Section
4 Propeller design
4.1 Minimum blade thickness
4.1.1 For propellers
having a skew angle of less than 25o as defined in Pt 12, Ch 1, 1.3 Highly skewed propellers 1.3.1, the minimum blade thickness, T, of the propeller blades at 25 per cent radius for solid
propellers, 35 per cent radius for controllable pitch propellers,
neglecting any increase due to fillets, and at 60 per cent radius,
is to be not less than:
where
|
L
|
= |
L
0,25, L
0,35, or L
0,6, as appropriate |
|
K
|
= |
|
|
G
|
= |
density, in g/cm3, see
Table 1.3.1 Materials for propellers. |
|
U
|
= |
allowable stress, in N/mm2
see
Pt 12, Ch 1, 4.1 Minimum blade thickness 4.1.2, Pt 12, Ch 1, 4.1 Minimum blade thickness 4.1.3, Pt 12, Ch 1, 4.1 Minimum blade thickness 4.1.4 and Table 1.3.1 Materials for propellers. |
|
E
|
= |
|
For aerofoil sections with and without trailing edge washback, E may be taken as
1,0 and 1,25 respectively.
For solid propellers at 25 per cent radius
|
C
|
= |
1,0 |
|
F
|
= |
|
|
M
|
= |
|
For controllable pitch propellers at 35 per cent radius
|
C
|
= |
1,4 |
|
F
|
= |
|
|
M
|
= |
|
For all propellers at 60 per cent radius
|
C
|
= |
1,6 |
|
F
|
= |
|
|
M
|
= |
|
4.1.2 The fillet
radius between the root of a blade and the boss of a propeller is
to be not less than the Rule thickness of the blade or equivalent
at this location. Composite radiused fillets or elliptical fillets
which provide a greater effective radius to the blade are acceptable
and are to be preferred. Where fillet radii of the required size cannot
be provided, the value of U is to be multiplied by
where
|
r
|
= |
proposed
fillet radius at the root, in mm |
|
T
|
= |
Rule
thickness of the blade at the root, in mm |
Where a propeller has bolted-on blades, consideration is also
to be given to the distribution of stress in the palms of the blades.
In particular, the fillets of recessed bolt holes and the lands between
bolt holes are not to induce stresses which exceed those permitted
at the outer end of the fillet radius between the blade and the palm.
Counterbored bolt holes in blade flanges are to be provided with adequate
fillet radii at the bottom of the counter bore.
4.1.3 The value U may be increased by 10 per cent for twin screw and outboard
propellers of triple screw craft.
4.1.4 For propellers having skew angles of
25o or greater, but less than 50o, the mid chord thickness,
T
sk0,6, at the 60 per cent radius is to be not less than:
The mid chord thickness, T
sk root, at 25 or 35 per cent radius, neglecting any increase due to fillets,
is to be not less than:
The thickness at the remaining radii are to be joined by a fair curve
and the sections are to be of suitable aerofoil section.
4.1.5 Results
of detailed calculations where carried out, are to be submitted.
4.1.6 Where the design of a propeller has been based on analysis of reliable wake
survey data in conjunction with a detailed fatigue analysis and is deemed to permit
scantlings less than required by Pt 12, Ch 1, 4.1 Minimum blade thickness, a detailed stress analysis
for the blades is to be submitted for consideration.
4.2 Interference fit of keyless propellers
4.2.1 The symbols
used in Pt 12, Ch 1, 4.2 Interference fit of keyless propellers 4.2.2 are defined as
follows:
|
d
1
|
= |
diameter of the screwshaft cone at the mid-length of the boss
or sleeve, in mm |
|
d
3
|
= |
outside diameter of the boss at its mid-length, in mm |
|
d
i
|
= |
bore diameter of screwshaft, in mm |
|
k
3
|
= |
|
|
= |
|
|
A
1
|
= |
contact area fitting at screwshaft, in mm2
|
|
C
|
= |
0
for turbine installations or electric propulsion |
|
|
= |
 for oil engine installations
|
|
E
1
|
= |
modulus of elasticity of screwshaft material, in N/mm2
|
|
E
3
|
= |
modulus of elasticity of propeller material, in N/mm2
|
|
F
1
|
= |
|
|
M
|
= |
propeller
thrust, in N |
|
T
1
|
= |
temperature at time of fitting propeller on shaft, in oC
|
|
V
1
|
= |
|
|
α1
|
= |
coefficient
of linear expansion of screwshaft material, in mm/mm/oC
|
|
α3
|
= |
coefficient
of linear expansion of propeller material, in mm/mm/oC
|
|
θ1
|
= |
taper
of the screwshaft cone, but is not to exceed  on the diameter, i.e. 
|
|
μ1
|
= |
coefficient
of friction for fitting of boss assembly on shaft |
|
|
= |
0,13 for oil injection
method of fitting |
|
v1
|
= |
Poisson's
ratio for screwshaft material |
|
v3
|
= |
Poisson's
ratio for propeller material |
4.2.2 Where
it is proposed to fit a keyless propeller by the oil shrink method,
the pull-up, δ on the screwshaft is to be not less than:
The yield stress or 0,2 per cent proof stress, σ0, of the propeller material is to be not less than:
where
|
δp
|
= |
proposed
pull-up at the fitting temperature. The start point load, W,
to determine the actual pull-up is to be not less than:
|
|
|
= |
|
4.3 Keyed propellers pushed up by an hydraulic nut
4.3.1 Calculations
are to be undertaken to show that the proof stress of the boss material
is not exceeded in way of the keyway root fillet radius. In order
to reduce the likelihood of frettage a grip stress of not less than
20 N/mm2 between boss and shaft is to be achieved.
4.4 Propeller boss
4.4.1 The forward
edge of the bore of the propeller boss is to be rounded to a 6 mm
radius. In the case of keyed propellers, the length of the forward
fitting surface is to be about one diameter.
4.4.2 Drilling
holes through propeller bosses is to be avoided, except where it is
essential to the design.
4.5 Fixed and steering nozzles
|