Light Displacement Plywood Sailboat
|Displacement||4,956 kg||10,930 lb|
|Sail Area||52.4 m^2||564 ft^2|
|Ballast||2,200 kg||4,850 lb|
Plywood boats can be cool. A single chine plywood boat can have style and performance, yet still be within the abilities of a competent amateur builder. The key is to carefully fair the forward chine, stem, and bow sections to achieve a shape that blends the bottom and topsides together at the forward end of the boat. A hard chine in the middle and aft parts of the hull does not detract from the boat's aesthetics, and can actually add to sailing performance. The hard chine increases form stability for a given displacement, allowing more sail to be carried. I'm of the opinion the chine itself does not add significant drag from midships aft, it's only in the forward sections that the chine becomes detrimental. It is possible to completely eliminate the bump in the forward part of the hull caused by the chine, but the boat becomes more difficult to build. A compromise can be made between an easy to build hard chine boat made with sheet plywood and a difficult to build but sleek cold molded hull. The compromise is to use double diagonal plywood panels for the hull skins. The panels can be full width plywood sheets aft, narrowing to 24 inch or even 12 inch widths in the forward part of the hull. There is nothing new about double diagonal plywood construction; it has been around for decades. Some very nice sailing yachts have been built with this method, including the well known "Infidel" which was first to finish in the 1973 and 1975 Transpac races after being renamed "Ragtime". I sailed on Ragtime in 1973 from Santa Barbara to Long Beach. I can say from experience there is not a cooler boat on the planet. Larry Drake designed many sportfishing boats with double diagonal plywood hull skins. I remember working on the construction of two 87 foot Drake sportfishers while I was at Knight and Carver in the 1980's. I built the plywood bulkheads and engine stringers for one of them, the "Notre Dame". The double diagonal plywood skins allowed the hulls to be built relatively quickly, yet they had plenty of flare in the bow and a reasonably deep forefoot, which cannot easily be done with sheet plywood. The Drake boats required very little fairing on the hull and had a first class yacht finish.
The K-39 is a design which takes advantage of the simplicity of double diagonal plywood construction. There is no point in making a complex boat with a simple hard chine hull as that defeats the purpose of building a hard chine boat in the first place, which is to save on building cost and time. Complexity is avoided with the K-39. All the revisions made
in the course of producing this design were to reduce complexity. Sometimes it takes a little more effort to simplify a design without sacrificing the quality of the design. My goal in the overall design effort was to get the proportions right. Beam, draft, displacement, sail area, and cockpit space should all go together without any one feature appearing to stand out or be extreme. The double ended hull with a graceful sheer give the boat a little style. The purpose of the design is to have a boat with good sailing performance to be easily handled by one or two people. The cockpit is over 7
feet long with full size backrests. The fractional rig and underbody with fin keel and spade rudder are designed with windward performance in mind. Cruising in Southern California means sailing to windward in the Santa Barbara Channel, so a weatherly boat with a comfortable cockpit is essential. The fin keel has the same section from top to bottom which is a good compromise between simplicity of construction and maintaining a low center of gravity for a given amount of ballast. The rudder is high density foam core bonded to a stainless steel shaft assembly and covered with fiberglass. The fin keel and rudder are swept back just enough to shed kelp and seaweed. Kelp is everywhere in California and it is nice to have a boat that can sail through it without catching it on the keel and rudder. Sailing the K-39 is easy. The fractional rig has swept back spreaders and does not require running backstays. The boat is designed to sail upwind with a headsail that has minimal overlap, and downwind with an asymetrical spinnaker ... no spinnaker pole.
The interior is basic and is similar to what is being done on many of the sport boats being built today, such as the J-92 or J-105. Elaborate cruising interiors are nice at anchor, but I have always preferred a simple interior while underway. There are two settee berths in the main cabin with the galley forward. The head is behind a privacy bulkhead and there is a double berth in the bow. That is all that is required for cruising with two people who get along with each other. Headroom is only 5'-6", but I'd rather not compromise the look or sailing performance of the boat to get standing headroom. I'm over 6 feet tall and once owned a 34 foot boat with only 5'-3" headroom, so I'm OK with it.
The hull construction is deliberately uncomplicated. Framing for the double diagonal plywood skins is longitudinal stringers supported by plywood bulkheads. The Alaskan cedar stringers are relatively heavy since they span over 6 feet and are on the "flat" rather than on edge. The cast lead fin keel is supported by laminated floors. System Three or WEST System epoxy is used as an adhesive and coating for all wood structural members. The structure has been engineered to meet the requirements of the American Bureau of Shipping's Guide for Building and Classing Offshore Racing Yachts. I have found that the meeting the ABS guide results in a structure that is stronger and a bit heavier than many plywood boats built in the 1960's or 1970's. The reason for this is the design pressures in the ABS guide result in relatively heavy internal members; stringers, frames, keel floors, etc. I believe the structure of the K-39 is about as light as it can be and still meet the ABS guide. I purchased a set of plans for the Thunderbird plywood sloop to help me plan the structural design of the K-39. The Thunderbird was designed in 1958, and many T-birds have been built by amateur builders. I borrowed some of the T-bird structural arrangements for the K-39. If you can build a Thunderbird, you can build one of these.