[Public-list] rambing on my knees (and chain plates)

[Roger L. Kingsland]

Albergers,

RE the thread on reinforcing the knees, I checked with the NA who surveyed my boat and learned FG tape has a strength in shear of 8,000 pounds per square inch.  The breaking strength of the 1/4" lowers is 4,700 pounds.  To insure the stays break first (comforting thought), the chain plates should hold 1.3 times the stay breaking load and the knees 1.5 times, or 7,000 pounds.  The load on the knees is transferred to the inside surface of the hull (which, like the main bulkhead, is a good, strong diaphragm) via the vertical FG tape on each side of the knee.  Assuming the knee is 8" high (I haven't measured) or a total length of 16", the thickness of the tape should be a minimum of 1/16" (7,000 pounds / 16 inches = 440 pounds/inch; 8,000 pounds / 440 = 1/18" tape thickness).

I plan to drill a little hole in the tape to check thickness but suspect it is greater than 1/16."  So, the knees are probably adequate but, for the belt-and-suspender folks, easily reinforced by simply adding new layers of tape to the existing.   

RE the chain plate size, I started looking into the shear strength of stainless steel and learned what a megapasquale is, and also learned its strength goes down drastically over 400 degrees so, let's hope global warming doesn't catch up with us too quickly. Then I realized an easy way to insure adequate chain plate strength is confirm that the minimum cross sectional area above or beside the turnbuckle fastener pin is at least 1.3+ times the cross sectional area of the SS stay the chain plate supports.  Since the area of the main stays is 0.20 sq. in. and the lowers is 0.11 sq. in., chances are the chain plates are more than adequate.

Our structural engineers tell me the majority of structural failures occur at the mechanical connections so, I suspect the weak link is the chain plate connection at the knees or bulkhead. Unless a friction connection is employed, the entire load will be placed on the upper part of the bolt holes in the knees and main bulkhead (shear connection).  A friction connection involves compressing the material between the chain plate and backer plate (or washers) sufficiently so the friction between the different surfaces prevents them from "slipping" (like Chinese handcuffs).

The Gougeon Brothers (West System) suggest friction connections under load (just about everything on a sailboat) be "bonded" by adding a layer of high compression, adhesive epoxy (West makes a slick powder additive) between the surfaces to be connected.  This insures friction across the entire surface area, not just the "high points."  They also point out that the bond must be rigid.  If the bond is soft and flexible (5200?), the load will cause movement which will degrade the friction into a shear load on the bolts.  Once this occurs, the bolt shafts work against the holes resulting in substantial reduction of load capacity and leaks (like the toe rail at the genoa track?).  

The wood knees on #148 appear to have the grain oriented vertically so the upward load from the stays runs parallel to the grain.  No big deal with a friction connection but real a problem with a shear connection because wood is much weaker parallel to the grain than perpendicular to it. The need for a good friction connection argues for backer plates as large as the chain plate and, perhaps even increasing the size of the chain plate.

Assuming the above actually makes sense (time for disclaimer; the author is not a structural engineer and all said above could be total BS; so, rely on it under pearl of tumbling mast), my plan to insure stays are properly fastened is to check thickness and quality of the fiberglass connection between the knees / bulkhead and the hull.  Then, make backer plates about the same size as the chain plates and fasten them with epoxy and a bunch of tension on the bolts.  My main bulkhead is in good shape but I have heard some are rotted, particularly if water seal between chain plate and deck has not been maintained.

Hope this helps sort out the issues, it did for me; but, then, I find it so easy to agree with myself.

As always (jealous of those those with boats in the water),

Roger
 
Roger Kingsland
Chief Boat Boy
PERFECT intentions, A30 #148
on the hard at N40° 33.945' W79° 51.260'
 
Author's Disclaimer; This email was produced exclusively by the sender and, in the interest of expediency, without the benefit of editing by others.  The sender, thank goodness, is a much better architect/sailor than speller/editor and, frankly, constantly laments an obvious flaw in "spell check," it does not know what the author is thinking.  Please accept the sender's sincere apologies for any "typos" that may appear in this document.  If present, they are certainly unintended and hopefully do not cloud the message, or spawn any unnecessary lawsuits.