dumping bilge through oar power

Discussion in 'Hydrodynamics and Aerodynamics' started by dcnblues, Jun 5, 2011.

  1. dcnblues
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    dcnblues Senior Member

    Thanks science, but again, I don't want to put additional energy into the oars. I'm looking for a solution elegant enough to use the existing energy the rower puts into the boat. That means using the existing acceleration of the boat to leave the bilge water behind, rather than having to carry it with you.

    In a self bailer, that's easy, but for a narrow and fast hull design, I can't afford to raise the center of gravity higher than the water line (which is why self bailers work so easily). Such a boat needs significant width to be stable, and I don't want that kind of drag.
     
  2. science abuse
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    science abuse Junior Member

    I think that removing the water is going to take energy, no matter what you do. In an efficient shell, there's not a lot of energy being wasted. To use acceleration, you need to produce acceleration, which means an impulse of additional energy. The nice thing about an oar actuated pump is, it works at any speed and any energy input level.

    Here's another option:
    http://www.youtube.com/watch?v=9BXxL9gztO8
    You could likely work something out to drain the water while you're airborne. ;)
     
  3. Submarine Tom

    Submarine Tom Previous Member

    How about just keeping the water out in the first place?
     
  4. dcnblues
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    dcnblues Senior Member

    No offense, but I find this kind of response frustrating. I'm designing my own boat, and while light, it won't be a shell. And if the bilge has water in it, then each stroke of the oars is sending the hull slamming into the inertial resistance of the water in the bilge. That's massively inneficient. And I'm producing acceleration with each stroke of the oars. It would be both elegant and efficient and a great design challenge to use ONLY THAT ENERGY to let the water flow out of the boat.

    I think it can be done, with a good enough design and execution using my concept. This forum is packed with expert boatbuilders, and no-one has given me a good reason why it can't. Viking North gets it, and made some realistic objections, but even those good points I can argue against. For example, in conditions where the boat is generally staying dry, but one freak swell breaks into the boat, dumping gallons of water in. I'm imagining being able to change course, head directly into the waves, and let their energy and my pull slide those gallons back into the sea, then resuming course when the boat is dry again.

    I think the key is going to be having a gooseneck in the tube that goes above water line, a scupper down close to the water line, and either a weighted pivot to keep following water from forcing it's way in, or an electronic sensor / servomechanism doing that job. It's doable.
     
  5. dcnblues
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    dcnblues Senior Member

    Now, finally, we're talking. This is the kind of response I was looking for. I'd love to know some more resources on how you got these numbers.

    I'm talking about building a custom boat, so I can design the stern anyway I want, and the scupper/throughull in any shape or any location I want.

    I'm guessing that a tube that shrank in size would indeed produce a venturi boost that could be used to get the water over a gooseneck above the waterline. But I don't know how to make the calculations. Let's say I design a bilge where the lowest point becomes a ten foot long tube (on an 18 - 20 ft boat) with a diameter of 8 inches. I can't imagine a good pull on efficient oars won't get that water moving with enough force to lift a cup of water (or more) over, say, an 8 inch high gooseneck. But how much to narrow the tube? If at all? I don't know how to begin to crunch these numbers. How about a bigger bilge tube? More water, more mass, more potential energy to lift up and out at a faster rate. I don't know...

    I'm also guessing that the flatter the arc of travel of the tube, the more efficient the energy transfer (every change in direction subtracting from the rearward acceleration efficiency).

    However, an ideal system would also have a flexible hose with variable height above waterline. Higher following waves, higher gooseneck to prevent inflow of seawater. Flatter water, negligible height of gooseneck.

    You'd need the gooseneck. The more I think about a weighted pendulum or even an electronic servo, the harder time I have distinguishing between acceleration from the oars, and acceleration from a following wave. What sensor could distinguish between these two? A following wave hits the stern, accelerates the boat forward. Bilge water slides toward the stern. You don't want your scupper opening in that condition...

    Easy enough to put a sensor on the oarlock, and send a signal during the middle (it would need to be tunable [and reliable]) of the power stroke. hmmm...
     
  6. science abuse
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    science abuse Junior Member

    Ehhh righto, good luck.

    The fact remains that the additional weight of the water requires additional energy to remove it: You accelerate with every stroke and decelerate between.
    You'll accelerate less (with the same energy input) with each kilo of additional bilge water.
    Thus, your ability to remove it via acceleration is reduced as the need increases.

    At some point you will inevitably need to do more work; it took Poseidons' energy to put the water in the boat, it will take energy to get it out.
     
  7. dcnblues
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    dcnblues Senior Member

    I question your physics.

    The oars accelerate the hull. I don't think much differently whether the boat is dry or has 20 gallons inside. Once the hull slams into the inertia of all that bilge water, it will slow down for sure, but avoiding the need to accelerate the bilge water is what I'm describing.

    If the water has a path of least resistance available through a hole in the stern, it will simply stay where it is as the boat accelerates away from it. There is no "additional energy" required. The bilge water's inertial resistance will lift it up and out of the boat. In fact, as I've said, the original oar power is all that's required.

