Page 230 - Introduction to Naval Architecture

PROPULSION

Figure 9.4 (a) View along shaft axis, (b) Side elevation

The blade outline can be defined by its projection on to a plane
normal to the shaft. This is the projected outline. The developed outline is
the outline obtained if the circumferential chord of the blade, that is
the circumferential distance across the blade at a given radius, is set out
against radius. The shape is often symmetrical about a radial line called
the median. In some propellers the median is curved back relative to the
rotation of the blade. Such a propeller is said to have skew back. Skew is
expressed in terms of the circumferential displacement of the blade tip.
Skew back can be advantageous where the propeller is operating in a
flow with marked circumferential variation. In some propellers the face
in profile is not normal to the axis and the propeller is said to be raked.
It may be raked forward or back, but generally the latter to improve the
clearance between the blade dp and the hull. Rake is usually expressed
as a percentage of the propeller diameter.

Blade sections
A section is a cut through the blade at a given radius, that is it is the
intersection between the blade and a circular cylinder. The section can
be laid out flat. Early propellers had a flat face and a back in the form
of a circular arc. Such a section was completely defined by the blade
width and maximum thickness.
Modern propellers use aerofoil sections. The median or camber tine is
the line through the mid-thickness of the blade. The camber is the
maximum distance between the camber line and the chord which is the
line joining the forward and trailing edges. The camber and the
maximum thickness are usually expressed as percentages of the chord
length. The maximum thickness is usually forward of the mid-chord
point. In a flat face circular back section the camber ratio is half the