[Public-List] Knee replacement
Roger L. Kingsland
r.kingsland at ksba.com
Tue Mar 31 14:50:10 PDT 2009
I did a spreadsheet calculating the required main stay chain plate bolt
strength (I can sent it to you) assuming the chain plates should be 1.3
times stronger than the stays (got that recommendation from the NA who
surveyed my boat). If my calcs are correct, which I do not certify or
guarantee in any way (my architect's professional liability insurance
carrier would drop me if I didn't say that), it looks like 3 - 5/16' bolts
would work for 1/4" main stay wire and 4 - 5/16" bolts would work for 9/32"
wire (that I understand some like to upgrade to).
This assumes the bolts would be tightened to 3,000 pounds tension each which
is 70% of their ultimate load. With 1-1/4" wide chain plates and about 3"
between holes, the compression (not tension) strength of the knee/bulkhead
material would need to be about 800 psi (call it 1,000 psi). This assumes
you use backer plates the same size or larger than the chain plates. If you
use 2" washers under each nut (several layers so they don't bend) you would
need compressive strength of about 1080 psi (call It 1200 psi). I imagine
the compressive strength of Starboard is at least as much as the tensile
strength (but check) so you should be OK (but don't take my word for it). I
assume you have confirmed the fiberglass tabbing to the hull will "stick" to
this Starboard stuff.
I am of the opinion that, since the chain plates function as a "connection"
where rigidity is necessary to prevent failure, they should have no flex
(the flex should occur in hull deck, wire, etc. to which they connect) so,
as long as you don't break something, they can't be too strong. I don't
know the size of the baby stay wire but could run the same calcs if I did.
FYI, Don Campbell and I, as we are want to do, have been having some
spirited off page technical discussions of late on this very subject and,
remarkably, we don't entirely agree. Therefore, Don, please chime in if you
think I am off the mark.
From: public-list-bounces at lists.alberg30.org
[mailto:public-list-bounces at lists.alberg30.org] On Behalf Of Hugh McCormack
Sent: Tuesday, March 31, 2009 2:41 PM
To: public-list at lists.alberg30.org
Subject: [Public-List] Knee replacement
Good day all,
The recent thread concerning chain plates and weak knees due to water
infiltration (water in the knees...no wonder we think of boats as living
things) and the parallel discussion about the strength of plywood or
hardwood prompts my questions.
Has anyone replaced their knees (and/or bulkheads) with new ones made of
King Starboard? If so, what is the relative strength of Starboard compared
to plywood or hardwood?
I have searched the list archives but can find no discussion of the use of
King Starboard except for replacement of the spacers under the jib or genoa
tracks. The King website says that the product's tensile strength is 4100
psi but they don't say for what thickness of material. If this applies to
3/4" material then it appears to be considerably stronger than plywood. It
also suggests that thinner material could be used in a repair/replacement.
Does anyone have any experiences or thoughts?
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