why keel bolts?
I think Paul has covered most of the big points. This is another one of those there are ''no one universally right answer'' type questions. In other words an argument can be made for either type of keel.
(For the record, I personally strongly prefer a bolt on keel rather than an encapsulated keel and for myself would not consider buying a boat with an encapsolated keel.) Here''s the way I see it.
Bolt-on keels tend to offer more performance since the ballast must be cast (rather than made up loose ballast and mastic) and without the keel stub skin thickness tend to be lower relative to the center of bouyancy. They also tend have significantly less wetted surface and frontal area making them theoretically faster on all points of sail. Bolt on keels are simpler to repair and generally can be repaired satisfactorily no matter how bad the damage. The keel structure of bolt on keels are generally a better engineering design because they require internal transverse framing in order to spread the loads out to the hull and so tend to have more sophisticated framing schemes. Proper transverse framing is often omitted even on expensive internal ballast boats. All other things being equal, the denser nature of a bolt on keel often means a larger sump volume than would have resulted from a similar displacement encapsulated keel.
On the down side they are more expensive to build; requiring precision casting, careful bolt hole drilling and a lot more hand fairing. They are higher maintenance requiring fairing every 10 years or so and depending in the materials used, new keel bolts at some point in the boat''s life.
Encapsulated keels are less expensive to build. There''s less labor and less precision required. Boat builders will often use less expensive forms of ballasting with encapsulated keels, such as iron or lead scrap cast in concrete, resin or other binder to further reduce costs. If they are not damaged in a grounding, encapsulated keels are less expensive to maintain.
On the down side, beyond being less efficient from a sailing standpoint, the real disadvantage of a bolt on keel is the difficulty in doing a proper repair when damage. Typically, in a hard grounding a number of things happen on an encapsulated keel. Typically the skin of the keel encapsulation gets ruptured and separates from the ballast at the point of impact. This allows water into the small cavities between the keel and the ballast and once wet it can mean the ''beginning of the end'' for the boat as this permanently wet fiberglass blisters itself from the interior and the wet areas spread out around the ballast. This is especially a problem on a boat that is hauled out during the winter in the climate permits freeze/ thaw cycles which can really pry the skin loose from the ballast. The problem gets worse when the ballast contains ferrous materials. Here the ballast begins to rust and can reduce the ballast into a loose mass of matrix and rusting iron.
Beyond that, in a hard grounding the ballast is often forced aft and upward as well. In an encapsulated keel the membrane of the hull is at the outside of the keel and the membrane above the ballast is often quite thin. In a bad grounding the ballast keel is often pushed through this membrane causing a serious and difficult to repair damage and what can be a major inaccessible leak.
In my family we grounded a boat with an encapsulated keel that we never could permanently fix for as long as we owned the boat. The problem would get worse with every year, spreading from a small dimple on the leading edge of the keel to an area that was much of the bottom and sides of the keel.
Lastly, further compounding this issue, it is very hard to lay-up the glass in the keel cavity. As a result the glass work in this vulnerable area of the boat is often inferior to the glass work else where on the boat with large lenses of unreinforced resin or improperly wet out cloth. So instead of being the strongest part of the boat, this vulnerable area becomes the weakest.
In the end this is a classic question of low maintenance vs long life. In theory, a well made boat with a bolt on keel can be maintained forever but over its life it will need a fair amount of maintenance. In theory an encapsulated keel has low maintenance but at some point it will delaminate from its shell which will pretty mean the end of the boat as a structurally sound entity.