fresh water ballast

Discussion in 'Sailboats' started by drnick, Dec 27, 2007.

  1. drnick
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    drnick Junior Member

    i am refitting an IOR 50' to cruise from the USA to NZ and in the process i need to add tankage. the thought has occured to me to add about 1200 litres of fresh water tankage per side around midships with 600 litres of fresh water carried on each side which is able to be transferred to the high side as needed ie aprox 1200 kg on the weather rail to reduce heel upwind. after searching the site under water ballast i found only one thread which looks at water ballast more directly:

    http://www.boatdesign.net/forums/showthread.php?t=9979&highlight=water ballast

    here the suggestion is to use about 25% of the boats ballasted weight as water, my keel weighs aprox 9500 lb giving me aprox 2375 lb of water to add. also the suggestion is to have the water ballast centre of gravity only half as high as it is in width away from the boats centre line. i have more room up high under the deck than i do further down in the hull.

    i am attaching a pic which shows the pipe cot berths amidships, waterline is below the lower cot. the questions in my mind relate to the boats stability. the first question is how the water ballast will behave in the event of a knockdown? the second question i have is how the boat will behave with the water split between the tanks port and starboard - how will this affect its propensity to roll downwind?
     

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  2. Kay9
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    Kay9 1600T Master

    The problem with wet ballast is the free surface effect when your tanks are either not full or not empty. I think your thought of transferring from one side to the other, will cause one of your tanks to be 1/2 full and the sloshing of that water might make your boat very uncomfortable.

    Im not a NA so I will let those guys talk on this subject some more if they wish, but you might want to think on this a bit.

    You could perhaps baffle the tanks enough to reduce this effect to a degree, but Im not sure.

    K9
     
  3. drnick
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    drnick Junior Member

    k9, 1200 litres total which is 600 litres per side when the boat is not heeled. when heeled it will all be put to the high side. baffled tanks are a must and the water would be distributed between several tanks on each side. a half empty tank is something that most boats will encounter so this would not be completely a unique situation however the shape and position of the tanks may be bigger determinants of comfort. but for me this is all speculation as ive never done this neither have i studied contemporary yacht design to see how far outboard fresh water tanks are reasonably placed on cruising yachts, or there shape or volume.
     
  4. Kay9
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    Kay9 1600T Master

    Drnick, dont take what I said wrong. Im certain with some good thought and maby some help from a naval engineer you can get this done. But the thought of 300 liters of water at 6 LBS per Liter sloshing in a tank on the high side of your sailboat would give you a moment of pause. Thats 1800 LBS on the high side of your boat my friend. I dont care how many tanks you put in in.

    Now if you only intend to fill one tank to 600 liters and keep the high side empty all the time, no sweat. But it sounds like you would like to use this water for drinking and such. So Im guessing here, as I cannot read your mind, but Im guessing, you want to fill both tanks to cap. ie. 600 liters port and stbd. and then as you use down your water, ballast to the low side of your boat. This is going to cause free surface in the tank/tanks on the high side of your boat. If the tank/tanks on the high side of your boat are half full that you will have, potentially 1800 + pounds moving rapidly fore and aft 4-10 feet and side to side 2-6 feet depending on your tank size. That would give me pause. If it dosnt you, then dont worry about it.

    My $0.02

    edited for spelling

    K9
     
  5. alan white
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    alan white Senior Member

    Whenever you add deck level ballast, the keel ballast is likewise reduced in righting effect. The degree to which the keel ballast effect is reduced depends upon distance of the two masses from the roll center, the further out having the greater effect, and the angular position of the masses.
    In this sense, the movable water ballast system isn't just nice to have on hand----- you HAVE to use it because if you don't, it works against you.
    To get around this problem, you might store ballast below the cabin sole instead of dividing it equally between the two deck tabks. Now your boat is actually stiffer even when you aren't employing the water ballast by filling the windward deck tank.

