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True and plumb tabernacles


Brent

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For those who have actually done this, what are some good techniques for setting the tabernacles that result in perfect angles, with the tabernacles giving two perfectly raked masts exactly 3* aft and perfectly plumb side to side?

 

I am visualizing a centerline string running from bow to stern for port to starboard alignment.  Then sighting down the tabernacle side faces to get the perfect 3* or so twist so that the masts fold next to each other. That leaves only the for and aft alignment which determines mast rake, the method for which seems less obvious to me.  The bulkhead the tabernacle mounted to are very close to plump with the waterline, but probably not sufficient to use in guiding placement of the tabernacles.

 

I remember measuring six or seven times when I place the mast tubes on my CS 20, with good results.  However I don't remember the details of how that was accomplished. I suppose one route is to get the tabernacle fore and aft alignment close, then tweak mast rake using spacers in the tabernacles.  I was hoping to get the tabernacles very close to true prior to making final adjustments with spacers between the tabernacles and masts.

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If you're building the boat, leveling her up should solve most issues. A plumb bob will keep her stick centered and some simple math will provide the rake.

 

As to lowering one stick to clear another, well this will require some setup, but generally I start with the masts in their lowered locations, to insure sufficient clearance. With this established you should be able to "pickup" the angle off the centerline for placing the pivot pin. Typically, this isn't much, a degree or two. It's entirely probable you'll have a good bit of slop as the masts are lowered, which can be helpful, if a little wobbly in strong winds. Trust the tabernacle to do it's job, which is to keep it relatively in line, as it comes down. This means it has to be stout, especially the "cheeks" which cradle the mast as it pivots down. The best tabernacles are metal and these are the least intrusive and have the least amount of windage. Metal tabernacles also tend to be lighter then wooden ones, mostly because you need a lot of wood to do the same job as metal.

 

If you get your measurements right, you should only need to "tune" the tabernacle the first time up. Once it's sitting on it's wedge (mast heel rest), you shouldn't need anything else. One last note, the mast should sit on the wedge, so loads go through it to the surrounding structure, rather then ride on the pivot bolt. Instead of wedges at the base of the mast, I use a keeper, which is usually just a bolt or pin. This hold the mast upright, but has enough slop so the weight of the mast is on the wedge, not the pivot or keeper. In fact, I usually drill the keeper hole in the mast a little bit higher, then the holes in the tabernacle sides. This places downward pressure on the heel, insuring it's resting on the tabernacle, not the keeper.

 

I hope this makes sense to you.

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When we built out 20 this is how we did it. Assuming the boat is nice and level. Port and starboard alignment I would say is probably the least critical but also the easiest to measure. 

 

We clamped in some "dummy" stub masts in the tabernacle maybe 3 or 4 feet long. I think we just used some nice straight 1x3 (anything straight really). With the aft tabernacle on the princess you almost wouldn't need much extension on it since it's already quite tall to clear the cabin. On the 20 we just slid the 1x3 through the thwart and stepped it into the mast step. On the tabernacle, we just clamped it to the inside face of the tabernacle to extend the inside lines. The alignment process, set the final position of the main tabernacle and mizzen mast step at the same time. We were in a basement so we couldn't measure up very high so we could only get a rake measurement over about 4 feet which really is plenty. if you make some accurate 3 degree wedges and hold them up as spacers then you can just use a level that you trust to get your rake. As PAR said we used some trigonometry to get a double check of our rake but I can't remember if we actually used a plumb bob. 

 

Getting them plumb port to starboard is just a matter of plumbing one and then sighting two straight references parallel as seen from the bow or stern. I think we did this one entirely by eye just like when you square a boat up bow to stern by putting rules across the sheer. If you have a 4' rule to sight its pretty easy to get them exactly parallel with some careful eyeballing. I would spend some time on that one because it looks the worst if your off a little bit. All the others could be out by a long way and be pretty hard to notice but that one will be seen every time you step the masts. If you have bad eyes, DONT WORRY. Just take a bunch of steady pictures of the two straight edges very close to one another from the bow or stern and then just pull them up on a computer and ZOOM IN to measure the difference as close as you want. 

