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

[Graham Stewart]

Your calculations are impressive but in the end you probably need to go with what you are most comfortable. The discussion has been interesting to me and I now think I would do it differently than I did last time when I drilled out the hole, filled with epoxy and then drilled new holes.

Option 1 to use slivers and epoxy and option 2 to drill larger holes are not mutually exclusive as I understand them and I think that combination is what I would go with. In that case I would want to maximize the proportion of wood slivers to epoxy. The epoxy would be only sufficient to seal the core and hold the slivers together. Not having the fiberglass overhanging the hole would likely reduce the damage to the skin should the plug pull out. The weakest link, I would think, would be the interface between the plug and the core material. Unless the core is damaged and the epoxy seeps into crevices I would think that the force required to pull the plug out would not be much greater than the force required to pull a screw out of solid wood under fiberglass. That is what I think but I could easily be entirely wrong. In this case you want a properly sealed but relatively weak bond. Using the slivers tightly packed strikes me as the best way to achieve this balance.

I don't know how you could abandon the old holes and drill new ones without moving the base. I would not do that. When I rebuilt my deck I removed the mast foot, repaired and then painted the step along with the entire deck. I did not make careful note of exactly where the plate was attached thinking that is was centered on the step. Nope, it has to placed exactly in the right place or you will not be able to attach the slider for the pop top to raise and descend properly. I ended up doing this install three times before I got it right. Each time I had to raise and then lower the mast. After spending 7 years restoring the boat you can imagine how frustrating that was to be stopped at the moment that I would be able to raise the sails and go.

The screws you have are, as I recall, much shorter than mine but if these are what Stan used I would stick with them.

Good luck,

Graham Stewart
gstewart8 at cogeco.ca


-----Original Message-----
From: Rhodes22-list [mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Chris on LBI
Sent: Sunday, August 2, 2020 7:40 PM
To: rhodes22-list at rhodes22.org
Subject: [Rhodes22-list] Reconnecting the Tabernacle Screws Following Sacrificial Failure

This topic is a continuation of a different topic titled “"Repair of Spider
Cracks at Mast Step Tabernacle”. The subject matter of that topic quickly
strayed from gelcoat repair to how to replace the tabernacle screws after
they pull loose. I thought it would be good to highlight this subject matter
with a new topic heading. I thank Graham Stewart, Rodger Pihlaja, Mike
Riter, and Rick Lange all of whom weighed in on the original thread.

Last week while lowering the mast, the three screws holding the tabernacle
to the block above the cabin let loose when the mast was at an angle of
about 45 degrees. A shroud had fouled and the resulting (unintended) forces
pulled the screws out. In reading on this list/forum, I was relieved to
learn that these screw connections were designed to fail to avoid more
expensive damage to the plate and the cabin top. It also seems that the
repair is very easy to accomplish. However, in asking advice regarding the
repair and in doing some research, I'm undecided as to how the repair should
be performed since I have conflicting information. I've started this new
thread to focus specifically on reconnecting these three screws to the block
below the tabernacle plate so that the sacrificial nature of the junction is
preserved.

The three screws on my boat are 3/16” stainless steel, 1” long, with 10
threads per inch. The mounting block below the tabernacle appears to be wood
covered with fiberglass and gelcoat. The combination fiberglass and gelcoat
appear to be about 1/4” thick. The stainless steel base of the tabernacle is
1/8” thick. The holes in the fiberglass are larger than the major diameter
of the screws.  Based on this information I estimate that about 5/8” of the
screws were threaded into the wood block. I read somewhere on this
list/forum that the block is typically pine. Using an online tool 

(https://www.engineeringtoolbox.com/wood-screws-allowable-withdrawal-load-d_1815.html
) that calculates pullout force, I estimate that each of the three screws
would pull out at about 70 pounds of upward force; 210 pounds total.