    Alan
     
  6. drnick
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    drnick Junior Member

    k9, i wish i was a naval architect (perhaps one who was wealthy and succesfull with an office of juniours) to say whats what and whats not, so i need you guys pitching in and maybe saving me from myself! to be honest i had not thought about the free surface effect beyond thinking the tanks would have baffles. my idea is to have two banks of tankage, one port and one starboard, each bank of about 1200 litres giving a potential total capacity of 2400 litres. but the tanks are only filled half way ie 600 litres each side to give 1200 litres of water carried. when the boat is heeled it can all be transferred up to the high side leaving the low side empty. this would be like having a complete rugby team sitting out on the rail and should reduce the angle of heel nicely and extend the range of wind a given sail can be carried in (providing the rigging is up to the extra loading).

    as far as the free surface effect.. im thinking that if the water is divided between tanks then you can limit the range over which the water can slosh about. if the tank is three feet in length and baffled, by one foot wide then the water can only move up to three feet rather than 10 feet in one enormous tank. im seeing that dividing the tankage is like having one big tank with completely effective baffles..

    but ill be the first to admit that im just talking out of my a$$ in as much as ive no experience in either refitting a boat to begin with or in setting up a working water ballast system, let alone one with fresh water which is concerved and the potential problem of any free surface effect. so any ideas on the size and position of the tanks are exactly what this thread is about.

    i suppose on the race boats with salt water ballast the free water scenario isnt an issue as the tanks are topped off when loaded.

    alan, there isnt enough room beneath the cabin sole to meaningfully utilise for any form of tankage so the tanks i will add must go elsewhere. i take your point that with the water tanks so positioned you have to factor them into maneuvering as they are acting on the roll centre with a different direction than the keel. at the minute there is a 30 gallon diesel tank behind the companionway ladder and thats it - no water tanks! it is a lovely boat though, german frers design with a lovely shape and 15' of beam amidships along with its pinched IOR ends:)
     
  7. Kay9
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    Kay9 1600T Master

    No sweat my friend. Ill tell you right now my experiance with water ballast are on essel very much larger then you are talking about ie 600' tankers and frieghters.

    I do see what your thinking, and in a tank or on paper it seems to make a lot of sense, but I keep thinking what happens if you suddenly jibe. Now all that wieght is in the wrong place quickly.

    Alan White is talking about the effect you will have on your CG and RM when you place all this wieght up high on a heeling boat, perhaps we can intice him to speak about this some more, in a way 2 dumb old sailors might understand ;)

    K9
     
  8. alan white
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    alan white Senior Member

    Here's an analogy: If you know how tires are balanced with different weights of lead positioned at various points around the tire's axis, it's easy to see how the roll center is like the axle, with the tire on edge.
    In the case of a keel ballasted boat, it is like a single large weight is creating an UNbalanced condition causing the tire to always settle into the same orientation with the weight down.
    But what would happen if the tire were balanced correctly by adding weights in two other places? Since it's round (like an IOR underbody midsection), it would have no reason to settle into an upright orientation.
    So by exaggerating the effect to it's etreme conclusion, where the boat is like a balanced car tire, unable to find its feet, we can see how any weight positioned above the roll center reduces stability to some degree.
    This is why I said that adding weight equally up at deck level would cause instability in passive mode, but improved stability in active mode.
    If you are storing the water at deck level, you are causing a less stable condition UNLESS you are actively using the system by favoring the windward side.
    I would be careful to calculate the effect of the system at rest first, and then I would also remind myself that the presumedly acceptable loss of righting arm isn't going to require me to pump constantly, that is, that the boat can still take care of herself and sail well even if the pump failed.

    A.
     
