[downeast450]

I have been "pondering" adding a tabernacle to Tundra Down for a couple of years now. With the help of Ed and Ellen Zacko I am on the verge of cutting out the pieces of aluminum that will be welded together and become the mast's new base. Part of the process that hinging the mast requires is careful alignment of the "hinge pin" that runs through the tabernacle and the mast and the top of the uppers' lower turnbuckles. That isn't my concern here. What I discovered when building a mockup of the tabernacle is that the attachment point for the lower end of the upper (at the chain plate) is not centered on the mast but is just 1" from the forward edge of the mast. I stretched a tight line between the opposite chain plates and found where it crosses the "base plate" (mast step) on the deck. (Red pencil in photo)

I have Bob Perry's plans for this boat. As one would expect it shows the upper centered on the mast, fore and aft. It is a drawing? I would like to move the center of the mast's base forward an inch or so in the new tabernacle. It would still be at least an inch forward of the mast's actual center line.

Moving the location of the mast's center forward an inch or so has other considerations. Wiring will no longer pass through the deck under the mast's base. It will exit the mast and I will use Bulgin hardware to route wires. The I-28 has an unusual solution for supporting the compression post. (I think the original design had the mast stepped on the keel?) Because of this, the post itself is oddly configured. I will need a stronger platform for the tabernacle because of the upward forces on the base involved in raising and lowering the mast. Increasing the number of anchoring machine screws from 2 to 4 will requiring a modification to the compression post's top plate. Routing wires inside the post is no longer an issue.

What do you think? I don't see a reason not to. Modifying the compression post is a must.

 

[Scallywag2]

Bob Perry would need to answer the question about the drawing. Moving a mast will change the CE of the vessel. I am not referring to moving forward or aft. Will this be to the good? Did the previous owner move the mast aft? No answers but questions for you to consider.

 

[downeast450]

Hoping Bob jumps in. I have no reason to believe the mast has ever been moved. The locations of the chain plates always seemed curious to me. The forward lower and the upper are so close it is difficult to insert a retainer in the clevis pin. Not sure why the upper is so far forward other than it fit the cabin configuration and the bulkheads? Could that be the case? I could move the uppers' chain plates, aft, if the aft lower wasn't in the way. I suppose I could relocate the aft lower and use the current lower for the upper? Or I just live with a hinge pin that far forward in the mast. It will be installed with a "bearing pipe" welded into the mast. I would feel more comfortable with the pin a little further back. Ugh!

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[bobperry]

1" will make no difference in anything so long as the steel pad beneath it is still taking the load.

 

[downeast450]

Thanks Bob! It doesn't hurt to have your "approval". The compression post will still be taking the load. I will need to enlarge the top plate to give me room for two more bolts. It now relies on two that sit off center to port so there is room for the hole for wiring. All it needs to do is hold place. The tabernacle will apply some upward forces stepping and un stepping so a more secure platform will be necessary.

I have run out of excuses! Ha! I can now remove the current base plate and the compression post to modify it. The wiring re routing can happen now, too.

I will give it a go. Some kind of crutch for the mast to rest on that adds to the strength of the bow rail will be necessary.

This is the current step.

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[paulk]

Just a question: are you doing this move because the mast won't pivot down with the chainplates where they are? I would want to be absolutely sure of that before going to all the work of redoing the wiring and compression post. We had a somewhat similar situation with our deck-stepped Soling mast, with the shroud bases a touch forward of the step. There was sufficient give in the setup to enable us to drop the 30' mast despite this. This month's Woodenboat also has a nice article on a folding mast refit that might be useful to you.

 

[downeast450]

Just a question: are you doing this move because the mast won't pivot down with the chainplates where they are? I would want to be absolutely sure of that before going to all the work of redoing the wiring and compression post. We had a somewhat similar situation with our deck-stepped Soling mast, with the shroud bases a touch forward of the step. There was sufficient give in the setup to enable us to drop the 30' mast despite this. This month's Woodenboat also has a nice article on a folding mast refit that might be useful to you.

Hi, and thanks for this help. It has been an interesting modification to plan. This post is offering some important considerations. I have been "copying" the design and operation of the set up Ed and Ellen Zaco have on their Nor' Sea 27.

The wiring gets done regardless of the location of the mast. Having the wires exiting the mast and connecting through the deck "outside" the mast makes sense. I suppose I could still route the wiring up through the tabernacle??? I will consider it. It would get it out of the way on deck? A Bulgin connector will be required either way.

I need to drill a hole through the mast to weld in place, at the ends, a piece of sch 40 .5 NOM pipe, I am so close to the front edge of the mast I just don't like the structural situation. A "washer" will get welded to the ends of the pipe and to the outside of the mast. This will provide a flat surface for the interface between the mast and the tabernacle. Moving the mast forward about an inch, perhaps an inch and a half, will make it a much stronger location for the swivel pin. The pipe is just a bearing for a .5" ss bolt that will act as the pivot. I will radius the leading "corner" of the mast to minimize the rise that results from rotating through the "normal" squared off end. The new location of the pin will align well with the end of the radius.

This is a photo of the tabernacle on an Nor' sea 27. I am shooting for this.

I need to strengthen the compression post anchor situation because of the upward force on it when stepping and un stepping. Once I have the current deck plate off I can mark the current bolt holes on the mock up from below and plan another two locations for machine screws. They might fit on the current top plate of the cp. If not, adding a little material isn't a big deal.

What happens with these mods on old boats is they lead to other things!! It will be time to deal with the ceiling next.

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