WATER BALLAST
First published by BOATBUILDER magazine, December 1993
Ballast is necessary to help control the heeling motion a boat experiences when under sail or exposed to wave action. A large amount of ballast, located as low as possible, lowers the center of gravity of a boat and increases stability. Dense materials, like lead, make good ballast because they can be positioned into small spaces far below the waterline. In the past few years several builders have introduced "water ballasted" boats. They claim that water ballast can replace the traditional lead, and then be drained out later for easy trailering. Like the proverbial "free lunch", it all sounds to good to be true.
To understand ballast in general, and water ballast in particular, lets examine the basic physics. Floating objects, like sailboats, displace an amount of water equal to their total weight, including ballast. The displaced water can be thought of as a single "buoyant" force that pushes up on the boat, causing it to float. This buoyant force acts vertically through what is called the Center of Buoyancy, or "CB". The CB is defined as the centroid of the underwater portion of the hull (including the keel), and will usually shift its position laterally when the boat heels. The amount of this lateral shift is determined by the shape of the hull. A round bottom hull will not have much CB shift. A catamaran hull will shift the CB a great deal at low angles of heel.
The entire weight of the boat can be considered to be concentrated at a single point, normally near or on the centerline of the hull, called the Center of Gravity, or "CG". The CG remains fixed at the same location, regardless of how much the boat heels, as long as no heavy parts of the boat (or crew) are moved, and always equals the same value as the CB. Under calm conditions, the boat will ride level, and the CB and CG will lie on the same vertical line. Since they are pushing against each other along the same line, they cancel each other out, and the boat remains level. If a wind gust heels the boat over to 20 degrees, the CB will shift away from the CG, and the forces will not cancel. This CB shift creates a restoring moment on the hull, which exactly balances the wind gust. A "stiff" boat will develop a large restoring moment at small angles of heel.
The horizontal distance between the CG and the CB is called the righting arm. The restoring moment, or "static stability", of a sailboat is equal to the length of the righting arm times the weight of the boat. When riding level, the CG and CB are directly in line, and the length of the righting arm is zero. If the boat heels due to a wind gust, the CB shifts, lengthening the righting arm. This creates a righting moment since the CB and CG are no longer in line. If the boat heels enough (usually around 40 degrees) it reaches a point where the righting arm stops growing and begins to shorten. When the heel angle reaches around 120 degrees, the righting arm returns to "zero", there is no restoring moment, and the boat capsizes. Only three factors control the magnitude of the restoring moment:
1. Underbody Shape: Hulls with a wide beam and firm bilge's will cause the CB to shift quickly at low heel angles, creating a long righting arm. A round hull, like a log, will have little or no CB shift, and be far less stable.
2. Center of Gravity: A low center of gravity always increases stability. At a given angle of heel, lowering the CG moves it out and away from the CB, increasing the righting arm length and improving stability. Moving the CG laterally away from the center of the boat will also improve stability. A common example of this is when the crew sits on the windward rail.
3. Displacement: The boats displacement (total weight) pushes up through the CB and acts on the righting arm to balance the heeling forces. The more weight, the more restoring force. Of course, the most efficient use of weight for stability is down low, where it also reduces the center of gravity.
Water can be used to improve stability by pumping it into tanks located outboard and low in the hull. The off center weight shifts the CG away from the centerline, increasing the righting arm from 17 to 24 inches. The total righting moment, which is the product of the righting arm and the displacement, increases by 50%. Its easy to see why this technique is favored by many high performance sailboats.
Any material can be used as ballast, its just that some are more efficient than others. A low density material, like water, can be used to improve stability by containing it in the keel cavity, and letting its weight act like any other ballast. The only problem is that water is not very dense, and this requires a very large volume to contain a reasonable weight. A large volume keel displaces a great deal of water and generates a buoyant force that tends to counteract the ballast. In fact, if water is used as ballast in a thick high volume keel, the buoyant force of the keel will equal the weight of the water ballast, and cancel its effect on stability. To minimize this buoyant effect of the keel, many designs put the water in tanks within the hull, but this raises the CG, and reduces interior volume.
To evaluate these various keel combinations, a set of lines were drawn for an average boat, and the stability calculated using several different keels. The thin keel, lead ballasted design was the most stable, under all conditions. The thicker keels containing water or a combination of water and lead did not improve stability much. The huge keel volume is creating enough buoyant force to counter most of the effect the lead has on the CG. Traditional narrow, full keel boats exhibit this trait, and are often ballasted with lower density materials like steel. The final calculation determined the stability of the example boat with the keel completely removed. The boat now displaces only 2200#, but its stability, which is all caused by hull form, is very nearly the same as the water ballasted design. This shows that the buoyant force of the keel volume is canceling the ballast effect of the water it contains.
So where is water ballast practical? One place is trailerable boats. Trailerable boats can be designed with internal ballast tanks that increase the weight of the boat, lower the CG, and provide some additional stability. The water is drained for trailering, which makes towing a lot easier. The additional weight also increases the inertia of the boat, improving its dynamic resistance to wind gusts and waves. Water ballast makes it "feel" like a big boat, and trailer like a small one. Unfortunately, it is difficult to put that much water in the hull without using up valuable interior volume or distorting the underbody shape.
One way around this problem is to put the water ballast inside twin keels that are located low and outboard. The twin keels can easily be large enough to contain a reasonable amount of water, Interior volume is not reduced, and the CG is lowered. The twin keels, which are set close to the waterline, are flooded to 500 lb. each. At low angles of heel, the buoyant force of the keels cancels the weight of the water ballast, and provides no stability improvement. At 15 - 20 degrees, however, the windward keel begins to pull out of the water, causing the CB to shift dramatically. In this example, the righting arm stretches to 24", which is similar to the stable lead keel design. ( Pictures of a twin keel, water ballasted sailboat)
Water ballast can make good sense for some applications, and should be given consideration along with more traditional keel materials. Be cautious, however, and be sure that the water is actually improving stability, and not just adding weight. Look at the underwater sections, and estimate how the CB shifts when the boat is heeled. The more stable designs will minimize the buoyant effect of the water by not altering the underbody sections much or using overly thick keels. The more stable designs will have firm bilge's and ballast tanks as low as possible. Ask to see a stability analysis, and compare it with other boats of the same size, and ballast type. For sailing in protected waters, a light weight, water ballasted design may be right for you.