Reduction of side windage on an electric catamaran

[JonathanCole]

I am working on the design of a solar electric live-aboard catamaran.

Such vessels must use energy very efficiently in order to have enough juice to meet the conditions encountered.

One important issue is windage. Headwinds can be somewhat mitigated by minimizing frontal area, smoothing all surfaces, and following aerodynamic principals as much as is practical.

Tailwinds are an energy advantage.

The issue for discussion in this thread is side winds. My original design included openings in the sides of the boat between the cabin and the hulls. (see attached sketch). Side winds are not simply an energy issue but also a control issue when energy is constrained. It seems to me that reducing side windage would be an advantage. The question is what would be the best way to incorporate pass-through openings between the hulls and the cabin. In the case of my boat, the hulls will not be used for living space, but only for liquid storage, and battery stowing. I have also included a more recent sketch of the boat.

Anyone have any ideas on how to approach this design issue?

[yokebutt]

Jonathan,

If you want to get cute about it, you could give the cross-section a gentle bulge, like a venturi, but I doubt it would generate a lot of lift even in strong cross-wind.

Yoke.

[SailDesign]

You may find it more beneficial to leave the skirts as a full section. The windage associated with grills and gratings is far higher than that for a flat plate. I'm not sure of where the cut-off is in terms of percentage of area, however, so some reearch may help.
Steve

[Alik]

SailDesign is right, small openings produce more drag than plain surface.
You have only very few means to influence windage drag, but probably You should consider daggerboards to prevent excess yaw?

[nero]

When testing my cat model. I noticed that the crosswind would push the boat forward as well as in the direction of the wind. There were no daggerboards on the model at that time.

Aerodynamics is about directing the flow of wind and the forces created from that flow.

[JonathanCole]

Interesting comments. In light of the comments by Nero, SailDesign and Alik, perhaps it would be worth considering adjustable vertical flaps in the openings that would convert windage into forward thrust. Depending on the angle of the side wind, they could automatically adjust their angle to convert the vector. It sure would not be difficult to do mechanically. Any thoughts? I would also be interested in any leads on where to research this subject.

Thanks,

Jonathan

[yipster]

Jonathan,
i think it was in the swath thread that Tom Speer mentioned a book about windage i still want to get someday, maybe its in there?
dont think there is much advantage in lift or thrust to get from the openings
but can imagine howling and perhaps (savorius) windmills bild into them?
closed sides may not only give better windage but also usefull inboard acces to the battery's.

[JonathanCole]

I looked in the SWATH thread but found no mention of a book. In any case, the point of having greater onboard access for batterries stored in the hulls is definitely a consideration. Although, the batteries can be stored in the connecting struts and on deck.

I think fitting windmills to such openings would not gain you much since the power in the wind varies as the cube of the wind speed and proportionally to the swept area of the rotor. r is the air density. (w = 1/2 r A v3) You are not going to get high wind speeds so low and the opening limits the size of the rotor.

So if you were to convert energy it probably makes the most sense to redirect whatever winds could be channelled into such openings to directly help propel the boat as a sail boat does. Since it has been stated here that a smooth sided hull has less windage penalty than openings would have (although I believe that to be too general a statement to apply to all situations), then if the openings are filled with vertical shutters, then the smooth face can be achieved if they are closed.

In the right circumstances, with a wind straight from the side and shutters deployed at a 45 degree angle, this must convert to thrust because of the deflection factor. If the hull sides were shaped to form a concentrating duct to the openings then the wind hitting the entire side of the hull could potentially be involved. If there is a windage on the flat faced hull of say 1 meter x 12 meters, that is quite a bit of area. A 12 square meter sail would certainly add thrust when encountering a stiff wind. Maybe with low wind speeds so close to the surface, this would be negligible. On the other hand, the entire side of the boat would tend to act as an air dam, increasing the density and velocity of the air moving through the side opening. At the low speeds of such a boat (< 12 kts) I don't believe that the aerodynamic drag introduced by open vanes would offset the wind energy captured by such an arrangement.

If the wind is a 3/4 tailwind such vertical axis airfoils could also capture useful power from the wind. This is potentially a way to combine windpower directly with photovoltaics without having the sails over the boat that put the PV in shadow. Or maybe a magnus effect sail would work in this situation.

Has anyone ever measured the wind that passes between the hulls of a catamaran? That might power a small wind generator and be in an out of the way location.

[JonathanCole]

Anyone interested in understanding more about the power in the wind can check out:
http://www.energy.iastate.edu/renewa...-08_power.html

[Plumbtex]

I don't know about "slots" but if you drive a truck load of pipe it will use less fuel if you tarp the front, than ifyou leave the pipe open , makes me think that overall "smoothness" is more important.

[yipster]

Jonathan, from the swath thread i was refering to: "According to Hoerner's "Fluid Dynamic Drag", a sharp rectangular shape can have its drag cut by 80% with a corner radius that is only 20% of the height of the body. That still leaves 60% of the flat side untouched."
once looked that Hoerner book up, still have to buy it

[tspeer]

Hoerner is probably the best place to look, as he has empirical data for all kinds of situations. They are aimed at aeronautical applications, but they might get you in the ballpark. There may be some ESDU data sheets that might be relevant.

Unfortunately, for the kind of application you have, these methods won't be capable of showing the difference in minor design details. This is why companies spend the big bucks to do wind tunnel tests. You're looking at separated flow around the struts, etc., and it takes really sophisticated CFD to predict drag from first principles for configurations like this.

A possiblility might be to visit a university with a low speed wind tunnel. Perhaps you could work out a cooperative project with a prof and students to do the testing. Instead of using the same old standard model for an undergraduate wind tunnel lab, you might be able to convince them to use your model. The students would get valuable experience and you'd get some data.

[yokebutt]

Jonathan,

Go find yourself a copy of "Life in moving fluids" by Steven Vogel, It may have some things of value to you, but in any case, it's informative and very funny.

Yoke.