 

Ok, now for the twisting. I think that one is fairly easy. If you make 2 boards that fit snugly into the top of the tabernacles and you clamp them in horizontal so that they are pointed at one another then you can sight down them from the bow and should immediately see if they collide. Just tweak the tabernacles little by little until you can sight the edges of the boards in line with the outside edges of tabernacles. DON"T FORGET that the top of the mast is going to be NARROWER and the mast won't be as tight a fit as the board either. We used some thin UHMW plastic washers on the inside of our tabernacle on either side of the bolt. 

 

Anyway, that's how i'd do it. 

Hope that helps. 

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Brent:

 

My understanding of the plans has both tabernacles vertically aligned with the centerline. The mast rake is taken care of with wedges inside the tabernacles at deck level. These half round wedges conform to the butt of the masts, which nest snugly inside these wedges to eliminate fore and aft and side to side movement, but once you pin the masts in place, the rake angle should be right. In tuning it up, you keep trimming the wedges until you get it right.

 

The angle to offset the masts as they fold down in the tabernacles is the tricky part and as I envision it, is dealt with by placing a thin shim on the back of one of the tabernacle side pieces as they are being glued into place on the bulkheads. That mounts them at a slight angle, such that when the mast is lowered, it comes down at a very slight angle so it rests alongside the other tabernacle. Not sure how thick that shim has to be*, but it is not much. Thickened epoxy fills any gap, which won't be much.

 

*(Edit: I calculate the shims to be about 1/16th inch or 2.5 mm to be more exact. That is about the width of one standard stirring stick. You could either shim it, or leave one side 1/16th proud when you glue up the tabernacles. That is the starting point and I'd dry fit them in place, then test them with either the actual masts, or some fake PVC masts. We will also have to fabricate some type of receiving bracket for them to land on. I"m thinking a piece of wood, with a dinghy sized rudder pin and gudgeon that can be removed and stowed when not in use.)

 

I had not thought of the issue Paul (PAR) raises with the vertical weight of the masts riding on the pivot pin. Will have to give that some more thought.

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I like the stub mast idea, but as Paul discerned it probably isn't needed for the mizzen tabernacle. Totally agree esthetically getting both masts parallel is very important. Getting the first plumb will require running a string from the top of the stub mast to gunwales on both sides or something similar. The boat is not presently resting dead level but doing something about that will make the whole operation easier, then the plumb bob will be useful.

Getting the twist in the tabernacles is easy. As said simply sighting down the tabernacle checks gets it close. I ran a string from inboard starboard cheek on the main tabernacle to outboard port cheek on the mizzen. Then used a straight edge on the cheek face to line up with the string, keeping in mind the mast taper. Howard I am right there with you in maybe a 1/16" shim being plenty.

The rake alignment is the dimension I had been struggling with the most. I recognize I can trim the mast wedge to fine tune rake but being a little compulsive wanted to get the tabernacle mounted as dead on as possible. Obviously getting the boat level fore and aft will make this easy, thanks for the reminder.

PAR's, I completely agree with using the lower wedge to take the bending and heeling forces off the pivot bolt. Are you also suggesting passing the vertical forces on the mast into some type of wedge at the mast base? I think that to be a good idea. I am imagining maybe cut the heel face of the mast at an angle then build a support plate tied to the tabernacle presenting a face matched to the mast heel angle. The angle being such that when the mast is rotated it lifts clear of the support plate... I need to think a little more on that.

I am planning to run the mast bases say 6 inches below deck to help spread the bending moment into the deck. I think Paul's cs20 main is done this way also. My tabernacles are built from high density foam and will get a couple of layers of glass. Even without the glass they are quite stiff...and not so heavy.

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I seem to recall quizzing Graham about the amount of bury the butt of the mast has in the tabernacle. It looks like there is an awfully long lever in the form of a mast and a very short piece of mast beyond the fulcrum of the tob of the tabernacle, such that the leverage force would be quite high. But as I understand it, once the butt of the mast mates up inside the base, and is firmly pinned to it, the leverage force transfers all the way to the base of the tabernacle and to the keel. I believe the relatively shallow bury of the CB in the CB trunk works the same way........the leverage force transfers to the doublers, all the way to the top of the CB trunk.

