well, I had a few more moments and found the following text from this website http://www.stenulson.net/rcflight/watrfly2.htm. I gather that the step does introduce air into the water which makes the rudders more effective and it also increases the incidence, while in displacement mode, of the forward 'planing' portion of the hull so that it will climb out of the water and plane sooner (i.e. sitting still, the forward section is angled upwards and out of the water).




Task 1: Staying afloat (hopefully upright!) Requires about 130% of "Neutral Buoyancy" Displacement, and adequate setup dimensions as far as float length and float tracking width to remain stable in all, including crosswind handling conditions.

Task 2: Maneuvering, Low speed; requires effective Water Rudder Design; This task requires the ability to steer effectively at speeds below where the air rudder is effective, and in windy conditions, without inhibiting the Drag Reduction Task later... see the water rudder design section for more insight on this subject.

Task 3: Maneuvering, moderate speed, in what is referred to as "Displacement mode", where the floatation provided by the float's displacement carries the weight of the floatplane. (Wing does not generate noticeable lift at this speed, except for the upwind half of the wing magically jumping into the air when turning crosswind... (grin)) In this mode, the tails of the floats sit deep in the water, possibly completely submerged; if float tails are narrowed to a very narrow wedge, they will allow water to be drawn in along the sides of a water rudder installed behind the tail, providing very effective steering. Squared off "boxy" float tails begin to develop "Cavitation" in this speed range, and water rudders located behind the tails on this type loose steering authority before the air rudder is really more than marginally effective. (This has led many to design water rudders which extend well below the bottom surface of the float tail... fine for this specific task, but a serious liability in later phases when drag reduction becomes the critical task....)

Task 4: TRANSITION from displacement mode to Planing Mode As full power is applied, the propeller exerts all of the thrust it can at low airspeeds. The water seems to be clinging to the floats, not wanting to let them plane out and free. The wing does very little to help in this phase; it's not generating any noticeable lift for quite a while yet! To get this task done well, the float bottom forward surface design becomes the most critical operating area; step height and shape are also at their most critical in this task, as is step location relative to the balance of the entire plane. Here's what has to happen: the bottom of the forward float hull, working with the thrust generated by the propeller, and working against the drag of the water, has to "climb up" onto the surface of the water, getting the water to shear away from the sides of the floats, and then out from under the afterbody of the floats, leaving only air underneath, behind the step. We're fighting our main battle with the capillary action of water here; this capillary action helps a well designed water rudder stay effective at increased speeds, as it loves to follow rounded surfaces and edges, clinging rather than letting go too easily. Sharp edges are best for float bottoms, and sharply defined edges well under 90 degrees are better, while rounded edges are bad news in this effort to break free of the water's hold. Sidecut is also very helpfull; that's where the top of the float is narrower than the planing surface, and as little as 5 degrees has proven to be very effective.. Triangle rails along the very outer edges provide a sharply defined edge, either "tunneling" water under the float hull bottom surface, or cleanly shearing it away from the sidewalls, enhancing the effectiveness of the float planing surface, and are arguably the most effective edge treatment we've seen.

Step design and location also comes heavily into play in this task, and the next. If the step is located too far forward, porpoising can be the result- pitch instability and a failure to plane stabily; you will have to be on the elevator stick all of the time. If the step is too tall, this can also result in less stability in many aspects of operation, especially in the transitions. If the step does not have a clean, sharp rear edge at the rear edge of the float hull's planing surface, it will be more difficult to get the afterbody of the floats well clear of the water, which is essential for the next task. Too low of a step could also result in difficulties in getting the float afterbody clear of the water, but we're talking about an average step height on 40 size floats at 1/2" working well on a good design; an average step height of 9/16" works well on my 60 size floats. NOTE: if the planing surface is too narrow, it can't be fixed with a taller step; adding the triangle rails will help marginal floats. Raw brute power may make it work when an adequate engine can't get the job done, but the real answer is to have adequate planing surface width to do the job right in the first place. Remember, you want to have good handling characteristics on the landings, too.