Subject: Re: [baidarka] Baidarka on a Diet
From: Mark Haller-Wade (digidog@frazmtn.com)
Date: Fri May 28 2004 - 23:49:44 EDT
Thanks for all the great great advice. I wish I had asked this before I
started building, but honestly I didn't think of it until the deck was
finished, and it's weight got me thinking about finished weight, and
doing all this work on a boat that in the end I can't lift. In answer
to some questions: I'm using Wolfgang Brinck's plans from his book, I
ordered #10 cotton duck canvas for the skin, and my gunwales are 1.75 x
.75. Is polyeurethane floor finish the lightest way to seal the
canvas? How well does boiled linseed oil do?
Since I hate to start over (not to mention the clear doug fir was not
cheap), I'll thin out my deck beams as Wolfgang suggests, and maybe
even drill some holes in my gunwales if you don't think I'll compromise
them. Interesting suggestion by Kurt about using
1/4" white oak for ribs, since straight willow is not readily available
in my area. Can spruce be use for ribs as well? What about ash? I may
find myself on subsequent boats trying build as light as boat as
absolutely possible, like Peter talked about.
Thanks for helping a novice builder with a bad back everyone.
Mark.
On Friday, May 28, 2004, at 05:56 PM, Peter Chopelas wrote:
>> I'm working on a Brinck baidarka. I've completed the chine and deck
>> beams, but now I'm getting a little worried about weight. I've had
>> several back surgeries and can't lift much. .... But I bought all
>> Doug fir
> for my baidarka, and I'm wondering about how much the finished boat
> will
> weigh with Doug fir. Moreover, I'd like suggestions from the more
> experienced out there that can make my boat lighter, and that I can
> still
> apply at this point in the
>> process.
>
> I will give you my take on this issue as an experienced structural
> engineer,
> and a kayak builder: I have worked with Doug fir, hemlock, and red
> cedar
> frames. Frames I built entirely from low density western red cedar,
> which is
> weaker than most woods, have yielded excellent results and are very
> light.
> Red Cedar is relatively inexpensive in the Pacific NW and a single
> clear
> 2x10 or 2x12 (16' long) can give you all you need if you have access
> to a
> table saw. It is easy to work, very light, and rot resistant, it also
> has a
> good strength to weight ratio but same size members of this compared
> to Doug
> fir are not as strong. Doug fir has a very high strength to weight
> ratio as
> well, but is very dense. If you use the same size members using Doug
> fir it
> will be heavier (perhaps 5 or 6 pounds for the whole kayak), but a lot
> stronger and stiffer, than if you use WRC or spruce. If you use the
> DF, you
> could either drill large 1-1/2 to 2 inch holes down the center line of
> each
> gunwale for the full length without noticeable loss of strength, or
> you also
> could thin the gunwales by about 20 percent (the issue here would be
> worry
> of splitting out the rib mortises). I have used gunwales of clear,
> defect
> free WRC as small as 0.75 x 1.5 inches on a smaller kayak for my wife,
> though I would not suggest this for a full sized one, it was also
> somewhat
> flexible, but plenty strong. You likely can go as small as .75 x 2.25
> for
> the gunwales in knot and defect free DF with no noticeable effect
> except
> making it a little more flexible (which can actually make it ride a bit
> better in rough water).
>
> This is what I would consider: use the Doug fir for the gunwales
> (perhaps
> thinned or lightened a bit), these are the most critically important
> structural element in the whole frame, and then use WRC or spruce for
> all of
> the other components. An alternative would be to simply thin
> everything
> down since the Doug fir is so strong, your deck beams are not really
> that
> heavily loaded (except the one right behind the coaming where you sit
> on it
> to get in), so these could be perhaps half as think as the plans call
> for.
> If you do this however, watch out for defects in the wood, splits,
> checks,
> sap pockets, knots, grain runout and other grain irregularities. Also
> make
> sure you provide adaquate edge margine for dowels or lashing holes.
>
> You can also make the bow and stern plates much thinner, and drill
> large
> lightening holes in them as well (these parts are not particularly
> heavily
> loaded either). I made a laminated mastic out of veneers I ripped
> from Doug
> fir, and used thick wedges of WRC on each end. It was only about 5/8"
> think
> in the center arch, with each end about 2.5 in. think, it is very
> strong (by
> laminating it has no grain runout despite the thin curved shape) and it
> weighs about 1/3rd as much as a solid sawn one. I am considering
> making
> laminated beams for the whole kayak on the one I am starting now. You
> could
> also increase the rib spacing by about two inches or more each, keep
> them
> closer together only at the cock pit where the weight of the paddler
> sits.
> I think most plans and books have the rib spacing much closer than
> necesary,
> all of mine I have used this wider spacing just to save building time
> (and
> is saves some weight too).
>
> Most frames are much stronger than they need to be, as long as you use
> clear, straight grained wood for the stringers, you should be fine
> using
> much thinner parts. Also, if you need blocks, wedges or spacer, use
> scraps
> of red cedar or other lightweight wood. Keep away from hardwoods,
> they are
> not any stronger than doug fir (except as rub strips on the keel), and
> are a
> lot heavier.
>
> If you use nylon or polyester skin instead of canvas you will also save
> weight. Though not as durable you might consider going to 6 oz nylon
> or
> polyester, it will hold up fine you will just need to watch for
> wear-through
> on the keel line. There is no reason to use heavier fabric than 8 oz
> nylon
> or polyester (I have used it for years with good wear and strength),
> Dyson
> and Co. in Bellingham sells this for about the same cost as cotton
> canvas
> (it also lasts longer than canvas). Also, three coats of polyeurethane
> floor finish would be lighter than 4-5+ coats of paint. You also might
> consider using 8 oz for the hull from the gunwale down, and use 4 to 6
> oz
> fabric on the deck (I would use a sewing machine to sew then together
> with a
> double lap felled seam at the top of the gunwale, and stitch the deck
> center
> line closed in the normal way). And then only use two light coats of
> sealant on the deck. This will save some wieght and keep the hull
> durable.
>
> You also might consider making it a foot or so shorter than normal,
> depending on your planned usage. For general recreational paddling in
> most
> calm conditions you will never miss the extra length, and this should
> save
> several more pounds.
>
> Keeping the frame and skin fairly light should yield a finished weight
> of
> about 32-36 pounds (depending how big it is). Using full sized
> members of
> DF with paint and canvas should put it in the 42-45 pound range (or as
> much
> as 50 if you go overboard with making it extra strong). One of the
> very
> cleaver, and rather advanced, features of this traditional design is
> the
> overall redundancy built into it. Except for the gunwales, failure of
> no
> one member will render the kayak unseaworthy, many people find they
> have
> been padding around for years with broken ribs, stringers or beams and
> never
> even knew it. Therefore do not be afraid to make these parts lighter,
> just
> keep a careful eye on the those gunwales for grain flaws, especially
> in the
> center third of the length (the loading is much lower in the outer
> thirds of
> the length).
>
> I have considered using these methods to see how light I can make a
> kayak.
> I would optimize each member to the actual load requirements, and use
> the
> aircraft weight 4 or 5 oz polyester fabric, and laminating up all the
> members to get maximum strength from the smallest possible members. I
> suspect I can get a strong and safe, almost full sized kayak with a
> wood
> frame to weigh in at only about 20 or 22 pounds. Though I suspect I
> will be
> reskining it more often than most.
>
> Good luck,
>
> Peter
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