expanding on what I typed in haste yesterday ...

William R. Watt ) writes:

...I also calculate numbers
with two hull design programs but they are not the same as measurements
from actual in the water tests.

I wrote a computer program which, like KAPER, accepts dimensions and
calauclates areas, volumes, and other numbers. Unlike KAEPER this program
uses analystical geometry to do its calcuations. My program is only for
flat bottom skiffs. It was inspired by a clever geomertical analysis of
teh dory hull by Barend Migchelsen of Dorval, Quebec who developed an
simple, elegant method of designing and buidling dories based on geometry.
This appreoach is pretty accurate. The program I wrote produces a tabel of
offsets which is the usual way boat hulls are described for computer
analysis and for boatbuiling. However, whe I input a table of offsets from
my program into the two hull design program I use there is quite a
variation in the areas (wetted surface) and volumes (displacement)
displayed by all three programs. The bigger the boat the more they
diverge. From 7% on a 12 ft skiff to 17% on a 20 footer. the
discrepenciews arise from the different assumtions and formuale used by
the different programs, adn by the way the two hull design programs accept
teh data. they both interpolate between stations and the both produce
different numbers depending on which order you type in the stations.

The program I wrote is on my website under Boats and Design. It is not in
the public domain but it is open source. Anybody can use it an modify it
so long as they don't attempt to sell the result.

So what I'm saying is design numbers are only a guide to boatbuilding. To
verify the numbers you have to test the boat and collect data. I've always
assumed that Winters' numbers were test data. I've also assumed his KAEPER
program was verified against test data. Often a scaled down model is
tested in a tank but even then there are assumptions made in the scaling
and testing apparatus. I've seen them explained in wind tunnel tests for
sails as well. Failures result when the design, despite teh best efforts,
is not good, and there are failures in real life, some quite expensive.
I'm sure some canoe and kayak designs are not very good despite the use of
computers.

BTW, the following figure shows what I explained in a previous post
but which you claimed was not correct.

http://www.greenval.com/fig3_1.gif

I've seen it. I'm familiar with it. It does not.

in your previous post you claimed minimal total hull resistance occurs
when the frictional and wave-making resistance are equal. if you'll notice
on the graph the minimal total resistance occurs when the frictional
resistance is about 1.5 lb and the wave-making resistance is 4.5 lb. There
is a local minimum but it's not the simple intuitive tradeoff you've
claimed.


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