 

I've been trying to recall the issue Travis had that caused the tabernacles on Pilgram to split. Somehow I think it related to too loose of a fit at the pivot pin, either side to side slop or something else. I also seem to recall orientation of the grain on the wood also had something to do with it. Now would be a good time to get that settled.

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A tabernacle needs to take the rig loads and place them into the hull shell. This is the crux of it. If the mast rides on it's pivots, it tends to rack the tabernacle, testing it's toughness. You can compensate with a really stout tabernacle or insure the loads pass though the tabernacle into the partners and step. Be step I mean where the tabernacle lands on the keel. To do this you almost always need a slightly angled mast heel, where it bears on the tabernacle. This is because it has to follow the radius, caused by the pivot, as it swings into the tabernacle.

 

On some I've seen elongated slots for the pivot, so the mast rotates but then rides up as the mast heel comes home into tabernacle. The amount of elongation doesn't need to be much, maybe an 1/8". With this arrangement the locking pin (what I use, though a gate can be employed) can just hold the stick in the tabernacle. Slop in the pivot/mast interface is also quite slight. A 1/16" would be more than enough.

 

I have a really stout tabernacle with a tall pivot and a 25' mast. I use a 2:1 purchase on a winch to lever the mast up and down. The purchase on the mast is just above the heel and pulls through a hole on the tabernacle, at the deck. The tall cantilever helps with the hoist, though the initial pull is significant, so I walk her up, with the mast on my shoulder, while someone else grinds away, keeping up with my movement. I could put temporary weights on the mast heel, which would ease the hoist effort a lot, but then I've got these weights to stow, so I've never done it.

 

As to the offset to clear the mizzen, you'll need about 2 degrees on a CS-17. I don;t have dimensions for the CS-20, though I'll be it's about the same. At 2 degrees and assuming the pivot is as high as practical and about 9' away from the mizzen, you'll have about 1 3/16" from the side of the main tube to the side of the mizzen tube. The lower the pivot, the harder it is to hoist, especially if using tackle.

 

A buddy of mine has a cute hoist on his Catalina 22. He uses a small 12 VDC motorize trailer winch. He has a 5' long paddle with a lanyard on the handle, which gets tied to the mast very loosely several feet above the gooseneck. The jib halyard id run through a groove in the paddle's big end and strung taunt to the winch, so it sticks straight up from the mast. The winch pulls on this as he helps the stick up, eventually the halyard falls out of the groove and the paddle falls away, sliding down the mast. Simple, easy and he can use the paddle to get home if his engine takes a dump. There's no tabernacle, per say, just a pivot pin on a deck mounted bracket. This is the traditional way to hoist, though he's using a motor, instead of a snatch block on the stemhead fitting like most do. This isn't practical on a cat ketch, though the trailer could be used, maybe with a hunk of 2x4 and a sheave on it top, clamped to the winch tower on the trailer. I've seen this down with beach cats.

 

In the end, you'll figure it, just give some though to the loads going up and after it's up with wind pressure in it.

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I used to use a "gin pole" type of arrangement attached to the winch stand on my trailer to raise the unstayed mast on my 17' Marconi rigged catboat. As far as the load transferring the load to the heel of the mast, this is mostly important on a stayed mast where the loads produced by the stays are transferred directly down the mast into the mast step.

 

Raising a mast with these methods that is not in a tabernacle requires some way of supporting the mast so that it doesn't fall off to the side. Tabernacles are GREAT!

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  • 2 weeks later...

Re: Howard's last comment on my main tabernacle issue...the pivot bolt was too loose and did not transfer the side-ward pressure on the tabernacle from the leeward side of the tabernacle to the windward side. The bolt must be tight enough to equalize the load on both sides of the tabernacle for maximum support.

