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

[Jonathan Adams]

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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