Yup.  That's definitely what I was missing.  A hinged mast with a sleeving tube, eh?

Suddenly the world makes a bit more sense.

If I went that route, I'm guessing that it would be something like:

1) Order a mast the same length as if were a normal keel stepped mast.

2) Cut the mast above the deck at about 10% of LOD.  Fabricate an internal hinge.

3) Source another section of mast with an ID = the mast's OD.

4) Sleeve this over the mast, fair the top somehow, and secure.

The numbers would look something like:

It's about 120 lbs of added weight with COG 4 feet above the deck, not including the hinge materials. I wouldn't have to pay somebody to design it - I can do that myself. Sourcing the sleeve might be straight forward.  The mast is tube (OD = 10"), the sleeve can be 10" Schedule 40 pipe.  Pipe ID = 10"   My mast is the max most places provide (10" OD).  I think most pipe comes in 6061-T6, so that's way more than strong enough.

I'm guessing the sleeve would cost maybe $1000 USD.  Figure another $1000 for the weld work fitting the mast hinge.

Doable. 

What d'yall think?  

Well, I'm one step closer to abandoning the tabernacle.  Besides cost, I'm having trouble getting comfortable with the potential for chaffing and hard points on the boom, and on the battens when reefed.  If I understand Pete Hill's tabernacle design for ORYX, much of the foot of the mast is actually below deck level, so the boom doesn't contact the tabernacle at all.  I don't have that luxury.  The tabernacle would have to be almost round all the way from the aft side, across the port side, and the across the forward portion in order to have a truly fair turn that doesn't create a hard spot.

Or perhaps I'm lacking practical experience.  It may be that only the aft port corner of the tabernacle need be of a nice long radius, because for any points of sail in which the boom and battens are forward of the port side, I'll be on a run or starboard broad reach and the pressure will push the sail and battens away from the tabernacle.  I'd still worry about chafe and flogging running in light winds, I think.

Am I missing something?  Who's got a boat with a tabernacle that extends above the boom position?  I'd love to hear them weigh in with actual experience.

Scott Dufour wrote:

David Tyler wrote:

... it would be good practice not to make the tabernacle to be a close fit through the deck and to rely on a thick flange, bolted and bonded to the deck, to carry the horizontal loads.

 

David, 

I got lost in a combination negative/and clause there.  Are you saying:

1) Don't make the tabernacle a close fit to the deck

2) Instead, rely on a thick flange bonded and bolted to the deck.

3)  Don't bolt to the tabernacle:  weld it instead to avoid stainless-to-aluminum batteries on the tabernacle itself.

I'll still end up with the stainless/aluminum contact on the bolts through the deck, though. Acceptable risk, perhaps?

Yes, all of the above. 

You have to fix down aluminium deck fittings somehow, and everyone uses stainless steel fasteners because there's nothing else suitable. If the sealant squeezes out around the fastener so that water containing gaps are eliminated, that's almost the best you can do. The best would be to use isolating plastic washers and bushes, in combination with sealant, but in practice nobody bothers. There is likely to be some corrosion eventually, but it will be visible from the top.

Aluminium only corrodes badly when there is a more noble metal very close to it and there is a small gap between the two in which salt water can lie, forming a battery cell. So it would be good practice to use stainless steel bolts only where necessary - that is, to weld the flange to the tabernacle and to bolt the flange to the deck. Also, it would be good practice not to make the tabernacle to be a close fit through the deck but to rely on a thick flange, bolted and bonded to the deck, to carry the horizontal loads.

I would first make up the U shaped tabernacle, then drop it into the hole in the deck and drop the flange over it, and then tack weld the two together or at least mark the tabernacle where the flange is going to be welded. It's best to leave a gap of about 3mm under the flange, so that when it's bolted down onto a bed of flexible sealant, the deck is very slightly pulled upwards. I can see no need at all to be able to withdraw the tabernacle for inspection, if done this way. 

Hi Scott,

I agree with David Thatcher on this one. They use adhesive sealants to glue aluminium aircraft together in combination with rivets, the adhesive is at least as strong as the rivets and not prone to corrosion. That is good enough for me.

 As David suggests I have had problems with corrosion when mixing fiberglass and aluminium, the mix may even cause corrosion of the aluminium as does carbon fiber.

All the best with the project. David.