"mislav" wrote in message
...
I also would be very interested to see replies to this question.
I do suspect that watching an experienced boatbuilder do this is
probably the only way to learn, and it's an art as much as a skill.
I have seen it done once, and not done ideally at that.

I finally transfered the lines of my rowing shell into the computer and
wanted to share the experience. Like I said previously it involves
trilateration measuring technique used by GPS. I set 3 separate reference
points (put some small nails through the wooden fence in my backyard to
which I could attach the end of my 10m long tape measure). Previously I
supported the shell with sawing horses not caring to level anything. The
shell was turned upside down. One reference point was designated 0,0,0, the
other lied on y=0,z=0 axis, the third only on z=0. It was only necessary to
measure all distances between nails, in fact the sides of a triangle
(a,b,c), and then using Heron's formula find coordinates of the third point:

y|
|
|
y3 |--------P3
| a . | . b
| . | .
| . | .
P1-------|-----------------P2-----
/ x3 x2=c x
/
/
z/

P1= 0,0,0
P2= x2,0,0
P3= x3,y3,0

where y3 = (2*A)/c
and A = sqrt(S*(S-a)*(S-b)*(S-c)), S=(a+b+c)/2
then x3 = sqrt(a*a - y3*y3)

Once you have coordinates you can use
http://en.wikipedia.org/wiki/Trilateration formulas to calculate coordinates
of any point in the surrounding space by measuring distances from that point
to all three referent points. However this technique is not without it's
drawbacks. Basically any measurement error will multiply in the results
because of the second and fourth power in formulas, so you have to be very
careful.

I marked and measured around 120 points in total on keel, chine and shear. I
set up a spreadsheet from trilateration formulas that instantly calculates
coordinates from my measurement data. Since this is a plywood boat made from
panels with straight edges it is reasonable to assume that all points on a
certain edge lie in the same plane. Therefore after importing points in CAD
I draw 3 planes through all points taken from keel, chine and shear, then I
projected all points on their respective plane (projection means creating a
new point that lies on the intersection between a plane and the pependicular
that goes through the source point), then draw splines through all new
points that lie on the plane. Note that some final fairing of the individual
lines (keel, chine and sheer) was also necessary.

This technique could be used on round bottom boats too. It would only be
necessary to mark and measure keel and waterlines, since all points on the
waterline naturally lie on the same plane.