Re: [baidarka] Baidarka on a Diet

Subject: Re: [baidarka] Baidarka on a Diet
From: Peter Chopelas (
Date: Fri May 28 2004 - 20:56:23 EDT

> 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,

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