[PCW] Hull design

[Malcolm Tennant]

Bill,

You are quite correct concerning the shape of the knuckle being critical. The
other critical feature is the height of the knuckle above the water. We use a
knuckle that is curved in cross section .It is generally around 150mm [6"] in
width tapering toward the bow with the curvature being "longer"ie: extends
over a greater vertical distance, so the increase in buoyancy is slower.
Similarly the knuckle generally sweeps up in a curve, to varying degrees,
towards the bow. If the knuckle is too low it slaps and slams. If it is too
high it just sprays water over the boat..

As you say the purpose is to give an increase in buoyancy, act as a spray
deflector and increase the interior space in the hulls. We generally have a
knuckle on both the inboard and outboard sides of the hulls which increases
the interior space by approx 300mm [1'] The curved wingdeck, or the chamfer
panel , comes off the inboard knuckle and further increases the interior space
in the hulls.

One builder "improved" our design by adding a further 150mm to the knuckle for
most of the hull length. This was in the form of a fender and, on the surface,
seemed like a good idea. Unfortunately it had the effect of making the motion
more violent as the waves came up under the wider knuckle and moved it
violently upwards. So, yes the width and cross section of the knuckle is of
critical importance.

When dicussing space in the hulls of a power catamaran you need to keep in
mind that generally you can have space in the hulls, or space up on the
wingdeck, but not both. Until the boat gets to be around 20m in loa you can't
stand up in the hulls under the wingdeck [see the Adriatic]. So to stand up in
the hulls you lose space out of the cabin. The semi-raised pilot house is a
good solution to this dilemna in the smaller [around 14m +] vessel.

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