Subject: Re: [baidarka] lowering the cocpits aft end
From: Bill (bill@billmercer.com)
Date: Wed Sep 17 2003 - 17:24:43 EDT
While I'd agree that the flat deckbeam will be only a little weaker than
an arched one when you're sitting on it (the only situation where an
arched beam, especially one carved from straight-grained stock, will be
stronger), I do thik 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. Unlike a Greenland kayak,
where the gunwales carry virtually all stresses,
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 bow deck stringer may not matter so
much, since it's under tension till the boat bends, 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. If you lower the rear
stringer so that it's not above the gunwales (or just istn't there at all),
you might get too much bendiness in the tail of the boat. That said,
Jack Gilman's system of using a triangle to tie the rear stringer to the
gunwales at the cockpit looks like a good plan to me--the stringer can
still contribute to stiffness (I think), since it looks like bending the
tail of the boat up would tend to compress or flex the stringer..
Of course this is just hypothetical, since I'd need to build two boats,
identical except for the rear deckstringers, to really tell. Not to
mention measuring deflection. But it makes sense to me.
Bill
n Wed, 17 Sep 2003, Peter Chopelas wrote:
> Dick wrote:
>
> > At the moment I am contemplating lowering the aft end of my baidarka's
> cockpit to facilitate better rolling. The idea is to use a straight deckbeam
> as an aft cocpit support and get the deckridge to run up to the next
> deckbeam that is of traditional height. ... In theory it weakens the
> structure of the boat somewhat.
>
>
> Dick,
>
> It will not weaken it even in theory. The fact that the a normal curved
> deck beam is cut across the grain means you risk cross grain tension on the
> under surface of the beam when you sit on it (as when getting into the
> cockpit). A pure parabolic arch does have signivicant strength benefit by
> putting the beam stress in compression rather than bending, but I do not
> think there is enough of an arch to make much difference (I have done the
> calculations before, but they are real messy and complicated). There could
> not be much arch effect anyway since this requires the arch supports to take
> a very large outward reaction (larger than the vertical load infact). The
> gunwales DO NOT have much strength in the outward direction anyway (and the
> skin is not nearly stiff enough to stop the outward spread before the deck
> beam would fail).
>
> Go ahead and make a strait deck beam, you could make it a bit deeper if you
> are worried about strength (but I would not bother). It is probably
> stronger if it is cut strait with the grain anyway. The reason it is curved
> I think is to increase storage volume and make aft access easier. I make
> these kind of changes all the time, all the strength of the hull comes from
> the gunwale size, and there will be no performance difference if no shape
> below the water line changes (excpet you will find your rolls a lot smoother
> and eaiser!). All of the other members are mostly there to just hold the
> skin in place in the right shape, any other strucural benefit from the other
> lighter members is incidental and compared to the gunwale strength.
>
> Peter
>
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