[Rhodes22-list] Reconnecting the Tabernacle Screws Following Sacrificial Failure

[ROGER PIHLAJA]

Chris,

This a Statics problem in that during a normal mast stepping/unstepping, the mast should never achieve a rotational velocity wherein momentum is a significant issue.  So, you just have the weight of the mast and all the force vector algebra associated with pulling on it from a changing angle.  Because of the nonlinearity of the trig functions associated with these changing angles, the shear force on the mast tabernacle changes in a nonlinear manner as the angle changes.  I would do the Statics vector math problem in increments of 10 deg from fully stepped to lying on the mast crutch.  You will be able pick the peak off this curve to get the maximum design shear force.

Roger Pihlaja
S/V Dynamic Equilibrium 

Sent from my iPhone

> On Aug 3, 2020, at 10:49 AM, Chris on LBI <cknell at vt.edu> wrote:
> 
> Roger,
> 
> Thank you for that thought regarding uneven loading of the screws. I suspect
> that you are correct that the screws would actually fail in some sort of
> sequence. It sounded like a single "pop" when they gave way, but it easily
> could have been three pops almost on top of each other.
> 
> Unfortunately I don't really have a target pullout force to base the design
> of the repair around. As I speculated in the original thread, under normal
> mast stepping/unstepping process, the screws should never have any upward
> force acting on them; it should all be shear force when the mast is
> horizontal. So I don't know whether 70, 210, 500, or 1500 pounds is "right".
> 
> All I really have is a guess at how the factory may have constructed it to
> begin with (how long were the factory screws? what diameter were the factory
> screws? was the fiberglass hole larger than the screw itself?) and my boat
> with 1" screws and oblong holes that are larger than the screws. I don't
> know if or how many times my boat has already been through previous repairs.
> 
> Also, I don't know what the designer's (Phil Rhodes?, Stan?) original
> motivation was for this sacrificial joint. Was it intended to fail safe to
> protect the cabin top (and what would be the maximum allowable force) or to
> protect something else that finds itself in the path of the lower half of a
> descending mast?
> 
> Thanks,
> Chris
> 
> 
> 
> 
> 
> 
> -----
> Long Beach Island
> --
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