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

[sousa, stephen (ENG)]

Roger,

The breaking strength of 1 X 19 stainless wire is as listed below: I
upgraded the upper shrouds and forestay/backstay to 9/32" which will accept
a 1/2" pin in the turnbuckle. I can't remember what I did with the lowers, I
can measure these next time on the boat. The other upgrade if you are doing
chain plates, we had the shop use 316 stainless 1/4 inch plate which was
passivated along with the new port frames. These were all laser cut so I do
not have any seams from welding. Hope this info helps.

3/16"	4000 pounds
7/32" 5350	
1/4 " 6900
9/32" 9400

Stephen

  


-----Original Message-----
From: public-list-bounces at alberg30.org
[mailto:public-list-bounces at alberg30.org] On Behalf Of Roger L. Kingsland
Sent: Thursday, August 19, 2004 4:13 PM
To: Alberg30
Subject: [Public-list] rambing on my knees (and chain plates)


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.
 
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