PAR, what is your comment? This is the tekst from one American boat designer
TWIN KEEL SAIL BOAT DESIGNS
Twin keeled sail boats have long been popular in areas with a large tidal range due to their ability to sit level when grounded. This feature is also valuable for minor maintenance chores such as bottom cleaning, painting, and zinc replacement. In theory, twin keel boats have some performance advantage when heeled over on a beat, since the leeward keel is in a more efficient, nearly vertical, position. Some designers have attempted to enhance this effect with asymmetric keel cross sections and/or adding several degrees of "toe in". A series of twin keel ocean racers (all named Bluebird of Thorne) were built in Britain shortly after WW II, and achieved moderate success. They appeared seaworthy, but performance off the wind was limited by the increased wetted surface of the extra keel. By replacing the heavy lead ballast with water I hope to reduce the wetted surface penalty by having the option of pumping out the water under light air conditions. The initial results are very encouraging, promising the dynamic stability and rough weather performance of a mono hull, and the speed and static stability of a multihull. As an "extra bonus", the interior volume is high, allowing a wide variety of arrangements and living space.
STABILITY CONSIDERATIONS:
Each twin keel is designed to displace half the total ballast, and is located as far outboard as possible This arrangement lowers the center of gravity and maximizes stability at low heel angles, since the flotation effect of the windward keel is quickly lost if the boat heels enough to pull it from the water. This is exactly like a catamaran. Filling the keels with water adds thousands of pounds to the boat's weight, sinks it down in the water 4-6", and increases static stability by about one third..
To quantify this increased effect, I calculated the stability of two 36' boats having the same displacement (15,000 pounds) and ballast (5,200 pounds). The single keel design had a 12' beam, lead ballast, and 6' draft. The twin keel had a 16' beam, water ballast, and a 3' draft. The water ballasted design was four times more stable at 10 degrees of heel, three times more stable at 30 degrees, and twice as stable at 50 degrees. The two boats had equal stability at around 80 degrees, and the twin keel capsized at 115 degrees. The single keel held out until 130 degrees. The twin keel design has a solid stability advantage under 80 degrees of heel, and in the event of a capsize, the water ballasted boat will not flood and sink. The water ballast is also much easier to contain structurally, compared with lead.
Boats only capsize under the most violent action of wind and water. Resistance to these forces requires dynamic stability as well as static stability. Dynamic stability determines how quickly the boat will responds to a wind gust or wave action. A wind gust or steep wave can introduce very large forces, and a light weight boat will pitch or roll violently, increasing the probability of a capsize. Dynamic stability can be increased by moving heavy components away from the center of the boat. These twin keel designs put the ballast around 7' from the center line, resulting in several times more dynamic stability than the single keel (just imagine how hard it would be to roll a catamaran that had both hulls full of water). The bottom line is that a water ballasted twin keel design can have much more static and dynamic stability than traditional multi or mono hulls. The down side is that if these wide beam designs capsize, their inverted stability is also very high, and they will want to stay upside down. However, if the twin keel design does capsize, it will not sink, even if damaged. The water ballast can also be drained out, providing extra flotation (or drinking water if you planned ahead).
01-23-2010, 01:06 PM
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