Water Ballast for small boats

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

  1. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think he means we have to make sure the tank is full.
     
  2. safewalrus
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    safewalrus Ancient Marriner

    Guillermo/Raggi - you got it, either fill thwe tank up or keep it MT (empty) or you going to get all sorts of interesting things happen (I ain't talking about 'flume' tanks or other patented systems here - after all it was originally about a simple cheap system to gety a bit of ballast into the boat)

    Gerrry cans (5 litre orange squash containers) fill em with water before you go and chuck it away as and when! bit like using bags of beach I guess save it can be reused, trouble is it fills the bottom of the boat up with containers, nowhere for your feet!
     
  3. mflapan
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    mflapan Junior Member

    A caution on water ballast

    I had an 'interesting' experience in a water ballasted trailer sailer that I would like to share with you.

    The boat was being tested for 'boat of the year'.

    The test started by operation under motor with the builder highlighting how fast the boat went (though she trimmed excessively).

    We stopped and the builder advised that he was filling the ballast tanks in preparation for sailing by opening a valve. This was noteworthy in itself as there were warning notices on the boat stating that the boat should not be operated without the waterballast tank full. Is it possible that he had operated unballasted under motor to impress with the best possible speed?

    I queried how he knew the tanks were full. He replyed "By waiting 5 minutes". I mentioned the potential for free surface to remain but he assured me that it was not a problem.

    In gusty 20 knot conditions, we hoisted the main. I was down below at the time as the cockpit was full. I felt the boat heel more than I would have expected. To find out what was going on, I stuck my head out of the companion while standing on the stairs. As soon as the jib was set, she went over on her beam ends with the boom in the water. As she heeled, I stepped onto the galley bench and then the inside of the topsides. My colleagues in the cockpit did not fare as well with them clinging to whatever they could grab and one dropping into the water.

    We let go the sheets and fortunately she rounded up and very slowly righted. After a rather long and awkward period of near silence as we motored back to the marina, I spoke with the builder. He believed that the cause had been the ballast tank not being completely filled.

    I subsequently had further dealings with this boat and it is my belief that the water ballast was not the only reason for its stability problems. As is so often the case, there was a chain of causation.

    However, it did highlight the potentially unforgiving nature of water ballast if not properly applied, especially on a vessel operated by persons who might not have an understanding of the risks involved.

    Regards
    Mori
     
  4. Claus Riepe
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    Claus Riepe Junior Member

    Mori:
    An interesting story, thank you for telling it. It is absolutely creditable, and it is awkward, especially for the builder and/or designer and the crew.

    But it's not the end of waterballast in small boats, because that is an absolutely superb new technique, when designed and handled with appropriate care and skill. But that applies -or should apply- to small boating generally.

    But you are right, waterballasting adds complexity, compared with fixed built-in ballast, there are several more grips to do, handles or levers to operate until things are set, but then it works just as well as fixed ballast.

    And even better.
    For one, because you of course have the freedom of choice, at any moment, to have ballast in, out or in again. With every different course, wind drop or increase you can always weighttrim your boat for best performance and/or safety. Fixed ballast does not give you that choice. You are boring safe -and slow- all the time.

    But there is yet another very important reason for waterballast vs. fixed ballast, where waterballast actually adds safety over fixed ballast.
    If you are familiar with the stability testing details of ISO 12217, you know the dilemma.: The RCD (ISO) appreciates stiff boats, but the stiffer a boat is under normal circumstances, the more difficult it will become to right it from complete inversion, which is also a point in the focus of the ISO.

    The only perfect answer to the conflicting ISO requirements is actually .... waterballast.
    That -alone- can both ascertain self-righting stability of the upright sailing boat, AND it can also de-stabilize the inverted boat so that it can be righted again easily, meaning with minimum crew.
    I do not wish to go into the details here, naval architects should grasp the idea themselves very quickly, it adds quite another dimension to the possibilities of small boat design under the restrictions of ISO.
    It is the way to go in the future.
    I am quite sure your guy in Australia who did not get it right first time yet will require very little time to come out with a much improved -maybe even foolproof- solution.
    He clearly made a bad mistake (free surface, sloshing ballast water), but he did not disprove waterballast in general.

    Claus
     
  5. mflapan
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    mflapan Junior Member

    Duct keels

    Dear Claus

    Thank you for your reply to my posting.

    Yes, water ballasting opens a lot of possibilities. My comments were not to stifle innovation. Rather, they were to highlight that water ballast arrangements need to be designed and operated with care taking into account the relevant risk factors that might include operators with limited knowledge and potentially very serious consequences if they are not used properly.

    While on the subject of water ballast, I saw at the last Sydney Boat Show a number of aluminium recreational fishing boats from New Zealand of about 6 metres length that were fitted with a duct keel. The duct would be flooded when the vessel was at rest. The duct was arranged so that when underway, the water would drain from the duct. The concept was that when stationary (a common occurrence on a fishing vessel), the added weight of water would reduce motions and when underweigh (often incorrectly spelt underway), the boat would benefit from the reduced displacement. It sounded like a good idea, though I did not see it actually in action.

    Has anyone had practical experience with boats having this confuration?

    Best regards
    Mori
     
  6. Crag Cay
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    Crag Cay Senior Member

    This was a feature of all the Avon Searider RIBs from about the mid 1970's onward. I think it was one of the first features built into the RIB concept being developed at Atlantic College in the 60's.

    It certainly improved 'at rest stability', but drastically increased the power needed to get the boat onto the plane.
     
