46
$\begingroup$

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?

Catalina (original image from https://upload.wikimedia.org/wikipedia/commons/2/20/PBY_Catalina.jpg)
(wikimedia.org)

$\endgroup$
58
$\begingroup$

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.

$\endgroup$
  • 8
    $\begingroup$ Re the drag in flight, an obvious question would be whether anyone had a retractable fairing behind the step to solve that problem? I hit Google and found a few places speculating about that (including US patent US6042052A in 1998), but no evidence of designers actually using one. $\endgroup$ – Graham Apr 6 at 23:41
  • 2
    $\begingroup$ @Graham I think the demand for better sea planes died around the time the Sea Dart and the Caspian Sea Monster. (Both of which incidentally had a different solution to the lift off from water problem than the step.) So lots of cool ideas but nobody to pay for actually using them. $\endgroup$ – Ville Niemi Apr 7 at 20:16
  • $\begingroup$ @VilleNiemi Yes, that was my thinking too. It did seem like something they could have tried before then, but I guess no-one did. $\endgroup$ – Graham Apr 7 at 20:47
  • $\begingroup$ A retractable fairing would complicate the design: the fairing has to seal perfectly against the step when it's extended, otherwise you're creating a giant scoop. It has to be sturdy enough not to be ripped off by the water. etc. $\endgroup$ – Hobbes Apr 8 at 8:55
  • $\begingroup$ No, the angled part allows for rotating. The step reduces water drag. $\endgroup$ – bogl Apr 8 at 8:58
28
$\begingroup$

enter image description here

It's called a hull step. Below is with and without:


enter image description here

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

$\endgroup$
9
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.