Join Date: Jan 2008
Location: Seal Harbor, Maine
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Re: Mast tabernacle project
"The other complication to this (and any system relying on athwartship support) is that ideally the center of rotation for both the mast pin and the supporting rigging ("A" in the diagram above) should be in line. Does he have shrouds attached to rigid 'tall' chainplates? or is he using the stanchion as a temporary one? it's not clear on the sketch."
You are correct. The most important consideration when locating the mast pin and the swivel point in the upper is that they be as close to perfectly in line as possible. He uses a clever set of nesting shackles on a tall jaw / jaw turnbuckle. It allows the shackles to remain as permanent parts of the rig and it only requires connecting the two A lines which also have turnbuckle ends for adjustment. If you look very closely at the A lines you will see a triangle plate connecting it to the upper and the chain plate. That is another way to hook them up.
Getting it close enough to finalize vertical alignment with the turnbuckle won't be hard. That will be a function of the turnbuckle's length, the fittings and where I cut the upper. I have located the line between the two upper chain plates across the deck and with a model of the base of my mast on the deck have the fore and aft measurement on the side of the mast for the pin. Now I just have to pick a height off the base of the tabernacle for the hole. I am thinking it will be centered at a height equal to the width of the mast which is 3 3/4". I will have a couple of 1/4" cheek plates welded onto the outside of the base of the mast, transfer the measurements and drill a couple of holes. That will take a bit of care but I have a portable drill press I can use. I will install a compression tube that will carry a 1/2" pin. Cutting the leading edge of the base away will decrease the vertical travel as the mast rotates through 90 degrees and reduce the effort required a bit. I hope to step the mast onto the tabernacle with a boom truck this next launch. The rigging mods happen after the mast is up. It will be interesting.
I can certainly attach a line over the mast's base to manage any upward moment. I don't have the bow pulpit the Nord'Sea enjoys.