Catamarans - symmetric, assymetric - can someone explain the difference?

I've been looking at cat design recently and notice that sailing cats usually have symmetical hulls but power cats often have assymmetrical hulls. Why is this? Is there any particular performance reason?

And following on, for something low powered (even human powered) which could be better? perhaps we might say for the same given displacement?

 

The choice between symmeritc and assymmeritic is a speed-length-seperation distance tradeoff.

Power cats tend to be much longer than their beam and high speed would force choked flow between the hulls if they were symmetric. Therefor, the inner waterlines have less curvature to relieve this problem.

Sailing cats tend to have beams approaching their lengths. In fact because driving force increases with the beam squared, some racing cats have beams greather than their length. With this wide seperation and the transfer of floatation volume from windward to leeward, the effect of the flow around the windward hull is minimal on the leeward hull. However, some sailing cats have assymmetric hulls to generate lift off the leeward hull and despense with center/dagger boards alltogether.

For a human powered cat, I'll be willing to bet that the hulls have a L/B >> 10 and a seperation distance of >> 10 B. In that case, unless the seperation distance is much less than 0.5 L, there is not much influence between the two hull and symmetry really won't matter.

 

Here's a thread I started some time ago which may throw a bit of light on the subject http://www.boatdesign.net/forums/showthread.php?t=14269

 

It seems to depend on what part of the world you are in. US = asymetrical power cats, Australia= symetrical power cats, South Africa = combination ("bobcat"), as examples. Some say that asymetrical demihulls tend to lean inwards in turns, some do, some don't. What is true though is that a vertical surface submerged in water creates no vertical lift only drag, I favor symetrical cats (unless they are foil assisted, that's another game). My experience is that there are plenty of efficient sail cats but very few efficient power cats (they usually run at the wrong Froude number). The sail cats are mostly symetrical.

What you want for a low power, low speed cat is a very slender, round bottom, symetrical hull and good separation of demihulls, possibly with an out-rigger hull rather than a same size second demihull. Or why not just go for a kayak.

 

In a very small (less than 14') sailing cat, is symetrical or asymetrical best for turning response? And can you have a symetrical hull that uses no dagger/centerboard?

Thanks,

Mark

 

Its not the symmetry that matters for turning, its the rocker shape, assuming you don't have a daggerboard which is always the best for turning.

Yes you can have a symmetrical hull that uses no dagger/centerboard.

The G-Cat sailing boat is a good example of a production sail boat with no boards or keel of any sort, that also has symmetrical hulls.

Gareth

 

Gareth,

Thanks for the reply. I am not a designer, so could you please explain or send me to references on what rocker shape is?

Thanks.

 

The bottom from stem to stern is rounded instead of flat. ie. if you set a hull with rocker onto a flat surface, it will rock fore and aft.

 

To put it in laymans terms, when you don't have something to pivot around like a board, you need to make the underwater shape something that is easy to turn or spin in the water.

Obviously a sphere is the easiest shape to "spin" around in the water. By contrast Catamarans have long thin hulls that like to go in a straight line and so are hard to turn, by having alot of rocker you are turning the underwater shape into something that more closely resembles a sphere, so it is easier to turn.

Also the a large rocker can concentrate the centre of effort into one spot a little more like having a centre board.

Sorry I don't have references. Can anyone else give a better explanation.

Gareth

 

Gareth,

That was a great explanation. I get it. Thanks.

Mark

 

Just another example of course are the “deep V” designs of James Wharram