DRP #2 Propulsion
This topic contains 39 replies, has 11 voices, and was last updated by Anonymous 6 years, 2 months ago.
November 24, 2008 at 7:07 am #757
I will be writing up the summary on fresh water and posting in the next couple of days.
The next topic, propulsion, is actually not very well researched in “the book”. The requirement for propulsion is to slowly move around. For starters we will pick 2-3 knots. There are a number of open questions:
– How is the propulsion done? Kites, Sails, Propellers, etc.
– For propellers, how many? Where? How big? What power source?
– For sails? What kind? How big?
– For Kites? How?
This topic is going to run for two weeks, since in the US, this coming week has the Thanksgiving holiday to sort of distract people from seasteading.November 25, 2008 at 3:57 pm #4334
By mounting horizontal fins on a spring-and-joint device, one can harness waves’ vertical motion to produce horizontal forces. MIT and Hitachi Sozen Corp. have built and tested a proof of concept 20 ton boat that sailed at 2 knots under the sole power of local waves earlier this year: http://tech.mit.edu/V128/N11/long3.html
The improved boat, Suntory Mermaid II, a 30′ catamaran, sailed from Hawaii to Japan from the 18th of March to the 4th of July this very year, demonstrating wave-power as a reliable, credible (if slow: 1.5 knots average) propulsion system: http://www1.suntory-mermaid2.com/english/index.html
See the photos for a good look at the fins themselves: http://www1.suntory-mermaid2.com/english/sm2_press.html
The tech itself is 19th century, but greatly benefits from modern materials, and it works day in day out, plus it dampens heave motion. I’d say it would be very well supplemented by the occasional set of sails.November 25, 2008 at 8:56 pm #4338
Jesrad’s research on Wave-power looks like a great concept, especially with the heave dampening.November 25, 2008 at 10:18 pm #4340
I was wondering how strong currents can get, and so how much speed is needed just to stay in place (provided we cannot simply anchor).November 25, 2008 at 10:43 pm #4342
Looks very simple. If I understand it correctly it´s just a spring loaded flipper that angles up and down due to the wave motions and thus pushes water in the same direction on both the up and down stroke.
I wonder what kind of waves would be neccesary to make it useful, and whether you need to have a vessel that bobs up and down a lot or if it works just as well with a boat/sesatead that has very small vertical movements (like a spar, perhaps).
If we put a lot of these things all around the circumference of a spar and put a computer with GPS in charge of the whole setup, could we use this for dynamic positioning?November 26, 2008 at 2:08 pm #4351
Sails are an obvious choice, being the well developped, most proven of all and low maintenance sea propulsion tech it is, in addition to not requiring fuel. However it adds a constraint on the design of seasteads, that they need to have enough keel surface (fins or otherwise) to grip on the water. I think this surface has to be higher and higher as the speed gets lower in order to keep some efficiency, so sails may be entirely unsuitable to some of the big and round seastead designs.
Any seasoned sailor should be able to give a good estimation of the sail surface for a given displacement target, as well as tips on the best assortment of different types of sails (which may work better with side or back wind) depending on the anticipated travel path through known windws and currents.November 26, 2008 at 2:20 pm #4352
Simple physics lead me to think a wave-powered propulsion system will get most of its efficiency when the boat heaves the least in the waves, which is when the vertical efforts on the fins are small relative to the inertia of the entire thing, so the heave-dampening might not be all that great if we want it to give out a decent amoount of push.
The device should be ideally tuneable to a given frequency of waves so it makes use of the heaving forces affecting the boat to counter the vertical forces on the fins, getting both the most heave dampening and efficiency, though. I think this is what was done on the Suntory Mermaid II, which could explain their putting the fins completely at the front of the boat. Maybe putting a second set of fins at the other end might help, too. Or sets that can be variably spaced, so there’s always half a wavelength or a full wavelength between them ?November 26, 2008 at 7:32 pm #4354
Looked to me like most places the current is 0.5 MPH or less. Some places like the gulfstream can get 3 MPH.
I think if your seastead can do 2 MPH and you plan your route you can be fine.
— VinceNovember 26, 2008 at 7:54 pm #4356
Thanks for pointing out that I had not mentioned wave power.
I suspect that after people start hanging out on the ocean for extended periods, we will see some truly ingenious adaptations. Harnessing wave power is likely to be one of them.November 27, 2008 at 7:32 am #4360
What are people expecting in terms of how much movement seasteads are going to do? In my very rough imagining, the movement is going to be something like:
1) deploy from land to some predetermined location, presumably where some other people also are.
2) stay put for a relatively long period of time.
This suggests that it’s more important to have the ability to do short, relatively fast movements, as opposed to slow long-term continuous movements.
But maybe other people have a different idea of how the seasteads are actually going to be moving around.November 27, 2008 at 8:04 am #4361
There are two basic scenarios:
1) Stay in one place. This scenario just needs to overcome movement due to ocean currents and wind. I suspect that 1-2 knots is good enough for this case.
2) Migratory. This scenario migrates the seastead north and south to stay in “nice” weather. There may be some east and west movement based on the local currents. I suspect 2-4 knots is good enough for this case.November 27, 2008 at 3:44 pm #4363
I think that migratory speeds can be under 1 MPH. They are also going to be with the current and with the wind, so you really don’t even need 1 MPH.
If you are trying to hold a position 50 miles off California and there was a storm blowing toward shore you might need propulsion equal to more than 2 MPH to keep from ending up on the rocks.
— VinceNovember 28, 2008 at 1:41 pm #4368
Agreed that sails are a great idea. They’re known to work well. Some commercial cargo ships have been using sails for many years to reduce their fossil fuel costs.
That said, fabric sails are archaic. A much better sail system is a rigid vertical sail, essentially a wing. An interesting version of this is like vertical venetian blinds, where multiple parallel wings are pivoted about a vertical axis, usually in a horizontal frame. Unfortunately I can’t find a reference handy, but working models have certainly been built already. The Spring Sail used on windmills is nearly the same idea, except that the pitch of the shutters is controlled by spring force instead of manually:
The wings need to be pivoted so they can lay horizontally when thrust is not wanted. When up in the air, they produce lift or drag to move up or down wind.
Here are some general arguments in favor of wing sails by Jef Raskin. Interestingly a flapped-wing also may also make for a very efficient keel-rudder:November 28, 2008 at 3:45 pm #4369
Use whatever boats are available to tow the seastead. Any old boat with an engine should be able to do this to a certain degree, right? Then when you reach a good spot you just drift for as long as you can without getting into trouble, then repeat.
I´m not sure whether this would be just mildly inefficient or extremely expensive fuel-consumption-wise though…November 29, 2008 at 2:33 am #4370
>I´m not sure whether this would be just mildly inefficient or extremely expensive fuel-consumption-wise though…
For most boats it would be extremely inefficient. I started a wiki page on this:
There is not much difference between using an existing boat as a tug boat and putting an equivalent outboard motor on the seastead. If all the propeller had to do was get a seastead in and out of harbor, and the long distances were propelled by kites or sails, then the efficiency of the propeller would not be a big issue. If the long distances are covered using a propeller then it had better be efficient or someone will be wasting a lot of money somewhere.
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