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Subject: TWL: Multihull performance
From: Robert Bryett (rbryett@XXX.XXX)
Date: Tue Nov 05 2002 - 23:09:38 EST
>>>>While on the subject of fuel efficiency, which is to say hull
efficiency, what's the story on catamaran hulls? They appear to be very
efficient. Catamaran sail boats obviously go much faster than full
displacement ones despite having the same power source.<<<<
Oh, you've done it now! I'm a veteran catamariner and tripilot and I could
talk your ears off about the wonders of multihulls...
First of all, with the exception of the few racing hydrofoil multihulls,
sailing cats and tris ARE full displacement boats. Many power cats have
planing hulls, but the hulls of sailing multis are intended to pass through
the water rather than over it.
Cat and tri hulls generally have lower wave-making resistance than monohulls
because their waterline beam is far smaller in proportion to the LWL. A fast
multi's hulls are likely to have a length to beam ratio greater than 10:1.
This long skinny hull has to "push aside" less water, generates less of a
wave system and so requires less energy to drive it through the water.
Light weight is also a factor that improves the performance of multihull
sailing boats, but this is more complex that it at first appears. Multihulls
don't have a weighted keel to keep the boat upright against the wind force
in the sails. This makes them lighter, gives them a high power to weight
ratio, and ALLOWS the use of the long, narrow, shallow hulls mentioned
above, because the hulls don't need the volume and buoyancy to support that
big lead udder hanging under the hull.
However, there's a dark side to this. Cats and tris not only CAN be light,
but MUST be light and kept that way. First you have to build them light,
while keeping them strong enough to withstand very high loads. Then those
long, narrow, small water-plane hulls often have nearly vertical sides and
short overhangs, so they don't have much reserve buoyancy and are very
sensitive to changes in load. If you pile on the cruising gear and supplies,
the hulls sink deep into the water. That's bad in itself, and also brings
bridgedecks, crossbeams etc. closer to the water where they can slam or
catch wave tops. Worse still, a multi relies on reserve buoyancy of the
leeward hull to resist the heeling force of the wind in the sails.
Compromising that buoyancy increases the risk of every multihull sailor's
nightmare - CAPSIZE.
Don't get me wrong. It's perfectly possible to design a cruising multi. It's
just that the whole thing is a bit more "fine tuned". I've read designers of
monohull displacement powerboats suggesting that having a boat float six
inches below her designed waterline on launch doesn't matter. Robin
Knox-Johnson's "Suhaili" started her voyage round the world loaded deep
below her marks with a year's supplies. You have to be much more careful
about loading and waterlines on cats and tris.
I think I've raved on long enough. Some of the factors I've mentioned above
don't really apply to a powered multi, but I hope this is of some interest
to the list.
In the meantime, how cool are these?
http://www.nigelirens.demon.co.uk/nid_power.htm
http://www.nigelirens.demon.co.uk/nid_power2.htm
Frigid! Of course getting a marina berth might be a bit difficult, not to
mention passing through locks...
For a complete change of pace, go here: http://www.wharram.com . James
Wharram might be a bit stoneground, wholemeal and cosmic, but an awful lot
of ocean miles have passed under his designs, and they prove that you don't
need to go all hi-tech. You could build one with twin small diesels and
ditch the rig... Build a wheelhouse on the bridgedeck... Hmmm...
Best regards, Robert Bryett,
Sydney, Australia.
mailto:rbryett@XXX.XXX
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