Just a couple of comments on Dennis' submissions. I agree with his comments
on the L:B ratio of the hulls for a displacement power catamaran. The range
he quotes does appear to be optimal in terms of hull resistance for the
sorts of speeds we are usually talking about ie: a top speed around 25 to 35
knots and a cruising speed of around 15 to 20 knots.Our experience has shown
that moving too far outside this optimal range increases the hulls
resistance as indicated by increased fuel usage. Obviously there are more
variables than this but it is important when considering displacement hulls.

However I do not, of course, agree with his comments on design "slop". If
the designer is given all the information and performs a complete weight
analysis[ and is given the time and the money to do this], and the
builder/client does not change anything during construction,then it is
possible to get the boat to trim spot on. Or at the very least for the
designer to predict how it will trim in various loading conditions. This is
evidenced by a number of our recent designs that have floated with in 3mm of
where they were supposed to in full load displacement. However I will admit
that it becomes increasingly difficult to maintain a level trim in all load
conditions when the fuel load becomes a very significant part of the total
displacement as will often happen in long range ocean cruising boats. In
this case it will probably be  necessary to pump fuel in that situation to
maintain level trim. But I would never suggest the fitting of tankage fore
and aft to control trim problems that "might occur" except in the direst of
circumstances. This is because they will constantly effect the trim of the
vessel as their level changes. More serious is that they will increase the
longitudinal moments of inertia of the vessel. All tankage should be located
more or less centrally in the vessel as much as it is possible.

As myself and many other designers are always stressing. The secret is to
design the boat completely before construction commences and then do not
change it. Unfortunately this very seldom happens.

Regards,

Malcolm

Malcolm Tennant Multihull Design Ltd
PO Box 60513 Titirangi,
Auckland 1007
NEW ZEALAND

ph +64 9 817 1988
fax +64 9 817 6080

e-mail malcolm@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com

Malcolm,

I understand the need to get variable loads such as fuel as close to the
center of buoyancy as possible so the trim doesn't change as the load
changes, and to minimize the longitudinal moment of inertia.  With fuel
tanks for example, you have the option of building the tank low and long
in the hull to keep it's center of gravity as low as possible.  On the
other hand, you can build it more vertically to position that load more
precisely over the desired point in the hull and reduce its moment.

In monohulls the designer almost always wants to get that load as low as
possible for stability reasons, but with cats that's not such a strong
driver so the load could be stacked more vertically if the interior
plans allowed for it.

How do you balance those considerations in your designs?

And many thanks for your insights on this board!

Bob Deering
Juneau Alaska