Designing hulls for efficient slow motoring

[JonathanCole]

Originally Posted by mackid068

Is submerged legal? (Totally submerged, probably not in small channels)

SWATH is legal.

Well, let's assume neutral bouyancy for our submerged but afloat hull!!

But seriously, anyone ever heard of floodable/clearable ballast tanks being used on a boat in order to have adjustable draft, for those times when water depth is less than optimal?

[JonathanCole]

Originally Posted by Skippy

A trimaran with deep narrow hulls has a small waterplane without the pressurized tanks, and has room for solar panels. I like a mostly submerged hull for motoring, in fact I was suggesting that for the human-powered boat in an earlier thread. Maybe you could even combine the two ideas, with some kind of T-shaped geometry. But you do have to watch out for roll stability, since you're putting flotation down very low.

That's an interesting thought. You probably end up with greater wetted surface than a twin hull, though. The t shape is really a SWATH hull, or am I misunderstanding?

[marshmat]

BillyDoc mentioned earlier about controllers that modulate pulse timing, voltage, length, etc. Solar race cars use a solid-state controller and brushless DC motors; the controller can vary the pulses in forward, reverse and regenerative states based on analog inputs from variable resistors that we hook up to the go and stop pedals. If you want to let your props freewheel in a current when you're at anchor this would be ideal.

On the semi-SWATH thing: I like the adjustible draft idea, but the compressed-air system might take a fair bit of energy to use. As skippy says, the roll stability is going to be difficult in a small SWATH boat. They have great seakeeping in most conditions but when you get to seriously big waves the stability gets difficult. For solar, maximizing deck space is the big factor and a tri or thin-hulled cat (of which SWATH is but one variety) is probably best.

[JonathanCole]

Originally Posted by marshmat

BillyDoc mentioned earlier about controllers that modulate pulse timing, voltage, length, etc. Solar race cars use a solid-state controller and brushless DC motors; the controller can vary the pulses in forward, reverse and regenerative states based on analog inputs from variable resistors that we hook up to the go and stop pedals. If you want to let your props freewheel in a current when you're at anchor this would be ideal.

That certainly is cost-effective if you only have to add some control circuitry to capture otherwise lost energy.

Originally Posted by marshmat

On the semi-SWATH thing: I like the adjustible draft idea, but the compressed-air system might take a fair bit of energy to use.

Using a modern portable compressor of the type used in the construction trade, it takes 6.66 watt/hours and about 13 seconds to produce one standard cubic foot of of air at 40 psi. If you have two 50 foot hulls with a 45 foot 8" id pipe in each hull for a ballast tank that gives you nearly 32 cubic feet . When filled with air you will get added bouyancy of almost 2000lbs. So actually you could make do with even smaller tanks. To blow the 32 cubic feet of water ballast would require about 213 watts and using my nail gun compressor would take less than 8 minutes. Maybe more boats could use this idea in place of fixed ballast.

[Skippy]

"The t shape is really a SWATH hull, or am I misunderstanding?"
No, I was thinking one submerged hull only, with conventional amas for balance. But the SWATH approach is the other picture I had in mind and was getting ready to suggest.

[Raggi_Thor]

Jonathan wrote: and super low wavemaking...

With low speed there is no wavemaking :-)

[mackid068]

What about water ballast? I've heard of remarkable successes with it.

[JonathanCole]

Originally Posted by Raggi_Thor

Jonathan wrote: and super low wavemaking...

With low speed there is no wavemaking :-)

There is always wave making. Even a SWATH hull being close to the surface must be raising some water. It is just a question of whether it can be overcome with the energy source you have. With a solar-electric boat you want to squeeze every last drop of efficiency so that you get greater range and performance. If a 50 foot 10:1 cat hull only raises and average of one cubic foot of water one inch, within in a few miles that wave requires one horsepower or about 750 watts of energy. So to make a motor boat that can go long distances without fuel, we want pure horizontal propulsion and as little water lifting as possible. A narrow, deep, subsurface hull with a flat top or maybe a sharp irregular top edge. Hey this is starting to sound like a fish.

