It's my understanding that the width between the hulls becomes more
important as speed increases in order to minimize the wake interference  between the
two hulls.  I suspect this is one reason that the  Endeavour 38 (16' beam) is
slower with the same power than their  heavier 44 with its 19' beam.

Henry Clews

In a message dated 2/24/2006 1:53:47 P.M. Eastern Standard Time,
mark424x@yahoo.com wrote:

To  expand on Bob's question, I'd be curious what other factors effect the
beam  decision (that is beam of the boat, not each hull).  It appears there are
stability rules, obviously interior room/layout considerations.  Are  there
any hydrodynamic considerations? e.g. is there some distance between the
centerlines/hulls below which you shouldn't go either as an absolute number or  as
a ratio relative to hull waterline  beam?

" It's my understanding that the width between the hulls becomes more
important as speed increases in order to minimize the wake interference
between the two hulls."

There's a freeware software package out there called Michlet that
forecasts hull resistance if you input a hull drawing.  It breaks down
the resistance to its various components such as bow wave, skin
friction, and for catamarans, the interference due to the wake from the
other hull.

It's interesting to play with some of the variables such as hull
separation, displacement, and speed to see the affects.  The results
aren't as intuitive as one might expect - just making the boat wider
doesn't guarantee less interference across the board.

As a sidenote, Dennis Raedeke told me that on his Wildwind IV powercat,
he experienced significant "slamming" on the underdeck towards the
stern.  I would have expected wave impacts towards the bow, but after
thinking about it, I think the cause of that slamming was from the two
wakes colliding under the hull and creating a vertical jump before they
cleared the back of the wing deck.  Haven't confirmed that yet with
Dennis.  I'm guessing that too is a function of hull separation, length,
and speed, as well as displacement and hull shape.

Bob Deering
Juneau Alaska

Here is a link to the Michlet Web Site....

http://www.cyberiad.net/michlet.htm

Regards,

Alan Schaaf
Green Cove Springs, FL

-----Original Message-----
From: power-catamaran-bounces@lists.samurai.com
[mailto:power-catamaran-bounces@lists.samurai.com] On Behalf Of Robert
Deering
Sent: Friday, February 24, 2006 7:44 PM
To: 'Power Catamaran List'
Subject: Re: [PCW] Beam (was Stability)

" It's my understanding that the width between the hulls becomes more
important as speed increases in order to minimize the wake interference
between the two hulls."

There's a freeware software package out there called Michlet that forecasts
hull resistance if you input a hull drawing.  It breaks down the resistance
to its various components such as bow wave, skin friction, and for
catamarans, the interference due to the wake from the other hull.

It's interesting to play with some of the variables such as hull separation,
displacement, and speed to see the affects.  The results aren't as intuitive
as one might expect - just making the boat wider doesn't guarantee less
interference across the board.

As a sidenote, Dennis Raedeke told me that on his Wildwind IV powercat, he
experienced significant "slamming" on the underdeck towards the stern.  I
would have expected wave impacts towards the bow, but after thinking about
it, I think the cause of that slamming was from the two wakes colliding
under the hull and creating a vertical jump before they cleared the back of
the wing deck.  Haven't confirmed that yet with Dennis.  I'm guessing that
too is a function of hull separation, length, and speed, as well as
displacement and hull shape.

Bob Deering
Juneau Alaska

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