  7. Claus Riepe
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    Claus Riepe Junior Member

    Well, not quite true. Such a waterballast system does not increase the power needed to get the boat onto the plane, that is a bit simplistic. In reality, the boat can plane with the same power, but it cannot do so within seconds, as it first must shed the ballast water. That can be done through a self-bailing device which starts to suck from around 5 knots upwards through the suction created by the speed of the hull through the water.
    Meaning, once the boat starts to move above 5knots, the ballast water load continually decreases, and parallel to this the power required to plane the boat is reduced. After a few seconds -or a minute- more the boat reaches the same planing speed as the unballasted boat, and with the same installed power.

    Claus
     
  8. Crag Cay
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    Crag Cay Senior Member

    Well maybe, but as a professional user of these boats back then, I stand by my original statement. In reality, the load carrying of these boats was quite restricted and always required more power to get the same load on the plane than the identical boats that had the water ballast system glassed off. It's no coincidence that this system did not find universal adoption either across all of Avon's RIB range or with other manufacturers. Its specific advantages came at a price.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    Toot's objective is to improve stability in a small boat at standstill so the deck is more usable without a risk of capsizing by using water ballast.

    If water ballast is added the CoG is lowered but so is the freeboard. The ballast must be in the lowest part of the hull or pendulum stability is impaired; when the hull rolls enough to bring part of that ballast above water the rolling resistance stiffens rapidly; however for a a typically shaped boat (i.e., not a raft) the people on the deck have already slid overboard.

    At more moderate roll angles stability increases slightly due to the increased waterline beam as the hull, er, sinks. That assumes no tumblehome of course. However, a safer and easier method is to attach pool noodles a few inches above the water line to improve stability, or add floats on outriggers. I don't recommend sailing in this condition however.

    In the real world of course water ballast is added inside the hull but as a mind experiment you can think of adding water either inside or outside the hull. Add it inside and the boat drops in the water as above. Add it outside and nothing much happens apart from changing the lines until the ballast breaks the surface.

    Water-ballast has some advantages on hulls that are designed from the outset as such, but this isn't something you can retrofit.
     
  10. benmww
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    benmww Junior Member

    I'm guessing this stability came from the high polar moments from the ballast tanks on opposite sides.
    did this system work to an equivilant extent when beam on in a sea way?
    also what was the ballast/displacment ratio of this system?
     
  11. Claus Riepe
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    Claus Riepe Junior Member

    Ancient Kayaker,
    You have made the two key points:
    1. The water ballast really kicks in most efficient -apart from it's passive effect from the hull lying deeper in the water- as soon and as much of it breaks over the outside water surface as the boat heels.
    and
    2. For that, the boat really would have to be designed for Waterballast from the outset.

    Meaning, the waterballast should be distributed as near to the outside as possible, as close to the outside water level when on even keel, and not be sloshing to lee.

    When done like that, it really does miracles to the stability, my current boat is proof.

    Claus
     
  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    Thanks for the confirmation Claus. I was theorizing and have no experience of (deliberately) admitting water into a boat. I would guess that boats that use water ballast are shallow draft and beamy, unless the ballast is movable. I think it would be useful in a catamaran, unless one wanted to get the weather hull out of the water to reduce skin drag.
     
  13. BOATMIK
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    BOATMIK Deeply flawed human being

    Hi All,

    The great New Zealand designer Jim Young has a number of cabin trailer yachts that are designed for home building that use water ballast.

    The system is gravity run and simple.

    Put the boat in the water - wait for the water level to appear in a plastic tube that is attached to the ballast tank - then turn a valve to keep it there.

    Before the boat is put on the trailer the valve is opened and the water pours out as it is winched aboard.

    His boats sail well enough to burn off quite a few of the high tech trailer sailors.

    He also designs innovative and very quick high tech and medium tech trailer sailors.

    But the beauty if his water ballast system is that it is so simple and has been used successfully for around 30 years in hundreds of boats

    Michael Storer
     
  14. Claus Riepe
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    Claus Riepe Junior Member

    Water ballast must be lifted above surface.

    Yes, that is the point, and that is why waterballast has a special relevance for small boats. It only temporarily kicks in as righting ballast when being lifted above the water surface in a heeling boat, it 'vanishes' as righting ballast when under water, then it is just mass.
    I myself had difficulties understanding this mechanism. What explains it is the 'potential energy' model: Every mass lifted from and above the surface represents 'potential' energy, which you can harvest from as it sinks back.
    Somewhat topheavy stuff this 'energy theory' explanation, I admit, the beauty is how simple it works in reality and practice.

    In my boat, a Swallowboats 'Searaider', as it's both a rowing and sailing racer, the use of waterballast is temporary and arbitrary. In addition to a slower gravity feeding you can quickly scoop the ballast water in through opening a reverse-positioned self-bailer, and you can sail it out again through opening three correctly positioned ones. This system works very quickly, through the scooping effect when taking the water in. Which is important, as the boat goes through moments of reduced heeled stability while the ballast tank is only part-filled, so the ballast water can slosh about and especially to lee. Therefore it is vital the filling -or emptying- process takes as little time as possible.

    Can't wait to use it again as soon as spring -hopefully- arrives back here again.

    A Happy New Year and good sailing to all in 2007!

    Claus
     

  15. BOATMIK
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    BOATMIK Deeply flawed human being

    Howdy Claus,

    There is perhaps one more thing to grasp. The change doesn't occur when the ballast water is lifted above the level of the body of water outside.

    ANY rise in the boat's centre of gravity as a whole is what makes the difference.

    And using water or any other ballast to reduce the height of the centre of gravity means that heeling makes the centre go up.

    If the water ballast is used to move the centre of gravity up high - like at the top of the mast - then as the boat heels the centre of gravity moves down so it makes it easier to heel the boat.

    So the water ballast is usually there to lower the centre of gravity!

    Best regards

    Michael Storer
     
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