There are many materials boat builders use for ballast but not all ballast is created equal or deployed in the same way. You should know what types are out there and what the pros and cons are of each of them.
With all other things being equal, the lower the center of gravity is in a boat, the more stable it will be. That is the reason ballast is usually added to a boat as low down as the designer can. This is where density comes into play. The denser a material the less room the same weight will take up and the lower down it can be placed in the boat—lowering the center of gravity. Here are the most common materials and their densities:
- Cast Lead—708 pounds per cubic foot.
- Cast Iron—450 pounds per cubic foot.
- Concrete —140 pounds per cubic foot. (An average of normal aggregate mix)
- Sea Water—64 pounds per cubic foot.
- Fresh Water—62 pounds per cubic foot.
If you do the math, you would need 11.4 cubic feet of fresh water, 5.1 cubic feet of concrete, and 1.6 cubic feet of cast iron to equal 1 cubic foot of lead. Since you are trying to get the weight as low as possible in the boat, you can see that less dense ballast has a direct effect on stability.
Common Ballast Arrangements
Because it is the densest material available, the most common ballast used in small and large sailboats is lead. Lead keels can come in all sorts if sizes and shapes. You can look over our article on Keels, Centerboards & Daggerboards for more information on shapes. Some lead keels are cast—meaning they are molded to be the finished keel and then bolted onto the hull. Lead ballast can also be encased within a fiberglass keel.
Cast Iron Swing Keels were a very popular thing to do in the 70’s and 80’s for many production boats. A few points:
- The benefit is that the builder can include a sizable keel with significant weight but still be able to launch it on and off a trailer at a standard launch ramp.
- When you move you ballast up and down on a winch, you will change the balance of the boat. The boat will not sail the same way with the keel up as it will when the keel is down so make sure that the draft when the keel is down is appropriate for your sailing area.
- When new, the cast iron was well sealed with either epoxy or encased in another metal or even fiberglass. However, as the boat is used over the years, this coating can get pierced and water gets in. Nothing rusts quicker that cast iron—so be careful to keep the coating intact or you will have one nasty looking keel in no time.
Centerboard with Water Ballast are still in use today on some production boats. This involves tanks (usually contained in the hull) that are filled by opening a valve after the boat is launched. Once the tanks are full (the water rises to the water line) the valve is closed and keeps the water from moving about. Several points should be made:
- The best thing about water ballast is that you can leave your ballast at the ramp and not trailer it home.
- Water ballast tanks are almost always up in the hull. As we discussed earlier, the lower the better so this type of ballast is already at a handicap.
- Water ballast tanks are usually filled up to the water line of the boat. Only when the boat is heeled over, is the water lifted above the water line and begins to act as ballast. Even at extreme angles of heel, less than half of the water is resisting the heel action of the boat.
- Because of point #2 and #3, more water ballast (in lbs.) is needed than the equivalent boat with a lead keel, or, more likely, will carry less sail. Because of the increased weight and/or smaller sail area, water ballasted boats tend not sail or point as well as other designs.
- Many water ballasted boats tend to have very quick initial heel—meaning it does not take much to heel the boat over. The boat’s heeling action will slow down and the boat will “stiffen up” at a certain point where the water ballast begins to do its job.
- The boat is less stable (possibly even dangerously so) if the tanks are empty OR are less than full. If there is substantial air in the tanks, water will slosh back and forth during tacks creating a shift in weight from one side of the boat to the other and then back.