2.1 * SQRT [ LWL ]

The formula is 1.6 times that of a multihull. Granted there
are many form factors. This is as good an approximation
as the one for monohulls.

Figure 60% faster than a monohull of equal LWL. A
passage would take about 60% the time a monohull
would need.

A Laser on steriods, like an Open 60 monohull would
do nearly as well and handle rougher weather better.
The benefit of a big Cat is more comfort, a kinder sea
motion.

Where things seem to start getting interesting is
heavy weather.

"DSK" wrote
"Bart Senior" .@. wrote
What is the formula for the hull speed of a typical multihull?


It's the same, just use a higher multiple (ie speed-length ratio).


Bart Senior wrote:
Here is another more specific question.

What speed could you expect to do in a multihull with
a 56' waterline?


How is it rigged & how heavily is it loaded? Who is sailing it? How hard
is the wind blowing? Is the water choppy?

The usual hull speed formula is SQR RT(1.34*LWL)
in words, multiply the waterline length by 1.34 and then take the square
root of the resulting number, which is the hull speed in knots. The
multiplier "1.34" is the 'speed-length ratio' mentioned earlier, most
modern boats will beat that pretty easily. For an average fin-keel
racer-cruiser, 1.5 is a better number to use (doesn't sound like much
difference, but try it). Multihulls can hit speed-length ratios of 1.8 or
higher.

A hull that is very narrow, like a Navy destroyer or a Hobie cat, will not
suffer as much drag from making waves. The cut-off point seems to be a
length/beam ratio of about 8:1 so a really fast 56 foot catamaran will
have hulls narrower than 7 feet (this sound wide, but think how many big
cats have athwartship berths in the hulls).

So your 56' cat might have a realistic sailing speed of 10 to 13.5 knots
without doing anything exceptional, you could plan on passagemaking speeds
in this range!

For cats with good performance, the speed-length ratio might be 1.9 or
higher. A better way to approximate the speed potential is a formula that
includes displacement & sail area: take the hull speed formula & multiply
it by the ratio of sail area's cube root to the displacement's fourth root

{(SLR*LWL)^0.5}*{(SA^0.333)/(DISP^0.25)}

It's interesting to play with these numbers and see the huge difference
between conventional racer-cruisers and modern hi-tech sportsboats &
multihulls.

Fresh Breezes- Doug King