Keel types compared: modern or old school, which is best?

Fluidodynamics. With this little magic word we can summarize much of the scientific studies on the world of sailboats and keels in particular (although the major field of use of this discipline is aeronautics). The shape of keels, drift blades, and ballasts crucially determined the development of sailboats and their design even with striking cases. Think, for example, of 1983 in Newport, when the famous “flaps” on Australia II’s keel interrupted 132 years of Stars and Stripes victories in the America’s Cup thanks to the intuition of Ben Lexcen, designer of the Australian boat. But it is wrong to think that these studies are only important for the racing world; the cruising world has also improved dramatically through the evolution of keels. Safer, more stable boats with greater righting moment and capable of sailing more effectively in all gaits, as well as being “steadier” even at anchor: the world of cruising sailing has received an indispensable contribution from design evolutions first experienced in racing.

Originally there was a long keel. Safe, one with the hull, very solid, but poorly efficient in a wide range of situations. Rather high center of gravity, poor lift in light wind, little righting in strong wind, little stability in the stern in wind and wave: for an endless series of reasons the long keel has been overtaken by history. Today if we take a tour of a boatyard among the boats on dry dock we will see basically three types of keel: the trapezoidal fin, typical of boats of the 1970s-80s and part of the 1990s, the boot or L-shaped keel, and the inverted T-shaped keel. If the trapezoidal is used somewhat less today the other two types and their variations are reality. Not forgetting additional variations such as canting keel or lifting, or true canting keel and retractable keel.

SHORT OR DEEP DRAFT, WHICH ONE TO CHOOSE?

Let’s start with a concept: what are the differences between the systems just listed (T, L, or trapezoidal)? The positioning of the center of gravity is the striking difference. In ascending order, with the same draft, the one to have the lowest center of gravity is the T-shaped keel, followed by the L-shaped keel and finally the trapezoidal keel. If we look at a modern racing keel we will notice a sharp, thin drift blade, at the end of which is positioned a very tapered torpedo shape. The deeper the keel, regardless of its shape,-or the more weight concentrated at the bottom-the more the righting moment increases, and in a directly proportional manner the greater the draft the less the bulb weight can be to achieve a righting objective. It follows, therefore, that a basic requirement of a good keel is to have a low center of gravity; on the side of cruising boats, which generally have L-shaped or trapezoidal keels, draft depth also becomes crucial. So if you are wondering which keel to choose for your new boat in the options offered by the shipyard, short keel or long keel, if your goal is to sail well, comfortably and efficiently, have no doubt: deep keel is better, it will lower the whole center of gravity of the boat.
What is the use of having a low center of gravity? Undoubtedly to skid less, and that is no small thing. But also to reduce drift and allow a more generous sail plan to be drawn, and last but certainly not least, the boat will also be more stable at anchor. Deeper keel equals less weight needed in ballast, so indirectly the whole boat will weigh less.

HOW DO DIFFERENT TYPES OF KEELS PERFORM?

Taking trapezoidal, L-shaped, and T-shaped, each has weaknesses and strengths, although in terms of pure performance there is no doubt that the T-shaped one is the most effective. But performance is only one aspect; in the diverse world of sailing there are many others that matter a great deal and perhaps more. The trapezoidal fin, for example, is much easier to construct given its simple geometry and is particularly strong in case of collisions given the large attachment surface area on the hull. Rather tenacious also the L-shaped one, while the T is certainly the most fragile in case of impacts, since often the chord of the drift blade, i.e. how wide (not thick) the blade is, is very restrained to improve speed.
The trapezoidal fin, having a smaller righting moment, subjects the boat’s structure to less stress, while with a T- or L-shaped keel the loads are significantly greater. For this reason, boats equipped with a blade and torpedo will have special construction features to absorb the loads imposed on the hull by the appendage and in most cases will cost much more.
Then there is another important factor to examine, the material of construction. The most common ones are lead and cast iron, which have markedly different behaviors and yields.
The specific gravity between the two materials is different: that of cast iron is lower, around 7 kg per cubic decimeter versus 11.34 for lead. What does it mean? It means that more cast iron than lead is needed to achieve the same weight characteristics: as a result, the keel made of cast iron, with the same weight and draft as the one made of lead, will have a larger volume and this will result in greater hydrodynamic resistance therefore generally a slower boat. But there is another element in lead’s favor, the shock reaction. Cast iron does not deform, and any impact will be discharged on the entire boat structure; lead, on the other hand, changes its shape with a violent impact and thus, at least partially, cushions the blow. Basically if you find yourself faced with a choice between cast iron and lead the latter should definitely be preferred.

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