Modified 75 Square Meter Sailboat
|Displacement||9,800 kg||21,600 lb|
|Sail Area||79.3 m^2||854 ft^2|
|Ballast||4,700 kg||10,360 lb|
The square meter classes, or Skerry Cruisers are fascinating boats. They are long, sleek and easily driven with a minimal amount sail area. They achieved their greatest popularity during the 1920's. The 30 square meter and 22 square meter classes are still active in Europe.
Skerry Cruisers are built to the Square Meter Rule which is still administered today. The rule limits length, beam, displacement and freeboard for a fixed amount of sail area. There are nine classes based on sail area, the most well known being the 30 square meter and 22 square meter classes. In addition to the 22 and 30 square meters, there are 15, 40, 55, 75, 90, 120, and 150 square meter classes. Each class races boat for boat and there are no time allowances. Sail area is taken as mainsail area plus 85% of the fore triangle area. There are a lot of restrictions on the rig measurements which is why Skerry Cruiser rigs have a distinctive look with tall mainsails and relatively small fore triangles. Headsail overlap and spinnaker girths are unlimited. Because of the unrated sail area of overlapping jibs and large spinnakers, some of the boats designed to the Rule were disproportionately long and heavy. The trend towards "freak" sails and long, heavy boats in the 30 square meter class was a disappointment to L. Francis Herreshoff, who introduced the 30 square meters to the U.S. The Rule was first adopted in 1908 and had a major re-write in 1925. I believe the current Rule is more or less the same as the 1925 version. The Rule allows designers to increase length by adding displacement, beam, and freeboard.
It's an interesting concept. Take a given amount of sail area and design the most efficient hull possible. This is the polar opposite of what is being done today with many sailing designs, both racing and cruising, which have large rigs that are overpowered in 15 knots of breeze.
The K-60 is a modified 75 square meter class. The hull, cabin, and sail plan are true to the Square Meter Rule. Although the hull is optimized for the Rule, I kept the proportions within reason to avoid the issues that so concerned Francis Herreshoff. Strict adherence to the Rule requires a keel of a minimum length based on boat length with the rudder attached to the keel. The K-60 deviates from the Rule by utilizing a fin keel and spade rudder. My primary motivation for the fin keel and spade rudder is construction management. The lead keel and rudder can be built offsite separately from the hull and deck. The canoe stern is for style. This is a yacht after all, and a yacht should be attractive and distinctive. L. Francis Herreshoff designed a 30 square meter with a canoe stern (Oriole US-1) and so did George Owen (Yankee US-6). The K-60 mast has swept back spreaders and jumper struts to provide fore and aft support. Running backstays are used for headstay tension, but they are not critical for mast support.
Interior accommodations meet the standards put forth by the Square Meter Rule. There is 6'-2" headroom in the galley, which is nice to have in such a low freeboard boat. The K-60 is a little too big for tiller steering in my opinion, so the boat is fitted with pedestal wheel steering. The sail plan is a bit small by modern standards, but that is the appeal of the design. Get as much speed as you can with a fixed amount of sail area. This boat will be easy to sail singlehanded or by a crew of two. The sail area to displacement ratio of the K-60 is 17.6. This is not low for traditional sailing yachts. The Kettenburg PCC's have a sail area to displacement ratio of 17.1 and they have plenty of sail area.
Comparison ratios like sail area to displacement and displacement to length must be used with care. The ratios can vary considerably depending on what load condition is used for displacement. Is it IRC empty weight, minimum operating condition, maximum operating condition, half load condition, or a number put out by the designer to fool the competition? All of the above are used, so published displacement figures must be taken with a grain of salt.
Construction of the K-60 is engineered to meet the requirements of the American Bureau of Shipping's Guide for Building and Classing Offshore Racing Yachts. The Square Meter Rule has its own scantling requirements, and I believe the Rule construction weights are similar to a boat designed to the ABS Guide. The K-60 hull is cold molded wood with diagonal planking supported by longitudinal stringers that are in turn supported by transverse frames. Heavy frames amidships support the transverse loads from the keel. The deck is plywood over transverse deck beams. The cockpit and cabin are a one-piece fiberglass molding using E-glass and PVC foam core. The ABS Guide is conservative, particularly for wood construction. Design pressures are based on length and canoe body draft, with draft being weighted more than length. Under the ABS Guide, a wide shallow boat will have lower hull pressures than a narrow deep one of the same displacement and length. So, a narrow and deep boat like the K-60 is subject to relatively high design pressures. The net result is a structure that is heavy in comparison to other cold molded boats. This is fine with me.
Another aspect of the ABS Guide that has engineering ramifications is its treatment of the cold molded laminate. ABS requires that the allowable stress of a cold molded laminate be reduced to 22% of what it would be for other wood construction such as strip planking. This is because the wood fibers are oriented about 45 degrees from the supporting structure. The ideal is 90 degrees to the supporting structure. However, the diagonal adds some support to both the longitudinal stringers and transverse frames. This is from the US Coast Guard Guidelines for Review of Structural Plans for Wooden Vessels (H1-13):
"(W)ith cold molded shell and deck plating, the cold molded wood plies in which the grain runs parallel to the ply, are generally laid at plus and minus 45 degrees to the longitudinal axis of the boat. Tests conducted by ABS Americas have determined that the ultimate strength in the principal axis of a panel laid up in such a manner is about 22% of the ultimate strength in the principal axis of a panel in which the plies are laid parallel to the principal axis. Thus, without the builder conducting material tests on the cold molded wood laminate, the modulus of rupture value to be used in the design calculations must be 22% of the particular wood speciesí established modulus of rupture."
The bottom line is the structural engineering of the K-60 is primarily driven by the big strength reduction of cold molded
wood required by ABS.