## [ksurf] Re: Fwd: KitePhysics 102

Expand Messages
• I d asked Dave Culp: Welcome to this discussion. Are you POSITIVE that this statement is ... their ... To which he replied:
Message 1 of 1 , Oct 2, 1999

<< >Welcome to this discussion. Are you POSITIVE that this statement is
>incorrect?: For "land-sailing or ice-sailing vehicle...a beam reach
>--...happens to be the course where apparent-wind speed and pull are at
their
>highest possible levels while sailing" >>

To which he replied:

<< Well, it's not all that black and white, Tom. It depends on the boat.
A slow monohull, incapable of sailing faster than the wind, will see
it's highest apparent wind when sailing upwind--likely even hard on
the wind (40 degrees, true), if it's a powerful boat, able to make
its maximum hullspeed on that course. OTOH, a land yacht, or iceboat,
capable of sailing at 4-5 times the true windspeed, will see its max
apparent wind on the same course they get their maximum speed--about
100 degrees off true.

For fast boats which are *not* iceboats, like kitesurfers, the point
of max apparent wind likely depends on the specific multiple of true
windspeed the craft is capable of--I've never run the numbers, as max
apparent wind isn't really of interest. It might be interesting to
chart this, for various different types of boats, just to see what
the curve looks like. Doing so would need a lot of data points, for
many different boats.

Any sailcraft's max speed (and perhaps its max apparent wind speed?)
is attained at 90 degrees, plus the craft's total drag angle. An
iceboat has a minimum drag angle of maybe 7 degrees, a landyacht
nearer 12-17; a really hot catamaran (Cogito--C-class cat) about 24
degrees, a kitesurfer nearer 35-40 degrees. Thus a kitesurfer might
hit his max speed around 125-130 degrees. I'd need to run a number of
examples, from real-life kitesurfers' performance, to tell you
whether they see their max apparent wind at that course, or something
closer to wind.

I can tell you, definitively, that the statement above *might* be
true, for some boat (or boats), and is certainly untrue, for others.
No help, huh? Almost without question, the authors made some (many?)
presumptions, then ran the numbers, and now believe they have a
"general" case. They are mistaken. However, they *might* have a good
number, for some, or even most kitesurfers. I really don't know,
without knowing what presuppositions they made. >>
Your message has been successfully submitted and would be delivered to recipients shortly.