Catching no air

Boats designed to compete in the BOC singlehanded around the world race have, understandably, become more specialized with each race. The trend now is toward shallow-bodied, wide-beamed, “skimming dishes” with deep bulb keels and towering rigs.

These boats also tend to have twin rudder - like a pair of inverted tail fins. For example, Michael Carr's 60-foot Class I boat, named Imagine, nearing completion at Howdy Bailey Custom Yachts in Norfolk, VA, is built of 70% recycled aluminum, has a 100-foot rig, a 20-foot beam, a 15-foot-draft bulb keel, and, naturally, twin rudders. Since racing boats should be as light as possible and have minimum wetted surface area, why carry two rudders when one can do the job? The answer lies in the complex dance of compromise required of any design, even one as seemingly unlimited as a BOC racer.

When the tall powerful rigs on these boats are set with thousands of square feet of sail, a counter force is required to keep the boat on its feet. One way to do this involves using a deep keel ending in a ballast bulb. Another method that has evolved with BOC racers is to increase the vessel's beam and to use water ballast tanks. Wide beam has two advantages: it allows water ballast to be placed far outboard, increasing its effectiveness, and it gives the boat added form stability. In effect, the buoyancy of the wide hull "pushes" against the water and helps to hold the boat upright.

These benefits clearly argue in favor of a wide hull. However, everything in boat design involves compromise. In this case, the compromise comes with the rudder. A single rudder mounted on the centerline won't work because of a pressure effect called rudder ventilation. When a rudder ventilates, it loses lift and the helmsman (or, in the case of a BOC race boat, the autopilot) experiences a loss of control. Depending on the situation, this can result in anything from the boat slowing down to a wild broach. The name ventilation provides a clue as to what's happening during this phenomenon: air is sucked down one side of the rudder, playing havoc with the established water flow.

To get an idea of why that water flow is important, let's look at the job rudders and keels do on a sailboat. The keel and rudder on a modern sailboat are generally symmetrical foils. When the boat is properly trimmed and the keel has the correct angle of attack (due to leeway angle), a pressure differential is created between the two faces of the foil: high pressure on one side and low pressure on the other side - or, a pressure side and a suction side. The suction side translates into lift that helps pull the boat to windward.

A similar process occurs with the rudder. When the angle of attack is correct, lift is produced. This lift helps to keep the stern up to windward, allowing the boat to track.

However, when a boat heels, the top of the rudder, where it meets the hull, can be exposed to air and ventilation becomes a possibility. Depending on the shape of the hull, the bow can also settle, causing the stern to rise higher out of the water. Some IOR boats, for example, had this problem due to their narrow bows. Anything that allows the top of the rudder to come out of the water can lead to ventilation.

This arcane process works something like this: High-pressure water on the leeward side of the foil bulges upward into lower-pressure air. On the suction side of the foil, however, atmospheric pressure is greater than that of the low-pressure water. This causes air to bulge downward and form a depression in the water (see accompanying diagram). If the boat increases speed, pressure on the suction side decreases. This draws the air pocket deeper down the face of the rudder. This process can continue until a large pocket of air is sucked down to momentarily envelop the face of the rudder. This gulp of air disturbs the water flow and can cause a sudden loss of control - not recommended for any boat, never mind one with a 100-foot mast that is blasting along at 15 knots.

Ventilation is a special problem for BOC-type boats due to their wide beams. When Imagine, for example, heels at its optimum angle of eight to 10 degrees, the top of a centerline rudder would be exposed to the air.

It is to remove the chance of ventilation that Imagine is equipped with two rudders that are offset six feet from the centerline. "The rudders are designed so that the leeward foil will be vertical at the approximate optimum heel angle when going upwind," said Hal Whitacre of Kaufman Design in Annapolis, the company doing the design work on Imagine. "When the boat is heeled, the leeward rudder will be immersed and free from any ventilation problems."

The leeward rudder will be immersed, but the windward rudder will not, only part of its area will be in the water. Since the top of the rudder will be only partially immersed, it will suffer from . . . you guessed it, ventilation. "Due to only being partially immersed and due to ventilation, the weather rudder's effectiveness will be reduced," said Ken Court of Kaufman Design, "but it will still be doing useful work." For any rudder, there are two primary factors that determine its overall effectiveness: how far aft it is placed and its surface area. The farther aft a rudder is placed, the more effectively it can turn the boat. That is one reason that transom-hung rudders, while not as efficient as high aspect spade-type foils, work well because they are as far aft as possible. Area is the other factor: A large rudder can use smaller helm angles and thus produce less drag. A small rudder has to use larger angles and that results in induced drag as turbulent water detaches from the foil and forms eddies. "At angles of 12% and above," said Court, "the flow around a rudder breaks down and the rudder stalls."

There is no free lunch, however, because a larger rudder means more wetted surface area and more skin friction. This drawback is greatest when Imagine is sailing downwind, since both rudders will be in the water, producing drag. According to Court, however, this is an acceptable given the ventilation protection that the twin rudders provide.

In order to get maximum lift to drag numbers, a deep skinny foil is best. However, Kaufman Design and the Carr Campaign weren't free to design any radical shape they wished. Overseeing the design and production of Imagine is the American Bureau of Shipping (ABS), a private, not-for-profit organization that sets standards for design and construction of all types of vessels, from large commercial ships to yachts and offshore race boats. An ABS certification ensures that a vessel meets tough standards for strength and durability. ABS inspectors verify the materials used in construction, they check on welding procedures and the qualifications of the welders, and ensure that the vessel is being built according to its plans.

While Kaufman was able to design a high aspect foil, they also had to meet the ABS requirements. These rudders are strong enough to withstand being broadside to the water flow when the boat is traveling 30 knots. Presumably, Imagine won't ever be in a situation where it's traveling on it beam ends at that speed.

Of course, there is another person who wants a light boat, but who also wants a sturdy boat: its sailor, Michael Carr. "There have been failures of rudders because they were lightly built," said Carr."The whole mix of weight, speed, and competitiveness has to be put in context. You have to look at the materials available, what you're trying to accomplish, and the race course. In the Southern Ocean, in surfing conditions, it's possible to reach 20 knots. Hitting a small piece of ice at those speeds could seriously damage a lightly-built rudder."

In addition to strong rudders, the rudder's shape must be optimized. The most efficient shape in terms of lift to drag is an elliptical foil. However, building an elliptical foil correctly can be time consuming. A faster, more reliable method is to build the foil in a trapezoidal shape using design parameters called taper ratios (taper is the change in width of the foil from root to tip, or top to bottom). Using the proper taper ratio will allow a trapezoidal shape to assume elliptical loading. It will act like an elliptical foil and provide high lift to drag numbers.

So, if the rudders are designed and built correctly, they will be light, yet strong enough to survive high speed collisions with ice; they will have enough area to provide control, but won't have so much that they create unnecessary drag; and they will be constructed in a trapezoidal shape, but, if all goes right, will perform like elliptical foils.

We haven't even discussed how the steering cables will be run, or how the rudders will be driven by a sophisticated autopilot setup, or how the rudder quadrants will be cross-linked, or how the wheel will be clutched in and out, etc. The rudder details alone are staggering. Imagine, when completed and under sail, will be an impressive combination of speed and strength; an offshore vessel destined to sail the wildest, loneliest, most challenging stretches of ocean on the planet.

By Ocean Navigator