Graham was sailing the boat and we were on a beat with single reefs set when we noticed the cracks down both sides of the tabernacle. I took several turns on the bolt and snugged it up and kept sailing, though with some trepidation.

The repair included ripping out the splits and epoxying in a "feather" in each saw cut and then wrapping the tabernacle in 12 layers of epoxy and glass tape. This was three years ago and I've done some hard sailing since then with no more issues.

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Travis:

 

Thanks for the info. Good timing as well, as I'm getting ready to assemble my tabernacles. To clarify, the board that split was the middle board that backs to the bulkhead? Do you also recall the grain pattern of the wood? Vertical grain or flat sawn? I would think a flat sawn board would be far more likely to split than one with vertical grain, splits generally running 90 degrees to the rings. Beyond that, glassing the tar out of it along with a close fit.........bordering on snug............ to take all the side to side slop out of it.

 

Howard

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Howard,

 

I studied Travis's tabernacle carefully and you were right. The centerpiece was quarter sawn, it would have been a lot better if it was plain sawn. As wood varies in density and strength even in the same species, it is worth carefully selecting good strong piece of wood for this critical part.

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If left to my own devices, I had planned to use vertical grain wood for all three pieces of each tabernacle. My reasoning being, whenever I see splits in wood........either lumber or splitting firewood, wood seems to want to naturally split in an orientation that is 90 degrees to the growth rings. It seems to me that if vertical grain is used for all three pieces, the center piece would be more likely to resist side to side splitting and fore and aft bending.......and each side piece would resist side to side bending and fore and aft splitting.

 

In the photos, mine would look more like the one on the right than the one on the left. The one on the left already has a check (split) running up and down it's length. Option C is to rip these boards down into staves or thin strips and laminate them together.

 

Regardless of which is used, if splitting at the top of the tabernacle center piece is an issue, would it make sense to also insert a wooden dowel rod, or even a piece of small diameter (say 3/16") threaded metal rod entombed in epoxy, at the top end of each center piece in the side to side axis to share the load? That would be similar to what they do with scaffolding lumber to pin the ends to prevent splitting. A dowel rod or threaded rod could also extend beyond the center piece and tie into the sides. Once it was assembled, it would be hidden from view. With either a dowel or metal rod, the whole tabernacle would have to blow apart before it would fail.

 

 

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Yes, pinning the tabernacle box together at the top makes total sense to me. As for how, you might consider running some 1/8" copper rod through the back and sides, then peening the ends over roves or small washers. Elegantly strong and not bad looking, either.

 

cheers,

Lynn

This and similar practices with no peening were common place in the days before epoxy and plywood.  Some times called drift pins.  McMaster-Carr is a good source of copper rod and Jamestown Dist. carries copper nails, roves and burrs.

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Run some through bolts through the back plate on the tabernacle. This is my usual method on wooden assemblies. Bond them in place with thickened goo. Theoretically, you could probably just bond some all-thread into the back plate, maybe covering the ends with a bung to finish it off neatly.

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The key to this is in keeping the hinge-pin bolt tight in order to balance the mast load on each cheek piece of the tabernacle...mine was too loose causing the leeward side of the tabernacle to have to bear too much load, thus the split. Happened to BOTH sides.

An oversized washer (Maybe fab your own) on the outside of each end of the hinge bolt would help in spreading the load across the top of the tabernacle.

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I agree with Scott. I would rather have my tabernacle pin shear load perpendicular to the growth rings. However, with a large epoxy bushing and close tolerances I don't really see an issue either way especially if the tabernacle is fiberglassed all the way around. The tabernacle will bend a little though. 

 

For the center piece plain sawn only makes sense. The darker summer grown in the rings is always more dense and stronger than the spring growth. Quarter sawing that piece doesn't take advantage of the denser material in the grain the way that plain sawing would. 

 

The centerpiece could probably be made of 3/4 plywood for the CS20, not for the longer tabernacles. 

 

A drift pin or long through bolt would be a good backup for the centerpiece but why not just throw some fiberglass across it. Spread the load out a lot more, keep everything sealed up and never worry about it. 

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