Tabernacle

[Lakesailor]

The reason I lowered the mast was because the compression post was 'sinking'. It was sinking because the 'core' on which it was placed was actually solid mahogany which had rotted over many years of freshwater running down the inside of the mast.

Anyway, having dug out the **** I then replaced it all with a shoe box sized series of West Epoxy layers. The end result was very satisfying. I faired it, painted it and replaced the compression post and raised the mast.

The next day I discovered the fecker was sinking again! Why? None of us here (some extremely better qualified than I) know. Perhaps it is air in the epoxy, exothermic heating (the amibient temp here was about 35 degrees), perhaps the last layer had insufficient hardener - we just don't know.

Anyway, the whole thing has to be re-done, and I have ordered a s/s plate to sit at the base. A pain in the arse, but clearly a pain in the arse due to the SIGNIFICANT loads placed on the compression post through that diddly little mast thingy.

So, LS, come out here and give us the benefit of your wisdom. I might even pay for your flight.

You are describing an entirely different problem. This thread has taken to usual forum divergence into discussing something which has no relevance to the OP's query.
The tabernacle has no significance to the compression loads. The shoe or step the mast sits on is purely to prevent lateral or longtitudional movement of the mast foot.
I have had two boats in which the coach roof structure has given way. In each case I installed a compression post between the deck head (through to the mast shoe actually) and the keel to take the compression loads. In one case I made the post adjustable and managed to extend the post and regain the coachroof profile to the state it should have been.

 

[Tranona]

I am not talking about my Vertue. I am talking about my tabernacle stepped LM26 which is kept on the Algarve!

With the compression post sinking the mast weight plus the rig pressure would deform the coachroof to such a point that it would crack and require major, major surgery to the balsa core sandwich.

At least that is what my Dutch surveyor told me. Perhaps you know better?

Besides, as you know, we have moved on from the Op discussion; remember:

"The tabenacle merely provides a base for the mast to sit on"

Which is what I am querying.

Think the point is that Clint's boat is nothing like yours. Not sure whether it has a compression post or just a beam that takes the load - but that load will be a fraction of what your bermudan mast generates. As Lakey says, the tabernacle is just there as a pivot point and although there is a risk that the mast will twist when being raised, the risk is very low as the mast is relatively light and short - just look at the photo. That is one of the benefits of a small gaffer as it is easy to raise and lower the mast to make trailing more practical. You will find wooden tabernacles very common on small gaff rigged boats.

 

[Clint]

So as l now understand it, the tabernacle is a receptacle which allows a mast to pivot from horizontal to vertical, all stresses/compression are born by the couple of square centimetres which make up the masts foot. The mast itself is held in place by ropes and stays?

 

[Tranona]

So as l now understand it, the tabernacle is a receptacle which allows a mast to pivot from horizontal to vertical, all stresses/compression are born by the couple of square centimetres which make up the masts foot. The mast itself is held in place by ropes and stays?

Exactly. You will have either a compression post, or more likely that curved beam across the deck and under the mast step takes the compression load. The stays hold the mast vertical in both planes and keep the mast in compression.

 

[William_H]

As said the base of the mast takes very minor sideways loads ie stops the base of the mast skidding sideways which is a small force and unlikely. The real load as discussed is downward when sailing. this should be taken by the plate under the foot of the mast not by the pivot bolt. The pivot bolt should be so sloppy that there is no load on it when the mast is up. Test it by removing the bolt and replacing.
However when lowering the mast. There can be a significant force pushing the base forward. Assuming a heavy mast and use of a tackle and gin pole(s). Even when raising by hand lift there will be some force pushing the base forward. Now if you look at Lakey's photo the base is very low profile and so pivot is close to the deck. The deck itself takes the force of the push forward. If the mast should lean sideways in its traverse down (ie at 45 degrees) the twist on the pivot bolt will almost certainly wreck the pivot mount because the leverage is so great. It is important therefore to not allow the mast to swing sideways, with people guiding it or stay wires.
Now looking at the wooden support posts (tabernacle) at the sides of the mast base on OP boat. Firstly we have significant height of the pivot above the deck. This will mean e loads trying to push the top of the posts forward as you lower the mast. Also if the mast should swing sideways in traverse the mast will have a huge force trying to force the 2 posts apart. Again it is important to not allow the mast to swing sideways. But not really a concern on a short mast raised by getting a shoulder behind it.
Looking at OP,s photo the real strength is in the post between the uprights. However because of it's height above the deck the side posts need to be well attached to the centre post to avert any tendency for the mast base to move sideways. Do not however try to hold the mast vertical with these side posts.ie by tightening the pivot bolt. That is the job of the stay wires
My guess is that the side posts will be fine when sailing and when raising the mast. All the OP has to do is to raise the mast and get someone to watch the 2 posts for any sign of distortion or distress. reference the later fred drift, of course the deck and base of the mast must be strong (pushing up) to take the down load of the mast which can be quite a lot. This is usually done by a post down to the keel from a solid point under the mast. ie not compressable deck material.
good luck olewill

