Rowing the water out of a rowboat

  1. I'm looking for a hydrodynamic analysis of a concept to design a custom rowboat hull with a smooth tubular bilge that could let water slide up above waterline, then down and out of the boat under the acceleration of the oars. Basically, I'd like to be able to row in choppy ocean swells without having to stop and bail water out manually.

    I'm curious about dimensions: whether a larger diameter bilge tube would be better, or smaller. About whether narrowing the tube would provide any kind of venturi / Bernoulli increase in pressure that could help the water climb above water line before flowing out. I need some help calculating how much acceleration would lift how much water how far, and under what conditions.

    I'm pretty sure that would work, provided the tube was full of water. A smaller tube would then be more efficient, but only get so much water out with each stroke of oars. A larger tube would be more efficient should the boat really get swamped with dozens of gallons of water. (I'm thinking of a boat with enough reserve buoyancy that one could sit and row even if swamped).

    Some details: it would be composite, I'm considering a wave piercing hull shape, but regardless it would be low windage, 18-20 feet long is optimal for speed v drag, sliding seat design, probably around 100 lbs, narrow, and with a low CG for stability (so the bilge would definitely be below water line).

    I'm imagining a flexible hose for the gooseneck, something which could be lifted for higher waves, or lowered to negligible height for smooth conditions (it would slide inside a tube leading to the draining scupper).

    There are other details, and I have a thread on boatdesign.net here which would be a convenient place to give me some numbers / resources. I don't know where to start to figure this out, so any help most appreciated. Thanks,

    << Link deleted by Moderator >>

    *crap the title got eliminated for some reason. Could a moderator change the title to "Rowing the water out of a rowboat?"
     
    Last edited by a moderator: Apr 13, 2012
  2. jcsd
  3. DaveC426913

    DaveC426913 16,439
    Gold Member

    An interesting idea. I like it.

    Are you aware though, that it will not be free? The rower will experience resistance on the oars, making him exert more effort, diverting it away from rowing? (It's the same principle as a generator-powered night-light on a bicycle.)

    Not that it means it's a bad idea, but it raises the question of priorities. I think it's worth it if you could disable it and enable it with the flick of a relief valve. It means you cold of you needed to, bail your boat without stopping rowing.

    I reported the post to get the title changed. (See the [REPORT] button?)
     
  4. Yes, I reported it too, thanks.

    And while it will take energy to move the water out of the boat, it will essentially be free because the work to get the water out is also moving the boat in the desired direction.

    I'm looking for elegance here: pulling on the oars moves the boat. Be nice if pulling on the oars also moved helped water flow out of the boat.

    But I need to know how to calculate how much you need to accelerate water to lift it. And what shapes and volumes would be optimal for a single human powered rowboat.
     
  5. DaveC426913

    DaveC426913 16,439
    Gold Member

    No, it will take more energy. The oarsman will feel resistance on the oars as he rows, (i.e. even if he isn't dipping his oars in the water).
    i.e. with regular oars, he might expend x watts of energy to cover y metres. With your oars, expending x watts of energy will only get him y-a metres, (or to travel the same y metres, he'd have to expend y+a watts).

    Have you ever had a generator-powered light on your bicycle? It takes effort on top of the pedaling effort to move.

    So, no free work.

    What you do get though, is economy of movement and tasks. The rower doesn't have to switch back and forth between rowing and bailing. And that is a genuine savings.

    As for calculations, I would think that there are enough variables that you would do better to experiment rather than worry about equations. I'd be trying different tubes, pump mechanisms and geometries.
     
  6. Sigh. I worry that I'm a poor writer sometimes. So few seem to get what I'm saying.

    There are no 'regular' oars vs 'my' oars. My desire is to AVOID adding a generator, motor, pump or other mechanism that takes additional energy. This is essentially a thought experiment: using ONLY the energy expended to row the boat (which will certainly be more if the boat is flooded), can a hydrodynamic path be engineered which will allow water to flow out as it's inertial resistance would prefer?
     
