True and plumb tabernacles

[Howard]

Not entirely sure I understand the source of "shear" load, so I drew up some sketches of this to better visualize what is being suggested.

 

As I understood Graham and Travis's comments on what happened, if the pivot pin is not tightened to take out all the side to side slop, when the mast bends under load, only one side of the tabernacle takes the load, thus transferring most of the leverage force to the leeward side of the tabernacle. Essentially, the top of the tabernacle then becomes the fulcrum, and the end of the long lever that is the mast then ends at deck level. Under the load of the long lever, the entire face of the centerpiece is in tension and if enough load is applied........easy to do with the vastly increased leverage.....the fulcrum is what gives way and bends, taking a segment of the centerpiece with it when the centerpiece splits.

 

(Howard's Edit: Side to side slop does not appear to be the primary issue. Loose bolt is. A tight bolt shares the push on the leeward upright with pull on the windward one. Loose bolt does not. Side to side slop is still an issue as the push and pull levers against the fixed centerpiece to the side with a racking force or turning moment........close fit is still needed).

 

If the pivot pin is tight, both side uprights share the load equally. The force in tension transfers from the centerpiece to the outside edge of the windward upright and the whole apparatus starts acting more like an I-beam. The top of the tabernacle then remains an extension of the mast, and the fulcrum moves down to deck level, with the end of the long lever going all the way to the keel.

 

Not sure where "shear" load factors into this. I can see how a flat or plain sawn centerpiece would resist side to side bending better than vertical grain or quartersawn, at least in the middle, but I would not think that to be the case on the ends.

 

The affect from several wraps of glass around the joint essentially works to immobilize the joint, tying both sides together much as the pivot pin does. The strength of the glass handles all the forces of tension and compression to hold it together?

 

Lastly, there may be an additional force I have not considered and that would be in a vertical plane acting on the sides.. Perhaps when the mast bends, there is a compression force pushing the pivot pin down on the leeward side and an equal force in compression pushing up on the windward side as the mast attempts to rotate. The epoxy bushings take that load and spread it over a wide area of the upright.

 

But back to those "shear" forces, I'm still not certain where the loads are coming from that a flat sawn or plain sawn board is going to help overcome?  Not doubting, just trying to understand.