Encapsulated or Bolt On Keel?
(This is a long one)
It''s not too hard to explain how I can keep on saying bolt on''s are equal to an internals. Lets start with the basics, niether bolt on keels or encapsulated keels are inherently stronger or weaker. It all comes down to how any given example of each is actually engineered. They each have to deal with a variety of loads they both have to spread these loads out into the hull and they both have to deal with the trauma of a heeled boat dropping off a wave or running at speed into an immoveable object.
If you look at the way side forces work, the maximum force occurs where the keel stub hits the hull and not necessarily where the ballast meets the hull. The keel is a simple cantilever and maximum bending moment occurs at the fold in the hull where it turns down toward the keel. What happens structurally at the bottom of the keel is almost irrelevant to the discussion.
So it is that the area that turns down into the keel needs to really have a lot of strength and that area needs to be reinforced to distribute the loads out into the hull. In a properly designed hull, there are a series of athwartship frames that take the load out into the hull and often there is a longitudinal frame that distribues these loads fore and aft. It does not matter whether we are talking about a bolt on keel or an encapsulted keel, these framing elements are critical to achieving the necessary strength in this area of the boat almost no matter how thick you make the fiberglass.
And these athwartship members have to be connected to the ballast keel so that they take the sideward moments (bending forces) from the ballast keel and connect them to the rest of the hull.
Here''s where it gets harder (but not imposible for an encapsulated keel). If you visualize a boat with an encapsulated keel at an extreme angle of heel, the weight of the ballast keel has a lot of leverage pushing against the sides of the encapsulation. The high side of the keel where the ballast ends is trying to pry the encapsulation away from the ballast. All that glues the ballast to the encapsulation typically is a polyester resin slurry. Polyester resin is not a great adhesive especially to lead or iron, and so oved time that adhesion fails.
You might think that is no big deal. The ballast is in there and where can it go. But lets go back to our earlier discussion. As I said earlier it is critical to transfer the side loads from the ballast to the athwartships frames. In a properly engineer encapsolated keel, this is done through a series of bonds.
To explain, once the ballast keel is ''glued'' in place in the fiberglass keel cavity, the top of the keel is glassed over. Normally this layer is pretty thin and is only intended to keep water out of the keel and if there athwartships frames, provide bonding for the athwartships frames. Then the athwartships frames are bonded to this membrane. Done right and still intact, the ballast transfers its loads to the sides of the keel cavity, which transfers most of the loads to the athwartships frames through the membrane at the top of the keel. And there in lies the problem, once the bond between the F.G. keel and the ballast has loosened the ballast is prying the top of the keel membrane away from the side of the keel cavity and with that goes a weakening of the athwarships connection to the hull.
Walk around most any boatyard with a lucite hammer and tap out the area about 6" to a foot below the top of the ballast on any older boat with an encapsulated keel and you will find that many, if not most, have a void in this area.
Making this problem worse is that this is an area that is next to imposible to repair because once water gets in there (and it does) it is very hard to re-establis a connection between the ballast keel and the hull.
Now then run that boat aground. There''s a couple things at work here. First of all, it is very hard to do a proper glassing job at the bottom of the keel. You are laying up glass in a narrow cavity that a person really can''t get into very well. Although there are lots of tricks to doing the work in these tight areas, having repaired quite a few of these in my day, the glass work at the bottom of keels is very heavy, from the laps in the cloth, but not very sturdy. There are usually large lenses of unreiforced resin and sections of dry cloth.
When you run aground hard, the force generally crushes the cloth and drives the ballast upward against the membrane to the top of the keel and the athwardhips frames and pulls downward at the forward end of the keel. Now you have a leak. Maybe small maybe large, but not one that can be easily repaired to full strength,
So now for the other side of the story. In the case of a bolt on keel you really do have greater maintenance. The fairing materials tend to be short lived. This is a pain in the neck process and even using epoxy it has to be done every 7 to 10 years. (That said it is no worse than fixing blisters that come from moisture trapped between the keel cavity and the ballast working its way out through the not so great glasswork in the keel cavity. But not every encapsulated keel boat gets blisters while every bolt on keel boat sooner or later needs refairing.)
At some point, on most production boats with bolt on keels the SS keel bolts need to be replaced. (I have never understood why for just a little more cost the boating industry does not use Monel Keel bolts but that is another story)
When we talk about the strength of a bolt on keel we are really discussing depending on the ability to make a mechanical connection vs than a glued connection. I''m a mechanical connection kind of guy. Also The stub is generally shallow enough that you can get really good glass work and the overlaps occur so that you have the thickest amount of glass where you need it most, right at the top of the keel. Often, on better built, low volume boats, the athwartships frames are often glassed right in as the hull is being laid up. (On less expensive boats and mass produced boats the frames are generally molded separately or as part of the liner and glassed or glued in.) The keel bolts then pass through the outward turned tabbing or flanges on the frames making a solid mechanical connection.
In the same grounding as above, the metal (especially lead) absorbs much of the shock of the impact before ever distributing the loads to the keel bolts. The bolts have to distribute all of the loads in a compact area. Here is where engineering is critical and why there is nothing worse than a poorly engineered bolt on keel, and nothing better than a properly engineered one. In that impact, the aft end of the keel pushes up. In a properly engineered bolt on keel, that upward force is distributed through a solidly engineered frame at the aft end of keel which carries the loads into a wide area of the boat. Often there will be a bulkhead (seat face or other cabinetry) or massive longitudinal member tied to this frame that distributes the loads fore and aft, and a athwartships bulkhead that distributes the loads into the hull and deck. (That same structure should be there in an encapsulted keel but it absolutely needs to be there on a bolt on.)
At the forward end of the keel there needs to be a similar frame as well. This is the frame is perhaps easier to engineer. It only needs to have sufficient bearing area to not slice down through the hull and to withstand the withdrawal forces of the forward keel bolts. Lastly there is shear but that''s the easy one. All it takes is enough surface area on the bolts and enough bearing plate area to prevent the bolts from acting strictly in single sheer.
Of course a proper bolt on keel requires better engineering and is probably a more expensive keel to build. The keel casting must be made carefully. The bolts, and bearing plates are expensive. Great care must be taken in boring the holes for the keel bolts and there is a lot of labor and handwork in fairing the keel.
This is why a lot of manufacturers take the short cut of doing an encapsulated keel. This is especially true of smaller production yards where the cost of precision tooling a keel casting can be exessive.
In the end it is a trade off between the maintainable and the low maintenance. Encapsulated keels are low maintenance until they can no longer be repaired.
To address your final line, while it is true that the keel cavity is integral with the hull, the ballast in not integral with the with the keel cavity. There it comes down to whether you trust an not extremely good glue (polyester resin) or mechanical fasteners.
(Heck, once upon a time, wooden boats had internal ballast, but they went to bolt on keels for the same reasons outlined here.)
For those of us, like myself, who tend to own older boats, I would think that the ability to repair a bolt on keel far outweighs the potentially lower maintenance of an encapsulated keel.
At least that''s how I see it.