Thank you everyone for all the responses, and humor, and a special
thanks to Jeff for the Team Philips link.
Definitely a thumbs-down response to my suggestion. I agree it is an
unusual combination. Perhaps I should have emphasized the wave-piercing
aspect rather than make mention of SWATH.

I'll try and explain further. Reserve buoyancy is important of course,
but it is a handicap when a large amount of that buoyancy has to be
pushed thru the water. By dedicating only an adequate/optimal amount of
reserve buoyancy to the hulls (let's call them amas) allows them to
efficiently do the tasks they are intended to perform, i.e. stay afloat
and progress thru the water. For a limited class of vessels, I believe
it is possible to make a better arrangement without pushing empty
buoyancy thru the water, and thus improve performance. Such a vessel
might be a coastal cruiser. Hence my post in this forum.

My suggestion was for 100% buoyancy per ama, providing 200% of the
total displacement of the boat, this being adequate (just) and still
confers stability and sea-kindliness. Elevating the main hull by 5 or 6
feet and in a central location provides the remainder of the reserve
buoyancy (but only when the vessel finds itself in conditions that need
it).

Here's a very basic schematic:
m
m
Side view: a ----------------
a
s cabin
s
t ----------------
t

2 amas - 0000000000000000000000000000000000000000000000


Because the buoyancy in the amas is minimal but adequate, the wetted
surface area of the amas combined is not excessive. Consider each ama
as a 3 ft diameter "cylindrical" at 80 ft long, with little rocker, and
50% immersion, equates to approx 750 sq ft wetted surface. This is
only marginally more than an 80 ft monohull (670 sq ft) but its 60+ sq
ft keel makes them almost equivalent in this regard. So wetted surface
area should not be a drawback (in this length range).
Total buoyancy for these amas amounts to 16+16 = 32 tonnes. This
allows an operating displacement around 15 tonnes, allocated equally to
amas and cabin contents/superstructure (5T each), thus keeping CG low
(but 4 ft above water). Frontal area is a mere 6 sq ft per ama, making
an excellent wave-piercer. This is quite an improvement over say a
similar catamaran, since less water is pushed aside and the shortest
distance is thru the waves, not over them.
Lateral resistance of the amas amounts to over 160 sq ft, and although
shallow, there should be little need for a keel. (This also makes for
good directional stability, though could present some difficulty in
turning). Two masts perhaps, allowing smaller sails for easier
handling by crew.

Thanks again,
Sailnut


On Oct 24, 9:53 pm, DSK wrote:
wrote:
Reducing the scale of these military vessels down to regular sailboat
sizes would create a very tender boat since each hull would be only 50%
buoyant. So beefing up to perhaps 100% in each hull would be a first
design step.??
Please explain further. Perhaps you mean the reduced volume
available for reserve bouyancy?

Apart from this pre-requisite, I see no serious drawbacks
to creating a superior performance and wave-piercing catamaran.You don't think that the lack of reserve bouyancy, and the
concomitant stability, is a "serious" drawback? Everybody
else does.

Fresh Breezes- Doug King

...Perhaps I should have emphasized the wave-piercing
aspect rather than make mention of SWATH. ...

A narrow water plane and light weight are key to high speed in
catamarans. The A class cats have been leading the way in very low
volume, very fine hull shapes. They are very much wave piercing
designs. The new C class "Object 2" by Steve Killing is a good example
of the current state of the art in this. Many cruising cats have been
designed with very fine bows and narrow water planes often expanding at
a distinct chine to get reasonable volume in the living spaces. Some
have even had wave true wave-piercing bows extending beyond the main
hulls. However, the trend in dedicated cruising boats has been to
towards wider, simpler hulls. When Alexander Simonis was designing the
Moorings 4500 he did a study and found that Fn's rarely exceeded 0.6 in
the cruising fleet and concluded that ideal cruising cat hulls should
place more emphasis on wetted surface and load carrying and less on
wave drag. His hulls ended up being around 1:8 length to beam and
quite full and round. The advantages of simple tooling, high volume
and good load carrying are hard to argue against in a dedicated
cruiser. In the racing fleet very light, slender, wave piercing hulls
have proven themselves. Sadly, there seems to be comparatively little
demand for cat designs that take the middle ground.

-- Tom.