Jerry mentioned that 40 years ago 3' was figured to be the right
dimension for bridge deck clearance.

Well the can of worms has now been opened.

There are so many types of hulls and so many blue water speeds and uses
that a flat statement of a dimension is not possible. The variables are
so great there is no answer.  Some of the variables are:    Width
between hulls.  Flat area between hulls. Length fore and aft of wing
deck.  Position of wingdeck fore and aft.  Shape of wing deck.  Speed of
the vessel.  Weight of the vessel. Basic hull design. I am sure someone
will mention more.

The answer might be like the Radison Diamond which has a wing deck
clearance of about 30'. I, once in St. Thomas , ran my dinghy under her.

I sometimes think that the higher the wingdeck clearance the hard it
hits when the right situation occurs.  I have been on a Roger Hill cat
with very little clearance and the pounding was small.  In these same
waters cats with a larger clearance would have been pounding.  In a
sailing cat we all worried about surface friction, but in a power cat
this may not be so bad.

In the end each situation needs its own solution. I think all cats will
pound at some time or other.

Dennis Raedeke  Wild Wind IV  Pachoud 60

Georgs Kolesnikovs.

Dear Georgs,

I agree with Dennis 100%. There is an enormous number of variables. Which
raises the point as to just how you are going to get a ratio for the height
of the wingdeck based on both the LWL and the between hulls beam
simultaneously? Certainly both of those parameters are important. Indeed the
distance between the two hulls impacts very much on the hull resistance if
you are talking about a displacement catamaran. too narrow a space and the
resistance increases measurably. How narrow is too narrow? here we go again
with lots of variables. It depends on the speed/length ratio of the hulls,
the displacement/length ratio and exactly how fast you are actually going:
ie: just where abouts you are located on the hulls resistance curve. In
other words, you may possibly reduce the wave impact by reducing the span,
but you may very well increase the boats resistance by so doing. Research
has shown that with the trimaran and pentamaran hull configurations it is
possible to decrease resistance by the correct positioning and spacing of
the hulls relative to the bow wave of the main hull.

The displacement [weight] of the vessel is important as Dennis says but of
possibly more importance is the location of the weight because we are
talking about the longitudinal moments of inertia. The Naval Architects
seeming obsession [ in my case anyway!]with the weight and the position of
that weight in all parts of the vessel is firmly rooted in reality. The
designers insistance in keeping the ends of the vessel light are because
weight in the ends increases the longitudinal moments of inertia. The
effects of this are that the vessel is slower to respond to the upward
acceleration imparted by the waves and may well bury. Of course the hulls
will ultimately accelerate upwards and when they reach apogee and do
accelerate down again they will impact with much greater force. This is
exacerbated by the generally finer waterline plane of the displacement
catamaran. So keep the hulls long and the ends empty in the interests of
minimising impact.

Basically when designing a serious ocean going power catamaran we try to
make the hulls as long as possible and the wingdesck as short as possible
and as high off the water as possible. I think that this approach is
epitomised by one of our recent designs. The Domino is quite a "small" boat
as it only accommodates a couple and some occassional guests but it is
20metres [65'] long. "Big" and "Long" are not the same thing with catamarans
and as usual there are any number of compromises.

I got a bit carried away. But it is a very complex topic.

Regards,

Malcolm Tennant.  ARINA  MA

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

Dear Georgs,

To answer your questions: You can see the Domino on our web site but here
are the figures you were looking for:-

LWL 61'
Distance between hulls [minimum] 11' 10"
Wingdeck height off water at minimum height which is 26' back from the bow
is 3' 10" in full load displacement of 39 tonnes. 4' 10" off the water in
light ship displacement.

As I have mentioned in a number of published articles, you do need to look
at the wing deck height differently for sailing cats and power cats [and
displacement.v. planing]. It is very seldom that a sailing cat will be going
head on into a sea way whereas a power cat may be punching straight into it
a lot of the time.

We used trampolines in the bows on some of our early power cats because if
the waves impacted on them there was really very little effect. However we
did find that it was very wet when the boat was punching into a seaway, the
helm station windows were constantly running with water. So as a compromise
we substituted a "solid" deck for the trampoline. This was still right up at
gunwale level and was largely non structural. If it was ever damaged then
the effect, in terms of possible water egress, would be non existant or
minimal. The wingdeck itself still swept up to the gunwale well back from
the bow.

Regards,

Malcolm Tennant.

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