[Public-List] Structural strength of knees
Gordon Laco via Public-List
public-list at lists.alberg30.org
Sun Feb 7 08:29:31 PST 2016
Right on Michael - thanks from me too, Anders.
I learn a lot here and am grateful for it. It's occurred to me in the past
that if there's any loss of integrity in the structure our rather short
chain plates are bolted to, there is a great risk of a single bolt taking
all the load if the others work their way through the material. I read
once that in machine threads only the top few turns take the load, and that
made me think about how chainplate bolts share the load down through the
ranks... I guess as near perfect a fit for each bolt in it's drilled hold,
and strong hard material to be bolted to are critical.
I look at SURPRISE's chain plates and their structure and whistle to myself
sometimes. I think I've written before that my wooden Folkboat's chain
plates are more than two feet long and are bolted through special inter-ribs
that are longer, thereby transferring the rigging loads to a large area of
the boat's skin and skeletal structure. I think one could pick up that
three ton boat by it's chainplates, and reckon that's the way it should
be...
SURPRISE's shroud chainplates are just like in the apres 400 series boat
pictures being shared in this thread. I recaulk their deck penetrations
regularly to defend against water damage below... And watch the caulk
material fearfully for any sign that the plates have moved up... They never
have even after the most extreme episodes in heavy air. I think I'm going
to reinforce them anyway.
Thank you.
Gord #426 Surprise
On 2016-02-07, 11:17 AM, "Michael Connolly via Public-List"
<public-list at lists.alberg30.org> wrote:
> Anders,
Nice analysis. We are all learning that this major structural
> element on our boats is a combination of several components. These components,
> if sound, work in harmony with each other to keep our rigs up. A weak link
> can be caused by deterioration of any single component; and, as you have
> currently pointed out, ...........................strengthening any one of
> these components can cause a shift in what will become the new weak link.
> Interesting, thanks for your thoughts.
I endeavor to be all business when
> it comes to maintenance and seaworthiness of our boats. When it comes to the
> actual sailing part I tend to be, for better or worse a "seat-of-the-pants"
> sailor. What I find delightful about fellow owners on this list is the
> divergent perspectives of how to solve problems combined with the common Love
> for the boat.
Michael #133
----- Original Message -----
From: "Anders Bro
> via Public-List" <public-list at lists.alberg30.org>
To:
> public-list at lists.alberg30.org
Sent: Sunday, February 7, 2016 9:18:36 AM
>
Subject: [Public-List] Structural strength of knees
I have had a lot of
> edukatin as I have read all the contributions about
the knee
> structure/bolts/tabbing. And although I am absolutely no
authority in these
> matters, there are a few points I would like to
present for contemplation?
> First of all, my background is as an engineer
(rock engineering and rock
> testing) and we in the rock engineering field
are very concerned with shear
> strength, as it is what keeps those big
blocks of rock in place so as to
> prevent land slides, dam abutment
failures and the like... but I digress.
>
My first concern is that the term modulus has been brought up, and the
>
various values of the materials involved (steel, epoxy, wood). The
modulus
> is generally associated with the elastic properties of
materials, and as such
> typically involves very small (elastic ie
recoverable) strains. I expect that
> we are really dealing not with
elastic properties but rather yield. As such,
> the strength of the
materials is was governs the failure of the structure,
> not the
small-strain elasticity. So it might be more appropriate to evaluate
> the
relative strength of the materials and not the moduli.
Then there is
> one point that might be relevant - the plastic (yield
deformations) that
> result in the sharing of the loads. If say the wood
is replaced by a very
> strong material, then the shear strength on an
individual bolt may be
> exceeded before the stress on the next bolt
begins to build. So it might be
> better to have a weaker (although not
weak) foundation rather than a super
> strong one?
This concept of sharing the shear load among the bolts may also
> be
misplaced. In the field of rock engineering, we use rock bolts to
>
stabilize large rock masses. However these bolts are never used in shear
but
> in tension. The idea is that the tension applies a normal component
of stress
> to the interface and thereby strengthens the shear interface
(by means of the
> coefficient of friction of the interface). So in our
case of the chain-plate
> interacting with the knees, if the normal force
applied by the bolts becomes
> small (due to rot and compressibility of
the underlying wood) then there is
> very little shear resistance that
develops and the bolts (rather than the
> interface) are placed in shear -
their weakest mode of failure. Again
> referring back to the rock
engineering field, we actually do not orient the
> bolts normal to the
shear plane, but rather at an angle so that if a small
> shear
displacement does develop, then the bolt tensile load increases,
> thereby
increasing the normal load and thus the shear strength of the
> interface.
If the bolts are oriented perpendicular to the place, no normal
> load
increase can develop due to a shear displacement. In light of this
>
experience, one possibility would be to angle the chain-plate bolts
>
downward?
One idea has been proposed to really beef up this junction. That
> is
certainly one approach, but I expect that the weak link in most of our
>
boats is (obviously) rot, and the weak bond between the plywood and the
>
tabbing. (I had a Southern Cross 28 in which the wood was totally rotted
>
underneath the tabbing, but not in the exposed portion of the bulkhead.
I
> excavated the rotten plywood back to good, and then laid in numerous
layers
> of fiberglass and then extended the tabbing about about 6".) I
expect that if
> the bond and the plywood is in good shape, there the
structure is probably
> adequate (?). I personally like the idea of going
with the 5/16" bolts, but
> expect the problem is low stress on the 1/4"
bolts that leads to the bolt
> shear. The problem I can see is that as the
wood shrinks and expands, the
> bolt stress will become low and so one may
need to rely on the shear strength
> of the steel rather than the shear
strength of the knee/chainplate interface.
>
My 2 cents...
Anders
On 02/07/2016 08:14 AM, Don Campbell via
> Public-List wrote:
> Stephen:
> Don't rely on your impression when it
> comes to engineering
> strength. You might want to read some of the Gougeon's
> book on boat
> construction for an explanation in more lay terms than
> engineering
> texts, but the strength is in the truss system which relies on
> the
> core being structurally sound and fully bonded to the laminates on the
>
> three sides for the knees. Polyester lay-ups are not strong on their
>
> own, and epoxy lay-ups are not really much better on their own.
> Don
>
>
> -----Original Message----- From: Stephen Gwyn via Public-List
> Sent: Sunday,
> February 07, 2016 12:32 AM
> To: public-list at lists.alberg30.org
> Subject:
> Re: [Public-List] Public-List Digest, Vol 3193, Issue 2
>
> Hi,
>
> I just
> pulled, cleaned and rebedded half my shroud chainplates (the three
> on the
> starboard side). It took about 5 hours. Everything was very
> wet. But
> all
> the metal was all rust-free and the wood was unstained and quite hard
> when
> I poked it with a knife. The chain plate for the upper shroud was
> quite
> damp, but as far as I can make out, it was damp with salt water
> from
> the
> last time I had the rail underwater. Lots of shiny crystals. Probably
> too
> salty for rot. I used 3M4000 (polyether). Much better handling and
> much
> faster cure time than Boatlife polysulfide. We'll see how long it
> lasts. I
> gave the port chainplates a hard look and they appear much the
> same.
>
>
> My impression of the fibreglass/wood knees for the lower shrouds is that >
> the wood is just there as a spacer. The strength is all in the
> fibreglass.
>
>
> SG
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