Designing hulls for efficient slow motoring

[cyclops]

Can we agree on long and skinny in good weather and rough water at the speed of 5 to 8 mph? It must have good harbor handling?

[JonathanCole]

Originally Posted by cyclops

Can we agree on long and skinny in good weather and rough water at the speed of 5 to 8 mph? It must have good harbor handling?

That sounds about right to me. Maybe long thin twin hulls (cat-style), and possibly a submarine ballast tank hull in between for adjustable draft (could be a pipe with a fine bow and stern). Should be able to turn on a dime and parallel park directly sideways. Actually I like the idea of GPS control and maintaining station without an anchor. Although using energy to go no place is not too efficient.

[mackid068]

Well, a small boat: say, .25 feet, is perfect for 1 gerbilpower. So slow is NOT the idea, well, with such a limited power supply, you can't go extremely fast. Upper range of speed would be approaching 12 kts AT MOST, with lightest possible hull and least friction wave form

[mackid068]

Water-ballast is often successful, but this boat CANNOT be completely submerged at any time (unless it is sinking ) to comply with maritime laws, however, as long as there is a fair amount of boat present above the water (I'm not sure about how laws pertain to Europe, but I'm pretty sure that in the US a boat that completely submerges ie submarine is heavily regulated). Torpedo-shaped/trans-ocean pedal boat styling sound appealing?

[Skippy]

"Do we need the hull to change shape as the speed increases in order to stay on the peak of the efficiency curve?"

Yes. That's one reason it's common to make the aft end of a hull wide & flat, and the bow deeper & narrower. In addition to the issue of heeling & weather helm, on a dinghy at least, you can shift your weight forward to go slowly in displacement mode, and then move aft when you're able to plane on the flat stern.

[JonathanCole]

Originally Posted by mackid068

Well, a small boat: say, .25 feet, is perfect for 1 gerbilpower. So slow is NOT the idea, well, with such a limited power supply, you can't go extremely fast. Upper range of speed would be approaching 12 kts AT MOST, with lightest possible hull and least friction wave form

OK that sounds great. We shoot for 5-8 kts cruising speed and 12 kts when we kick in the over drive.

So with a 16 meter length we use maybe a 20:1 LB, twin hull design, 0.5 Cp, fine, narrow and deep with the widest section 2/3 of the way aft. I have been thinking about a 6 meter beam, but is there some optimal hull spacing that will create waves at the resonant frequency of the hull and actually help the efficiency?? Anybody know about that stuff?

[JonathanCole]

Originally Posted by mackid068

Torpedo-shaped/trans-ocean pedal boat styling sound appealing?

Ya got any pictures?

[Skippy]

"is there some optimal hull spacing that will create waves at the resonant frequency of the hull and actually help the efficiency??"

Wow, that's interesting. The only way for the bow waves to partially cancel out would be to stagger the hulls so the waves are out of phase. But I wonder if that's part of the reason a cat does better than it's hull speed so easily? Even a little above the hull speed, where the wave from each bow is bottoming at the corresponding transom, it's still peaking somewhere near the transom of the other hull! That doesn't apply to the outboard sides of the hulls, and of course also not when a sailboat is flying the windward hull. But still, it would be interesting to know if that really made a difference.

The beam is usually set to balance pitch stability against roll stability, and to trade off pitch stability against turning ability and strength/weight.

[grob]

"is there some optimal hull spacing that will create waves at the resonant frequency of the hull and actually help the efficiency??"

In that case more hulls are better, wave cancellation has been studied in quite some depth on this site.

http://www.cyberiad.net/hydro.htm

see this page for images
http://www.cyberiad.net/wakeimages.htm

and in particular this page for a fine animation
http://www.cyberiad.net/waketet.htm

All the best

Gareth
www.fourhulls.com

[dionysis]

Keep in mind though, that the more stuff you have above the water, the more resistance you will have from contrary winds. This may lead towards a more compact and streamlined shape: a monohull for open water. In canals a cat may be ok, since they would be protected from the wind.

[Skippy]

Following Grob's 1st link:
Small, Low Drag, Solar-Powered Monohulls and Multihulls

[BillyDoc]

About 40 years ago my Father was doing some research for the US Army trying to come up with a means of reducing the “Whup-Whup-Whup” sound that a helicopter rotor makes. He reasoned that the noise was due in part to the fact that when the chopper is moving forward the blade traveling forward is traveling faster than the one on the opposite side. So, he was looking for a simple way to change the lift characteristic of the rotor blade airfoil dynamically without changing the angle of attack.

He did a very simple experiment. He attached an elastomeric cover to a length of plywood about half a meter in chord and 1.5 meters long so that the edges were air tight, but the elastomer was free over the top surface of the plywood. At one end of this “wing” he affixed a venturi (I believe from an old carburetor) and piped the vacuum from this to the area under the elastomer. At the other end he attached a simple “bleeder” valve. He then attached the whole thing to the side of his car with a small angle of attack, out the right window, with the venturi on the outboard end facing forward, and got a colleague to drive.

He found that by adjusting the bleeder valve to some particular setting and leaving it so, the effect of the balanced venturi and elastomer was to create what looked like a “perfect” airfoil shape over the top surface of the wing which would self-adjust with speed.

Remembering this has made me wonder if this sort of approach might somehow be applied to a boat hull to reduce wave-making. Would there be an advantage to a “soft” hull form? It does seem to work for a porpoise. Any ideas?

[cyclops]

Soft fish shapes are running in contests. Eff. is very high along with speeds.

[mackid068]

What about those companies that will rent you a boat to travel along canals in europe?

[JonathanCole]

Originally Posted by dionysis

Keep in mind though, that the more stuff you have above the water, the more resistance you will have from contrary winds. This may lead towards a more compact and streamlined shape: a monohull for open water. In canals a cat may be ok, since they would be protected from the wind.

Here is an interesting idea from the Australian mathemeticians designing for a solar boat race (the link above that Skippy noted):

• Using the solar panel as a lifting surface in the wind to reduce displacement may or may not be allowed under the rules. We view it as a drag reduction mechanism rather than as a propulsion device which is prohibited under the race rules.

In our 16 meter length, 6 meter wide twin hulled boat the roof will be covering nearly the full footprint (yes, we walk on water here). So if there were a way to shape the panel structure into a wing like lifting device with a self correcting angle of attack, one might lift the hulls part way out of the water while underway and reduce drag/wetted surface that way. It probably would not have to lift much in order to make a noticeable difference. But the wing might have to be able to morph its shape depending on the direction of the wind. Probably impractical.

By the way, those Australian mathmeticians seem to be backing up the general consensus emerging in our thought experiment, that a cat is the way to go. Especially when you compare vessels with equal solar panel carrying capacity. The monohull just doesn't cut it here because in order to get the same beam and footprint as a cat, the monohull needs to be huge and heavy.