Water Ballast Analysis (From another thread)

Don12364 · Jan 6, 2010

If it helps think of it as if the water ballast were solid ice. Then visualise it as a lump of something solid but having the same density as ice.

BTW: During the days of the IOR we went through a period when some of the quarter tonners had a buoyant wooden keel. This was to make them unstable for the inclining tests and so reduce their rating.
The secret was the stability was mainly provided by some lead pigs of internal ballast - and 5 hefty crewmen!

Maine Sail · Jan 6, 2010

Most humans float in water yet place them on the windward rain and you have ballast...

Port and stbd water tanks of equal size, below the water line, fill the port one and what happens..?

MoonSailer · Jan 6, 2010

water ballast works!!! It is a tank of water at the bottom of the boat. When the boat heels the bottom moves upward. This upward movement is conteracted by the weight of the water. Sure water ballast is not as effective as lead. But it works and is great to use on a trailerable boat. Instead of hauling 2000#s of ballast down the interstate you empty the tank. This results in great savings in gasoline and wear and tear on the tow vehicle. If I ever buy another trailerable boat it will most probably have water ballast. Even with a small sailboat the extra 650#s of cast iron is a pita to tow. Also in case of a grounding it is possible to pump out the ballast tank and float free. It is hard to get rid of a cast iron or lead keel. You forget hull form when thinking about water ballast. The water tank is normally deep and in the center of the hull.http://h260.com/water_ballast/water_ballast_paper2.html

Maine Sail · Jan 6, 2010

druid said:
The humans are in air, not water (ie above the waterline).

Ballast of any type or weight, inside a floating hull, above or below the water line, is ballast. This could be beer, lead, water, batteries or humans. Remember it is inside the floating/displacing vessel thus the weight of it counts. While you need considerably more water volume to equal that of lead, it is still ballast.

druid said:
As for the two water tanks - one is full of air and floats, so will exert an upward moment to that side.

It is inside the hull thus it is not floating and adds no negative weight, float or buoyancy. Your whole boat is full of air. It will however add weight/displacement as the weight of the plastic tank, even empty, is added weight to the total weight of the vessel.

One could place every life jacket and 100 styrofoam coolers on the port side, below the waterline, but if their total combined weight was 10 pounds you would technically have 10 pounds of added weight/ballast on that side of the vessel not added buoyancy.

druid said:
The water-filled one works great once it's above the waterline.

Does not matter where the weight is placed above or below it will still make the boat list because it is inside a displacing hull that moves the center of buoyancy when it heels. In a static position, not sailing, weight closer to the center-line you'd see less list and list closer to the beam more list. My tanks are below the waterline. If I forget to leave the balance/equalizing valve open I get a list, despite the "air" in one tank.

Joe · Jan 6, 2010

Excellent! Thanks for that, Moonsailor.

Sumner · Jan 6, 2010

http://en.wikipedia.org/wiki/Sailing_ballast

A common type of ballast for small boats that avoids many of the problems of high density ballast is water ballast. While it seems unintuitive that placing water in the hull (which is, after all, the same density as the water outside the hull) would add any stability, in fact, adding water ballast below the vertical center of gravity increases stability. When the boat heels, it must then lift the ballast clear of the water, at which point it is obvious that it does provide righting moment

If you carry a 5 gallon jug of water onto your boat does it have weight? If you place it below the water line is that weight still there? Now carry 30 of those jugs onto your boat and go below water line with them. Does the boat sit lower in the water? If you put them all on one side will the boat start to list to that side even if they are still below the water line? They have weight even if they do take up more space than lead.

One has to remember that this water is captive in the ballast tank, so if the boat starts to heel it has to lift the ballast tank towards the surface on one side. The bottom of the boat now becomes one side of the boat that is pointed towards the bottom. The real bottom with the captive weight of the water is now being lifted up on one side of the boat and is opposing the heeling moment.

If you just put water in the bottom of a boat and it wasn't confined as the boat heeled it wouldn't do anything as it would remain at the bottom of the boat, not the real bottom, but the surface pointed straight down.

Large ships use water ballast quite effectively.

Stability is often maintained in large ships by moving water around in the vessel’s ballast tanks. This is often used to ensure that the ship stays upright and does not adopt a heel (lean) to one side or the other if the cargo is loaded asymmetrically, or if fuel is taken from a tank on one side of the ship. Water ballast is frequently carried to maintain stability in an otherwise empty ship

http://74.125.47.132/search?q=cache...last+for+large+ships&cd=5&hl=en&ct=clnk&gl=us

I'll say one thing, if we take our Mac out and don't fill the ballast tank it doesn't take long to realize something is very wrong,

Sum

Our Trips to Lake Powell, UT - Kootenay Lake, Canada - Priest Lake, ID

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Joe · Jan 6, 2010

If you read Moonsailors piece and carefully analyze Maine's comments you will conclude that it is the distribution of weight within the hull's displacement area that affects it's righting ability. Think about it.....with another brandy... and it will make sense. You can affect your own boat's trim, fore and aft or lateral, by simply moving things (ballast) around inside the boat. Adding water ballast increases the overall weight of the boat without changing the volume. Think about an oil tanker... when it is filled with crude why does it ride so low in the water? We all know that crude is lighter than water, but it is much heavier than air.

walt · Jan 6, 2010

I just looked at the old 1990 Mac 26S and seems the very top of the water tank is maybe 5 inches below the water line.

