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Subject: TWL: RE: Multihull performance
From: Robert Deering (deering@XXX.XXX)
Date: Tue Nov 05 2002 - 23:54:10 EST
There are two separate issues in this discussion:
1. Why a catamaran requires far less power than a typical displacement
monohulled boat of the same length to travel at the same speed; and
2. Why a displacement catamaran can travel at higher speeds than a
similarly sized displacement monohull.
Just last weekend I had a discussion with a catamaran designer about
these very topics. Here's what I gathered from it.
Power requirements are a function of beam and displacement (along with
other things). In general, a fatter, heavier boat requires more power
to push all that water in front of it aside. A catamaran may have a
length to beam ratio (for each hull) of say 11:1 compared to a typical
trawler that is probably under 3:1. That's a lot less water that has to
be moved to let a catamaran past. A cat will generally be much lighter
than a monohull of the same length as well.
The speed of the hull is a function of Froudes Law (the old 1.34 times
the sqrt of the waterline length). Froudes Law applies to cats as well
as monohulls, so a cat of a similar length as a monohull will have the
same hull speed characteristics. The difference is that with a
displacement boat the bow wave is so big that it becomes impossible to
climb over it. Since a cat doesn't push that huge wall of water with
its skinny hulls it's able to climb the modest bow wave and keep
accelerating.
The designer I talked to said that power catamarans run into the same
power hump at the calculated hull speed, and to get past that a whole
lot more power has to be added. More power is a relative term though -
his 55' displacement cat reaches hull speed with about 80hp (total), and
it'd take about 300 hp (total) to push it at 15+ knots.
He stated that most power cat designs will make the aft hull flatter to
help keep the stern from squatting while climbing over the bow wave,
just like a semi-displacement hull. He also stated that cats under sail
don't seem to run into the same power hump. His theory is that the
forward force from the sails/mast tends to press down on the bows and
raise the stern, and that counteracts the stern-squatting tendency of
the power boats.
I could have my facts all wrong here, so any hull experts out there feel
free to enlighten me.
Bob Deering
Juneau Alaska
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