A lively discussion, so far....

I would like to add that perhaps the most persuasive issues in my
estimation  are weight and more importantly weight distribution.  All
boats are subject to performance penalties of various sorts when their
weight increases significantly from that intended by the designer, and
more importantly, when the balance the designer built into the boat is
dramatically degraded by adding more and/or different weights out at the
fore or aft ends.  Cruising catamarans are much lighter than their
monohull displacement peers as they neither have nor need ballast.
Great efforts are expended to minimize their overall weight both
throughout the design and in later admonitions to the buyer.  First,
catamarans (particularly the semi-displacement kind) have very slender
hulls and trade off great roll resistance at the cost of very much more
pitch sensitivity.  And second, these catamarans with their slender
hulls have tiny flat planing surfaces on their bottoms aft, and the
orientation to the water surface of this lifting surface is pretty
critical.  Both these considerations make the fore-and-aft distribution
of weight really critical.

The worst sin is to add too much at the ends, such as heavy diesels aft
and/or a heavy genset forward.  The designer has worked long and hard to
get the weight concentrated amidships and also to make the fore and aft
ends as light as possible in order that the hull can react better to
pitching in rough water.  It is a matter of rotational inertia, or
moment, and at the same time a lever problem.  Imagine if you will a
seesaw with a pair of sumo wrestlers on it.  If the wrestlers both sit
way out at the ends of the seesaw plank it could indeed be in balance,
but their large inertia would make it hard to move the seesaw ends up or
down.  The big guys would have to move their considerable weight a great
distance up or down to get the plank to move -- say maybe twenty
degrees.  Now place our imaginary sumo-dudes right next to the fulcrum
(pivot point).  It could still be in balance but now they don't hardly
move up or down at all to get the plank to tilt our twenty degrees.  The
sumo-moms propelling this activity from out at both ends of the plank
don't have to work nearly as hard.  Same goes for pitch sensitive
catamaran hulls.  Their bows need to rise and fall easily to track the
waves.  I suppose that wave piercing hulls might work with long skinny
hulls containing massive inertia in the bow and/or stern, but so far
that is just my speculation.  I have no direct and very little indirect
information about the pitching performance of a full sized wave piercing
catamaran.  However, does anybody know what happens when the speedy
little beach sail cats bury a bow (or two) in a wave?  It's called
pitchpoling, and is not very pleasant.

The point of all this jabber is that the Prowler designer probably
worked pretty hard to properly balance the fore-and-aft trim of the hull
in order to get optimal pitching performance and to achieve the desired
attitude to utilize the planing surfaces on the bottom.  Simply
increasing the weight of the engines is bad, unless they can be moved
far enough forward to restore the original trim.  Clearly the outboards
hang way out behind, and are not exactly lightweights themselves, so
perhaps it could be done.  Sorta like moving the heavier sumo wrestler
much closer to the fulcrum than the sumo baby he replaced.  Gensets (and
sumo wrestlers too) need to be carefully placed in a boat to get optimum
performance.

This stuff may or may not be apparent to everyone, but I for one think
it is a significant issue

More from that old drone Mister Science
AKA Gary Bell

As a practising designer I would just like to endorse Gary Bells piece on the
importance of the longitudinal moments of inertia in the area of boat design.
Two of the most important variables in naval architecture are [1] full load
displacement and [2] the concept of moments. When we  are calculating the mass
and the position of the various centroids we end up with many  thousands of
items and somewhere between 40 and 60 pages of spread sheet calculations. But
the result of this is that we can expect our boats to float within 3 to 4mm of
where they are supposed to and that we can predict with certainty what will
happen to the trim of the vessel as the tanks are filled, or emptied ,of the
many thousands of litres of fluids required for long distance travel.

When calculating all the various centroids LCG, VCG, TCG, CB and CF it is
important to keep in mind that they are all sums of moments. ie: mass X
distances, volumes X distances, areas X distances. What this means in practice
is that moving a large mass a short distance has exactly the same effect on
trim as does moving a small mass a big distance and goes a long way toward
explaining why designers are so fussed about keeping weight out of the ends of
the boat.

I have been told by clients that I am obsessed with weight/mass.Probably true.
Certainly in March 2004 I wrote an article about my concerns for Professional
Boat Builder called "Power Catamarans and the LCG [a cautionary tale]" this
was accompanied by a case study for a 19.6m power catamaran that illustrated
what can go wrong when you start "improving" the original design and moving
mass around.

If any of the members of the list would like a copy of this article send me
your e-mail address and I will e-mail you a copy.

Regards,

Malcolm Tennant.

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

Natale Ramondeti.

Dear Natale,

I need your e-mail address. The Catamaran List does not accept attachments.

Regards,

Malcolm Tennant.

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