: I wonder what
: kayak I should build for my 10 year old boy? I thought maybe the little
: Auk but I read it might be a little wide for short arms.

Depends on how much he weighs, how tall he is and the same statistics
read off a child growth chart for some time in the future (how long do
you want the boat to last ?). Don't be afraid to take an existing design
like the Great Auk and simply scale it to get an appropriate displacement.
Kids cope with narrow boats in the sort of waters they'll start off paddling.
If you put him in a boat that's too big, it will float high, have a short
narrow waterline, and feel unstable even if the secondary stability is fine.

I built a Great Auk scaled to 5/6 of the size in Nick's book for my daughter.
She is 8, and in the bottom 1% of children's sizes. I gues she will be about
1.55m and between 40 and 45 kg as a young fit adult (in twelve years).
Geyrfugl is a little big for her now as a dayboat, but is fine with some gear
stowed fore and aft. She will be about right as a dayboat for her as an adult
(my wife is 1.6m and 45kg and finds her OK in a slight to moderate sea, but
she has not been paddling that long - by the time Sarah is that size she will
have been paddling a decade and will appreciate a fast and narrow boat). A
friend's 11 year old daughter recently paddled Geyrfugl in the Conwy ascent
(a tidal river with tidal flow at 7 knots at the start - see pic below) and
found her just fine. I can even paddle her myself at a pinch (and I'm 1.7m
and 75 kg) so don't be afraid to build low volume and low beam !

Others have mentioned the Grebe, which was developed from the Cormorant with
a few changes to keep her a little more stable. I built a scaled down
Cormorant (a little under 95% of full size, 52.3 cm beam) which I find
really fun to paddle as a dayboat, and I have managed to get six days'
gear in her and still paddled off Northwest Scotland in force five and
a steep chop. I added a retractable skeg which helps in steep following
seas and tailwinds, but my design wasn't perfect - it is a bit prone to
jamming - so I know I can paddle the boat without.

Remember if you are scaling that the displacement goes as the cube of
the linear scale, but the boat weight goes more like the square, unless
you use correspondingly thinner materials. From a rigidity/strength point
view, thinner materials are OK (you are putting less weight in the middle
of a shorter beam, so the stress is much reduced) but with kids in particular,
thicker glass on the bottom of the hull is a good precaution against the
inevitable abrasion and encounters with sharp rocks... You can do a straight
linear scale for any design (S+G or strip), but if you want to scale one
dimension differently from another, strip gives you more flexibility.

Andy