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

[Michael D. Weisner]

Chris,I understand your issue of pulling out the entire epoxy plug and damaging the fiberglass over the epoxy that has flowed outside of the original hole where you removed damaged wood.Can you make a wood plug that is the diameter of the hole, made of the same wood used in the tabernacle structure, preserving the correct grain? If you can, why not set this wood plug into the epoxy and set the screw into the wood plug? You will have replicated the original structure and release properties for the screw. You shouldn't need to worry about the epoxy pulling out if the screw releases first.   Mike   s/v Wind Lass ('91)   Nissequogue, NY
-------- Original message --------From: Chris on LBI <cknell at vt.edu> Date: 8/2/20  7:39 PM  (GMT-05:00) 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 SpiderCracks at Mast Step Tabernacle”. The subject matter of that topic quicklystrayed from gelcoat repair to how to replace the tabernacle screws afterthey pull loose. I thought it would be good to highlight this subject matterwith a new topic heading. I thank Graham Stewart, Rodger Pihlaja, MikeRiter, and Rick Lange all of whom weighed in on the original thread.Last week while lowering the mast, the three screws holding the tabernacleto the block above the cabin let loose when the mast was at an angle ofabout 45 degrees. A shroud had fouled and the resulting (unintended) forcespulled the screws out. In reading on this list/forum, I was relieved tolearn that these screw connections were designed to fail to avoid moreexpensive damage to the plate and the cabin top. It also seems that therepair is very easy to accomplish. However, in asking advice regarding therepair and in doing some research, I'm undecided as to how the repair shouldbe performed since I have conflicting information. I've started this newthread to focus specifically on reconnecting these three screws to the blockbelow the tabernacle plate so that the sacrificial nature of the junction ispreserved.The three screws on my boat are 3/16” stainless steel, 1” long, with 10threads per inch. The mounting block below the tabernacle appears to be woodcovered with fiberglass and gelcoat. The combination fiberglass and gelcoatappear to be about 1/4” thick. The stainless steel base of the tabernacle is1/8” thick. The holes in the fiberglass are larger than the major diameterof the screws.  Based on this information I estimate that about 5/8” of thescrews were threaded into the wood block. I read somewhere on thislist/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 screwswould 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 thendrilling holes in the epoxy to receive the screws. Since there was some softwood 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 theholes in the fiberglass.  That is, some of the epoxy plug would beunderneath of the fiberglass. This led me to wonder at what force the plugwould release from the surrounding wood because if it did release, it wouldtransfer the load to the fiberglass above the plug. Apparently the way toestimate this force is to take the vertical surface area of the wood/epoxyplug and multiply it by the shear factor of a wood/epoxy joint (obviously anestimate). A few sources online use 800 psi as this shear factor. So using a5/8” deep hole with a diameter of 5/16”, I get a pullout force of about 500pounds per plug; 1500 pounds total. I assume that the pullout force of a screw from an epoxy plug is greaterthan 500 pounds but I can't find a way to estimate this. Although, if itwere a steel pin (without threads), the shear factor would be estimated at1600 psi. So with the threads the shear factor would be much great than thisand thus the screw withdrawal force would be much greater than the plugwithdrawal force. That is, the plug will pull out well before the screwpulls out of the plug. Unless of course it is a shorter screw. But without away to calculate it, I would not know what length screw to use to achievethe desired pullout force.While a few days ago I was all set to start mixing up a batch of epoxy, nowI'm not so sure. Using these calculations based on the assumptionsindicated, the planned approach would change the original design in two verysignificant ways. First, the total pullout force would increase from 210pounds to 1500 pounds. Second, with the epoxy repair, in the case offailure, the load would be transferred to the underside of the fiberglasswhich I assume the original design is intended to protect. That is, theoriginal 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, BiggerHoles, New Holes, Shorter ScrewsWood SliversThe idea would be to partially fill the holes with epoxy, and then pack theholes with wood slivers that would intentionally weaken the epoxy with thehope that the epoxy/screw junction fails before the wood/epoxy junction ofthe plug. That is, the screws would pull out without damage to thefiberglass. Obviously this pullout force can't really be calculated, but itseems like it should be less than the wood/epoxy junction of the plug, butthere is some risk that the plug could pull out first and transfer the loadto the fiberglass.  One very important unknown to this approach is thequality of the wood/epoxy junction. If the wood surrounding the epoxy isweak, the plug may pull out of the hole before the screw pulls out of theplug. This would transfer the load to the fiberglass. So there are risks tothis approach. Filling the hole with epoxy and encapsulating the slivers inepoxy should prevent water infiltration.Bigger HolesThis idea is to drill larger holes in the fiberglass, so that if the epoxyplug were to pull out, it can without damaging the fiberglass. The holeswould have to be sealed to prevent water infiltration, perhaps with gelcoat.The force to pull the plugs out is likely much greater than the originalpullout force (1500 lbs vs. 210 lbs), but maybe this is not a bad thing; ifa minor accident occurs (less than 1500 lbs), everything remains intact andI still go sailing that day. It would seem that this approach would protectthe fiberglass as intended (although maybe a stainless steel rail gets bentor broken).New HolesThe original holes could be filled with epoxy and abandoned. Three new holeswould be drilled in the tabernacle base plate and the block, being sure tomake holes in the fiberglass larger than the major diameter of the screws.This would closely replicate the original design. Care would have to betaken to prevent water infiltration. It would be interesting to getconfirmation from Stan that the factory drills holds in the fiberglass thatare larger than the screw diameter.Shorter ScrewsThe holes would be filled with epoxy, drilled, and new, shorter screwsthreaded into the epoxy plug. The problem with this approach is that I don'thave 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/forumfor advice. I suspect that in a case like this, group-instinct trumpscalculations.-----Long Beach Island--Sent from: http://rhodes-22.1065344.n5.nabble.com/