[Public-List] chain plate spacers

dickdurk at atlanticbb.net dickdurk at atlanticbb.net
Mon Mar 23 17:19:13 PDT 2009

Makes sense to me, Roger, except there is a history of the 
original (1/4"?) bolts distorting-something known to 
surveyors familiar with Albergs. Shock loading perhaps? I 
don't know how you engineer for that.


On Mon, 23 Mar 2009 17:31:44 -0500
  "Roger L. Kingsland" <r.kingsland at ksba.com> wrote:
> Gordon et. al. Albergers,
> Since properly connecting the rig to the boat is A), so 
>important, and B),
> something I need to do soon, I am going to jump on my 
>friction vs. shear
> soapbox again hoping others might confirm the logic.  I 
>have attached a copy
> of an epistle I wrote back in 2004 (please note 
>disclaimer) below.  
> The basic argument is, by putting enough tension 
>(tightening) on the bolts,
> the chain plate is forced against the knee/bulkhead with 
>enough force that
> it will not slide in the vertical plane (direction of 
>load).  This
> "friction" connection is quite strong and is the design 
>used on most bolted
> connections for steel building structures.    
> The problem with shear a connection is it relies only on 
>the strength of the
> weakest connection material over less than half of the 
>surface area of the
> bolts.  This makes the shear strength of the bolts 
>irrelevant because the
> shear strength of the wood thru which the bolts go is 
>less; the bolts would
> elongate the holes in the wood long before the bolt 
>would break from shear.
> Also, with a shear connection, only bolts in exact 
>alignment with the holes
> in the chain plate would achieve their full shear 
> With a friction connection every bolt applies friction 
>so their strength is
> cumulative.  I checked the tensile strength of a 1/4" 
>304 SS bolt and it is
> about 3,700 pounds.  Assuming a 1.3 safety factor (see 
>below) and a 6,900
> pound breaking strength of a 1/4" SS wire mainstay, 
>sufficient friction
> strength could be achieved with 3 - 1/4" bolts if each 
>were tightened to 80%
> of maximum load.  So, it appears 5/16" bolts are only 
>necessary if one wants
> to move up to 9/32" mainstay wire.  In fact, larger 
>holes would reduce the
> area of the chain plate and possibly degrade the ability 
>of the plate to
> distribute the needed compression load across sufficient 
>area (the plate
> could bend at the narrowest part beside each hole).  
> Another issue is the compressive strength of the 
>bulkhead (marine plywood)
> and knees (hardwood).  This could be field tested by 
>torquing down a bolt
> (does anyone know how to convert tensile strength to 
>bolt torque) with a
> couple of layers of wide washers on each side and 
>observe if the wood
> crushes.  To me, this is perhaps the most important test 
>since several
> owners have mentioned water leaks and water damaged 
>bulkheads/knees would be
> the weak link in any connection detail.
> Does this stuff make any sense or is my version of 
>reality drifting yet
> further away from the norm?         
> Best - Roger 148
> Aug, 2004
> 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 
> 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 
> 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 with boats in the water),
> Roger
> ________________________________
> Architects/Planners/InteriorDesigners/ProjectManagers
> 3441 Butler Street
> Pittsburgh, PA 15201
> N 40° 27.8344'  W79° 57.9831'
> 412-252-1500 ext.101 
> 412-779-5101 cell 
> 412-252-1510 fax
> r.kingsland at ksba.com
> www.ksba.com
> -----Original Message-----
>From: public-list-bounces at lists.alberg30.org
> [mailto:public-list-bounces at lists.alberg30.org] On 
>Behalf Of gordon white
> Sent: Monday, March 23, 2009 3:04 PM
> To: public-list at lists.alberg30.org
> Subject: Re: [Public-List] chain plate spacers
> Washer-type spacers are a bad thing, if I understand the 
>description as to
> how they are used. They apply a bending moment to the 
>bolt rather than
> straight shear. Reminds me of a vintage racer whose rear 
>axle was a couple
> of inches wider than  his suspension setup. He used 
>spacers much as you
> describe to bolt the axle into the car. He was running 
>on a race track just
> ahead of me when one of the bolts broke, letting the 
>car's chassis dig into
> the track. The car flipped over and killed him.
> Not a good thing.
> But if you put a shim the full width of the chainplate 
>you can mostly
> correct the situation, even though I would not recommend 
>it if it could be
> avoided. Certainly up-size the bolts.
> - Gordon White
> Brigadoon II
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