[Public-List] Testing knee/bulkhead compression

Roger L. Kingsland r.kingsland at ksba.com
Wed Mar 25 10:42:30 PDT 2009


Don,

I certainly agree with you and guess you missed the last paragraph of my
3/23-17.32 post; 

"Another issue is the compressive strength of the bulkhead (marine plywood)
and knees (hardwood).  This could be field tested by torquing(sp?) 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."

I figured I would use a torque wrench for "the test" but don't know how to
convert nut torque to bolt tension.  Do you, or any other Albergers, happen
to know?

All the best,

Roger 148



-----Original Message-----
From: public-list-bounces at lists.alberg30.org
[mailto:public-list-bounces at lists.alberg30.org] On Behalf Of Don Campbell
Sent: Tuesday, March 24, 2009 10:03 AM
To: Alberg 30 Public List -- open to all
Subject: Re: [Public-List] Chain plate spacers - role of friction


    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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