Water Ballast for small boats

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

  1. Toot
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    Toot Senior Member

    Here's what I'm envisioning based upon Gilly's and your own inputs.

    Take a large PVC pipe, capped at both ends. Add maybe 3 cups of slurry to it (to make it just a little bit buoyant), let it cure. Then fill it with water. It'll weigh a lot when full of water, but will just barely float.

    Now imagine a clasp, like you have on the trunk of your car- it has two parts, one fixed, and one that is attached to a spring to keep it closed. Attach two or three of those to your keel.

    Press the PVC tube sideways into the clasp and it shuts. Go boating around with a few hundred pounds of watery ballasted goodness.

    When you want to remove the ballast, pull on a few small wires to open the clasp and the "ballast" will float on the surface- not a lot as it still weighs perhaps 55-60lbs/cu.ft., but it'll get to the surface. Now pull the rope attached to the ballast to bring it to the stern. Then unscrew one end of the PVC pipe and tilt it downwards and all the water will run out. Haul it on deck, and bring the boat ashore.

    Total cost: PVC pipe, some .1" thick metal to make brackets, a couple of springs, some slurry, a few nuts&bolts, and a rope.

    It'd be pretty cheap to make, but it's a complex little task. Perhaps a bilge pump would be the way to go afterall....? But anyway, even with the bilge pump setup, you'll still need an air vent line to get air inside. I'm thinking that to prevent backflow, you would have something like a plain check valve. Close the valve, and no water or air is going to run backwards. Open it up and you can start pumping the air in, or the water out- either way.




    Crag- sounds like a perfectly good and simple system for a motor boat. Would be quite the challenge on a sailboat though, I would think. But your description of its benefits is exactly what I am imagining.
     
  2. Toot
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    Toot Senior Member

    Ok, this is along the same lines. I admit it wouldn't be as functional and it wouldn't work once the keel is out of the water, so it would only be good for balancing the boat, not righting it.

    What if you made something sort of like a drogue chute...... But instead of being tied to a line, it's attached to the hull... basically trap a bunch of water in a sack underneath the boat to keep it stable while standing still?

    argh... there's the complexity problem again, I'm afraid. I can't envision a good way of retrieving the "chute" or getting it out of the water without getting very wet in the process.
     
  3. Toot
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    Toot Senior Member

    Ok how bout this.... A little unconventional, but bear with me...

    Twin keels with a space between them. At the aft portion of the boat, you have a tunnel between the keels which slopes downward toward the bow. Then you've got a hinged bracket with a collar at the pivot point for the pipe and you've got the locking bracket near the bow. The collar prevents the pipe from slipping into the water and the front locking bracket allows the pipe to attach solidly before launching the vessel.

    This allows you to stand at the stern and lift the tube up into the boat, allowing the ballast to self-drain as you lift. Then, once the pipe is angled upward into the boat, you pull it up and aft, slowly drawing the pipe as you slide it out of the water.
     
  4. frosh
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    frosh Senior Member

    If this is so good why don't existing boats have it?

    Hi Toot, Do you recall the original stuff you used to put at the bottom of all your early postings on this forum. Here I will remind you. I thought that I could find it on some of first postings but they have all been changed to
    "Are we off topic yet?"

    Anyway you will remember it much better than me!

    Something like --"I don't even know enough yet to understand if I have any knowledge about boats" Sorry but that is not exactly it.

    The thread so far apart from your first posting has been about the methodology to get the water ballast in and out of the keel section as simply as possible.
    Somehow it seems that you and most other respondees have already accepted that the original hypothesis will work, and it now just a matter to determine how to do it.

    I must strongly disagree with the original premise and suggest we go right back to beginning and first try to determine if in fact there is something to gain by incorporating water ballast in the first place.

    I have never tried it of course, and I am not a NA, but I have studied university level physics, amongst other things.

    In my humble opinion the relative bulk (not weight) of the water being pulled through the sea, lake whatever is going to slow the boat down dramatically. Even worse, I dont believe it will really add to stability at all, just increase the inertia that will be needed to be overcome when the hull starts to heel.
    Inertia is only an issue related to the speed that a body changes it's orientation. Sure, with the water keel, and a gust hits the boat it now will respond more slowly to heeling forces, but if crew weight in relation to sail area and moment of force between CLR and CE of sail is insufficient to keep the hull fairly flat in the water, there will be no improvement ultimately to this imbalance with a water filled keel no matter how big it is.