[Raggi_Thor]

Of, course, I meant that in low speeds the energy used to wave making is so much smaller than the friction, so the most important parameter become wetted surface.

[Skippy]

Remember he's talking about slow motoring, not slow paddling. Substitute in "moderately fast sailing", you'll have enough wave drag to make it worth worrying about. And this is a mid-size cruiser, so the surface/volume ratio will be fairly low. I'm not sure how wave drag scales with boat size, but those big cats like Orange II always seem to have high length/beam ratios (20+), so they don't seem to be worried about wetted surface too much.

[rxcomposite]

Originally Posted by JonathanCole

Is there any information out there about hyper-efficient hull forms? I want to build a slow motoring boat that is extremely fuel efficient. I know long slim cat hulls and perhaps SWATH or semi SWATH with a not too small waterplane have been discussed on this site. What if you wanted to make a world record in efficient motoring. What are all the elements, both conventional wisdom and more obscure for designing for motoring efficiency?

The basics of Naval Architecture tells us that a slow boat is efficient if it is short, fat, and light. As we go faster the hull becomes thinner, center of gravity moves aft, and sports a transom much like the planing boat.

Speed will dictate hull form. Wetted surface area is the dominant factor as it is the "wet friction" that slows down the boat. Second is the wave making capability of the boat. A slow boat does not create much wave.

But it seems the theory collapses on the development of newer hull forms. The bulbous bow "extends" the length of the ships, the submarine does not create bow wave, the wave piercing SWATH vessel developed the idea around the submarine. All are designed to be efficient when going fast.

Newer, more efficient boats seems to use this hull form. The trend seems to be very slim lined elliptically shaped wave piercing hulls. Because the hull is so narrow, it is mostly designed as catamaran or trimaran with very small AMA's.

Because of the design, the hull will tip over when used as a monohull. As the powering data implies, it seems to be the most efficient hull but impractical to use as a monohull.

[mackid068]

Slow boats are the goal, so a monohull is probably the goal. What about a light weight dory hull? Check what Ted Brewer has to say.

[SAQuestor]

Originally Posted by rxcomposite

[snip]

Speed will dictate hull form. Wetted surface area is the dominant factor as it is the "wet friction" that slows down the boat. Second is the wave making capability of the boat. A slow boat does not create much wave.

[further snippage]

Hmmm... Perhaps "conventional wisdom" is lagging a bit?

See the attached pdf file and then Google find out more about Nils Lucander's pioneering work.

It seems (from his research) that wave making resistance is the primary resistance that limits hull speed. Frictional resistance from wetted surface are is secondary - at least according to Lucander.

His hull designs - as someone mentioned earlier - seem to indicate the fullest part of the hull about 2/3 of the way aft is more efficient than conventional displacement designs.

But, as always, YMMV.

Best,

Leo

[Skippy]

mk, a slow boat is not the goal. The goal is a boat that goes as fast as possible with a limited power supply. Imagine if you had to design a boat that would go as fast as possible, and the only power source you were allowed was your pet gerbil running inside its little wheel....

[JonathanCole]

Originally Posted by Skippy

mk, a slow boat is not the goal. The goal is a boat that goes as fast as possible with a limited power supply. Imagine if you had to design a boat that would go as fast as possible, and the only power source you were allowed was your pet gerbil running inside its little wheel....

Skippy has it almost exactly right. Except if 1000 gerbils equal one horsepower, we've got more like 10,000 gerbil power to work with. In a pinch we can add another 5 0r 6000, with the backup generator. But this is electric gerbil power with full torque at all RPMs yielding thrust which is 3 or four times greater than internal combustion of the same horsepower according to most electric drive manufacturers.

We want to go as fast as possible, yet maintain the greatest range per horsepower. In other words energy efficiency is primary. Do we need the hull to change shape as the speed increases in order to stay on the peak of the efficiency curve? That is an interesting thought because it makes you think about the cross-over points (of speed) where one hull form would need to change to another.

Another thing. there's the wave you make and the wave made by mother nature. In some cases nature's wave is a much greater impediment than the hull made wave. But for the purposes of the super-efficient hull we are assuming modest inland and protected waters.