Water Ballasted, Twin Keel Sail Boats

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        I have been developing water ballasted twin keel cruising sail boat designs from 13’ LOA to 24’ for a number of years. They are all beamy, light weight, stable, and have a large interior volume.  With the water ballast drained, these boats offer improved light air performance, which helps compensate for their larger wetted surface. The bottoms have a deep vee to reduce pounding and provide additional water ballast volume.  Centerboards, dual rudders, and a variety of rigs are possible. All designs can be safely beached for maintenance.

        These boats started life in the same mold, but the modified twin keel version appears to be "evolving" and is very comfortable ashore.  These boats are identical, except for the twin keels, and have been tested together under a variaty of wind conditions.  The twin keel design  tracked very well and felt very stable when the keels were full.  Speed was slightly less then the unballasted centerboard sister ship, due to the increased wetted surface.  The twin keel boat could use more sail and is presently being modified.  My gut feel is that the increased sail area will make the boats about equal under light conditions and the water ballast will greatly help when the wind is above 15 knots or so. (back to water ballast article)

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 penality by having the option of pumpimg 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).

Grenada 14

        The Grenada 14 evolved from the 13' test boat described above.  The hull shape was flattened, with harder bilges and slightly more beam, the amount of water ballast was increased, and the sail area raised 15%.  Water ballast is carried in the hull as well as the keels, which allows the keels to be thinner, resulting in reduced drag at high speed.   The hard chines make this boat perfect for "stitch and tape" construction, which greatly speeds up the construction process.  The deck is varnished wood strips, which helps make up for the ugly hard chines.  The waterballast system is very simple, one valve for each keel.  Open the valve and the keel fills(or drains if your out of the water).  Close the valve and the water is captured.  If you want more performance, especially in light conditions, pumps may be added to transfer ballast.   Each keel contains roughly 20 gallons of water, so the pumping system needs to be sized accordingly.  The base of each keel is covered with a steel plate which allows the boat to be draged over rough surfaces without damage.  A 5hp outboard can be mounted next to the rudder.

  Water ballasted sailboats are becoming popular in part due to their low trailering weight. With the water drained, these boats can be pulled by a small car, and are one solution to high slip fees. Trailering also opens a vast cruising area, from the Pacific Northwest, Florida and the Bahamas, Chesapeake Bay, and the Great Lakes. All just a "freeway away"!

    The GRENADA 24 combines the advantages of twin bilge keels and water ballast. The boat can be beached safely, gains stability by placing the water ballast low in the twin keels, and is light enough to be trailered. She is designed to accommodate four people on trips of one to two weeks duration, and is suitable for coastal cruising and exploring inland waters.

    The GRENADA 24 can be built by anyone with average woodworking knowledge, and is designed to require a minimum amount of special tools or skills.  Plans include a building manual, Five pages of drawings, and nine full size patterns.

        Traditional wood strip (or DURA CORE) and fiberglass construction is recommended for these designs. One unusual feature that greatly speeds up construction is that the deck is built first, upside down, over simple cambered forms. After fiberglassing the deck, the hull is formed using temporary building frames set directly on the finished deck. No sheer clamp is required, and the hull to deck joint is fully fiberglassed (both sides) and cannot leak or flex. Computer generated full scale patterns eliminate the need for lofting.

        The hull is stripped and glassed with a minimum of one layer of 24 oz. woven roving. The keels are built in a similar fashion, except plywood is used for the sides. Each keel is finished and glassed before mating with the hull. After attaching the keels to the hull, the joints are faired and reinforced with additional layers of fiberglass. After turning the boat over, the insides of the hull and top surface of the deck are cleaned up and fiberglassed. The top of each keel is fitted with a heavy fiberglassed lid, and bonded into the sole structure.

        The final result is a wooden cored sandwich hull/deck assembly with a hull to deck joint that is fully reinforced on both sides. This process is fast and simple, and results in a stiff, strong, damage tolerant boat. The decks, cabins, and cockpit are fabricated using traditional plywood techniques and fiberglassed to the hull structure. 

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