  7. what dave is saying is that if you are trying to do something more with the oars than just rowing the boat, it is going to take more energy to actually use the oars

    there is no way to get around that
     
  8. DaveC426913

    DaveC426913 16,439
    Gold Member

    No. The energy used to row the boat will row the boat. If you wish to do additional work (such as lifting water), it will require more energy from the human.

    As Bob-of-the-shish points out, there is no way to get around that.
     
  9. However, one might consider that the weight of the water sloshing back and forth also consumes energy and might be put to better use, i.e. by using that movement to evacuate the water and lighten the boat, which of course would see a net drop in energy usage over time.
     
  10. sophiecentaur

    sophiecentaur 14,151
    Science Advisor
    Gold Member

    A standard self-bailer is designed to work at fairly slow speeds and they are pretty efficient. They work on the bernoiulli principle afaik. Very effective and cheap. The non-return valves just need to be checked now and then.
     
  11. DaveC426913

    DaveC426913 16,439
    Gold Member

    Yep. Basically a one-way valve near the waterline.
     
  12. jambaugh

    jambaugh 1,808
    Science Advisor
    Gold Member

    Let's see... Bernoulli's principle says the pressure drop equals 1/2 density times velocity squared. Water is 1000kg per cubic meter. So figure 500Pa per m/s squared. Round g up to 10 m/s^2 or 10N/kg. With water that means a pressure of 10,000Pa per m head pressure. Ah the problem is density independent. You get v^2/2g height of head pressure. Row at 1m/s and you have 1/20 m= 5 cm. Your outlet mustn't be deeper than 5cm. Double th speed and you quadruple the depth.

    You can get further advantage by channeling flow with hull geometry so flow rate at the valve is higher than the boat's speed. That v^2 factor means just a little extra flow rate has a magnified effect.
     
  13. DaveC426913

    DaveC426913 16,439
    Gold Member

    You know, perhaps the OP is barking up the wrong tree.

    An ounce of prevention is worth a pound of cure. Why expend so much active means to remove water from the boat? Why not use passive means to prevent water from getting in the boat in the first place?

    Very high gunwales will cause most waves to deposit themselves back in the sea rather than in the boat.
     
  14. sophiecentaur

    sophiecentaur 14,151
    Science Advisor
    Gold Member

    If this boat is for racing then it will be low profile and it's hard to prevent any water from getting in. Of course, the OP probably refers to an existing boat. Self bailers are magic. They get my vote and they probably perform better than any system we could come up with (first time round at least).
     
  15. Ahh, thanks so much Jambaugh. That's what I'm looking for. Now I need to know what typical acceleration numbers for a fast boat are.
     
  16. jambaugh

    jambaugh 1,808
    Science Advisor
    Gold Member

    If it were me, building my own boat, I'd build a mini-catamaran rower. Stable, sealed decks so no need to bail. Good performance.
     
  17. sophiecentaur

    sophiecentaur 14,151
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    That's fine if you don't mind getting yourself cold and wet. Give me a nice warm cockpit and a saloon where you can get dry and make a cuppa tea.
    Never did understand those barmy windsurfing types - very impressive though!
    Perhaps it's a 'climate' thing.
     
  18. jambaugh

    jambaugh 1,808
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    Gold Member

    Yea, but then again, why row? I'm a toasty cabin cruiser type myself. If I have to forego an engine, at least let the wind do the work. If I need exercise, I can do 16oz curls (of ice cold beer so as not to overheat!).
     
  19. sophiecentaur

    sophiecentaur 14,151
    Science Advisor
    Gold Member

    I get enough exercise rowing out to my mooring. Fighting wind and tide for 10 minutes is enough for me. And I then deserve a sit down and a drink - in the warm.
    Sailing is quieter and smells better.
     
  20. DaveC426913

    DaveC426913 16,439
    Gold Member

    +1

    Nothing like a high freeboard and a tippy dinghy to limber up those muscles.
     
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