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  9. drnick
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    drnick Junior Member

    alan, im starting to appreciate the advice given in the thread ive linked to here with my first post - to keep the weight lower and restrict it to 25% of existing ballast weight. i like your analogy, this is exactly what i need to picture and i can see that if the added weights were at 180 degrees from each other around the roll centre (in passive mode) then they would reduce the righting moment of the boat from vertical substantially - i see that they are acting even if not at 180 degrees as well but the lower they are the closer they are to the same rotational angle as the keel.

    perhaps an idea would be to separate the tanks on each side vertically as well as horizontaly - so that the 600 litres held each side in passive mode could be restricted in its ability to migrate upwards (beyond a lower tank level) and compromise stability ie in a rolling seaway.

    i think your advice here is exactly correct, i need to look carefully at the system at rest - in passive mode - first. i had not thought in terms of the water ballast exerting there own independent angular force/momentum about the roll centre and the effective reduction of the keels righting effect - although i new any rolling movement would be exagerated in extent..

    the merits of dumpabale salt water as ballast becomes easier to see, but at the same time i need to add tankage and this cant be put into the bilges and will be to some extent lateraly placed in the hull...

    i will try and post some pics of the interior layout, maybe you guys can help me out on this a bit more.
     
  10. drnick
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    drnick Junior Member

    more pics

    the first is of the interior looking aft from just in front of the companionway and the second is a pic of the interior arrangement plan. this gives an idea of the raw material i am planning with. the idea is to install around 1200 litres of tankage and not upset the existing layout too much. any ideas??
     

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  11. Guillermo
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    Guillermo Ingeniero Naval

  12. drnick
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    drnick Junior Member

    ok so there is no free lunch with this idea.. it would be nice to be cunning in devising a place for my tanks but i think they are simply going to go about amidships where there is space but lower down than i was thinking. i think that the initial idea in having them wide and high will compromise static stability more than i am happy with.
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    To just to add another aspect to water ballast issue. I mean using Water ballast as safety equipment against stormy weathers? When it's choppy some extra weight down by the bilge makes boat movements more tranquilled. Right?
     
  14. Omeron
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    Omeron Senior Member

    Carry some of your water in 5lt bottles. Besides there is a big advantage of keeping the water uncontaminated and fresh, you can store them wherever
    you want depending on sailing conditions. On a long passage, it will keep you
    busy and exercised, moving them from one side to the other. Also no problem of free surface.Keep empty bottles sealed, in case of an emergency, they may keep you afloat, or can grab a few of them, if you have to abandon the boat in a hurry. If you enjoy the stability they provide, you can fill the empty ones with sea water and continue shifting around.
    Just remember which ones are fresh and are not!
     

  15. Eric Sponberg
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    Eric Sponberg Senior Member

    Drnick,

    Water ballast has been used on round the world racers ever since the first BOC single-handed race around the world back in 1982-83. Back then, the rule of thumb was that you can have as much water ballast as would make the boat heel over 10 degrees either side. This guideline has never changed. It is considered prudent and safe.

    For a very good article on the subject, look at the following by my friend, associate, and fellow naval architect Geoffrey Van Gorkom, who designed a water ballast system into his Mount Gay 30 design. At the end of the article, it shows a schematic of the water ballast system. I know from personal experience that this system worked very well, and I have used a very similar one on my design, Bagatelle.

    http://www.vgyd.com/article_5_ballast.html

    In racing sailboats with water ballast systems, the free-surface effect is very small, so you generally don't have to worry about that. Also, it is indeed best to install the tanks as far outboard as possible for maximum righting effect. This means installing them up high under the deck. The added stability by virtue of their position outweighs any negative effect of the extra weight being up high and outboard.

    There is ONE BIG CAUTION, however: Added water ballast will increase the stability enough that you may overload the rig. That is, the boat's righting moment will increase significantly, and rigs are engineered to the boat's righting moment. So if you increase the boat's righting moment with water ballast, the rig will be under-designed. The tensions forces in the standing rigging and the compression forces in the mast all will be significantly higher, and you may end up with rigging failure, which can be particularly dangerous and life-threatening. Therefore, if you go forward with the water ballast modifications, have the rig checked by a naval architect and make any upgrades that are necessary. I cannot stress this enough.

    I hope that helps.

    Eric
     
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