Tabernacle

[benz]

I know that keel-stepping a mast is more reliable than deck-stepping--it kind of makes sense, but has anyone worked out how much weaker a tabernacle-stepped mast would be? One of those tabernacles that go up two or three feet and are firmly bolted through the cabin to a compression post. It seems that with a beefed-up hinge point on the mast and a properly-sized locking pin below, this might be almost as reliable as a keel-stepper?
This is not something in my immediate future--just an academic puzzle for me on a rainy day.

Incidentally--not a wooden tabernacle, but welded steel or aluminum--kind of like those flagpole tabernacles you see now and again.

[Mark Johnson]

The boat that I built previous to the last / current one, was a Seaclipper 28 trimaran. It had a massive tabernacle, made of a heavy aluminum casting. It was part of a mast kit, and I think that this tabernacle base was "one size fits all", so was ridiculous overkill for such a small trimaran. It had a 1" SS hinge pin, but with a loose enough fit, that the weight went on the mast bottom. The tabernacle hinge pin went through doughnut doublers on the mast that increased the mast wall thickness to about 5/8" at the pin. The base was bolted down to the deck, (with its doubler square underneath), by 6 @ 3/8" bolts. Under this doubler, was a 3' long 4X4 compression post, that delivered the stresses to the top of the CB trunk, which really spread out the load.

The fractional cutter rig was a dirt simple double triangle, with no spreaders, or removable feature to the lower staysail stay. John Marples has used this concept many times, and if the extrusion is large for its length, and the wires amply sized, as was mine, it is as strong as a tank! One just designs the rig to take the tabernacle's weak link into account.

I was a younger (= wilder) man then, and had the boat hovering at almost 20 knots on several occasions. (Full main & genoa in 30 knots) Spectators told me once, that they could just see the centerboard exit the bottom of the main hull! I pressed this boat hard, including the worst pounding I ever had at sea, and still consider the rig the strongest, proportionate to the boat, that I ever saw.

I only had occasion to once, but I could stand the mast with lines, tackles, a "push stick" and a helper. If it was needed often, I would've refined my system considerably.

To dismiss the tabernacle out of hand as weaker, is simply an improperly designed and built tabernacle rig... IMO.

Mark

[Ian McColgin]

Any column held rigidly at two points and supported up high by stays, as a keel stepped mast is also held at the partners, is more able to stay in colum than one held rigidly at one point and then with stays. You can look in any good NA text to see the difference in scantlings for a deck stepped versus a keel stepped mast.

A tabernacle is just a pivot for raising and lowering the mast. It is not a structure that even approxomates the strength of a reinforced by the partners hole in the deck. A mast on a tabernacle should be of the same heft as any other deck stepped mast, as that's what it really is.

Especially in small boats and with the reality of standard extrusion sizes it's not unusual for the keel stepped mast you actually have to be a bit over-strength and perfectly workable if you cut off a few feet and deck step. That simply reflects what people build, not engineering possibility.

G'luck

[Brion Toss]

Hello,
The question was, I believe, whether a keel-stepped mast is "stronger" than a deck-stepped one. The answer is no. However, a keel-stepped mast is definitely stiffer than a deck-stepped one, and that, as Ian points out, is information that any engineering manual can confirm. So if you want to save weight, and there's room below, basically keel-stepped is the way to go. If you want to raise and lower the mast, or play with extremes of rake under way, or want to build a boat more cheaply, or any number of other considerations, then go deck-stepped, and up the scantlings accordingly.
Fair leads,
Brion Toss

[benz]

Thanks all, for the input. I don't have deck-stepped now, but if I ever get tired of cruising and build another boat, it will be something a lot smaller, definitely with a tabernacle for the sake of space.
I was thinking mostly of strength (not breaking), and stiffness had not ocurred to me, so I'm grateful for that extra consideration.
Back to my musing........
Ben