Without exception, all of the floats seen on licensed civil seaplanes today are of the twin-float type. The single-float type popular with the U.S. Navy when it still had seaplanes, with small floats under the wingtips as on flying boats, has never been approved for civil operation.
There is another configuration that was in fairly common use, but so long ago that it has been forgotten by all but the history buffs. This is the three-float, or “taildragger,” arrangement. In spite of its antiquity, it is worth looking into for use on today’s homebuilts, particularly the ultralights.
The major advantages of the configuration are structural simplicity and decreased weight.
For the most part, the two main or forward floats were built as simple slab-sided sea sleds without the traditional hydroplane step. On most designs that used them, the rear ends of the mains were located slightly aft of the point where the step would be on longer floats, so the end itself acted as the step. The tail float only had to be large enough to support the weight of the tail when the plane was at rest.
Another advantage of the three-float arrangement is that it permits a higher ground angle (in this case water angle) and therefore more “rotation” on takeoff and landing to produce a higher angle of attack for the wing. This can result in shorter takeoff runs and lower landing speeds.
The three-float arrangement was used on all sizes of planes well into World War I, even on some twin-engine types. In the early days, the weight advantage of the arrangement was a prime consideration – the planes were so underpowered and marginal on lift that they could barely fly on wheels with a practical payload, much less take on the added weight of floats. The planes had so much drag then that the slight increase in drag of three floats, their associated struts and wires, over the two-float configuration was negligible. As power loading came down and aerodynamics improved, longer floats came into vogue and the “taildragger” concept disappeared.
Since ultralights are taking us back to the aerodynamics and loadings of the pre-World War I years, use of float concepts from that era makes very good sense, if only from the weight standpoint. The successful use of fore-and-aft floats on canard designs — another revival from antiquity — is another plus.
With their hand-pull starters, the ultralights have the problem of engine starting when afloat licked. The bigger ships without starters may or may not have problems. On a design like the low-wing monoplane “Fly Baby,” the pilot can stand on the float ahead of the wing, flip the prop from behind, and then step up on the wing and into the cockpit. On high-wing cabin types with a forward-opening door, or on a biplane, he has an access problem that can best be resolved by the installation of a starter.
OH BABY! A World War I Sopwith “Baby” seaplane with the “taildragger” float arrangement. Note the location of the rear ends of the forward floats and the use of a water rudder on the tail float. The plane is not an amphibian – the wheel is part of the separate beaching gear.
V-BOTTOM: Most sea sled floats were flat-bottomed, but this U.S. Navy Thomas-Morse S-5 of 1917 has a slight V-bottom. No water rudder on this one. The main purpose of V-bottoms is to reduce the shock of the landing impact, but this shouldn’t be a problem for ultralights.
FIRST SUCCESSFUL WATER TAKEOFF: Canard designs are not usually taildraggers, but they can use fore-and-aft floats. This is the French Fabre pusher that made the first successful takeoff from water in March 1910 (but not a successful landing). Note the shape of the floats and the use of rounded water rudders.
THE POWER OF THREE: A more conventional canard was the French Voisin of 1912. The floats follow the Fabre pattern, but use three instead of two under the main wing. Note the high angle of the flat bottoms relative to the centerline of the airplane.
ELABORATE DESIGN: A more elaborate float structure was used on this 1914 French Nieuport, floating “three-point.” Note the additional fin under the tail – even in those days it was often necessary to add fin area when converting a landplane to floats.