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I noticed there is a little offset in a seaplane's hull (highlighted in the following picture). I suppose this is useful as it exists in all the seaplanes I know.
It appears to be neither a hydrodynamic nor aerodynamic feature. I could not find clue by myself as I don't know this feature's name.
My question, what is its name and purpose?
That's called the step. Without it, you'd have to fight against the buoyancy of the rear end of the hull when you rotate for takeoff.
However, a seaplane float or hull must be designed to permit the seaplane to be rotated or pitched up to increase the wing's angle of attack and gain the most lift for takeoffs and landings. Thus, the underside of the float or hull has a sudden break in its longitudinal lines at the approximate point around which the seaplane rotates into the lift off attitude. This break, called a "step," also provides a means of interrupting the capillary or adhesive properties of the water.
The water can then flow freely behind the step, resulting in minimum surface friction so the seaplane can lift out of the water. The steps are located slightly behind the airplane's centre of gravity, approximately at the point where the main wheels of a landplane are located. If the steps were located to[o] far aft or forward of this point, it would be difficult, if not impossible, to rotate the airplane into a pitch-up attitude prior to planing (rising partly out of the water while moving at high speed) or lift off. Although steps are necessary, the sharp break along the float's or hull's underside causes structural stress concentration, and in flight produces considerable drag because of the eddying turbulence it creates in the airflow.
It's called a hull step. Below is with and without:
It reduces water drag. As the plane gains speed and the aft body is lifted, only the forward hull will be in contact with the water.
Source: Laté 631 Replica - Chapter 3 - Hydrodynamics
As everybody has said, it's called a 'step'.
But it's nothing to do with buoyancy, it's to do with the opposite effect - water suction. Without the step you will never get the airplane off the water simply due to the suction of the water clinging onto the airplane.
The step forces a break in the water-suction, in the case of the Catalina above probably halving it, which then allows the lift of the airplane to overpower the remaining water suction.
Landing and taking off is done on the step. During the takeoff, the aircraft is rotated forward until it is planing on the float or hull portion forward of the step; less surface is in contact with the water. A notable increase in acceleration can be felt as the aircraft rises and rotates onto the step, and then a further slingshot effect felt as the remaining drag ends as the float or hull separates entirely from the water.
Land is the reverse; the landing is made on the step, or the section of the float or hull forward of the step; high speed water taxi is done "on the step," and low speed taxi is done by reducing power until the aircraft sinks deeper into the water in a condition known as "plowing."
The step is a hydrodynamic feature which causes a break in laminar flow along the float or hull; with an adequate increase in speed this serves to decrease drag along the portion of the hull or float aft of the step. For simplicity when flying seaplanes, operation on the forward area is simply referred to as "on the step."