    There's a timing issue as well. The peak of the power stroke on the oars needs to coincide with the moment of highest elevation gain of the wave under the boat. You need to piggyback on the lift energy of the wave (which imparts a lifting vector to the bilge water as well).
     
  8. Milehog
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    Milehog Clever Quip

    A series of one way flappers spaced along a trough in the bilge? Hinged at the top would make fabrication simple though they'd need a grate above to prevent damage.
     
  9. Submarine Tom

    Submarine Tom Previous Member

    I think you're on your own for this one dcnblues.

    Let us know what you come up with.

    Why hasn't this been done before?

    Best of luck.
     
  10. dcnblues
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    dcnblues Senior Member

    Beats me. At first thought, there's no reason these principles wouldn't work for a shallow draft powerboat, or sail. There'd have to be serious trust in a scupper / throughull so close to water. I think that's probably why...
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    This is an expanded version of my post http://www.boatdesign.net/forums/boat-design/designing-fast-rowboat-14250-92.html#post544068 -

    The amount of water that has to be moved out of the boat in a surfing situation is a lot more than I originally envisaged. Auto-bailers are good at moving large amounts of water quickly but require more speed to work than a rowboat is likely to achieve.

    Using the acceleration of the rowing cycle is an ingenious concept, but one problem is the transom of a rowboat is raised above the bottom. Even as little as 3" requires a speed differential of at least 4 fps to overcome even with 100% efficiency and I don’t think any rowboat would have that much speed variation. These numbers come from this simple formula relating gravitational acceleration G to the speed V acquired as result of falling height H : -
    v^2 = 2GH where G = 32 ft/sec^2

    Some kind of venturi device to exchange speed for pressure somewhat on the principle of a steam-powered water injector may work to eject water, but I am not hopeful as there is so little speed to work with. The principle of the water injector is described at http://en.wikipedia.org/wiki/Injector and is identical in principle to the hydraulic ram effect described in dcnblues' link.

    As far as the energy required to make this work, there is no such thing as a zero energy way to do this, energy has to be provided and it must come from the rower ultimately. So selecting the method is simply a question of finding something that works, if anything, and if there is more than one candidate then the most efficient can be selected. The universe is so arranged that only politicians can get free lunches . . .

    I prefer alternative approaches. Like keeping the water out in the first place, as kayakers do with their fully decked craft and cockpit skirts, although that would not be compatible with a sliding seat rowing system. A conventional self-draining cockpit using a raised floor is not an option for a rowboat; to raise the floor the center of gravity increase would reduce the boat’s stability and to restore that would require increased beam and wetted surface that would use significantly more energy, not a good idea for a surfing boat.

    Using the angle of the boat while climbing up a wave is also ingenious. I’m not sure of the physics involved, whether the boat effectively has a forward acceleration component from its angle WRT the gravity vector or is merely not decelerating fast enough to avoid reaching the top of the wave. There are interesting and complex water motions under the surface of a wave that on analysis might apparently yield free energy, but in practice I believe it will be subtracted from forward momentum of the boat.
     
  12. Submarine Tom

    Submarine Tom Previous Member

    I had another thought that might frustrate you.

    A small paddle wheel spinning a little pump.

    No water in the bilge, no load, little resistance.

    You could also run a tiny generator for LED nav lights.

    Again, no load, no resistance.

    Silly I know, but maybe...
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    - many years ago slow reving generators were made inside the hubs of cycle wheels. The were called dynahubs in the UK and were made by manufacturers like Sturmey-Archer who also built gear hubs. Sometimes one can find one of these generator hubs on an old bike; they are very efficient and have hardly any drag.

    Caveat; do not dissemble one to clean it as the permanent magnet will lose its power unless a soft-iron keeper ring is used.
     
  14. philSweet
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    philSweet Senior Member

    Okay, maybe something along these lines. Keep the volume in the bilge area as small as possible. if you need a foot well, then make a foot well, but that is all. Everything else sufficiently raised to dump out a large aft scupper in a hurry. That leaves you the footwell left to drain. Run an enclosed pipe aft out the footwell to an up pointing tee. On the up side of the tee, install a caged ball float backflow prevention device. It lets air in or out, but won't let water up and out because the water floats the ball up onto the seat. Aft of the tee, reduce the pipe size to about half and put in a vented riser loop. Just high enough to do the job. Maybe swivel it so you can adjust the height according to conditions. Then exit the pipe out the stern at the waterline or a bit above.

    The way this works is that air is sucked in the tee, partly filling the tube to the footwell. When you pull on the oars, the water in the bilge pipe accelerates and the air escapes. When the air is gone, the ball closes and the water hits the reduction and is hammered over the loop and out. I'd start with 1/2 inch PVC and neck to 3/8 after the tee and see how it works out. you could recover some pressure if the pipe expanded nicely after the riser loop.
     

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    Phil: sounds like an idea to try. It could be tested with a bowl of water in the bottom of the boat to see if it works before modifying or rebuilding the boat. I'd combine it with partial decks to reduce the amount of water getting in . . .

    Getting back to the spray skirts that kayakers use, a lot of them prefer to use a boat sock that fits inside the boat, sealed to the cockpit rim, which limits the amount of water that can get in if the boat rolls. It is cooler than a full skirt.
     
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