Sorry, here it is without the other messages just in case it didn't go
through, Pat

With the Manta 44 powercat, stability was a point of emphasis with the design.
We found for a serious cruising boat the perceived benefits of a flybridge
design did not out weigh the benefit of a lower center of gravity obtained
without the flybridge. The big problem with Powercat design is you are
fighting the opposing forces of interior headroom, bridgedeck clearance, and
freeboard. For an offshore boat the emphasis should be high stability and
bridgedeck clearance and low freeboard, unfortunately you can't sell boats
people cant stand up in, so it is a balancing act and compromise. As cats get
higher off the water, the VCG becomes a factor in ultimate stability,
additionally the higher the freeboard, the more leverage for wave induced
capsize. Another factor of course is beam, many designs today are made narrow
for the sake of getting into marinas. Underwater profiles can be a factor in
the ability to slide or skew down a wave, many designs are using keels are fin
skegs to protect propellers which detract from this. The Manta has a very low
VCG compared to most designs, because of lack of flybridge, and all fuel
located below the soles. With a 2:1 length/beam ratio, and very shallow
underbody, stability is maximized, and the ability to slide down waves
enhanced

"With a 2:1 length/beam ratio, and very shallow
underbody, stability is maximized"

Pat,

How does length affect stability?  If a cat has a 2:1 L/B ratio and it's
40 feet long, how does its stability change if it's stretched to 50 feet
long, yet keeping the beam constant, assuming everything else remains
constant?  Better, worse, or no change?  Is there a standard way to
calculate that?

Thanks,

Bob Deering
Juneau Alaska

To expand on Bob's question, I'd be curious what other factors effect the beam decision (that is beam of the boat, not each hull).  It appears there are stability rules, obviously interior room/layout considerations.  Are there any hydrodynamic considerations? e.g. is there some distance between the centerlines/hulls below which you shouldn't go either as an absolute number or as a ratio relative to hull waterline beam?

I'm surprised that the PDQ and Scimitar are ~ 16' beam, it would seam that if you constrained it to 14' you'd open a lot of slip options at cheaper prices.  Is there something magic about the 34' length that they wanted to stay under?

In this article:
http://www.pdqyachts.com/ver2/pdfs/otheVoices/SAILdesign06.pdf
Melvin says:"The heavier a cat becomes, the wider each hull needs to be. But performance improves as the hulls get longer and skinnier. The same holds true for centerline-to-centerline beam (the overall beam measured from the centerline of each hull). Long and narrow is faster than short and wide. The narrower you can design the boat (while preserving overall stability) allows the boat to be lighter, faster, have a better motion, and so on."

Why wouldn't I design a 42' x 14' cat?

Robert Deering deering@ak.net wrote: "With a 2:1 length/beam ratio, and very shallow
underbody, stability is maximized"

Pat,

How does length affect stability?  If a cat has a 2:1 L/B ratio and it's
40 feet long, how does its stability change if it's stretched to 50 feet
long, yet keeping the beam constant, assuming everything else remains
constant?  Better, worse, or no change?  Is there a standard way to
calculate that?

Thanks,

Bob Deering
Juneau Alaska

Power-Catamaran Mailing List

Here's a fairly simplified explanation of terms such stability,
righting moment, buoyancy, gravity et al:

http://powerboat.about.com/library/weekly/aa061903a.htm

--Georgs

Georgs Kolesnikovs
Power Catamaran World
http://www.powercatamaranworld.com

" Regarding your question about stretching a hull and if the stability
would
increase.  The displacement of the boat would increase due to the
additional length.  This higher displacement multiplied by the same beam
between hull
centerlines would result in  and increased righting moment."

Cort,

Thank you for the explanation.  It makes sense to me in a static
situation, or perhaps for a sailing cat where the tipping force is from
the sails.

In a power cat I'm assuming that it's not the wind that will tip her
over unless the wind is really extreme and the boat has a lot of
superstructure (sail).  Rather, as Dennis implied in his recount of his
Pacific trip, I'm thinking it'll be wave action that would do most of
the damage, either from a breaking wave or from burying the bows.

If a cat is lengthened, its displacement increases.  But at the same
time its hull surface area (excuse me for not using the proper naval
architecture terms to describe this) also increases, giving more area
for a wave to apply force to.

Take my question to its extreme - stretch the Manta 44 to 144 without
changing the beam.  Have you still made it more stable?  Or is there a
point where it became less stable as the L/B ratio increased?

Again, thanks for helping a simple guy like me understand this.

Bob Deering
Juneau Alaska