[PCW] Catamaran Motorsiler with DynaRig

[Malcolm Tennant]

My experiences with free standing masts housed in the cabin structure of
catamarans has made me a little cautious about their application. They do
reduce the compression load from the standing rigging of a mast but a
"normally" rigged mast on a catamaran has low compression courtesy of the
staying angles of 30 to 45 degrees that are obtainable because of the extreme
beam. The normal stayed rig is therefore considerably lighter and much, much
cheaper. But of course the stayed mast cannot rotate 360 degrees to depower as
can the free standing one. On the Maltese Falcon they are talking about a
bending load of around 13.28 MILLION lbs ft at deck level! This is some load.
Brian is proposing to handle this cantilever load with a carbon compression
structure. We used a Kevlar tension structure which is potentially lighter.
The mast we had experience with was our  motor sailer of 1992, the Revolution,
in which an 18m diameter propeller was attached to an 18m freestanding spar.
This drove two in the water propellers via a shaft and hydraulics. Today this
could be replaced by off the shelf components of a wind turbine driving
electric motors. The cantilever load was not in the same order as that of the
Maltese Falcon but it was enough to continually destroy the roller bearings
used as collars at deck level. This gave me a real appreciation of the loads
imposed by free standing masts when you have a relatively restricted bury as
occurs with the deck cabin of a catamaran.

A phenomenon occured which we hadn't anticipated,. Though in hindsight perhaps
we should
have. The rotating propeller had a major gyroscopic effect and the boat had a
tendency to want to keep going in a straight line.

Maybe the free standing bi-plane rig might be a better option as it would have
a deeper bury and would only have half the height. Two side by side Dynarigs
perhaps, or as on team phillips?

The major problem with the Revolution was that it did not have the "romance of
sail" even though it was wind driven. This would appear to be a real stumbling
block to acceptance  [see the Walker wingsail also]



MALCOLM TENNANT MULTIHULL DESIGN LTD
PO Box 60513, Titirangi.
Waitakere 0642
NEW ZEALAND
Ph: +64 9 817 1988
e-mail: malcolm@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com