    Don't take me word for it, and check with a NA, but I would be very surprised if he disagreed with my hypothesis. :eek:
     
  5. Toot
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    Toot Senior Member

    The software does that automatically, it's not my own doing, I assure you. I'm guessing it just saves storage space if the computer only needs to "fill in the blank" with "x" rather than save all those words many times over. I changed it because people took that to mean I know nothing of anything whatsoever, and that became somewhat annoying, as I do have a knowledge of structures, aerodynamics, composites, and a touch of metals, but am defficient merely as pertaining to boats in particular. Some people failed to pick up the tongue-in-cheek nature of my statement. So I changed it. Screw it (but not with drywall screws, eh?)

    Again, the first sentence of my first post explains that this is for a slow, safe, sailing boat that is intended for use by children and will spend considerable time standing still. I am not considering this for a long range cruiser of any sort. Neither efficiency, nor speed are of particular concern here. It goes back to the old idea of "everything has its use." It may not be right for 99% of the boats out there, but I'm questioning whether it might be good for a particular application.

    I fully realize that this offers little advantage while underway. However, as I said in a later post, I'm thinking that the moment-arm of the weight at the bottom of the boat may allow a small boat to better accept asymmetric loadings- perhaps even extend the deck beyond the hull to provide more usable space without the cost of a longer or wider hull. Again, this isn't for a cruiser. I'm fully aware that there are better ways of doing this as far as efficiency is concerned. However, in terms of ease of getting the hull in and out of the water, this seems to be a reasonable approach.

    I am primarily questioning the effects of the water-ballast while standing still. On a small boat, obviously, standing far off to one side may cause a capsize. However, when you do that with a water-ballasted keel, as the boat rotates around its CoG (on the longitudinal axis), the water ballast and the person's weight form a moment arm, and so long as the moment arm for the water ballast is greater than that of the person, the boat ought to remain upright. This means it may be possible to extend the deck out farther without increasing the size of the hull. Now, of course, you'd ask, "Why would a smaller hull be beneficial? Why isn't efficiency while underway important?"

    The answer is that this is intended to be a light boat that can easily be lifted in and out of the water, but can give another "fun" option to someone who wants more than a sail-trainer, but is not interested (or incapable of using) a larger, heavier boat. It's a niche market, for sure.

    As to your suggestion that this is wholly impractical and without benefit, I point you to Guillermo's post that gives a specific example- the MacGregor 26 which uses a design based on the very same principles that I am discussing here. Further more, I point you to Crag Cay's post where he describes his personal experiences with a system which is not unlike the one I describe.

    You are thinking very much like someone focused on speed, efficiency, and sailing performance. These three items are of only secondary importance for the application I am discussing. I see your point about inertial-attenuation as being the main thing here. I agree. And you may be right. Nevertheless, I do appreciate your input and am still thinking about it. Thanks!
     
  6. Guillermo
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    Guillermo Ingeniero Naval

    Well....I'm afraid static transversal stability has nothing to see with inertia, but with forms and vertical position of the CG. In fact when studying the static stability curve (the righting arms or GZ curve, or, to content Vega, the RM curve, whatever) we asume heeling is produced in slowly infinitesimal motions (So inertia is taken out of the game!).

    In fact a fresh water tank in the bottom of the boat has the same effect on stability than the kind of ballast tank discussed in this thread. Empty it and the boat will be more tender; fill it and it will be stiffer.

    Of course fresh or sea water, being not very dense, has less effect, for the same volume, than lead or iron, i.e., but for the same weight and CG position it has exactly the same. In a small boat some dozens or a few hundreds of kilos down at the bilges will have a definite effect on stability, absolutely, be them water, lead, or feathers. Of course they'll occupy different volumes, but asuming we can locate equal weighted iron pigs or feathers cushions having both the same VCG, both will have exactly the same effect on static transverse stability: If VCG is low enough they will increase it.

    Of course inertia around a longitudinal axis also increases with the added mass, and, as you point, the effect of this in real life is to slow down roll movement at its beginning, and, on the other hand, increasing the amplitude of the roll angle at the end of the movement. This makes the total angle bigger than what strictly would have corresponded by what the GZ curve dictates. Movement is not infinitesimally low, and inertia plays it's role.

