**
Sec. 25.533 - Hull and main float bottom pressures.**
(a) *General.* The hull and main
float structure, including frames and bulkheads, stringers, and bottom
plating, must be designed under this section.

(b) *Local pressures.* For the
design of the bottom plating and stringers and their attachments to the
supporting structure, the following pressure distributions must be
applied:

(1) For an unflared bottom, the pressure
at the chine is 0.75 times the pressure at the keel, and the pressures
between the keel and chine vary linearly, in accordance with figure 3 of
appendix B. The pressure at the keel (psi) is computed as follows:
$$

where --

*P*k=pressure (p.s.i.) at the keel;

*C*2=0.00213;

*K*2=hull station weighing factor, in
accordance with figure 2 of appendix B;

*V*S1=seaplane stalling
speed (Knots) at the design water takeoff weight with flaps extended in
the appropriate takeoff position; and

βK=angle of dead rise at keel, in accordance with
figure 1 of appendix B.

(2) For a flared bottom, the pressure at
the beginning of the flare is the same as that for an unflared bottom, and
the pressure between the chine and the beginning of the flare varies
linearly, in accordance with figure 3 of appendix B. The pressure
distribution is the same as that prescribed in paragraph (b)(1) of this
section for an unflared bottom except that the pressure at the chine is
computed as follows:
$$

where --

*P*ch=pressure (p.s.i.) at the chine;

*C*3=0.0016;

*K*2=hull station weighing factor, in
accordance with figure 2 of appendix B;

*V*S1=seaplane stalling
speed at the design water takeoff weight with flaps extended in the
appropriate takeoff position; and

β=angle of dead rise at appropriate station.

The area over which these pressures are
applied must simulate pressures occurring during high localized impacts on
the hull or float, but need not extend over an area that would induce
critical stresses in the frames or in the overall structure.

(c) *Distributed pressures.* For
the design of the frames, keel, and chine structure, the following
pressure distributions apply:

(1) Symmetrical pressures are computed
as follows:
$$

where --

*P*=pressure (p.s.i.);

*C*4=0.078 *C*1 (with
*C*1 computed under §25.527);

*K*2=hull station weighing factor,
determined in accordance with figure 2 of appendix B;

*V*S0=seaplane stalling
speed (Knots) with landing flaps extended in the appropriate position and
with no slipstream effect; and

*V*S0=seaplane stalling
speed with landing flaps extended in the appropriate position and with no
slipstream effect; and β=angle of dead rise at appropriate station.

(2) The unsymmetrical pressure
distribution consists of the pressures prescribed in paragraph (c)(1) of
this section on one side of the hull or main float centerline and one-half
of that pressure on the other side of the hull or main float centerline,
in accordance with figure 3 of appendix B.

These pressures are uniform and must be
applied simultaneously over the entire hull or main float bottom. The
loads obtained must be carried into the sidewall structure of the hull
proper, but need not be transmitted in a fore and aft direction as shear
and bending loads.

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as
amended by Amdt. 25-23, 35 FR 5673, Apr. 8, 1970]