Tabernacle mast - seaworthy?

[Benz]

Originally Posted by Don C L

You know, my question about that, and this is not a criticism, is that isn't it really one pin that is taking the strain? I mean for both to take on the strain, one of them is going to have to slip a tiny fraction of an inch, but still, isn't one the main carrier? I have thought about that for chainplates as well and wondered if it isn't better to have chainplates with horizontal bolts rather than vertical. I have seen boats with chainplates like that. With a tabernacle that would mean a second locking pin horizontal through the mast.

I'm sure that the upper pin, on which the mast hinges, does take the burden of the strain. But the locking pin--which is identical and has the same reinforcements, cannot be pulled out while the shrouds are tight. So once the compression sets in, it takes some of it, though in what proportion I don't know. If nothing else, were the upper pin and holes inclined in the slightest way to deform, the lower would immediately receive more strain and take some load off the top. Every material has some elasticity/compressibility.
As for chainplates, I think the multiple-bolt vertical design is because unless you have perfect tolerances in drilling and zero elasticity in the fabric of the hull, whether wood or fiberglass, things are going to stretch and flex and elongate and compress, and having a series of bolts means that each will take up the slack the one above it suffers, preventing it from suffering more.

[Don C L]

Originally Posted by Wckoek

I see it is possible to have a few horizontal bolts to secure and maybe stiffer than a pin, but I tend to see tabernacle more than hinged of these types.
Maybe a tabernacle is easier, or drilling through deck is undesirable?

When the pivot point is higher off the deck, as the mast comes down it stays above all features of deck or house that may prevent it from coming down to lay flat or on some kind of a frame to support it. A hinge can do that too, but the hinge has to be long and strong if there are deck items to clear. On a smaller boat you can get away with a hinge since the mast is lighter and the mast may be more likely to be mounted on top of the house and so probably will be nearly flat when it comes down to the main hatch. I have also seen hinges used to allow the mast to drop forward.

[CassidyNZ]

@ El Pinquino

Yes I can imagine the chainplate extension as you show it would work to bring the cap shroud attachment point onto the same plane as the mast pivot but as you say, it would work on a 25ft boat, not much bigger.

But to be fair, the mast on a 25ft boat would be light enough that it can be pivoted up into place without much need for lateral support. Most trailer sailers would fit into that mould.

[Adelie]

Originally Posted by CassidyNZ

Is there a difference between “tabernacle” and “deck-stepped”? I guess on smaller boats where the rig may be folded down to facilitate transporting, a swivel pin may be present but on bigger boats the mast will be stepped into a casing above a compression post. In this case, whether it is deck-stepped or keel-stepped makes little difference to the mast, foot pressure is the same.
.....

Deck stepped just means that the bottom of the mast is at deck level. It does not indicate that the mast can be dropped with or without outside assistance but generally it is understood that outside assistance is required, or a crane.

There are arguments both ways for keel- or deck-stepped. Both can be equally strong but specifics and bending behavior for sail shaping will vary.

A tabernacle is a frame around the bottom of the mast and the mast pivots at some point significantly above the bottom of the mast, 1-3’. The tabernacle guides the mast as it raises and lowers.

A hinged step has a pivot at the bottom of the mast. Secondary steps need to be taken to stabilize the mast as it is raised and lowered. The photo in post 20 shows one such arrangement.

[Mainstreet]

Given that the mast is pivoting on a hinge pin so, out of necessity must be free of the mast step in Oder to pivot, there can be an issue of transmitting compression forces and support but if a double wedge is driven into the space between the mast base and the deck to transmit the load directly to the compression post then I see no issues with strength and stability. I’m sure that some vessels incorporate a tapered mast base that fits snuggly into a matching tapered base in the tabernacle in order to transmit the compression forces. These are old designs that were in use during the age of commercial sailing before pleasure yachting. My guess is going back to these old designs would teach us a lot about the correct way to use this rather valuable technology,

[Scaramanga F25]

So you all know the calculations for a deck stepped mast and a keel stepped mast are different.

[edlepera]

I have sailed a Contest 31 with a Tabernacle mast, a sailboat built for the north sea. Made the owner an offer he said no. But if he heard of another one for sail he call me. A Tabernacle mast sailboat would be great for doing the great loop.

[Scaramanga F25]

a "deck-stepped mast is much more likely to bend below deck level and take out a large section of the deck "
No, that does not happen but they do bend 5 or 6 feet above the deck. And that is a real problem as it not only requires to cut the rigging wires off but sawing off the stump before the spars put a hole in the boat.
A keel-stepped mast has more support which is why they are on race boats. One can get away with a thinner and/or lighter section.

[Mauruuru]

while not really a tabernacle mast step, I sail a Seaward 32RK which has a deck stepped mast pivot that accomplishes the same thing. It has sailed to carribean and back. The only issue I had was the aluminum mast required a stiffening collar at the base which US Spars recommended to the builder but had not installed. I put it in the first year I had the boat and have self raised and lowered the mast probably 30 times in the 10 years I've had the boat. Once the shrouds are tensioned they don't need released in order to raise or lower the mast....a really slick design.

[RaymondR]

Originally Posted by Scaramanga F25

a "deck-stepped mast is much more likely to bend below deck level and take out a large section of the deck "
No, that does not happen but they do bend 5 or 6 feet above the deck. And that is a real problem as it not only requires to cut the rigging wires off but sawing off the stump before the spars put a hole in the boat.
A keel-stepped mast has more support which is why they are on race boats. One can get away with a thinner and/or lighter section.

The Euler formula for the critical buckling load for a column is:

Pcr+C*2Pi*EI/L^2

Where C is a constant which depends on the end condition, E the elasticity of the material, I the moment of inertia, and L the length. Since L is squared ie exponential, it's effect on the critical buckling load is most significant and Pcr decreases rapidly.

However there are trade offs to be made. On my cruising boat the mast is not closely fastened where it passes through the deck so L is about 8' longer than it would be if it was deck stepped. If is was securely fastened so that the column (mast) was restrained the column would be the same length however and both the bending moment would come into effect and the end fixity factor C would differ benignly.

[Seymore]

Originally Posted by Wckoek

Probably a dumb question, so far I have only seen tabernacle mast on coastal sailboat, particularly catboats, but is it possible to have a tabernacle mast sailboat that is seaworthy, on the sense that safe to take it offshore?


Is there an example of a sailboat model with easily lowered mast of this kind that can cross ocean?

There were about 726 Westsail 32s built, which are noteworthy seagoing sailboats, and many had tabernacle masts. Reference is the Westsail 32 service manual, items A21 and A22 at WORLD CRUISER YACHT CO.

Bud Taplin is the Westsail guru and he sells Westsail parts, including things like tabernacles. If you get serious about installing a tabernacle, go to WESTSAIL - WORLD CRUISER YACHT CO. to dig up his contact information, and contact him.

[Mauruuru]

Seaward 32RK (or the 26RK) both have rear swept spreaders and shrouds and so as mast is pivoted down, shrouds fall to the rear and do not need to be loosened each time or readjusted when raised.....
in addition the lazy jacks have an extension "Y" cable that is used to keep the mast from swinging to the side while lowering or raising. A video exists on You tube of the rigging and raising/lowering procedure which gives a better view of the layout