I had planned to make the repair by filling the holes with epoxy and then
drilling holes in the epoxy to receive the screws. Since there was some soft
wood around the perimeter of the holes, I scraped it out as best I could,
and noted that the new epoxy “plug” would be larger in diameter than the
holes in the fiberglass.  That is, some of the epoxy plug would be
underneath of the fiberglass. This led me to wonder at what force the plug
would release from the surrounding wood because if it did release, it would
transfer the load to the fiberglass above the plug. Apparently the way to
estimate this force is to take the vertical surface area of the wood/epoxy
plug and multiply it by the shear factor of a wood/epoxy joint (obviously an
estimate). A few sources online use 800 psi as this shear factor. So using a
5/8” deep hole with a diameter of 5/16”, I get a pullout force of about 500
pounds per plug; 1500 pounds total. 

I assume that the pullout force of a screw from an epoxy plug is greater
than 500 pounds but I can't find a way to estimate this. Although, if it
were a steel pin (without threads), the shear factor would be estimated at
1600 psi. So with the threads the shear factor would be much great than this
and thus the screw withdrawal force would be much greater than the plug
withdrawal force. That is, the plug will pull out well before the screw
pulls out of the plug. Unless of course it is a shorter screw. But without a
way to calculate it, I would not know what length screw to use to achieve
the desired pullout force.

While a few days ago I was all set to start mixing up a batch of epoxy, now
I'm not so sure. Using these calculations based on the assumptions
indicated, the planned approach would change the original design in two very
significant ways. First, the total pullout force would increase from 210
pounds to 1500 pounds. Second, with the epoxy repair, in the case of
failure, the load would be transferred to the underside of the fiberglass
which I assume the original design is intended to protect. That is, the
original design sacrifices the wood/screw connection, the screws pull out,
and the fiberglass is left unharmed.

So now I am left contemplating four different options: Wood Slivers, Bigger
Holes, New Holes, Shorter Screws

Wood Slivers
The idea would be to partially fill the holes with epoxy, and then pack the
holes with wood slivers that would intentionally weaken the epoxy with the
hope that the epoxy/screw junction fails before the wood/epoxy junction of
the plug. That is, the screws would pull out without damage to the
fiberglass. Obviously this pullout force can't really be calculated, but it
seems like it should be less than the wood/epoxy junction of the plug, but
there is some risk that the plug could pull out first and transfer the load
to the fiberglass.  One very important unknown to this approach is the
quality of the wood/epoxy junction. If the wood surrounding the epoxy is
weak, the plug may pull out of the hole before the screw pulls out of the
plug. This would transfer the load to the fiberglass. So there are risks to
this approach. Filling the hole with epoxy and encapsulating the slivers in
epoxy should prevent water infiltration.

Bigger Holes
This idea is to drill larger holes in the fiberglass, so that if the epoxy
plug were to pull out, it can without damaging the fiberglass. The holes
would have to be sealed to prevent water infiltration, perhaps with gelcoat.
The force to pull the plugs out is likely much greater than the original
pullout force (1500 lbs vs. 210 lbs), but maybe this is not a bad thing; if
a minor accident occurs (less than 1500 lbs), everything remains intact and
I still go sailing that day. It would seem that this approach would protect
the fiberglass as intended (although maybe a stainless steel rail gets bent
or broken).

New Holes
The original holes could be filled with epoxy and abandoned. Three new holes
would be drilled in the tabernacle base plate and the block, being sure to
make holes in the fiberglass larger than the major diameter of the screws.
This would closely replicate the original design. Care would have to be
taken to prevent water infiltration. It would be interesting to get
confirmation from Stan that the factory drills holds in the fiberglass that
are larger than the screw diameter.

Shorter Screws
The holes would be filled with epoxy, drilled, and new, shorter screws
threaded into the epoxy plug. The problem with this approach is that I don't
have a way to calculate the pullout force of the screw from the epoxy. 

Since I am a novice at repairs of this sort, I again look to the list/forum
for advice. I suspect that in a case like this, group-instinct trumps
calculations.



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Long Beach Island
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