DRP #2 Propulsion
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December 1, 2008 at 3:40 pm #4379
The article by Raksin is very interesting. Knowing that a fabric sail gets about 0.8 Cl is very useful, as a high-lift airfoil can easily produce upwards of double this coefficient and even above 2. There is a good explanation of how to select a good airfoil for a given purpose here. Basically, we’ll want a laminar flow airfoil for the keel (which works at close to zero angle of attack at all times) and not for the sails (so we can adjust the angle of attack over a wide range to mitigate drag and lift against wind velocity). Here is a popular java-based airfoil calculator for testing purposes.
The much higher efficiency for both the keelboards and sails might make sails viable even for the larger and rounder seasteads. However in order to make use of an asymetric airfoil some means of flipping the wingsail around is necessary.December 2, 2008 at 3:03 am #4382
Clearly sails are a direct form of harnessing wind power and translating it into seastead movement.
An alternative is to have have a turbine capture the wind power and convert it into electricity with is then used to drive an electric motor that in turn drives a propeller. While there are losses in each conversion step, the losses are small. Using a propeller that is properly sized to move the seastead at exactly the desired speed might be more efficient that a sail though. In addition, if there is no wind, solar power and/or diesel power can be used to augment the propulsion.
Does anybody know what the various propulsion efficiency trade-offs are?December 2, 2008 at 3:05 am #4383
Are large propellers for slow movement an off-the-shelf item or are they too specialized for our application? It would be nice if they could simply be purchased.December 2, 2008 at 12:04 pm #4384
I agree, a small boat propeller would probably be very inefficient for a big and slow vessel like a seastead.
Another idea: What if a boat deployed a large sea anchor (an underwater parachute essentially) some distance ahead of the seastead. Then a winch on the seastead reeled in the wire, pulling itself along. then repeat the process. This way you would be grabbing a large area of water, similiar to a impossibly huge propeller, making for efficient transport.
Or, instead of a boat you could launch the sea anchor from a gun or with a rocket.December 2, 2008 at 12:11 pm #4385
There is a thread on submersible mixers for waste-water treatment on this forum. It´s an established industry apparently. They´re up to around one or two meters in diameter if i recall.
Of course given the slowness of seastead transport even bigger props are probably needed. I wonder if one could build something useful from sheet metal or if that would be too crude and thus wasteful. Like ye olde windmill sails but out of metal rather than fabric.
Then with really big propellers there is of course the problem of getting them to endure high seas without breaking off.December 2, 2008 at 12:18 pm #4386December 2, 2008 at 3:26 pm #4389
Yes, this idea has intrigued me. The trick would be doing it in a fully automated and reliable way. My favorite is a small robot boat out a mile ahead and two sea anchors that take turns going back and forth between the seastead and the small robot boat. Clearly they are collapsed when going toward the robot boat so it does not have to pull too hard.
It is a fun idea, as it might be very energy efficient. But my guess is that we would rather use a large propeller. Note that the sea anchor is sort of holding onto the same water, so once it is moving it is just the drag from other water going past that slows it. So it does have to be much bigger than a propeller which keeps pushing on new water. But that is easy enough to make a large parachute like sea anchor. But the reliability of a propeller seems much better.
— VinceDecember 2, 2008 at 4:16 pm #4390
The large propeller mixer is the best I have found. Getting the motor and propeller all together with the right sort of size and speed makes that very good for us.
You could use a propeller from a larger ship. There are large diameter propellers out there. They are designed for much higher Hp and higher rotations than we would want. They are also going to be heavier than we would like. So these would not be ideal. Could probably get one cheap from a ship that was taken out of service.
There is at least one company that makes custom propellers. So while maybe not off-the-shelf it would not be hard to get.
— VinceDecember 2, 2008 at 4:24 pm #4391
Powered by a nuclear reactor?December 3, 2008 at 1:23 am #4392
A serious company in Germany has already set up kite systems of various sizes for ship propulsion : http://www.skysails.info/index.php?id=472&L=2 , http://www.skysails.info/english/products/. It recently undertook sea trials with good success. The prices seem rather exorbitant, perhaps we could consider a DIY option.
Apart from its obvious application in propulsion, I believe kites have a role in harnessing high altitude power. There are some nascent efforts mentioned on the web to tap high-altitude wind for power generation : http://peswiki.com/index.php/Directory:High_Altitude_Wind_Power.
I think we could simply use the same kite in different configurations for both propulsion and some power generation as previously discussed : http://www.seasteading.org/interact/forums/engineering/infrastructure/harnessing-high-altitude-wind-powerDecember 3, 2008 at 9:27 am #4393
I’d think this solution is basically a linear paddle wheel.December 3, 2008 at 4:51 pm #4395
Powered by whatever power you use for everything else. Diesel with some solar or wind thrown in I suppose. Are you suggesting that this is inherently too expensive?
I figured a paddle wheel, while apparently not very efficient for boats, perhaps could be more practical at very low speeds. It seems a pretty easy way to build a propulsion device with very large surface area. Or, well, easier than building a similar sized propeller anyway.December 3, 2008 at 5:12 pm #4396
Interesting links as usual, Vince. Here http://www.inboardpropellers.com/ they mention the Kort Nozzle http://en.wikipedia.org/wiki/Kort_nozzle , essentially a ducted propeller, which is more efficient at low speeds.
A duct obviously adds complexity and cost but if you are already ordering custom propellers this might be better in the long run. Or perhaps on a seastead it is easier to increase efficiency by using a bigger prop. I imagine tugboats have less leeway in experimenting with huge propellers than a seastead.December 3, 2008 at 5:23 pm #4397
Yeah. It seems a very complex operation compared to a paddle wheel or a propeller though so it is probably stretching the boundaries of practicality a bit…
Maybe if you could make the sea anchor parachute thing self-propelled. Perhaps something like an automated torpedo or submarine towing it out in a collapsed state.
edit: ok, that´s pretty much exactly what Vince suggested one post back. Nevermind…December 4, 2008 at 3:45 pm #4401
I just had this image of a paddlewheel seastead ploding along the oceans at a relativiely high clip (8-10 knots) and the amount of power that would take just boggled my mind. Something much slower, like what was requested at the opening of this thread, would make a paddlewheel more plausible. I apologize for the near dismissal of an idea that has a great deal more merit than I originally thought. Thank you for all of your insight on these boards.
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