Water Ballast Analysis (From another thread)

[kloudie1]

Water Ballast

WHAT?? Ya mean it floats with the water filled end of the tube up?? I didn't know that..
I should have said "effective" CB of the void at the water's surface formed inside the shape? What I meant is the center of the attachment point of the magic string supporting the tube.

 

[woodster]

that is some interesting reading rodger .....

thanks
woody

 

[walt]

I'm worn out again from having to mentally carry all sorts of jugs of water and big chunks of lead in and out of the hull of the boat.

One of the things you hear even from water ballast boat owners is that the boats are initially tippy but stiffen up at some heel. This may come from some suggestion that the water ballast needs to get higher than the water level in order to work. Ie, you would have a "non-linear" righting moment from the water ballast.

The model in the original reference would say this is not true - the righting moment begins to increase immediately with heel and also will increase in mostly a linear fashion with heel. This is also what I feel from sailing one of these boats for three years now. The magnitude of the righting moment force is higher the lower the CG is (ie, deep lead keel) but overall behaviour of water ballast or lead keel is similar.

 

[Cayennita]

Here is another one. Most of the big (100"or so racing machines) have a ballast tank all the way in the back of the boat. When going up wind the tank is empty and depend on the swing keel for righting moment. Downwind in heavy air they will fill the tank as needed to keep the bow from going under. It is ballast in this case. Sometimes two tons of water gets added.

 

[walt]

Another aspect of water ballast sailboats.. Ive heard this before and would have to agree that sailing a 26 foot water ballast boat is somewhat similar to sailing an oversize dingy.

The point to make here is that pretty much all the water ballast boats have a deep high aspect foil for the keel (similar to most dingy's). The lift vector of the foil is mostly normal to the foil surface and with a high aspect foil, it tends to be fairly deep where it contributes a higher "moment arm".

So somewhat similar to a dingy, you are probably more using the righting moment of the dagger or centerboard lift vector than you would in for example a shoal draft lead keel sailboat which has a much lower aspect keel. Ie, the righting moment from weight of the lower lead keel is larger but the righting moment from lift is larger from the higher aspect foil.

 

[MoonSailer]

I was very surprised at how well my 1988 Mac 26 sailed. I had a Helsen 22 with a cast iron swing keel (simular to a catalina 22) and my wife wanted a better place to sleep. I wanted something lighter to tow. The Helsen was about 2600#s the Mac was about 1700#s a big difference towing over a mountain road behind a S10 chevy. Anyway the Mac would run circles around a Catalina 22. Also in high winds the Helsen's keel would sometimes come out of the water in gusts. The Mac never felt out of control . The Mac 26S is a decent sailing boat that is easy to trailer. The newer model with the twin rudders I don't like at all. Water ballast is great for trailering. I think Druid might be a troll.

 

[druid]

Water ballast is great for trailering. I think Druid might be a troll.

I have my opinions of how well the Mac26 sails (I've never sailed a 26 - had a 22 for years, but I've sailed past many of them and observed how they sailed). However, this thread is NOT about that, nor am I trying to "slag" water-ballast boats. I'm trying to understand the physics behind water ballast, with two competing models.

I think I may have to build a more precise model, with a long balloon for a "keel". My theory says that with water in the balloon, it will be much more stable than with air in it, but not significantly more stable than NOTHING in the balloon.

Something Roger said perked up my ears: when you add water, you're adding "buoyancy" as well as "weight" (and in the case of water, the two are equal). This may be the key to bringing the two models together.

druid

 

[Roger Long]

I think I may have to build a more precise model, with a long balloon for a "keel". My theory says that with water in the balloon, it will be much more stable than with air in it, but ...

It would be a lot quicker just to read my stability primer linked above.

Water in the balloon has no more effect than water outside the balloon as far as static stability is concerned. Until the balloon is lifted above the water with heeling, it's just a boat with a balloon on the bottom and it doesn't matter whether the balloon is inflated with water or shriveled up empty. The weight you are adding to the hull and the displacement you are adding exactly counterbalance for all amounts of water added or subtracted. The boat's dynamic behavior will be different, however, because the inertial of that water not forced to move with the hull is greater. The boat will be slower with more water in the balloon because water has to flow around a fatter hull.

When you inflate the balloon with air, displacement is being increased but there if virtually no change in overall weight or center of gravity. Stability will decrease rapidly as air is added to the balloon.

 

[Bob V]

I'll give this a try

Take a Clorex bottle and lay it on it's side with lead shot on the "bottom" (which used to be the side).

Exert a heeling force on the bottle and the lead shot will move without creating any significant righting force. Itt's just like the water except the water molecules roll even easier. It is the containment of either of those at the bottom that makes it ballast.

 

[Maine Sail]

I am fascinated that this discussion is still continuing.. Wow, is all I can say...

 

[MoonSailer]

I am baffled by the confusion. It all seems so simple to me. If you put 1000#s of water in the bottom of a boat fixed in place in a tank. Then the boat is harder to tip over. If you put 1000#s of lead the CG would be lower but the effect would be about the same. The boat would displace the same amount using either lead or water. Maybe it is because I used to teach high school physics. Maybe it is because I have read about boat design. But I suspect Druid is a troll pretending to not understand. http://www.marine-concepts.com/features.html#ballast Wow here is a great looking thin water sailboat. Twin water ballast tanks one on each side. This would seem like a great fishing boat for Florida.

