Pouring urethane:  some feedback for David

Following David’s useful suggestions I decided to fit the mast pole to the upper tabernacle  by using castable urethane to pour a clamp block in two halves. It is expensive stuff to use in any volume, and it might have been more economical to make the blocks from a combination of wood and epoxy, while still using the mast and tabernacle (in horizontal position) as a cavity mould. However, urethane is new to me: its flexibility appealed, as did the opportunity to learn something by trying a new material. I decided not to mess around trying to reduce the amount of resin needed by embedding triangular wooden blocks. The over-all result was  a (qualified) success. See pictures here: Two half blocks cast; and  The two blocks clamping the mast

I will give a run down on what I found, as it may be helpful if some else decides to do the same at some time in the future.

I made plywood dams and divided up the space between the mast and tabernacle in the manner suggested by David. But I was unable to fit them nicely, watertight and coved with modelling clay – because with the mast lying in place I could not get my fingers down into the space to do that. Instead I put plumber's silicone rubber around all the edges, stuck the dams in place and settled the mast in position, hoping the silicone rubber would set, hold everything in position and contain the urethane when I poured it. It was a mistake. It worked all right, but the silicone rubber stuck so well to the aluminium surfaces that it was a very tedious job to clean up afterwards, and also made the finished urethane casting a little rough around some of the edges. After the dams were set in place I used a rather expensive spray can product to coat the inside of all the casting surfaces with a release agent – and this worked extremely well, the castings later came away beautifully, by comparison with the silicone rubber which had stuck firmly to the dry aluminium as described above.

(As an aside, David pointed out the value of having the tabernacle sides flaring slightly to allow the castings to pop out. As it happens, due to slight inaccuracy, the sides of my tabernacle do flare out, by about 1mm. But it would not matter in this case. Due to the shape of the castings and the flexibility of the rubber, they can be deformed and easily removed from the mould. I suspect even negative flaring would not have caused a problem - in this case – though a solid block might not pop out so easily.)

I made the two half-blocks successfully (see photographs above). They are 150mm in length, and used about two litres of urethane to make the two halves. I actually also cast two spare blocks at the same time, but miscalculated and ran out of urethane before completion. I quickly mixed up the dregs from the containers it came in (without measuring) and threw that in to help - the result was some soft spots where the material did not cure properly. The lesson to learn from this is: this stuff needs to be measured accurately and stirred properly. Failure to do so does not just result in delayed curing and possibly slightly lower quality (as would happen with epoxy) – it simply does not cure at all in patches, and that simply means failure.

A word on the material used: I decided Spartite was too expensive, and PCM790 was not available locally, so I used what I assume to be a similar product called Simpact 85A (A “Smooth-on” Product.)

Its was claimed to be shore-hardness 85A, and very low viscosity for easy pouring. In fact the viscosity was quite resinous and it flowed quite slowly. But it did fill all the spaces and left no voids.

PCM90 is very slightly harder (90 on the shore A scale) – but neither is as hard as Spartite, which I found out later. Spartite is rated shore hardness 60 D – but this is a different scale. A chart showing how these two hardness scales relate can be seen here. All three products are close in hardness, somewhere between a rubber shoe heel and a supermarket cart wheel - and harder than car tyre rubber. But Spartite (60D) is the hardest and Simpact 85A is the softest. I think all three products will do the same job where they are in a confined space and subject only to compression forces. My feeling is they will all work out about the same for a mast collar arrangement. In hindsight I would have preferred something harder for a tabernacle– something like the V-blocks and rollers in boat trailers would be better, I think. Still, the mast will fit snugly when the tabernacle is bolted up tight, and there will be no shocks or rattling at that end of the tabernacle At the heel I want something hard and tough, and I am going to cast a solid block around the mast, made from wood and epoxy. (Interestingly, the reverse of what Pol is doing.)

I need the mast heel to be tough and hard, as I have decided now to follow another of David’s suggestions and face the tabernacle backwards, the mast pivoting at the deck (with no hinge.) More on that later.

Delrin. As a further afterthought, the material referred to by our anonymous friend (in one of his posts) called Delrin sounds to me to have the most perfect properties for both the top and bottom of the tabernacle. But I don’t think it is available in kitset castable form, and machining a solid block (even if I had the machinery and the skill) would, I think result in far too expensive a pile of shavings.

I would use urethane again, where shock absorbing, tight, accurate fit and slight flexibility are required. In most cases though, I would stick to the tried and trusted epoxy/wood/glass combination.

Thanks again for David's help and suggestions.

Making a design is, for me I am afraid, a rather long-winded process.

Each new idea that clever people give me, seems to suggest another, and sometimes the need to revise a decision previously made. One could always do it better next time.

Temporary mast clamp for jumper strut – and possible tabernacle hinge

Another matter which I now realise should have been more carefully considered when designing the tabernacle. If the mast is to be raised or lowered from the deck, then a temporary pole (I’m calling it a jumper strut) needs to be rigged at approximately right angles to the mast, a bit like a spinnaker pole.

