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

[ROGER PIHLAJA]

Chris,

During a “normal” mast stepping/unstopping, there should be no upward pulling force on the screws, only shear.  However, the shear forces are applied at one end of the screws.  This would apply a torque on the screws that would try to rip the screws out of the mast step.  Assuming some rot, this torque might cause the screws to fail by rotating thru the mast step, not shearing the head of the metal screw off.  Another disaster scenario you could calculate is to assume the mast is allowed to fall off the boat’s centerline by 10, 15, or 20 degrees as it is being raised. This off-center geometry would introduce a torque that would try to pull out the screws.  This is actually the sort of disaster scenario that the screws should be realistically able to withstand.  You might have a brief angular mast excursion, recognize it, and quickly correct it.  It would be nice if the system wasn’t so finicky as to fail in a scenario like that.  If you experience a stay or chainplate failure, well just hope no one gets hurt!

Roger Pihlaja
S/V Dynamic Equilibrium

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From: Chris on LBI<mailto:cknell at vt.edu>
Sent: Monday, August 3, 2020 12:39 PM
To: rhodes22-list at rhodes22.org<mailto:rhodes22-list at rhodes22.org>
Subject: Re: [Rhodes22-list] Reconnecting the Tabernacle Screws Following Sacrificial Failure

Roger,

I think you are saying that I can conceivably calculate the shear force that
the screws need to withstand. All of my analysis and comments up to this
point have been about pullout force, not shear. I have been assuming that
the shear forces acting on the three screws are trivial relative to the
strength of the screw, the tabernacle base and the mounting block. Perhaps
this is a mistake and this force is not trivial.

Now in thinking about it for the first time, if there is a non-structural
gap between the bottom of the tabernacle base and the epoxy plug (maybe
1/4") that is filled with gelcoat or some other sealant, this gap would
effectively transfer the lateral shear force into a rotational force on the
screw, a component of which would be pullout force and could be calculated.

Thanks,
Chris




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