- It helps to keep the boat from sliding sideways when the wind is not taken directly astern, and
- it keeps the boat from tipping over when the sails are full of wind.
Old sailing ships used rocks, held in the bottom of the bilge, as ballast. When they took on cargo, some of the ballast was jettisoned because the cargo held below the waterline would provide the righting moment, and because the ships were designed to carry cargo, after all, not rocks. (This led to the ballast spoils areas in many harbors that were heavily used in the 18th and 19th centuries.)
Modern sailboats have moved the ballast outside the boat, lower, where it can have a greater effect. Like the big kid and the little kid on the teeter-totter (have these all been removed from playgrounds? What will we use for the illustrative analogy in the future?), the more weight and the deeper it is suspended below the boat, the better: the greater the righting moment. This started with shaped blocks of lead bolted to the bottom of the keel in full-keeled boats. But it has evolved from there.
Internal or external?For a fin-keeled boat, there is really only one choice here: the keel is a shaped piece of metal bolted onto the boat's bottom. It is an external keel.
But for a modern fiberglass hull which is designed as modified- or full-keeled, a second choice is possible: internal ballast. In this design, the shape of the keel is part of the fiberglass molding of the hull - the keel is molded as part of the hull. And then the ballast is added to the inside of the molding. Typically, this is as metal shot combined with resin in a cement mixer and then poured into the keel. But it can also be done using shaped pieces of metal laid up in the interior of the keel and then encapsulated with resin.
- Internal keel pros:
- There are no keelbolts to worry about. Short of complete destruction of the hull, the keel simply cannot drop off the boat as some external keels have famously done.
- Maintenance of a water-tight seal between the keel and the hull is unnecessary.
- In a collision with a hard object, there is no fear of loosening the hull-keel joint.
- External keel pros:
- In a collision with a hard object, it is the keel which makes contact, not the hull. If the keel is a soft metal like lead, deformation of the keel at the point of impact will absorb a portion of the energy.
- Because the void space of uniform-sized shot is 33%, the density of the poured shot keel will be considerably less than the solid metal. Therefore the solid metal keel of equivalent shape will have a greater righting moment than an internal keel made from poured shot. This advantage is considerably less if shaped metal pieces are used.
(Question: Has any manufacturer explored the use of a range of shot sizes, designed to reduce the void percentage? Concrete manufacturers have been doing this with the aggregates they use for centuries. Seems like a simple improvement.)
- Iron keel pros:
- Iron is a lot cheaper than lead
- For a steel boat, there is less galvanic potential between an iron keel and the hull than there is with a lead keel.
- Lead keel pros:
- Lead is denser than iron. This means that for equivalent shapes, the lead keel will have considerably greater righting moment.
- Lead does not corrode in seawater; tedious and frequent scraping, sandblasting and painting of the keel is not needed. In fact, other than for protection against freeloading sea life, painting of a lead keel is completely unnecessary.
- Lead does not corrode in seawater; iron does. In fact iron swells as it corrodes - that is, iron oxide is greater in volume than the iron from whence it came. Therefore leakage is a serious problem for internal iron ballast, since the incoming seawater will corrode it and eventually cause the keel structure to burst.
- Already mentioned above: An external lead keel will absorb a portion of the impact energy in a collision with an underwater object. An iron keel will convey essentially all of that energy to the keelbolts and the hull at the top aft end of the keel.
- Internal lead
- External lead
- External iron
- Internal iron
But as any of you who have gone thru a boat search already knows, the emotional tug that a boat has on you overrules almost any of the analytical studies that you might have prepared beforehand.
At least it did for us. So I guess we are just lucky... but hey, this makes a nice rationalization, don't you think?