Water Ballast for small boats

Discussion in 'Stability' started by Toot, Sep 24, 2006.

  1. Toot
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    Toot Senior Member

    Hey folks. My recent thread on a mini kiddie boat got me thinking.

    Ballast is nice for boats as it lends additional stability, however, of course, it adds weight and on a smaller vessel that's intended to be hauled in and out of the water, the extra weight may not be terribly appreciated. So I got to thinking...

    What if you designed a "system" below the waterline that would allow water to flow in to a compartment through a series of open holes? Perhaps a hollow and water-tight keel with a few holes in it would be the best example. The holes would, of course, add a little drag so it would be wholly infeasible for a racing vessel (besides, water isn't that heavy). However, for a craft where ease of haul-out is more important than cruising speed, could such a system be a viable option?

    On, say, a 14 foot vessel, you could have a 1'x1'x12' box that would hold as much as 750 pounds of ballast and yet would add little or nothing to the haul-out weight. Ideally, you'd also be able to cap the water up in there while aboard so that in the event of a capsize, you'd retain the water to help you get the ship back upright. Maybe you could even just have a capped PVC pipe with a rounded nose hung a few inches below the keel.

    I'm talking about small boats here, obviously- things that come in and out of the water every day. My objective, really, is to provide more stability while at a standstill so that the deck of a small boat can be more usable without a risk of capsizing- perhaps even enabling you to extend the deck of a small boat beyond the limits of the hull itself without resulting in a big splash.

    Has this been done before? Is it absurd? What do you think?
     
  2. l_boyle
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    l_boyle ol' can of sardines

    Look at submarine... They uses water as ballast, it help them to dive. Luxury cruiser uses water as ballast in their hull..
    If you could have a pump the quickly fill the chamber, as well draining it.. Well, why wouldn't it work? Remember this, water in water have no action... Therefore it's netural. But, as uprighting the craft it may stop at the surface of the lake, ocean or which ever you sail in... I would of experiment with a toy boat or something simliar before getting into bigger thing like your own boat.
     
  3. Toot
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    Toot Senior Member


    Your reasoning is flawed and your examples are of expensive craft which are virtually the antithesis of what I am proposing.

    Water-in-water has no buoyancy. However, it does have mass- so it takes work to move it and it resists rotational forces. Fill a 55 gallon drum with water, fasten it to the bottom of a canoe and tell me that wouldn't make the canoe more difficult to tip. This is the same effect as a hydrodynamic keel on a sail boat, except instead of being a dynamic application of force to an ever-changing portion of the water (drag), the water is self-contained, thereby working better at low speed or while standing still (but being less effective at speed- hence my caveat that this would be inapplicable to racing boats).

    Just try lifting a 5 gallon bucket out of the water. It weighs a lot more than an empty bucket sitting on the beach! And if you are trying to lift it with a long stick, the closer the stick gets to horizontal, the harder it will be to lift the bucket (you've got a moment-arm there).
     
  4. Guillermo
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    Guillermo Ingeniero Naval

  5. Toot
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    Toot Senior Member

  6. safewalrus
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    safewalrus Ancient Marriner

    Guillermo you've done it again mate! Superb! :cool: the only thing is I think I'd like to bable to dump the water whilst standing still in the water - you may not always be able to 'steam' at five knots, would a two way valve on the bilge pump be sensible? pump either the bilge or the ballast? (or even shift it around the boat?) :confused: or am I being too complicated? I don't think so - your views would help! (not just Gilly's but yours as well Toot)

    Mike
     
  7. Toot
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    Toot Senior Member

    For low cost and simplicity, I think I would have the holes facing downward, with a small bit of tubing running up to the deck. With the tubing open, the air would vent and the ballast tank would fill with water. When you want to empty it, just hook up a bicycle pump and just keep pumping till you see some bubbles coming up to the surface. Once you've got it filled with air, just clamp off the hose and the air will stay there.

    Of course, you can design a fancier and more complex system, but for my purposes, I'm trying to keep it simple.
     
  8. Guillermo
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    Guillermo Ingeniero Naval

    Yes, why not? The only thing is you'd need a rather big bilge pump to be able to pump out ballast in a reasonable time.
    Let's say you want to empty the thing in 10 minutes; you'd need a 51 lts/min (806 US gall/hour) pump for that.
     
  9. Toot
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    Toot Senior Member

    Remember, it's not just about removing the water, you've also got to get air into there. Air will displace water, so I say pumping air will be easier than pumping water. Air weighs less. :)
     
  10. Guillermo
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    Guillermo Ingeniero Naval

    I'm afraid it will take you a lot of time and sweat to do that, if the amount of water is bigger than a few litres! (just think about the air pumped at every stroke of a bicycle pump). An electric pump or a devoted hand bilge pump such as a Whale Gusher fits better to the job.

    P.S. The Gusher 30 pumps out 117 lt/min (31 US gall): http://www.whalepumps.com/marine/product_list/7/48/
     
  11. safewalrus
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    safewalrus Ancient Marriner

    ?

    Still prefer the bilge pump rather than a 'bicycle pump' you wouldn't want your air vent too big would you? otherwise it could become a water vent with a decent head of water

    Your bicycle pump would be a bit small! and from experience a good bilge pump with a large capacity is needed for any boat (never mind the jokes about frightened men with 2 gallon buckets!)
     
  12. Toot
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    Toot Senior Member

    I suppose you are right. What about a "piston" that you can pull forward inside a long ballast tube.

    Behind the piston will be an air tube running to the surface. As you pull the piston forward, air fills the aft portion of the tube and the water ahead of the piston is displaced.


    Or a method of tilting the ballast tube upward to get air into one end, and the slowly lift it up above the waterline.

    Or maybe make it an external ballast tank on a tether and make just slightly buoyant. Pull a release switch to send the ballast floating to the surface, then pull the tether line to bring it to the transom where you can empty it and bring it aboard. Then, before the next voyage, just affix it to the keel and you're ready to go again.
     
  13. Guillermo
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    Guillermo Ingeniero Naval

    Have to leave now. Cheers, guys. :)
     
  14. safewalrus
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    safewalrus Ancient Marriner

    cheers Guillermo, catch you next weekend!

    Toot mate it would appear your getting away from your first idea - keeping it simple - you don't want to spend too much on this SMALL boat do you?

    Mike
     

  15. Crag Cay
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    Crag Cay Senior Member

    The original Avon Searider RIBS used water ballast to make them stable when stopped. The whole volume of the deep V hull below the deck would flood through two small holes near the bow. As you began to move forward again all the water would flow out of the larger hole in the lowest point of the transom.

    It provided excellent stabilty when at rest and they were great for sitting out at sea and 'watching' (if that was what you were paid to do). But other manufactures, as well as many operators of the Seariders themselves, found you needed much less power to get onto the plane, if you didn't have to drain the V hull first.

    You could also get up faster with a shallower V, so soon loads of competitors to the Searider appeared claiming 'more speed, better acceleration, with less power and cheaper' which is a hard marketing pitch to compete against. Especially when all you can really offer is 'better stabilty when stopped in extreme conditions'. But I appreciated it.
     
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