Scott wrote:
I've just finished doing the math for my boat; same problem, different scale. I elicited the input of a couple of marine engineers that I admire, and the same answer came back: make it out of aluminum or stainless steel. It'll be lighter, stiffer, and less bulky than a wooden one. Costs can be reasonably had. A good engineer can even work in lightening holes if it's designed in from the beginning. Though with your smaller scale, I don't think you'll need them, nor is there room.
For your scale, I'd lean toward in a simple u-shaped tabernacle, that becomes a closed box below decks. Scantlings need research, or a smart guy like Arne or David to chime in.
I had planned on making a “Pete Hill” type tabernacle, in fact I looked closely at the one he made for Annie, since my mast length, scantling requirements etc are similar to those of the SIBLIM design. However the source of timber that Pete used had dried up and I could not find an economical source of suitable wood, so I gave up on the idea.
My aluminium tube arrived a few days ago, and the idea has revived. I found a local engineer who can guillotine and fold aluminium sheet, and the cutting and folding to make a 160mm x 160mm channel is only of the order of about $30. Cutting and folding 6mm is certainly feasible locally, and maybe thicker, I am not sure. I was wondering if 6mm would be thick enough.
So I am now looking at the cost of the material, aluminium sheet (plate?). Starting with a length of channel and some pre-cut flat plate it should be possible to use hand tools and make something simple and square, similar to the wooden tabernacle, the only welding required would be lugs for bolting on the 4th side of the box, attachment points for turning blocks, and flange at the deck.
(I would add to Scott’s suggestion and make it a closed box above deck as well as below.
The hinge could be a simple temporary lashing to a horizontal round bar fixed at the top rear of the tabernacle – temporary because when the mast is erect it is enclosed by the bolted up 4th side of the box and no need of a hinge until next time it is lowered.
Or the tabernacle could be reversed, with open side facing aft, and the mast “hinged” at the deck with no fitting at all, as suggested by David Tyler. Once again, the 4th side of the box, above deck, bolted on after the mast is raised. Still considering the pros and cons of those two options.)
So the question is scantlings and I was hoping someone smart about these things would “chime in”, as Scott puts it. If the tabernacle can be made from sheeting light enough for cutting and folding, but strong enough for the Pelorus, then this could be a runner. (If not, the idea might still be useful for a smaller boat.)
I seem to recall Pete used the reasoning that the wooden tabernacle scantlings could be derived from the Hasler-McLeod scantlings for a hollow wooden mast (I can’t find the quote now) and I think he used 50mm timber. My hollow aluminium mast will be the same at the partners as SIBLIM’s (aluminium 152mm OD and wall thickness 5mm.) The height above deck will be about 9m and the boat displacement is about 3.5 ton or thereabouts. Could I use similar reasoning and fabricate an aluminium tabernacle from 6mm plate? It will essentially comprise a U-section channel with 4th side bolted on to make a box, both above and below deck, with rounded fillets moulded inside the box above deck to stiffen the structure a little, and snugly fit the mast. Does 6mm plate thickness sound enough?
I would be interested to see the numbers Scott came up with for his boat