[PCW] Air cushion between catamaran hulls

[Tim]

Georgs,
              A few years back we built racing hulls for a client.
We strunge these hulls apart using aluminium girders and a trampoline,
(in order to test the speed versus width using a 50hp outboard.)

Needless to say we were wet most of the time and this distracted
our testing so be bolted on hardboard to the girders.

Two things happended, we were dry and the second we immediatey
noticed a difference in speed, in the absence of the nose not digging
into the waves, especially down the wave.
We also seemed to virtually alleveate the broaching tendancies a we had
before.

Maybe this showed that there is a bit of a cushion, as we always believed, 
shown
in may cats when  she sneezes coming off a wave.

Tim Jordaan


----- Original Message ----- 
From: "Robert Deering" <deering at ak.net>
To: "PCW List" <power-catamaran at lists.samurai.com>
Sent: Wednesday, March 04, 2009 5:29 PM
Subject: Re: [PCW] Air cushion between catamaran hulls


> Georgs, I agree with you.  I sure miss Malcolm's analytical explanations 
> of
> catamaran dynamics.  This is an issue that needs more factual analysis and
> less subjective claims by boat marketers.
>
> Henry, I'm not saying there is zero cushioning effect from the air in the
> tunnel, but I do believe that it is negligible with the boats, and at the
> speeds we're talking about.  I suspect that your experience with the ride
> improving has more to do with the boat transitioning to a semi-planing
> condition and the hull rising higher above the waves.  I experienced that
> effect often in my planing catamaran - often the best ride was above 30 
> kts.
>
> Sealubber, for the cruising/recreational catamarans we're talking about 
> here
> I doubt that air injection would help much.  Most  planing cats have 
> limited
> planing surface area as compared to a monohull, so injecting air would 
> tend
> to worsen that situation, making it harder to stay on step.  The
> displacement cats aren't running so fast that air injection would seem to
> offer much performance advantage.  A racing hull is a different story, and 
> I
> would expect that air injection is used where it provides an advantage.
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