[TowerTalk] Re: An Engineering Question

[David O Hachadorian]

On Sat, 31 May 1997 13:32:01 +1200 Martin Ellis <jmellis@ihug.co.nz>
writes:
>Hi,
>
>Just reflecting on the problem of placing guy anchors
>closer to the base of the tower...
>
>Interesting how yachting designers plan their rigging to
>keep 100+ foot masts upright, when they are limited 
>by the beam of the yacht.  And how many square feet
>of sail area?
>
>Martin ZL1ANJ
>

That _is_ an interesting thought. There are some differences
between the two problems though.

I'm not a sailor, but I think the following observations are
at least partially correct:

Many of the sails are triangular, with the center of pressure
fairly low on the mast. Ham antennas have a big blob of
surface area at the very top.

The boat heels in the wind, spilling some of the air. You'd
better hope your ham tower doesn't do this!

The sails are not orthogonal to the wind in most cases. You
can't count on this for ham antennas, which are frequently
"sailing" unattended!

Isn't there a wind speed limitation, above which
the sails must be lowered? Pressure exerted by wind is a
nonlinear function:

 5mph = 0.1psf
10mph = 0.4psf
20mph = 1.6psf
40mph = 6.4psf
50mph = 10psf
60mph = 14psf
70mph = 20psf
80mph = 26psf
90mph = 32psf
100mph= 40psf

I think sailboats lower their sails just as the curve starts
to get interesting for ham antennas!

Dave Hachadorian, K6LL
k6ll@juno.com

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:			  http://www.contesting.com/km9p/search.htm