Builders choose whether to use a bolt-on (external) or encapsulated (internal) keel on a sailboat. Neither choice is necessarily right. The five main trade-offs are cost, maintanance, tankage, repair, and performance.
A bolt-on keel is a lead cast fin shape that attaches via bolts to a fiberglass stub molded with the hull. Of the five trade-offs, a bolt-on keel is easier to repair and gives better performance.
A bolt-on keel is easier to repair in the case of damage from grounding compared to an encapsulated keel. Firstly, fairing back a bent or dented bolt-on lead keel is easier than fiberglass repair on an encapsulated keel. With the bolt-on keel you can detach her and freely work on her. The fiberglass repair is difficult because you are stuck in the awkward space around the keel cavity. An encapsulated keel molds integrally with the hull forcing in-place work. Fiberglass layup is often inferior here. The bolt-on keel is easier to work on because you can detach her. Secondly, with the ballast material in the encapsulated keel, water corrosion can be an extra fix. With an external bolt-on keel, water cannot seep inside. The keel is solid lead. Encapsulated keels on the other hand are full of mixtures of lead, iron, resin, or concrete. Once water gets inside, the iron if present rusts and swells bursting against the fiberglass into a rusty, loose matrix. The constant moisture blisters up the fiberglass keel and detaches the ballast from the fiberglass skin. These factors the working space and ballast material mean a bolt-on keel is generally easier to fix after serious groundings.
A bolt on keel gives better perfomance. Firstly, because of the posibility of sharper fairing than with fiberglass, a lead cast keel has less wetted surface. Less wetted surface for the same ballast and draft means a faster boat. The bolt-on have a fine tunable shape while the fiberglass is fixed after layup with more generous curves. Second, because a bolt-on keel has no fiberglass skin, the keel is more compact and lowers the center of gravity. The lack of a diluting skin of fiberglass encapsulating the ballast lowers the depth of the lead that small but important skin thickness. The center of gravity is lower leading to increased performance. These two factors, the fairing and skinlessness, make a bolt-on keel boat faster. Performance boats like Swan use bolt-on keels. Jeff_H on Sailnet is a great palimpsest of this same knowledge.
I talked to Cecil Lange, the builder of Cape George sailboats one time. He had moved on to become a surveyor in Mexico. He talked about the bolt-on keels of Beneateaus and other recent production boats without even a fiberglass stub. The builders just bolted the keel flat against the hull. Cecil was shocked these builders could be so bold. Without the “stubby,” fiberglass keels put tremendous stress on the hull and can clear break off. Or at least, he had seen on Hunters crazing all around the unsound hull where the keel was beginning to break away.
An encapsulated keel has a fiberglass cavity laid up with the original molding. The builder then fills the cavity with a ballast mix including a dense metal such as iron or lead and a glue such as concrete or resin. For example, Gulfstar used a concrete and lead slurry. An encapsulated keel is less expensive, less maintanance, and allows more tankage in the bilge than a bolt-on keel.
An encapsulated keel is less expensive. First, the work filling the cavity is less costly than the bolt-on casting process. Casting separately a keel is more expensive than filling a fiberglass cavity. Filling the encapsulating keel takes less skill and thought. Workers dump in the ballast mixture and fiberglass over. With the bolt-on keel, they fill a cast with hot lead to make the shape. The bolt-on is a more complicated process. Second, the ballast material can be less expensive. Builders use mixtures of low cost material such as lead or iron scrap mixed with a binder. Lead is necessary for the external, casted bolt-on keel though recent Beneteau’s have used external cast iron keels. The encapsulation protects and hides the ballast allowing the use of less expensive filler materials. These two factors, the complexity and materials, lower the cost of encapsulation. Most boats built in Taiwan had encapsulated keels to minimize cost.
An encapsulated keel requires less maintanance than a bolt-on keel. The encapsulated keel if undamaged by groundings requires no maintance. A bolt-on keel needs fairing and new keel bolts at some point say every 10 years. The encapsulated keel is the same as the rest of the fiberglass bottom but has no special maintanance like with a bolt-on keel.
With an encapsulated keel you can set the tankage easily in the bilge. Firstly, a bolt-on keel needs more internal athwartship framing to handle stress loads. This framing takes up space and interferes with tankage in the bildge. The framing is an obstacle that makes tankage more difficult to fit. Second, without keel bolts, a builder can cover the encapsulated keel over easier. You will not need space to reach keel bolts. You can place the tankage and block access. Cabo Ricos are a great example of this philosophy. The idea of internal ballast dovetails with their bilge tankage.
An encapsulated keel has cost, maintanance, and tankage advantages while the bolt-on has repair and speed. Racing boats clearly prefer bolt-on keels. With offshore and Caribbean cruisers, you will find either.