by Tad Roberts, naval architect, BC, Canada

The stern does a lot of heavy lifting in a full dis­placement hull, and one of the oldest stern shapes is the fantail. The photo above is of three fantail sterns, all slightly different due to the intended use of these boats. Fantails in classic powerboats are descended from the early fantails used on square-rigged sailing ships. This makes the form one of the oldest styles, while its efficiency with minimum power is undeniable.

A full displacement hull is one intended to slip through the water at relatively low speed, not exceed­ing a speed/length ratio of 1.34. Speed/length ratio is the speed in knots divided by the square root of water­line length in feet. The speed/length ratio of 1.34 is of­ten referred to as “hull speed” because up to this speed the major resistance factor is surface friction, while beyond this speed wave-making becomes the major resistance factor. Full displacement hulls are those designed to operate efficiently up to hull speed, but due to rapid increases in resistance have a very hard time going faster without excessive amounts of power.

A 30’ waterline at 7.34 knots is running at a speed/length ratio of 1.34. A 40’ waterline has a hull speed of 8.47 knots, a 50’ waterline 9.47, and a 60’ waterline 10.38 knots. This shows us how lon­ger hulls get easier to push through the water, and how longer hulls can run at higher speeds with­out creating a giant wake. At hull speed (below the speed/length ratio of 1.34), a full displacement clas­sic yacht will create very little in the way of a wake.

And a big part of achieving this minimum wake is a stern shape that allows the water that’s been pushed out by the hull’s bow and midsection to return to where it was with the minimum of fuss. The fantail shape, with its long gently rising buttocks and double-ended wa­terlines, is really one of the best shapes to achieve low resistance at low speed, and its shapely form is always attractive looking. The downside to this shape is diffi­culty of construction (it’s more complex than a flat tran­som), lack of interior volume, and poor pitch damping.

Gyrfalcon was designed by Harold Hanson in 1941 to conduct survey work in the Aleutian Islands. Chief requirements were strength, seaworthiness, and efficiency under power. Note that the “knuck­le” height of her stern is about halfway between that of Blue Peter and Tern, but her deck height is the highest of the three. The knuckle is the intersection of the bottom planking with the upper topsides. Mr. Hanson kept the deck high for increased buoyancy in case of being “pooped” by a big wave while running down-sea. And the high knuckle lengthens the aft overhang, creating a gradual increase in buoyancy as a wave lifts the stern and easing motion in a big sea.

Blue Peter’s fantail is certainly the most elegant of the three, as befits her luxury yacht status. De­signed in 1928 by Ted Geary for coast-wise cruising, seaworthiness was less a concern than outstanding good looks and efficiency under way.

Teal was also designed by Harold C. Hanson, this time in partnership with Leigh Coolidge, and launched in 1927 for Fisheries Patrol work in Alaska. Note how her fantail knuckle and deck line are the lowest of the three boats. This shape is a result of her inshore patrol work assignment, offering good effi­ciency with increased interior volume for storage, and the lower deck height aided access to small boats and low docks.