Tuesday, November 10, 2009

Keel? Why Is There a Keel?

(For those of you reading because you are attracted to the romance of sailing into the sunset, or the peace of being anchored in a quiet cove at dawn... There is no romance in what follows - it is the technology which enables the romance. But I promise: no equations or vectors, just common sense and analogies)

Why would you take a perfectly good boat, and then handicap her by hanging 6 tons of lead (in Eolian's case) from her bottom?

Two reasons. The keel actually performs two separate functions that, in a wonderful bit of convergence, work out to require the same structure.

First: Keep the boat from falling over.

A sailboat has a tall mast, and carries a lot of sail up there, The weight of the mast and rigging alone, plus the sails would make her unstable and likely to fall over without the weight down below the waterline. Then there is the tremendous force that the wind adds! The hull actually acts as a fulcrum on which the mast/keel arm pivots. As the mast descends due to weight and force of wind, the hull rolls and forces the keel to rise, like a teeter-totter (<-- I think that may be the first time I have ever typed that word... it looks strange to me). As the boat rolls farther, the keel is pulled out farther away from directly below the hull, increasing the roll resistance. At the same time, as the mast goes more and more toward the horizontal, it becomes less and less effective at catching the wind. therefore the system is stable and self-correcting. As the boat heels more and more, it becomes more and more difficult to increase the angle of heel. The greater the weight, and the lower in the water below the hull it is suspended, the greater the stability which results.

Second: Keep the boat from sliding sideways in the water.

When the boat is sailing in any direction except dead downwind, the force of the wind on the sails is not along the boat's line of motion. To make this possible, something is needed to create a force which will resist the hull's desire to drift downwind. Once again, the keel comes to the rescue. In this case, the shape and surface area are the controlling factors; weight is irrelevant.

In the early days of yacht design, it was the "keel-as-a-barn-door" theory that prevailed. The idea being that a barn door would be really hard to push sideways thru the water. And in fact this is true. It leads to the full and modified-full keel designs which have been with us from antiquity. These boats (including Eolian with a modified full keel) are additionally very stable and easy to hold on course.

Tho the full keel design works well at optimizing one parameter: lateral resistance, it fails badly at optimizing another: wetted surface. Dragging anything thru the water takes effort - an effort that is proportional to (among many other things), the amount of surface area submerged in the water. Given a constant propulsive force, decreasing wetted surface will result in an increase in speed.

In the middle part of the last century (that would be the 20th century...), the search for a keel design that would provide the needed lateral resistance with decreased wetted surface was on. This led to the modern fin keel, when it was recognized that the keel moving thru the water could be viewed as a wing (much as the sail can be viewed as a wing operating in the air above), and thus could be designed as a hydrodynamic lifting body, - long and narrow with an airfoil cross section, instead of a barn door. Fin keel boats are faster, but are a little less stable - it takes more steering to keep them on course. However, they are far more maneuverable in close quarters, like a marina, since the boat pivots easily on the narrow fin.


At first, the fin keels were just (nicely shaped) slabs of lead or cast iron. But then another conceptual breakthru came when it was realized that, if the keel material were strong enough, the bulk of the weight could be concentrated at the bottom in the form of a bulb, maximizing the righting moment that any given amount of keel weight could deliver.

Next, bear with me for a moment as we consider the keel as a lifting body. Consider the more familiar form of a lifting surface: an airplane wing. No matter how you describe the mechanism that makes it generate lift, it is a truism that the pressure on the bottom of the wing exceeds that on the top, if it is generating lift. Now what happens at the end of the wing? Yup... air flows out from under the wing and tries to fill the low pressure area on the top. This leads to the tip vortices that force air traffic controllers to space out flights at an airport. And it leads to a decrease in lift. The small vertical winglets seen on the wingtips of the most modern planes are effective at blocking this bottom-to-top flow.

Now back to the keel... it needs to lift from either side as the boat moves from one tack to the other., making each side of the keel alternately the high pressure and then the low pressure side. And yes, tip vortices rolling off the bottom steal away some of the effective lift of the keel. The same thinking that brings winglets to airplane wings has brought the wing keel - but with winglets on both sides. And in addition, the winglets are usually a modified form of bulb - that is, they contain a substantial fraction of the keel's weight. Because the wings increase the keel's effectiveness as a lateral plane, and because they allow weight concentration at the bottom where it is most effective, wing keel boats can have shorter keels than their fin keel counterparts with equivalent performance. Conversely, with the same depth and weight, a wing keel will increase both the boat's stiffness and its pointing ability.


Working in the opposite direction, it is also possible to divorce the functions of roll stability and lateral resistance into two separate structures. The shoal draft/centerboard boat is such a case. In this design, the ballast keel is increased in weight, but held relatively high up, giving a shoal draft configuration. Because this keel generates very little, lateral resistance is provided by a retractable centerboard, which is weighted only enough to keep it in place. With the board extended, excellent lateral resistance is generated; with it retracted the boat can venture into thin water (tho the keel alone will not provide enough lateral resistance to allow thin water sailing without making huge amounts of leeway, unless nearly directly downwind).

The Future

So, what comes next? Venturing into prognostication, I have seen some interesting developments that have been tried on the "any $$ for 1/10 of a second" boats. Some of these are:
  • Canting keels - they can be hydraulically tilted from side to side to increase the righting moment
  • Keels with tabs - flaps really, keeping the aircraft wing analogy. The flaps increase the lift that the keel generates
  • Boats with two steerable keels, fore and aft and no rudder
All of these ideas presume that something movable can be kept working in an environment that features barnacles, mussels, crab pot lines, and the occasional ill-placed rock. To survive in this environment, things have to be simple and robust - no delicate linkages need apply.

We'll see.

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