 

[Lakesailor]

the deck and base of the mast must be strong (pushing up) to take the down load of the mast which can be quite a lot. This is usually done by a post down to the keel from a solid point under the mast. ie not compressable deck material.
good luck olewill

Yes. On the subject of the thread drift, I dug out the material under the mast foot and cast some resin pads so the top of the compression post was linked directly to the mast shoe and bolted right through

I also made a small adjustable saddle so that I could apply tension to the original steel tube across the coachroof and move that back upwards without lifting the mast base.

The whole post could be raised using a thread at the base (yes, the tube was not squared-off at this stage)

 

[Scotty_Tradewind]

From my own experience on a previous boat .......

The weight of the mast is only a minor point, the forces down through the mast to the mast step, from sailing into a blow with heavy seas, is greatly increased.

Not only the tabernacle but the step below the mast must be strong and bonded through bulkheads/beams and often down to the keel to stop movement as Lake Sailor demonstrates..

The one thing that Lake sailors support does not show is a bonding to a bulkhead.
Lateral movement could still take place. If a larger boat were built like that, I would expect a 'soft' boat to demonstrate its failure in heavy winds and seas by showing the lee shrouds to go slack.

S.

 

[Lakesailor]

Yes. That 1" steel tube I showed in the pic with the adjustable saddle is part of a steel frame that crosses the cabin roof and down to the gunwhales.

 

[Scotty_Tradewind]

Yes. That 1" steel tube I showed in the pic with the adjustable saddle is part of a steel frame that crosses the cabin roof and down to the gunwhales.

That's sound, but..... does it then pick up and complete the triangle each side from the gunwhales back to the keel, either directly or via a bulkhead?

S.

 

[Lakesailor]

Not on that boat, A Foxcub. The Mk11 went for a compression post as standard. It's ony an 18ft boat so the stresses are not great. The damage on the Mk1s was caused by overtensioning the rigging.

 

[William_H]

From my own experience on a previous boat .......
.
The one thing that Lake sailors support does not show is a bonding to a bulkhead.
Lateral movement could still take place. If a larger boat were built like that, I would expect a 'soft' boat to demonstrate its failure in heavy winds and seas by showing the lee shrouds to go slack.

S.

I don't know quite what Scotty means by a "soft" boat. However he seems to imply that lateral movement of the base of the mast is the cause of lee shrouds going slack.
I doubt that very much. Most boats (even large) seem to find that the normal cabin roof structure is very adequate to locate the mast base fore and aft and sideways and there is never any doubt of strength laterally.
I think slack lee shrouds is inevitable from wire stretch and hull distortion in that triangle of loads mast down to keel out to chain plates then up the shrouds. I have never been too concerned about lee shroud going slack. olewill

 

[Clint]

After all the lively debate in this post, I have managed to find a beautiful solid brass tabernacle at the Titchfield Boat Jumble, but it is slightly to big for the mast, it has a gap of about 12.5 mm either side of the mast, how "snug" should the mast be against the tabernacle, obviously it has to be able to move, so that it can be lowered and raised.


http://s298.photobucket.com/user/ceemark/embed/slideshow/

Clint

 

[Lakesailor]

Just bond some bits of hardwood to the sides of the mast to take up the slack. You could make them the height of the tabernacle to prevent either the pivot point or the foot sliding about.
Or fit them to the uprights of the tabernacle. You'd need to drill through the side plates and get some screws to just catch the wood. You could grind down some screws so they don't poke through and bear on the mast.

 

[Tranona]

You will also need a packing piece behind the keep bolt. Tabernacle was obviously off a much bigger boat than yours and way OTT for your purposes!

 

[Clint]

Thanks guys, I didn't think about bonding wood to the mast itself, that's a good call, and the packing piece behind the keep bolt will solve the problem of what to do with bolt head. The tabernacle is probably from a much bigger bolt, he wanted £140 for it, but I got it for £110 in the end, so I was not going to say no.