Join Date: Feb 2000
Location: Annapolis, Md
Thanked 176 Times in 143 Posts
Rep Power: 10
Re: Bolt on Keel
I apologize that this is quite long and that I had written it for an earlier discussion, but it represents my take on this topic.
This is another one of those ‘no one universally right answers’ 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.) Here’s the way I see it.
Bolt-on keels tend to offer more performance since the ballast must be cast and without the keel stub skin thickness tend to place their vertical center of gravity lower relative to the center of bouyancy. They also have significantly less wetted surface and less frontal area making them theoretically faster on all points of sail. They are simple to repair and generally can be repaired satisfactorily no matter how bad the mistake.
On the down side they are more expensive to build; requiring precision casting, bolt hole drilling and a lot more hand fairing. They are higher maintenance requiring fairing every 10 years or so and 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 further reducing costs. If they are not damaged in a grounding, encapsulated keels are less expensive to maintain.
On the down side they are less efficient. Their real downside is the difficulty in doing a proper repair. 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. 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 around the ballast. This is especially a problem on a boat that is hauled out for cold winters where freeze/ thaw cycles 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.
In my life, one of our boats with an encapsulated keel nicked a rock and ended up with issues 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.
Beyond that, in a grounding the ballast is often forced 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 is pushed through this membrane causing a serious and difficult to repair damage and leaks.
Because it is very hard to lay-up the glass in the keel cavity, the glass work in the are vulnerable most vulnerable during a grounding is often inferior to the glass work elsewhere on the boat. In repairs that I have made when I worked in boat yards it ws not unusual to see ‘dry glass’ or lenses of unreinforced resin in this area. On the other hand, the glass often overlaps from either side of the encapsulation sides and the bottom of the keel is generally considerably thicker glass.
Much is made of boats losing their keels. This is a reasonably rare occurance and one which in my experience seems to happen equally with boats that have bolt-on keels and boats that have encapsulated keels (that is if you except race boats for the moment).
In the case of a bolt on keel, the engineering needs to include an internal framework that disburses normal service loads, and impact loads into the hull and surrounding structure. This kind of robust engineering has become reasonably routine in modern boats, but is not always as robust on ‘value oriented’ designs, and some racing designs as would be ideal, and was not fully accommodated in many early fin keel boats.
There is a perception that this level of framing is not required with an encapsulated keel. That is not necessarily the case. This perception is only valid if based on a variety of assumptions and those assumptions are true.
Part of the perceived basis for the belief in the encapsulated keels is that when the ballast keel is fully adhered to the encapsulation envelope, the ballast itself can serve as a part of the structure and help distribute service and impact loads. That is true when the ballast is properly adhered to the encapsulation. But the bond between the ballast keel and the encapsulation envelope is not always very well done, and over time that bond can often break down due to the leveraging of the ballast against the side of encapsulation envelope.
I personally have walked through a local boat yard and did an informal survey of boats in that yard with encapsulated keels. I basically sighted the keels and tapped on suspect areas, and took notes. More than 50% of these keels had voids between the ballast and encapsulation that were over a 1 ½ feet in diameter, and come had entire sides of the encapsulation delaminated from the ballast.
Another reason that encapsulated keels may not be a better option relates to the issue of internal framing and where failure mode in most keel failures. Surprisingly, bolt-on keels are rarely lost due to keel bolt failures. More often than not, the problem results from the failure of the fiberglass in the area surrounding the keel stub. The forensics in many of the most notorious keel losses showed that the hull itself failed beyond the are where the bolting takes place. These same areas of the hull receive similar kinds of loads on encapsulated keels. But while bolt on keels need and generally receive robust framing, that same level of framing is much rarer in encapsulated keels. Over time, these high stress areas are more prone to fatique and are reduced in strength, and in the absence of framing this is more likely to occur then with an encapsulated keel.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay