Topic: Compression Posts

I’ve surveyed five or six of these things and have never seen anything close to “oil canning” at the mast step.  Several of these boats had been heavily sailed.  One had been an OSTAR contender and another had done two Bermuda ‘round-trips and several years in So American waters.  The compression loads are transmitted to the main bulkhead by way of the arch on deck and not to the hull/deck joint.  The arch is used to span the open space.  If yours is showing any cracking, you have some breakdown of the internal structure of the arch or the interior cross beam – probably both.

Re: Compression Posts

Mine's fine... 

Look at the liner in front of the cross member on some boats.  Where the bulkhead posts are.  I bet that the liner is cracked where the cabintop has been pushed downwards.  Doesn't mean that the cross member is done, but it means that the cabintop moves up and down when you sail.

But I've heard of 2 that had cracked cross members.  I can see that the bulkhead is supposed to take some loading - but if you look at the way the thing is built its pretty shitty.  There's .750 clearance between the top of the bulkhead and liner/cabintop.  So the cabin top is not directly supported by the bulkhead.  The bulkhead sits on the floor.  The bulkhead is plywood, and fastened to the cross member with a half dozen wood screws on either side.  How much can this really take??  Maybe it's just me, but this doesn't seem to be the most desireable setup.  Compression posts would be far better. 

Based on what I've done, the cross member does transmit cabintop loading (mast or people...) to the hull/deck joint through the side decks.  The horizontals at the end of the cross member should be (but I'm sure on a lot of boats they were and aren't now) bonded to the bottomside of the deck.  I've got both bulkheads out, screw jacks supporting the cross member (acting as the bulkheads...), the new decking all bonded in place but havn't bonded the horizontals to the new decking.  Step on the cabintop, or even just sit inside and pull down on the hatch opening, and you can plainly see those horizontals pull away, down from the deck.  To me that means that some loading is transmitted to the deck and hull/deck joint.

Re: Compression Posts

It appears that all JJT boats were not created equal.  My bulkheads have no clearance at the cabin top or at the hull liner.  There appears to be some “hard” filler at the top contact areas to take up any small gaps. Both bulkheads are ¼-20 bolted (six per side) through to the molded verticals for the galley area that are glassed to the hull.  The upper edges, like yours, are scrooed to the molded crossbeam.  Since any loads here are in shear, that isn’t the best arrangement, but adequate. 

The posts that make up the doorway are fixed at both upper and lower ends and scroo fastened to the bulkheads.  These posts are 2 x 3” Teak stock that (because of cuts & slots) have probably 4 sq” minimum of cross-sectional area.  Given that their fastening to the bulkheads keeps them in “column”, and given that teak has a compressive strength of roughly 1200 psi, each post should be able to support better than 3000 lbs. (I used my phase-of-the-moon fudge factor here). The posts and the bulkheads together transfer all mast loads to the hull over quite a large area.

Re: Compression Posts

They built em with what they had I guess.  I like the sounds of your setup better, which was pretty much what I had in mind.  Either way, I'm building some new bulkheads from 1/2 ply, and they'll fit tight.  And be bolted through.  And the posts will be installed permanently, or mostly permanently with the intent to act as compression posts.  As they were, they just sat on the floor.  Real Great.....

Re: Compression Posts

Anyone ever given any thought to making the bulkhead posts permanent and act as compression posts?  The Contessa is one of the only boats I've ever seen that doesn't have a compression post or a keel stepped mast. 

Having my boat all apart, and I mean all apart, it's easy to see why the deck oilcans on these boats - it's very apparent that mast loads are transmitted through the glassed in "cross member" to the deck, and then to the hull.  The only transmission of mast loads to the hull (where you want them) is at the hull/deck joint, localized at the cross member.  There is no member in compression that supports the mast directly.

Building a mast support beam, as seen, would help, but only by strengthening the cabintop and transmitting more load to the side decks and hull deck joint - which would stop the oilcanning.  The bulkhead posts rest on the floor, and up against the liner.  I'd bet that on every Co26 that has actually been sailed, you'll see the liner  is stressed or cracked where the bulkhead posts sit.  Solution?  Thru bolt the posts to the cross member (sleeves, brass or otherwise thru the wood) and then cut away the floor liner and glass em to the hull.   At least I think so.  Ideas anyone?