Maine Sail · Jan 6, 2010

druid said:
Sumner, one more time: replacing AIR with something heavier than air reduces the bouyancy and therefore the NEGATIVE righting moment. As the boat heels, you do need a container to move the water, cuz F=ma. But once it's there, AS LONG AS IT"S BELOW THE WATERLINE, it provides no downward force, as illustrated when you remove the bottom. If the water is not exerting a force on the bottom, how is it pushing the boat down?

And as for the water-filled water bottles - put them underwater and see how much they weigh.

I'd love to do a mockup in my bathtub, with a removable water-filled tank, or one on a scale to measure what downward force, if any, it exerts. (Not I'm not talking about a scale in the air-filled hull)

EDIT: yes, increasing the mass creates stability, as in F=ma. But I'm talking about righting moment here - not the same thing.

druid

Druid,

You may want to read some books on yacht design such as Danny Green's Cruising Sailboat Kinetics. The bottom of a boat will always be there. If you remove it you sink. for real world results you have a displacement hull. If you place weight low in the vessel it will lower the CG thus lengthening the righting arm. If the water remains in the center of the boat and the CB moves when you heal it creates a righting force. Remember any discussion of ballast takes place inside the hull thus adding to the overall displacement.

Sorry, I don't know how to explain it any simpler, wish I did....

Sumner · Jan 6, 2010

druid said:
..........And as for the water-filled water bottles - put them underwater and see how much they weigh

You are completely missing the point, they are not under water they are in the boat!!

Take a liter plastic bottle. Empty it. Cut an opening in one side, the top of your new boat. Put 5-6 oz. of water in it and place it in a filled sink on its' side. Rotate it (heel it over). Does the water in the bottom resist your heeling and return the boat upright? NO! Now you just have a swamped boat (like the milk jug you filled with water).

Now take a couple pill containers or some other small plastic container that has a lid. Tape it inside your boat on the bottom and put the boat back in the sink. Heel it to one side and let go. Will it right itself. Probably if the plastic container you taped in there is heavy enough.

Now fill that smaller plastic container with water and retape it in the bottom of your boat. Heel it over. Does it right itself? Yes!! A water battast boat.

The thing that is hard to visualize and I had problems with this at first is that the bottom of our boats as they heel are no longer the bottom of the boat. The bottom is now the point of the boat furtherest down in the water. The bottom of the boat as we think of it is rotated up to one side and any weight there will want to rotate it back down if it gets the chance, i.e. the wind stops blowing. The water in the ballast tank is not free to go where it wants to, it is trapped in that tank and goes where the tank goes. If the tank moves up to one side as it will when the boat heels the water (weight) moves up. It takes force to raise it up. The wind on the sail.

Maybe lay off the brandy and look again at that web site

,

Sum

Joe · Jan 6, 2010

Druid.... what's your point? The answers to your questions have already been stated.. are you trying to re write the laws of physics?

LloydB · Jan 7, 2010

The analysis here: http://h260.com/water_ballast/water_ballast_index.html
bothers me cuz I can't find where it's wrong but I KNOW it's wrong. I think I may have at least some insight, but would appreciate more...

My 1st impression of your thought experiment as I imagined was to throw a bucket overboard with water in it and see it tilt and sink. Next take another and put lead in the bottom and see it float better. I went to the http 260 site and saw where that explanation seemed right to me in general. Read through the replies and thought Sum got it best even without explaining his latest Mac mod(which probably would have delayed my response until tomorrow)

. then another idea had i, how about a open pop can with water on the side opposite the hole instead of bucket. Only problem is how to keep water on bottom side without getting too technical. Came up with: put some water in a can add some oatmeal lay it on its side and write up experiment while oats setup.(BTW oatmeal is lighter than water) then test before sending this. I was surprised by the stability as i expected less with only about 3/8in. depth.

Roger Long · Jan 7, 2010

Wow. This has been an amusing thread to read.

If you put water into a hull and contain it so that it can't move around (full tank vs. partially full tank) it will have exactly the same effect on stability as a chunk of lead the same weight located at the center of the tank.

However, imagine a flat bottom skiff. If you add a false double bottom to it in the form of a tank built out of a material the same density as water and then fill this double bottom with water, the boat will end up having exactly the same stability it did before at normal heel angles. However, it will feel heavier, the motions will be different, and it will be less likely to pound since the bottom will be farther below the surface. The sail carrying power type of stability will be the same because you are adding buoyancy in exactly the same place and amount as you are adding weight.

kloudie1 · Jan 7, 2010

OK, here is something to consider: When ya “sink” anything the center of buoyancy of the "airspace" goes UP .. So when you fill the ballast tank, the CB rises (formerly empty space is not empty) and the center of gravity of the boat goes down. It is this change in stability parameters that makes the boat more stable. When you add lead to an existing shape, the center of buoyancy does not change, but the CG goes lower. OK here is another way to think about it.. Consider a long thin glass tube (closed end) that is empty.. It will float horizontally.. If you want it to float vertically, put water into it. As you do, the CG will go down and the CB will go up .. it will get more and more stable vertically .. as more water is admitted, it will sink more until the center of buoyancy is at the very top and the CG is pretty far below. It will get to be very stable before it goes under. You get less total buoyancy but the thing still floats.. You can make it stable by adding lead shot instead of water .. it will float higher but as long as the CG and the CB are the same distance apart, the tube will have the same static stability or righting moment.
Roger, forgive my misuse of the CB term.. just trying to provide a picture..

BobM · Jan 7, 2010

My head hurts. You budding naval architects can't do this to me before I have my second cup of coffee!

Roger Long · Jan 7, 2010

kloudie1 said:
So when you fill the ballast tank, the CB rises (formerly empty space is not empty)

That's exactly backwards.

This might help:

http://www.rogerlongboats.com/Stability.htm