Just a thought.

Since most of the discussion on hull design centers around long and
narrow - why not a hull design where below the water line the hull
meets that requirement but above the water line it is flared wider to
give greater design livability.
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I realize this would not make much difference under heavy weather
conditions, but most of us tend to stay out of heavy weather if we
can. There may be other issues but this was just a thought.

Another thing that I have observed in this forum, and that is assumed
usage of your boat.

First and foremost as anyone can see at any marina, most of the boats
never leave the dock.

Of the ones that do use their boat, day/weekenders are probably the
largest group.

With the smallest group being the very lucky cruising couple. with
this group being further divided into luxury and minimizing fuel
costs.

based on these obviously simplistic concepts - what is important to
each is very different. There is no right or wrong just different
usages.

I myself fall into the weekend with extended trips group. Which makes
me want more heavy luxuries on my boat ( costing more fuel to push it
through the water ) but I am wanting to enjoy my short periods of
usage and am willing to pay the fuel penalty. If I ever am able to do
a long extended cruise I am sure my needs would change. Bottom line
different designs for different usages.

I by no means feel these descriptions are anything more than an
attempt to maybe qualify our points of view, so they add to a readers
value. If we know their perspective we can better understand their
meaning.

By the way our  latest discussions have been wonderful and the reason
I wanted to be part of this group.

Jim Meader
jimmeader@mac.com

Jim wrote,

"Since most of the discussion on hull design centers
around long and
narrow - why not a hull design where below the water
line the hull
meets that requirement but above the water line it is
flared wider to
give greater design livability.
|            |
|----/----|  all I could do with a keyboard

I realize this would not make much difference under
heavy weather
conditions, but most of us tend to stay out of heavy
weather if we
can. There may be other issues but this was just a
thought."

You can see this 'expanded hull above the wl' concept
in several sailing multihull designs.  The corsair
(Farrier) trimarans, Chris White's Hammerhead 54 tri,
there are others.  It's especially prevalent in some
sailing tris as the center hull contains most (all) of
the living accomodations and there's not much there
unless you flare the hull.

The downside is too much "expansion" which will cause
slapping or pounding.

Malcolm Tennant's trademark powercat hullform employs
a similar feature with the pronounced knuckle above
the lwl.  This provides a skinny hull in the water
with reserve bouyacy when the knuckle is immersed, a
spray deflector, and an increase in interior volume
above the knuckle for accomodations.

Downside is similar; too much knuckle can slap or
pound, and probably the transition fron hull through
the knuckle to topsides is important.  If it's an easy
transition, noise and motion will be easier.

Bill

  ____________________________________________________________________________________

Never miss a thing.  Make Yahoo your home page.
http://www.yahoo.com/r/hs

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@tennantdesign.co.nz
www.tennantdesign.co.nz
www.catdesigners.com

Marks comments on draught are interesting. Generally when you are designing a
power cat hull there are a number of variables involved. If you have set the
LOA then you try to calculate the full load displacement, you are looking for
a particular hull speed/length ratio, a particular displacement/length ratio
and probably a prismatic coefficient within a given range. Basically what this
means is that once you set the LOA the draught is fixed if you optimise all
the other variables.

Of course you can do it the other way by setting the draught, optimising all
the other variables and then the LOA will be what it will be to give you that
draught.

Once you set one of your variables your choice of values for the other
variables becomes severely limited.

Of course full load displacement is the most important. Once you have that
everything else tends to follow.

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