The lifting line (halyard?) goes from somewhere around the mast head, over the end of the strut and down to a turning block placed as far forward as possible at deck level.

For me, this strut can be only about 2.5m (which means it might also serve well as one of a pair of drying-out legs, later.)

(An aside: A longer strut would be better, but we are a bit compromised with junk rig. The forward placed mast and lack of a bowsprit precludes a more ideal geometry for the lifting arrangement.

A rough back-of-an-envelope statics calculation (here) shows that at mast horizontal, the position of maximum tension, the tension in the lifting rope is less than 150kg. That’s still quite a lot. My windlass has a working load rating of 180 kg. However, I was going to use a 2:1 tackle anyway, just to slow down the rate of pull.)

I was going to make a rotating mount for the temporary strut, and have it carried by the same pin which also makes the lashing point for the mast. However I see now that with castable urethane technology, the temporary strut could, alternatively, have a “jaws” fitting and bear on the mast, like a spinnaker pole does. It does not need to be clamped as it is mainly only in compression, but a clamp is convenient, and with the casting technology suggested by David, it seems worth while to cast a “jaws” – or even make a full clamp – at the same time as casting the mast fitting blocks.

That being the case, we are now more than half way to making the other half of a proper clamp-on tabernacle hinge – a fitting which I had been intending to avoid by using a rope lashing.

It is time to stand back and review the entire assembly: tabernacle, fitting of the mast via blocks cast from urethane, hinge, jaws/clamp for jumper strut.

I got the tube welded to the top of the tabernacle yesterday (here it is) and then immediately realised how easy it would have been (and possibly still could be) for that tube to be part of a 12mm-pin butt hinge. The other part being a similar tube welded to a flat plate fastened to one side of a mast clamp – the same clamp could carry the jumper strut. The pin can be knocked out and all these parts removed and stowed away when not in use. It would take only a couple of minutes to re-assemble the clamp, insert the pin and mount the strut. Here is a rough sketch.

I wish I had made the tube a bit bigger now. It is 12mm ID with a 4mm wall thickness. It looks a bit light for a load-bearing hinge, and I wish I had made it heavier now. But maybe it is still strong enough to be part of a butt hinge.

Anyway, I think I will cast a couple of “halves” while doing the other casting. It will allow me to make a “jaws” and be still not too late to make a proper clamp-on tabernacle hinge if decided at a later date to dump the rope lashing idea and make a full clamp-on hinge fitting.

In my case now, I think there are still two options:

(1) the simplest most stripped down option – a rope lashing and pin for the mast hinge, and a jumper strut carried by a “jaws” bearing on (or clamped to) the mast

(2) a tempting alternative - removeable integrated butt hinge/strut mounting clamped to the mast

The flanged channel with its bolt-on 4^th side provides the obvious mounting points for halyard and other turning blocks. I think this, and the above, covers all the required functions of a tabernacle, but a little messy, and perhaps there are still some elements which could be removed.

Next time, always next time! I hope these lengthy and tedious questions and answers might be of some help to the next person who decides to make a tabernacle. It would be nice to be able to draw up a fully integrated simple design for tabernacle, hinge and lifting gear based on a folded aluminium channel, wood, and castable urethane fittings, with welding required only for the removable clamp-on butt hinge.

PS you can get lucky sometimes. It seems that there was a very slight inaccuracy in the folding of the tabernacle. The sides are not quite parallel, by about 1 or 2 mm as far as my eyes can determine, when wearing the right glasses. The open sides flare out very slightly.

Graeme Kenyon wrote:

And one more question David: PMC780 is available in NZ (evidently not 790). But it is expensive, I may decide to fabricate with wood and epoxy rather than cast. Still weighing it up. Question: can you confirm 100mm is long enough to spread the load, for the top and bottom blocks? And if casting in urethane, would there be any problem with reducing the volume of material required by permanently immersing 4 wooden triangular fillets in each of the blocks?

100mm is the depth of the partners that I used on Tystie, with a much bigger mast.
The problem with the wooden blocks would be to hold them in the correct position, but if you can solve that, I don't see any reason not to use them. Maybe they would be fixed permanently to the plywood dams, which would be left in place and coated with epoxy? If epoxy costs less than polyurethane, the wooden blocks would decrease the volume such that exotherm wouldn't be much of a problem if you poured the blocks with liquid epoxy.

Hi Graham,

don't even think of welding to the mast, as soon as the welder touches the aluminium you have lost almost half of the strength of the section at that point and at the most critically stressed location on the mast. Use a clamp made of two semi circular bent strap pieces with bolting tabs  at the ends, then weld your hinge pieces to this. 

David.

Yes, I can see how method 1, a square block around the mast, helps with making the hinge. I'd still go with poured polyurethane as being the quickest, easiest way to make this. It might be just a little too tight at the sides, as the mast is being stepped, but that would be easily fixed. Ideally, the sides of the channel would not be parallel, but would be spread apart a degree or two so that the square block touches the sides only at the same moment as it bottoms out.