What are the advantages of assymetrical cat hulls

[JonathanCole]

I have been looking through various threads relating to power cat design and have noticed quite a bit of talk about assymetrical cat hulls, but without much information as to their advantages and disadvantages. I ran across this quite attractive live-aboard power catamaran houseboat from Desert Shore
( http://www.desertshore.com/index.html ) that uses assymetrical aluminum hulls. The use of assymetrical hulls would probably change the shape and amplitude of the wave between the hulls. Are there other reasons to use assymetrical hulls?

[Deering]

Yes, the tunnel wave is modified - more water pushed outside than in. The other reported advantage is that the boat will lean into high speed turns like a monohull as opposed to retaining a flat stance as a symetrical hulled cat would.

Something to consider for that houseboat.

[D'ARTOIS]

A fan**************tastick houseboat. However, Jonathan, for Europe no go! Too high, wide and therfore useless. Not moveable, transportable etc.
Beautiful to have such a "Watercondo" on lake Geneva, but a bit unrealistic too.

[JonathanCole]

D'artois,

I'm not interested in having such a houseboat in Europe or anywhere else. I am interested in understanding the advantages and disadvantages of different approaches to designing electric powered catamarans, which must be very efficient in order to be practical. The cost of applied design intelligence is a great investment. The amount you can save over the life of a vessel in reduced fuel requirements simply by designing intelligently is substantial. But there doesn't seem to be anyone here who knows the answer to the advantages and disadvantages of assymetrical cat hulls. The houseboat in the picture is not meant to be a racing vessel I would presume, so there must be more than high speed cornering to account for its assymetrical hulls.

[Leo Lazauskas]

Originally Posted by JonathanCole

D'artois,

I'm not interested in having such a houseboat in Europe or anywhere else. I am interested in understanding the advantages and disadvantages of different approaches to designing electric powered catamarans, which must be very efficient in order to be practical. The cost of applied design intelligence is a great investment. The amount you can save over the life of a vessel in reduced fuel requirements simply by designing intelligently is substantial. But there doesn't seem to be anyone here who knows the answer to the advantages and disadvantages of assymetrical cat hulls. The houseboat in the picture is not meant to be a racing vessel I would presume, so there must be more than high speed cornering to account for its assymetrical hulls.

Essentially what I wrote in another thread...

Some people claim that there is some small drag reduction, but I've never seen any good experimental results demonstrating it. It's not clear that there is any significant resistance reduction by using flat inboard surfaces, especially after induced drag is included in the total resistance.

Prof. Ernie Tuck at the University of Adelaide recently gave a paper on this topic. The paper is at:

http://internal.maths.adelaide.edu.a...s/vortex04.pdf

What he found (and I have verified numerically as well) is that asymmetric demihulls might be beneficial if the demihulls are not at their optimum spacing for a particular speed. If the demihulls are located near their optimum position, it is unlikely that you can reduce the drag by using asymmetric demihulls, or by using toe-in or toe out.

Regards,
Leo.

[JonathanCole]

So if there is no drag reduction, does it make no difference if the demihulls are symmetrical or assymetrical? Perhaps there is an advantage in simplicity of construction? This aluminum twin hull from DesertShore appears to be completely flat on the inside. and quite flat on the bottom. Any comments about such a design for a houseboat? If the boat is not meant to exceed 12 knots would such a design be optimal?

[Deering]

My comment on high speed cornering for a houseboat... tongue in cheek. C'mon. If you had asked specifically about reduced drag for an electric drive in the first place you might have received different answers.

The photo you posted looks pretty obvious to me - that design is intended to reduce labor costs of building in sheet aluminum, not to minimize drag at low speeds. If you want to minimize drag resistance at low speed you go with minimum wetted surface, i.e. a cylindrical cross section. At the low speeds (especially with electric drive) and protected water you'd run such a boat, wave making between the hulls will have negligible effect.

[Leo Lazauskas]

Originally Posted by JonathanCole

So if there is no drag reduction, does it make no difference if the demihulls are symmetrical or assymetrical? Perhaps there is an advantage in simplicity of construction? This aluminum twin hull from DesertShore appears to be completely flat on the inside. and quite flat on the bottom. Any comments about such a design for a houseboat? If the boat is not meant to exceed 12 knots would such a design be optimal?

I was only speaking about reduction of the wave drag component. As Deering says in another post, reducing total resistance is the key. Sections that are (roughly) semi-circular would seem to be better in that case.

I'm not sure what the designer's intention is in the DesertShore vessel, so it's a bit hard to comment on the design. (The hulls look a little like skis to me).
I also can't comment on construction costs, or "simplicity" of construction, as I am a complete ignoramus on such matters.

Leo.

[tspeer]

Originally Posted by Leo Lazauskas

...
Prof. Ernie Tuck at the University of Adelaide recently gave a paper on this topic. The paper is at:

http://internal.maths.adelaide.edu.a...s/vortex04.pdf

The link doesn't work for me - would you repost it without the "...", please, Leo?

[Leo Lazauskas]

Originally Posted by tspeer

The link doesn't work for me - would you repost it without the "...", please, Leo?

Sorry, Tom,. Here is the link to Ernie Tuck's publications:
http://internal.maths.adelaide.edu.a...uck/index.html

The vortex paper is number 175, near the bottom of the page. You might also be interested in number 173, "Generalised Induced Drag".

Leo.