Monohull Design

     You might ask, how could I introduce a monohull description with a multihull symbol? Well, my monohull had twin keels.

     While I was working on the original aft-rig concept, I was also intrigued with the idea of homogenizing Bruce King's very successful twin daggerboard, SORC race boat design with the British twin keel cruising boat concepts. The 50' world cruiser,"Bluebird of Thorne", became my base model. I drew up my idea of a real blue water cruising monohull which made use of my mast-aft sailing rig. The twin keels offered the similar shallow draft exploration features of the multihulls, and the vessel could be beached in remote areas for cleaning the bottom and/or work on the prop, rudder, thru-hulls, etc.


Bluebird of Thorne



     Contrary to the use of symmetrical twin keels as popularized by Westerly, my design utilized asymmetrical foil keels which were placed at a 3 degree angle to the fore/aft centerline of the vessel. The asymmetrical foil shape was more efficient per square foot of keel area at developing the leeway reducing forces, thus the total wetted surface area could be reduced. The 3 degree toe-in was used to avoid the vessel having to crab sideways to its intended direction in order to develop any lift at all (symmetrical foils must be driven at some angle of attack in order to develop lift). The asymmetry and the pre-set angle of attack would act to significantly reduce the 6 to 12 degree leeway experienced by conventional symmetric-keeled vessels.

     The combination of the foil shape and the skewed angle of attack was accomplished with very little increase in total frontal area projection over a traditional single fat-foil keel. The two keels were attached to the hull such that as one became vertical upon heeling, the other assumed a more horizontal attitude and contributed to the righting stability of the vessel.


Mast-Aft 50' Mono



     Furthermore, the mast-aft rig's absence of a conventional mainsail imposed less leeway inducing forces so that the keel's surface areas could be reduced (extra wetted surface area was always recognized as a nemesis of twin-keeled vessels). The outer ends of both keels were tipped with small end-plates which further contributed to their efficiency ,and acted as grounding plates. And the overall lower center of effort of this sail plan allowed for less voluminous keels (less ballast required).

     This single-masted ketch configuration, with all roller furling, meant a sailing couple could handle up to a 70' version, and without having to leave the cockpit/saloon house cocoon in nasty weather.

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