The hinge

It seems that fitting out the square tabernacle to take the round mast boils down to two approaches: Build up the mast to a square section (the packing being part of the mast) – call this method 1.

Or, as David is suggesting, make a cast packing, in two parts, which acts as a clamp, one part attached to the tabernacle and neither part attached to the mast. – method 2.

Both are practical and (on thinking about it) neither is difficult to do.

I can see the beauty of method 2.

If neither approach has any potential disadvantages, then, before deciding, it is also necessary to consider the type of hinging system to be employed, because there are implications. It is a given that there will be no welding or drilling of the mast at this point, or pins through the mast.

Method 1 allows a simple and cheap hinging arrangement. For example (breaking the no-weld rule just once) a piece of heavy-walled tube can be welded across the aft upper end of the tabernacle, stiffening it, providing a roll-over surface to bear the flat aft of the built-up mast as it falls backwards over the tabernacle – and allows for a temporary pin to be pushed through, protruding each side, to make a temporary “sampson post” to which the mast can be lashed when raising or lowering. The lashing will rotate with the mast as it rolls back over into the lowered position, as the centre of rotation is the centre of the pin. I don’t think anything could be much simpler (apart from a suggestion David has made in the past which is to face the tabernacle aft and hinge the mast at the deck, with no fitting at all. I would certainly do it that way if I were not encumbered with a raised cabin preventing the mast from reaching the horizontal.)

(Method 1 allows the attachment of a normal type of hinge to the mast too, as there would probably be enough meat in the packing to take screws - perhaps applicable to a smaller set up. But I won't be doing that.)

David’s Method 2 does not, as far as I can see, lend itself so readily to such a simple hinge as the pin and lashing allowed by method 1.

I am inclined to think that if there are reasons why method 2 is preferable, then it would pay to cast an extra pair of semicircle packers, to enable the making of a temporary clamp to be able to put around the mast just above the tabernacle, and a custom made fitting which can make this clamp the second part of a two part hinge, the first part being the removable pin through the welded-on tube, as described in method 1. It needs to be in place only when actually raising or lowering the mast.

I think this would be very nice, but it might call for some more workshop engineering and a bit more cost and time. Just considering the hinge on its own, I think the rope lashing is a bit more KISS and I like it more.

My guess is that one method is as good as the other, and hopefully not much difference in cost, so perhaps it just boils down to personal preference and what skills and materials are at hand.

But I am not sure.

Method 1 requires a nice fit, with the possibility of some clamping in the fore-and-aft direction by the use of a gasket placed under the bolt-on 4^th side. (Thin polyurethane foam would be good here, I think.)

Method 2 simply allows a good tight clamp all round (though with immediate release the moment the front is removed and the mast allowed to rotate backwards.)

Are there any disadvantages to method 1?

Annie Hill wrote:

David, do you think there could be any issue with corrosion where you have water between the mast and the casting?

I don't think so, because the whole idea is to make a clamped fit, thus excluding water. Corrosion only occurs when you have dissimilar metals with salt water lying between them. It doesn't occur either 1) when aluminium has full access to air, or 2) when everything that might act as an electrolyte is excluded. 

The channel is looking good, and seems to be a very cost effective and rapid way of making a tabernacle.

I would go with option (3), as it is indeed quick and easy compared with the alternatives, and automatically achieves a good fit. 

I would aim to make two semicircular components at top and bottom, with a small gap between them when assembled such that they will act as a clamp to hold the mast firmly in position with no annoying rattle.

Here's a suggested way of proceeding. Lay the channel horizontal. Make eight thin plywood dams that fit into the channel, 85mm high, with a semicircular cutout to fit the mast just above the centre of channel, so with its centre 88mm above the bottom. Fix two of the dams in place with modelling clay, 100mm apart, at the top of the channel, and two more just above the where the deck level is going to be. Apply mould release wax liberally to the mast, at the points where it will lay in these dams. Now do the same again, in different positions, but this time, apply mould release wax liberally to the channel as well. Lay the mast into these dams, and seal around the dams with modelling clay. Pour castable polyurethane into the moulds thus formed, up to the top of the dams. Remove the mast, and the two mouldings that are going to fit on the open side. Screw these two mouldings to two pieces of flat bar which are then to be bolted firmly to the flanges when the mast has been stepped, with maybe packing pieces if needed, to clamp the mast. The anti-turning device can be a large screw through the lower, deck level clamp into the mast tube. 

I would use something similar to Smooth-On 770, 780 or 790. Since this has proved to be fully able to support a mast through deck partners and at the heel, rather than wooden edges, it will work well in this application as well.

Thanks David, your advice and comments are greatly valued.

That just about wraps it up.

I'll report back when it is done.

(Three hours later.)

These guys are great to deal with.

Here is the proposed section, to come exactly out of a 600mm wide half-sheet.

The CAD drawing is here

It comes out with an ID of 175mm square, and 44.5mm flange.

Now that we know how much to allow for folding, it looks slightly too big to me.

The sheet could be trimmed by a 25mm offcut - the dimensions would then be 170x170 with 39.5mm flange.

 However, I guess a few mm isn't going to make that much difference and simplest would be to run with it as it is, with no offcut.

What do you think guys? As it is, or trim it down? (The sheet has not been cut yet.)