 

[Sumner]

Take a Clorex bottle and lay it on it's side with lead shot on the "bottom" (which used to be the side).

Exert a heeling force on the bottle and the lead shot will move without creating any significant righting force. It's just like the water except the water molecules roll even easier. .........

Druid the above is what you keep doing in your head and in your modeling.

. It is the containment of either

of those at the bottom that

makes it ballast.

This is the key "containment"........

Hope the brandy wore off ,

Sum

P.S. The average PHRF of the Mac 26 S and D or in the 210 to 222 range, not all that bad , the 22's average is 252.

 

[caguy]

I am fascinated that this discussion is still continuing.. Wow, is all I can say...

Maine, I'm sure Stonehenge wasn't built in a day. It took the Druids years to figure out how to keep a bunch of rocks upright.

 

[Ross]

Ships travel with cargo or ballast, without either they lack stability. Many of the problems with invasive species has been because ballast water from a foreign port has been dumped in US waters. Therefore ships are now required to change ballast water in mid ocean. During colonial years the colonial governors required ships sailing in ballast to carry granite stones cut to a specified size and shape and then charged the shipping company to allow them to be unloaded. The stones were then used as paving. This is why most eastern US port cities had cobble stone paved roads and streets.

 

[COOL]

Center line water ballast is does add righting moment
as the boat is heeled, but it is a very ineffective form
of ballast. This is why the boats you see that use water
in the keel, as their primary ballast. have such diminutive
rigs and sail so poorly.
As you move the water further to windward of the
CB it begins to become more effective. Water ballast
in tanks out at the rail is very effective, which is why
it is so prevalent in short handed race boats.
Compare the sailplan of an Open 6.5 Mini to that
of a Mac 26, and you can discern just how effective
water ballast can be when used in outboard tanks
instead of a centerline keel tank.

 

[Ross]

The Vikings used round stones for ballast because when they would suffer a knock-down the stones would roll out of the boat and not sink it. Other boat designs have used sand bags as movable ballast.

 

[Paul Brinkley]

Ballast comparison

Might as well give this a shot. Consider 2 identical hulls- same size, shape, weight, draft, displacement (see sketch). So that they are equal, both have a tank in the bottom, as a water ballast boat. The tank will hold 1000 lbs of water, and have a uniform cross-section. Put water in one hull's tank and close it. In the second hull, insert a lead rod in the tank, such that the lead rod is the same length as the tank, and of sufficient diameter for the lead rod to weigh 1000 lbs. Support the rod so that its center of gravity is in the same position relative to the tank that the center of gravity of the water is in the first tank. We now have the two identical hulls, both still weighing the same (both just a 1000 lbs more than when empty); both with everything else remaining the same. Put both hulls in the water. They will behave identically- same draft, displacement, righting moment.

The only way for the lead to make a difference is to mount it on the bottom of the tank (or below) so that its center of gravity is lower than the c.g. of the water ballast. In that case the lead will produce a larger righting moment, not because it is lead, but because of its location.

In order for water ballast to not work, we would have to believe that 1. MacGregor, Hunter, and Catalina water ballast boats won't work; 2. cargo ships and tankers can't use water ballast to stabilize them when empty; and 3. submarines can't submerge by filling their ballast tanks, and surface by emptying them.

Hope this helps. -Paul

 

[Dan Hill]

The defining key to all this is that the water ballast is FULL and CONTAINED and UNMOVING (cant flow). As the boat heals, due to the tank design the CONTAINED water is partially raised ABOVE the water line. Fill a 5 gallon bucket and hold it underwater, not much weight right? Now lift it out and ABOVE the water line. Think about it.

 

[Sumner]

The defining key to all this is that the water ballast is FULL and CONTAINED and UNMOVING (cant flow). As the boat heals, due to the tank design the CONTAINED water is partially raised ABOVE the water line. Fill a 5 gallon bucket and hold it underwater, not much weight right? Now lift it out and ABOVE the water line. Think about it.

It doesn't even need to be raised above the water line. The water in the ballast tank isn't in the water outside of the boat. Since it isn't and it is contained as you pointed out it will work like ballast of similar weight placed at the same point regardless of material type.

It is explained very clearly here........

http://h260.com/water_ballast/water_ballast_index.html

................ why it doesn't need to be raise above the waterline to be effective.

c ya,

Sum

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[Paul Brinkley]

You're partly right, WrenchBender...

The defining key to all this is that the water ballast is FULL and CONTAINED and UNMOVING (cant flow). As the boat heals, due to the tank design the CONTAINED water is partially raised ABOVE the water line. Fill a 5 gallon bucket and hold it underwater, not much weight right? Now lift it out and ABOVE the water line. Think about it.

That the water ballast is contained and can't move is important. However, the ballast does not have to be raised out of the water to work. Take your 5-gallon bucket of water aboard a large ship, and carry it as low as you can in the bottom of the ship. Does it get lighter as you go down? Will it weigh less if you are on a deck that is 20 feet below water level? No, to both questions. It weighs the same as it did when you were above sea level. The water in a ballast tank is contained within and is funtionally a part of the boat, as you pointed out above; it is not free to move independently, as is the 5-gallon bucket held underwater. Incidentally, a 40-lb bar of lead will not feel like 40 lbs if you hold it underwater, either! -Paul