Re: [PCW] Air cushion between catamaran hulls
I think the air cushioning thing is a matter of degree, all cats trap air
between the hulls. The pressure goes up with the square of the wind speed and
the lift will depend on the area of the bridge deck. Yesterday, we were
running our 34' PDQ straight into 30-kt winds and I opened her up just to see what
would happen, we were showed 19.5 knots on our GPS. The ride was
exceptionally smooth considering the 3-4' chop. There was no doubt that we were getting
some cushioning effect from the air trapped between the hulls.
But the fact that we've owned this boat for three years now and have only
observed this effect a few times means that it's really not a big factor for a
boat like this. It is, however, a huge factor for those go-fast powercats
running at 40-60kt speeds.
Henry Clews
aboard Sno' Dog - in Stuart, FL (enroute back from a 3-week Florida-loop
cruise)
www.snodoglog.com
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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.
Put a different way...
A catamaran running at speed through waves has a lot of very complex forces
and interactions going on between the hull and the water (and the air).
Water is some 800 times denser than air, so the effects of the water on the
hull are going to easily drown out any effects by the air. A boat operator
running his boat in those conditions simply does not have enough of a
controlled experiment going on to be able to isolate the two effects such
that he can make any objective claims.
It would certainly be an interesting experiment to set up and perform, but
I'm not smart enough to know how that could be done.
Bob Deering
Juneau, Alaska
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@ak.net
To: "PCW List" power-catamaran@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.
Power-Catamaran Mailing List
Over the years I have investigated the "Air Cushion" theory with some of my
power cat designs. I have tried varying the height of the tunnels to allow for
less airflow through, which in turn "should" compress the air between the
hulls and in theory offer some lift or dampening of vertical acceleration with
a cushion of air. The problem associated with having the tunnel lower to the
flow of water is wetted surface area on the tunnel itself. As soon as a wave
passes through the tunnel it wants to suck the boat down to the water and
greatly reducing speed. Also in certain wave conditions a wave passes through
the tunnel the compressed air has nowhere to hide and so it is blown out the
front of the tunnel in a cloud of mist, AKA "Sneezing Effect"
In this same research I have found that the tunnel shape has more of an effect
over the boats performance than the height of the tunnel from the water. It is
my opinion that no mater what size the cat is or what speed it does, that the
shape of the tunnel has more attributes of a wing and that it provides lift.
IMHO the sensation of a cat using the hulls to compress air is simply the
tunnel acting like a wing and proving lift. This can be demonstrated in most
cats at a speed of 5 knots in a 50-knot head wind.
I have also found that lifting the tunnel in my faster offshore designs that
the airflow is cleaner out the back of the cat. I feel this helps to keep the
tunnel dryer and reduce wetted surface area and increasing the overall boat
speed. By lifting the tunnel I have also found that the dampening sensation is
also attributed to the increased internal freeboard buoyancy of the hulls. I
feel it is a slightly better solution than relying on the theory of compressed
air for a comfortable ride.
I do not discredit that the air compression does not play some part in the
performance of a cat. But I would only rate it as having 20% effect on any of
my designs.
Noah Thompson
Director / Designer
East Cape Marine Ltd.
Office: 77 Dunlop Rd. Te Puke.
Mail:
Po Box 705
Te Puke
New Zealand 3153
Phone: +64 7 573 8101
Fax: +64 7 573 4099
Mobile: 021 11 00 984
info@eastcapemarine.com
www.eastcapemarine.com
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