Help me design a new tabernacle/mast step

[whipper_snapper]

35' Wharram cat. Like all Wharrams, it is designed to move and flex and the cross beams attach to the hulls on flexible bushes, all other attachments across the structure are lashed.

The mast sits in a tabernacle and its loading is taken onto one of the 4 massive cross beams. If it slipped off the beam, it would punch through the deck in an instant. Shrouds, backstays and forestays are all adjusted by lashings. All very low tech!

However, the mast is secured in the tabernacle by a massive alloy pin which passes through tabernacle and mast. As the boat sails and everything moves, this pin walks out of its holes with an absolutely inexorable force. We have tried assorted normal things to retain the pin to no avail. We could drill holes and drop smaller pins through the ends of the big pin, but I am quite sure that if we did it would 'walk' until one end pin hit the tabernacle and then just destroy pin or tabernacle.

So I am wondering what the pin is for anyway? Why did the builder decide to lock the mast when the rest of the boat is designed to flex ?

I am therefore proposing to chuck it away and reinforce the tabernacle with stainless steel cheeks, with stainless steel pins NOT passing through the mast, but just holding the tabernacle together in fornt of an behind the mast, and hardwood packing pieces to hold the mast in place in the tabernacle but to allow it to move - especially to rotate slightly.

Pictures of the present situation below may help to clarify. Note the mast is not perfectly upright in the pictures, but it looks worse than it is as one hardwood side piece has been removed.

Any comments or suggestions or better ideas? My overwhelming concern is that while we must allow mast movement we must not allow it to jump out of the tabernacle or slip off the beam (for example in extreme weather of if a rigging piece broke, or after a big accidental gybe) that would be a total disaster and we would loose the rig in an instant. Also, we must not allow the mast to rotate too far, I don't see why it would want to as the rigging loads it more or less evenly from all around, but who knows! Finally we don't want it to slowly drill its way through the beam it sits on!


Thanks in advance

Below. Looking aft. You can see the pin sticking out on the right of the picture (P side of boat). In the picture it seems to lie on top of that big cable conduit.

Below. Looking fwd. You can see the pin emerging from the tangle of rope of the left of the picture.

 

[mainmarine]

Could you get a short length of Stainless steel or alloy pipe, say six inches long, of a larger diameter than the mast, cut a slot at the rear for the cable, have the pipe welded to a similar metal plate and bolt or screw this inside the original tabernacle, as a socket to locate the mast

 

[Bobobolinsky]

Help me design a new tabernacle/mast step

Hi, you could socket the bottom of the mast, preferably in ally not stainless and with drain holes, but you would need to step with a crane then, increasing cost

Robin

 

[Swinranger]

Hi,
Presumably the pin is to pivot the mast when raising and lowering. I don't think it's designed to carry loads once the mast is set up. Could you drive wedges or packing pieces under the heel to take the load off the pin which would then stop working it's way out?
You would still need to prevent the packing from working loose.

 

[Stemar]

Could you replace said pin with a SS bolt and nyloc nut?

 

[Salty John]

I'm with Swinranger on this; I don't think the pin should be carrying the mast load, which is typically close to the weight of the boat.
I would wedge the mast up to take the load off the pin. You then need to devise a way of keeping the wedges in place, or you could, perhaps, slot the hole that the pin goes through and allow the mast heel to settle back onto the deck. I assume there is a mast partner below this arrangement to transfer the mast load to the hull. Then put a batten fore and aft of the mast to prevent movement that way.
I'm not a naval architect, but the current arrangement just doesn't seem right.

 

[whipper_snapper]

Thanks for these comments.


The tabernacle does not actually function as a tabernacle! I probably should have mentioned that! The geometry is such that the mast cannot pivot on the pin; its function does appear to be solely to locate the mast and stop it going walkabout.

I don't think the pin should be carrying the mast load, which is typically close to the weight of the boat.

It doesn't ! It sounds odd, but when it is all lined up, the pin slides in and out perfectly freely. But as we sail, the mast moves minutely and slowly 'walks' the pin out.

Could you replace said pin with a SS bolt and nyloc nut?

If we replaced the pin with a monster bolt, my suspicion is that the head would either rip slowly through the tabernacle or the bolt would rip slowly through the mast. The forces are colossal. In previous mid-ocean attempts to keep it in we used big coach bolts into the wood to hold a ss plate over the end, it just slowly shoved the whole thing aside. I think we should not be attempting to fight the forces as they come from the inherent flexing of the structure.


I do like the ideas of making a big socket for it. We could use stainless but put a disk of hardwood (or even some HD plastic ? Teflon?) at the base to take the grinding and prevent corrosion. It could then be cross-braced to the beam and should be very secure. In fact, that amounts to making a new 'tabernacle'. I was hoping to be able to fix it without lifting out the mast which involves a crane and money, hence my thoughts about building a SS-backed hardwood lined cage around it....

 

[Swinranger]

Ok, the forces acting on the pin are like those of the plunger in a mastic gun, if you enlarge the hole in the mast and insert a nylon bush it won't have the same grip.
Then you just need to locate the mast heel more positively, the socket ideas proposed sound good.
regards

 

[Gordonmc]

I understand your concern. On a previous boat the deck-stepped mast sat on a tabernacle with no socket so the exstrusion end was subject to wear. Two M12 bolts held the shebang together but a sleeve which at one time went through the mast to locate the bolts disintegrated and the bolt holes were elongated.
The repair was straightforward enough with plates and new sleeves welded in. Instead of steel bolts I used 12mm alloy rod, believing the reaction of alloy/steel caused the problem in the first place.
Assuming the pin goes right through to the other side of the tabernacle could you drill holes in the pin at the point between mast and inside face of the tabernacle and use "R" clips to locate the pin?
The pin would then only travel until the R clip met the inner face of the tabernacle. The cip on the other side would be held off the mast.