stepped hull cat

[small cat]

Perhaps someone can help me with this question. It's my undestanding that the dry smooth ride of powered cats can be attributed to the cushion of air trapped between the hulls. I've seen pictures of racing cats with steps vented to the outboard side of a single hull (asymmetrical hulls) but I don't think I've seen a symmetrical deep vee cat with steps. Is this because the venting of the step (inboard) will adversely affect the air cushion that the boat is running on?

[gonzo]

Cats don't ride that dry. I think the air cushion is more of a myth. At the usual speeds and with the displacement of the boats the air under the tunnel has hardly enough lift. The most important thing that makes a cat's ride smooth is the small beam to length ratio. Racing cats are very different from the commercially available ones in power to weight ratio. The center of the deck, at very high speeds, does produce some lift. The "sea sled" design is a better example of a tunnel that produces lift. It works at lower speeds that a cat because it is narrower and lower aft. It was banned from competition shortly after it came out in the 20's. The US Navy tests in the 90's show that the design is equivalent in performance to a hydroplane wihtout the inherent unstability.

[JPigott]

Gonzo,

That is a unique design. I can see how it develops lift at low speeds. It's almost like a jet engine concept except the thrust creates hydrodynamic lift.

http://ivb-boats.netfirms.com/index.html

[Jimboat]

SmallCat: It would not be really factual to say that the dry smooth ride of powered cats can be attributed to the "cushion of air trapped between the hulls". The tunnel hull design does not really create a so-called "cushion", but rather, when properly configured, generates aerodynamic lift from deck and tunnel roof surfaces. This aero lift is enhanced due to the close proximitiy of the boat's "wing" to the water surface, which allows for more efficient lift (called 'ground-effect') and a much improved hydrodynamic lift from sponsons (and centerpod, if it exists). Since there is less wetted surface, there is less hydrodynamic drag, and less impact from water. The twin-hull configuration also adds substantial lateral stability that a vee-bottom cannot attain.

As for steps, there are powercats with stepped running surfaces using both symmetrical and asymmetrical hull shapes. The "venting" does not significantly affect the pressure in the tunnel or on upper "wing" surface at all, and thus does not materially affect overall aerodynamic lift. It can, if properly located and dimensioned, improve hydrodynamic drag during changes in dynmamic stability (accelleration).

The other thing to consider is that the running surfaces of tunnel hulls are usually fairly low deadrise, and so can often generate some fairly "wet" spray patterns, especially in heavy water. Spray rail design on both inboard and outboard sides of sponsons can be very important in the design of power catamarans.

It's all quite interesting, actually. Check out the website or the book on performance powercat design, or the design software for performance analysis. The book discusses both symmetrical and assymetrical hulls, and also step design - in both vee-hulls and tunnel hull designs. The software addresses hull design and step configuration also.