that is stiff, will not rot, and bonds well with fiberglass. Perhaps G10?

Any suggestions on a method to jack up the inner liner from the bilge?

SlowButSteady 09-06-2012 02:44 PM

Re: Mast Compression
 
I can't really speak to the way the liner works on your boat. On mine the liner and the hull look pretty independent in that area, so I assume that there is a reason for that. It might not make any difference at all; maybe I'm just being a bit paranoid. However, I do know that stiffer is not always better; fiberglass boats often flex a bit for a reason. On a 1964 model, that may not have been a consideration, but you would have to ask a real boat designer/architect to be a bit more sure.

I think one thing you really want to avoid is any sort of a "point load" on the hull. In other words, I wouldn't just transfer the load from the compression post to the hull without spreading it out over a pretty wide area. Having a little bit of the liner deform is one thing; having the hull deform would be far more problematic.

Simply stiffening the beam may be the way to go. That way the design essentially remains the same. How to go about that? Ay, there's the rub!

Thorp 09-08-2012 02:48 AM

Re: Mast Compression
 
I've come to the conclusion that I don't want to take the load from mast down to the hull. After a bit more investigation, I have a better understanding of how the loads are applied. The inner liner (or beam) balances the downward force from the mast and the upward force from the chainplates (attached to main bulkhead, not hull). The hull does not add much to the structure.

I think I will see if I can bring the compression post back into a perfect column. I will then support the inner liner with g10 laminated from underneath the liner if I can get the access needed. I will replace the beam at the top of the compression post with something more substantial.

Thank you for the input SlowButSteady!


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