    Cheers.
     
  7. Claus Riepe
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    Claus Riepe Junior Member

    Waterballast? -You are coming late.

    Frosh:
    >>If this is so good why don't existing boats have it?
    <<

    It is there. Swallowboats are already having it in their new raid boats ( www.swallowboats.co.uk), the aluminum 'Innovaar Alife' has it since last year, and another dutch boat is getting ready to show it off at Sail Caledonia next spring. It is in full swing already.

    Claus
     
  8. Crag Cay
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    Crag Cay Senior Member

    ".......If this is so good why don't existing boats have it?...."

    The emphasis there might be on 'so good'. Well, actually it's not that good. Anything with a density greater than water will be better as a given weight will sit lower in the boat.

    HOWEVER, there is nothing as convenient as water if you want to have some ballast in your boat, but don't want to trailer it home at at end of the day. Way more convenient than rocks off the shore, which were the traditional method.
     
  9. Vega
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    Vega Senior Member

    Water ballast is used in racing sailing boats and in some fast cruisers, but not on the keel. It is used where it increases more the RM (not at rest, but while sailing), on the upper side of the hull, on the opposite side of the heeling.

    These boats have two water tanks, and the only one that is full is the one on the opposite side of heeling. It works the same way as the weight of all the crew that sits there when the boat is racing. Weight in that place can increase RM a lot.

    The tanks can be emptied by gravity and they can partially fill by the pressure of the water created by the movement of the boat; a pump will finish the job.

    Of course this is only good for travelling big distances.
     
  10. tom28571
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    tom28571 Senior Member

    Craig,

    There are some Swedish powerboats that use an auto water ballast system to gain stability when still or running slow. They have a longitudinal chamber in a double bottom along the inside of the keel. As the boat picks up speed the water exits thru the stern. Lighter weight for planing and greater stability at the dock.

    Anything that weighs more than air low in a boat is ballast.
     
  11. Claus Riepe
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    Claus Riepe Junior Member

    Crag Cay:
    >>Well, actually it's not that good. <<

    Vega:
    >>Water ballast is used in racing sailing boats and in some fast cruisers, but not on the keel. It is used where it increases more the RM (not at rest, but while sailing), on the upper side of the hull, on the opposite side of the heeling. ...
    Of course this is only good for travelling big distances.<<

    Sorry guys, but you are quite misinformed.

    Waterballast works absolutely brilliant in small boats.

    I am NOT speaking about asymmetric waterballast here.
    This is about keel-symmetric waterballast.
    If done well, that is good for travelling short distances too.

    Any more misconceptions? Let them come.

    C.
     
  12. Crag Cay
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    Crag Cay Senior Member

    Yes I know. I think in another thread I wrote of my experiences in the 70's with the same system in the Avon Seariders.

    My only point in this thread was to try and quantify exactly how good is 'that good' ? It was in response to a statement that said 'If water ballast is that good, why doesn't everyone use it?"

    My suggestion is that there is nothing magical about water as a ballast material, EXCEPT you can move it easily in and out of a boat, move it around a boat and it will conform to any shape of available space.

    However if you don't need those qualities, then there are loads of materials that make better fixed ballast.
     
  13. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    As a rule, concentrate ballast as much as possible longitudinally, close to the lcb/lcg, but spread it out transversely, out in the bilges. In most boats we want to minimize the longitudinal roll period to get over waves and maximize the transverse period for comfort.
     
  14. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Jan Herman Linge is Norwegian :)
    he claimed to have a patent on this I think
     

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  15. FAST FRED
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    FAST FRED Senior Member

    We tried water balast in our 33ft loa 28lwl 10.5 beam Motor Sailor.
    The boat displaces 17000 lbs with 7000lbs in the keel.

    3, 40 gal water tanks are on either side , the difference between carring the water on the "high" side ,low side empty is not noticable.

    Perhaps with a boat with a far lower DL ratio the water would be more effective , but for a heavy offshore cruiser , hardly worth the extra pipe .

    A great advantage tho is the ability to use the 2in Edison pump to empty the tanks easily when chlorene and flushing are needed.

    FAST FRED
     
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