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

[Roger L. Kingsland]

Stephen,

I have to admitt I used the first numbers I found (at
http://www.jamestowndistributors.com/eserv/eclipse.ecl?PROCID=WEBDISP.WOEB.MAIN&ID_1=13&ID_2=1389&ID_3=2281&ID_4=2282&CLEV=4&TRACKNO=J1234015002)
for strength of 1x19 wire which are substantially higher than yours.  But
then I went here http://www.apsltd.com/Tree/d3000/e621.asp and found numbers
that agree with yours for 3/16", 1x19.  BTW, what is "passivated"?

Roger
----- Original Message ----- 
From: "sousa, stephen (ENG)" <sousa_stephen at emc.com>
To: "'Alberg 30 Public List -- open to all'" <public-list at alberg30.org>
Sent: Thursday, August 19, 2004 4:28 PM
Subject: RE: [Public-list] rambing on my knees (and chain plates)


> 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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