Subject: Re: [baidarka] lowering the cocpits aft end
From: Bill (bill@billmercer.com)
Date: Thu Sep 18 2003 - 20:04:50 EDT
Hmm. While I agree that the frame can't act as a proper truss (with all
bending loads taken as either compression or tension by some member(s), if I
remember correctly) because the lashings have too much stretch, when the
frame bends it does bend most members. Thus the keel at least does
contribute to overall stiffness (and the keel is as
deep as the gunwales).
On second thought, I do agree that the rear deck stringer probably cannot
act as a compression member, though not for the reason you gave. The
deckstringer is ends by butting against the backrest deckbeam (if you
follow Wolfgang Brinck's building plan), so in compression it is rigidly
fastened at that end (unless the deckbeam bows, but the beam is fairly
thick). The problem I think is that the tailfin couldn't
transmit a compression load to the stringer, because the fin isn't rigidly
fastened to anything. The lashings would stretch long before it could
compress the deck stringer.
Failure of the gunwales at the cockpit has got to be correct, though I'd
think that extreme bending of the gunwales would also flex the cockpit
coaming, since the coaming is fastened to the gunwales by a couple
lashed-in blocks that ought to be able to carry compression loads.
Anybody want to break a baidarka frame and take lots and lots of pictures
of the process?
Bill
On Thu, 18 Sep 2003, Peter Chopelas wrote:
> Bill wrote:
>
> > I do thick you risk losing a bit of longitudinal stiffness
> > because the rear deck stringer won't be held high enough above the
> > gunwales to help resist bending of the boat.
>
> I think you are over estimating the effect of deck stringer by assuming the
> frame is acting like a truss. Most of my professional work consist of doing
> structural analysis of wood framed structures (buildings, bridges, trusses,
> etc.) along with concrete and steel (among much other engineering
> experience). I have spent more than a few hours contemplating the action of
> the members of the biadarka structure (especially while planning and
> building them), and the problem you are overlooking is that the deck
> stringer has no capacity to take compression loads for several reasons: It
> is not attached in such a way to handle large compression loads and transfer
> them to other members at the coaming (or any other frame members, it is
> simply laced in place), there is no way to transmit "truss" action loads to
> it without either a shear web or a some kind of diagonal brace between the
> it and any other member lower in the frame. Without any members to transfer
> the shear loads to the deck stringer, there is no "truss" action as you
> imply. It is in simple bending only, the gunwales are about 27 times
> stiffer than the deck stringer, it will not even be able to contribute even
> a tiny amount to the stiffness since the gunwale would fail long before
> there is any loads transferred to it. It is possible that the skin could
> transfer some shear loads, IF you have no stretch in the fabric (doubtful)
> and if you have a perfect structural bond between the skin and all of the
> framing members (also doubtful). So while there may be some small amount of
> truss action simply from the wimpy lashings and skin friction transferring
> some small forces, I would not count on it contributing for much. It is
> much more likely you will get more truss action between the keel and the
> gunwales, though the amount of load transfer is highly speculative.
>
> >.. an Aleut boat is almost certainly flexible enough to spread loads
> through all > longitudinal members--when the boat bows, the gunwales, keel,
> and even
> > deck stringers all bend somewhat
>
> The fact that all of the members bend in flexure is proof that there is no
> "truss action", if you get significant bending in a truss you will fail
> members in buckling or tension. And since the size and orientation of the
> gunwales makes them 10 to 27 times stiffer than any other longitudinal
> member, it is doubtful they contribute very much. Except for a small amount
> of contribution from the skin friction, all the members are in simple
> bending.
>
> > ... but the rear stringer butts against the tail fin (lashed to the tail
> spreader) and the backrest
> > deckbeam and is above gunwale height, so when the back of the boat flexes
> > the rear stringer has to flex or compress too.
>
> But the attachment of the deck stringer to the deck beam precludes much
> compression load being transferred, if there was, this attachment would
> fail. Beside, the maximum bending force occurs right in the middle of the
> cockpit, so there is no meaningful contribution of any of the deck stringers
> to stiffness or strength since the hull will fail in bending right at the
> cockpit.
>
> > If you lower the rear
> > stringer so that it's not above the gunwales (or just isn't there at all),
> > you might get too much bendiness in the tail of the boat.
>
> If you have flexure, you have little or no truss action. there is little or
> no contribution from the deck stringer. It is there simply to hold the skin
> up.
> Stiffness in just the tail of the kayak will not prevent significant overall
> flex, nor will it add to the strength since a frame failure would occur
> forward of the aft cockpit deck beam.
>
> It would be interesting to measure how much each member adds to stiffness at
> you build up a hull, but that many members attached in such an uncoordinated
> manner, any overall truss action I suspect would be purely incidental to the
> simple bending of the members. I just can not see how you would get
> reliable load transfers between the members knowing the variety and manner
> in which the various "traditional" ways biadarkas have been built.
>
> Peter
>
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