[PCW] Air cushion between catamaran hulls

[East Cape Marine Ltd.]

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 at eastcapemarine.com
www.eastcapemarine.com




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