[PCW] Displacement

[Malcolm Tennant]

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 at tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com