DRP #2 Propulsion
December 4, 2008 at 4:27 pm #4404
Surfing oceanic waves may be the most efficient method of sea travel available, as it is fast and requires no fuel and (for most of the travel) no moving part. However this method of gravity-based propulsion using wave power requires that the vehicle be capable of:
- accelerating to the speed of the wave
- keeping up with the wave under its own power
There is little doubt anything big enough to live on will not be able to sail at the speed of any wave big enough to push it along. That does not mean this method couldn’t be used by onboard vehicles (even though there is little practical interest to it: waves mainly go in one direction only at a time).
Typical ocean waves move at 3 times their period, in knots. Formulas taken from here:
- speed = 1.56 x period (m/sec)
- wavelength = speed x period = 1.56 x period x period (m)
- full height = wavelength / 7 = 0.223 x period x period (m)
We see that ocean waves of average period (4-8 seconds, 25-100 m length) travel at 12-24 knots, a considerable speed especially for a fuel-less propulsion method. The higher the period, the larger the wave and the further it travels across the ocean: excepting internal viscous drag waves only start braking when the depth comes under half a wavelength. Fully developped, ocean waves have a 1/7th slope, or roughly 8 degrees average slope, or a little under double that slope over two 1/8s (front and back) of their wavelength.
The height of a wave decreases with the ongoing drag as it travels, so a surfing vehicle needs to be able to keep up with the wave even using a fraction of the fully-developped wave’s slope: it must be able to advance at at least the wave’s speed while gliding down its (slowly decreasing) slope, and ideally should be able to accelerate to that speed under its own weight within around half the wave’s period (2 to 4 seconds only) off fully developped waves (it may compensate with another propulsion system otherwise). To take advantage of the steepest part of the wave it needs to have a length under 1/8 of the wavelength (6 to 25 meters for exploiting our example typical waves).
With this, we should have all the constraints required for designing surf-capable vehiclesDecember 4, 2008 at 4:31 pm #4405
(just so that people don’t take the previous post seriously)
Waves only travel in one direction, the direction of the wind, and are only big enough when the wind is strong enough to create them in the first place, so wind power will always be a lot more practical as a propulsion method than surfing. But a surfin’ seastead would be a cool sight !December 4, 2008 at 11:07 pm #4408
Pondering the paddle wheel some more, there are obviously some problem areas. For instance, only a small part of it actually contacts the water at any given time. A propeller, by comparison, is fully loaded all the time. And the paddle presents a lot of bulk above the water that you can´t use for other things. It could be a hazard as well, to things floating on the water like boats or people. And a source of noise. The axle is above the water though so there is no hole through the hull needed. I´m not sure this is an actual advantage over a prop though. You can drive those with electric motors in pods outside the hull.
Thanks for the props (pun intended).December 5, 2008 at 11:45 pm #4425
Nerd.December 6, 2008 at 1:01 pm #4427
ThanksDecember 8, 2008 at 4:20 pm #4433
My first thought on the paddle wheel was, “what benefit might be derived from the surface part?”
Rather than thinking of it as an immediate negative, I wonder what that exposed part of the wheel could give us? Could some efficiency be sacrificed and practical gain realized by substituting every other paddle with a fishing net? Could it be used to raise water for washing the deck?
Nothing specific, just proposing a different way of looking at it. I’ve been reading a lot about permaculture lately, and one of the biggest considerations is multi-purpose/multi-function with lots of diffuse cross-benefits and reinforcements, even when it means smaller yields from a specific crop.December 9, 2008 at 4:08 pm #4446
I love the way this man thinks. Polyculture and Permaculture are going to be essential on successful seasteads.December 12, 2008 at 10:55 pm #4475
Kites seem to have trouble in the doldrums. Surprised?December 14, 2008 at 12:13 am #4478
I am not really recommending this, just had a good laugh:May 11, 2009 at 6:23 pm #5951
Would it be plausible to stagger the propellers with propeller-like curent generators? I was thinking that having one unit, similar in design to a wind generator, for each propeller could drastically reduce movement related net energy costs if not negate them completely. Just throwing the idea out there.
And a side thought:
Develop the kite idea into something like the system Jaba’s Sail Barge uses in Return of the Jedi?
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Written by Wayne-Gramlich