[Public-List] Chain plate spacers - role of friction

[Don Campbell]

    While a number of people  have discussed friction and the need for 
things to be rigid for friction to work  as designed on paper, no-one 
has mentioned that there will be some compression due to the core of the 
truss being wood. IF that center part of the truss is wet, and I have 
the feeling that many are or have been, then the structure of the truss 
is not as contemplated in the paper design!
    I replaced the forward lower knees this last year on #528 and found 
plywood core, very dark from water and total delamination on one side 
between the polyester resin  and the plywood. This resulted in a total 
failure of the design of the truss and the only thing holding was the 
glass tape, which was in extra tension because of excess torque from the 
hull wall to the holes. Wet wood or wood that has been wet maintains 
very few properties of the original material. The worst part is that you 
don't know how bad the system is and assume it to be as it was when 
built. This is not necessarily so.
    The net result is that as long as the knee is stronger than the wire 
or the joinery from wire to the fittings on the ends of the wire, 
something else will break first. That is a good thing because one does 
not want the hull to crack if it is the knee that does go first. If 
backing plates make things so strong that the knee cannot give even the 
3/4" through the bulkhead, then be ready to damage the hull if that part 
ever tries to let go (as in a collision where rigging is snagged with a 
shock load from external mass) There are times when failure is a good 
thing, and planned failure even better. .
    When it comes to rigging tension, I think some of you who use high 
tensions on your rigs will notice that you can deform the curve of the  
hull at the chainplates with the loads possible on the wire.  To me, 
this is excessive,  as it just forces the mast down through the deck 
beam. There is always excessive stress on knees and chainplate bolts in 
this rig tension too (and little wonder the original bolts deform).
Don #528

Jonathan Adams wrote:
> Thanks for the interesting discssion.
>
> I have an old style boat. I just re-did my upper shroud chainplates, and put two SS plates on either side of the bulkhead and sandwiched the bulkhead with plenty of epoxy, and then upped the chainplate itself to 1/4 inch with 5/16 bolts. The plates on either side are as big as I could get in the space going from Deck to top of shelf on both sides. On the after side, the backing plates handles all three bolts. On the forward side the plate goes from deck to top of shelf, and handles two bolts - the bottom bolt is on the other side of the shelf and has a big washer.
>
> Anything has to be better than it was. My experience was that the wood was old and although not rotted, it had been jerked around so much over the years - and my be especially last year :). There were fairly elongated holes.
>
> Interestingly although my intuition told me the chainplates were moving, you could not see it based on the caulking giving way and being visible on deck. It was not until I removed the bolts, that it was clear they were moving. The top bolt (1/4 inch threaded all the way) was worn and bent after one season. I had replaced the next one down at the beginning of last year with a real 5/16 bolt.
>
> I will post pictures when I get organized.
>
> Jonathan
> 197
>
>
>
>
> ________________________________
> From: David Tessier <dfjtessier at hotmail.com>
> To: Alberg 30 List Start thread <public-list at lists.alberg30.org>
> Sent: Monday, March 23, 2009 9:53:49 PM
> Subject: [Public-List] Chain plate spacers - role of friction
>
>
> Thanks all for the advice.
>
>
>
> I think that friction likely has a role to play.  This is considered 
>
> explicitly in descriptions of the pre-load which one applies when 
>
> bolting two metal plates together (and the resulting shear strength of
>
> the resulting assembly).
>
>
>
> Fibreglassed knees, and especially 40 year old ones, will
>
> be more compressible than the steel chainplates. I think this 
>
> indicates the need for backing plates, and SUBSTANTIAL ones,
>
> rather than washers.  How SUBSTANTIAL? I suspect that aside
>
> from having an area equal to that of the chainplates, the backing plate's
>
> thickness should be chosen to be sufficiently strong so as to remain 
>
> essentially planar as the bolts are tightened, thereby transferring 
>
> the above bolt pre-load force over a wide area of the knee.  Less pressure
>
> (i.e., force per unit area) but a more sure friction set owing to the
>
> relative deformable nature of the fibreglassed knee. 
>
>
>
> In terms of dimensions rather than forces,
>
> for example, if a standard flat washer were to deform the knee
>
> by 1.0 mm upon tightening the bolt/nut, then perhaps the alternate 
>
> backing plate should have sufficient strength to deflect outward less 
>
> than 5% or 1% of this value at the midpoint between the bolts.
>
>
> I plan to use very substantial backing plates, at least
>
> as wide and long as the chainplates, and plenty thick.
>
>
>
> The spacers that are present on my starboard cap and aft-lower shrouds 
>
> are not washers, but pads of sorts of the same area as the chainplates they
>
> support -- better than washers but not quite as solid as they could be if
>
> mated directly to the knee or bulkhead.
>
>
>
> I wonder whether I could post a photo of the spacer arrangement for discussion?
>
>
>
> Per the suggestion, I will certainly measure to see whether the positions of the 
>
> corresponding opposite chainplates are equidistant from the stem
>
> (before I remove the chainplates).  If they are equdistant, then somehow, 
>
> the starboard side of the main bulkhead and the aft starboard knee would 
>
> seem to be hav ebeen installed about 3/4" too far forward...
>
>
>
>
>
> Regards,
>
> David
>
>
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