Section
7 Propeller design
7.1 Minimum blade thickness
7.1.1 For
propellers having a skew angle of 25° or less, as defined in Vol 2, Pt 4, Ch 1, 1.3 Propeller skew angle definition 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 in each case due to fillets, and at 60 per
cent radius, is to be not less than:
where
|
L
|
= |
L0,25, L0,35, or L0,6 as appropriate
|
|
K
|
= |
|
|
U
a
|
= |
design allowable stress in N/mm2 derived from the
allowable stress U, by the relationship
|
| = |
U
a = φu
U
|
where
φu is a factor
in the range unity to 1,5 and is governed by the design operational
profile of the ship. If 50 per cent or more of the design life of
the propeller is to be spent operating at powers below 0,4P,
the value of φu is governed by the anticipated design life number
of revolutions (n) that the propeller will experience within the power
range 0,75P to P of the propulsion machinery.
As such, for bronze and stainless steel alloys:
If n ≤
107 revolutions
If 107 < n ≤
109 revolutions
If n > 109 revolutions
For other operational profiles, the value
of φu must be specially justified.
When
high damping alloys are used, φu is to be taken as
unity for all values of n.
|
E
|
= |
|
The coefficient Z is given by Table 1.7.1 Values of Z
.
Table 1.7.1 Values of Z
| Per cent radius
|
25
|
35
|
60
|
| Fixed pitch propellers
|
0,5
|
—
|
0,36
|
| Controllable pitch propellers
|
—
|
0,53
|
0,46
|
7.1.2 For solid propellers at 25 per cent radius:
C = 1,04
For controllable pitch propellers at 35 per cent
radius:
C = 1,72
For all propellers at 60 per cent radius:
C = 4,17
The value of W is to be taken as 0,16
for fixed pitch propellers and 0,12 for controllable pitch propellers.
The parameter V is the centrifugal bending moment
lever acting at each of the Rule stress sections, in mm. For linear
distributions of rake along the blade, the value of V can
be determined from the following relationships:
When non-linear distributions of blade rake are used,
the value of V must be calculated individually for each
stress section and the supporting calculation submitted along with
the other information required in Vol 2, Pt 4, Ch 1, 5.2 Plans 5.2.1
For optimum free-running propellers, the values
of λ
T and λ
Q can
be taken from Table 1.7.2 Values of λT and
λQ
Table 1.7.2 Values of λT and
λQ
| Per cent radius
|
|
25
|
35
|
60
|
| Fixed pitch
propellers
|
λT
|
0,45
|
—
|
0,14
|
| λQ
|
0,62
|
—
|
0,13
|
| Controllable pitch
propellers
|
λT
|
—
|
0,38
|
0,15
|
| λQ
|
—
|
0,48
|
0,14
|
7.1.3 For non-optimum and tip reduced circulation propellers,
the values of the parameters λ
T and λ
Q are to be derived from the following expressions
for 25, 35 and 60 per cent Rule radii, as applicable:
and
where
|
ξ |
= |
a non-dimensional
radius between the integration limits |
|
x |
= |
the Rule non-dimensional
radius, 0,25, 0,35 or 0,6 whichever is appropriate |
|
x
h
|
= |
either the boss or hub non-dimensional radius depending on whether
a solid or controllable pitch propeller is being considered |
|
F’T
|
= |
elemental thrust forces acting on the blade sections |
|
F’Q
|
= |
elemental torque forces acting on the blade sections. |
7.1.4 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.
7.1.5 The
value U, when used for determining U
a,
may be increased by 10 per cent for twin screw and outboard propellers
of triple screw ships and craft.
7.1.6 For propellers having skew angles of greater
than 25°, but less than 50°, the mid-chord thickness. T
sk0,6, at the 60 per cent radius is to be not less than:
|
T
sk0,6
|
= |
|
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:
where
The thickness at the remaining radii are to be joined by a fair curve
and the sections are to be of suitable aerofoil section.
7.1.7 Results
of detailed calculations, where carried out, are to be submitted.
7.1.8 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 Vol 2, Pt 4, Ch 1, 7.1 Minimum blade thickness 7.1.1 but maintaining the required
value of U
a, a detailed stress analysis for
the blades is to be submitted for consideration.
7.2 Fluid channels in propellers and blades
7.2.1 Where
it is required to emit air or other fluids from the blades of propellers,
then the channels conducting the fluid are to be arranged such that
they pass through low stressed regions of the blades.
7.2.2 Full
details of any fluid channels in the propeller and its blades, including
the method of manufacture and the details of any closing plates together
with any required welding processes and procedures, are to be submitted
for consideration together with supporting stress calculations. Consideration
is to be extended to the method of transferring the emission fluid
though the propulsion system to the propeller and the safety devices
provided to accommodate the effects of a failure of the fluid transfer
system.
7.2.3 In cases
where it is considered necessary to introduce holes, passing from
the suction to pressure surfaces of the blades, in order to control
cavitation in the blade root sections, the details of these arrangements,
together with supporting calculations, are to be submitted for consideration.
Such holes are to be designed with blending radii from the hole to
the blade surface but need not be of constant profile. Furthermore,
if any throttling or other arrangements are required to be fitted
within the holes, full design calculations and fitting details are
to be submitted.
7.3 Interference fit of keyless propellers
7.3.1 The
symbols used in Vol 2, Pt 4, Ch 1, 7.3 Interference fit of keyless propellers 7.3.2 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 |
|
k3
|
= |
|
|
l |
= |
|
|
p
1
|
= |
|
|
A
1
|
= |
contact area fitting at screwshaft, in mm2
|
|
B
3
|
= |
|
|
C |
= |
0 for turbine
installations or electric propulsion |
| = |
|
| = |
for 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 °C |
|
V
1
|
= |
|
|
α1
|
= |
coefficient
of linear expansion of screwshaft material, in mm/mm/°C |
|
α
3
|
= |
coefficient of linear expansion of propeller material, in mm/mm/°C |
|
θ
1
|
= |
taper of the screwshaft cone, but is not to exceed  on the diameter, i.e. θ1 ≤ 
|
|
µ
1
|
= |
coefficient of friction for fitting of boss assembly on shaft |
|
|
= |
0,13 for oil injection
method of fitting |
|
ν
1
|
= |
Poisson's ratio for screwshaft material |
|
ν
3
|
= |
Poisson's ratio for propeller material. |
7.3.2 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, σo 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:
|
W
|
= |
|
7.4 Keyed propellers pushed up by a hydraulic nut
7.4.1 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 fretting, a grip stress of not less than
20 N/mm2 between boss and shaft is to be achieved.
7.5 Propeller boss and hubs
7.5.1 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.
7.5.2 Drilling
holes through propeller bosses is to be avoided, except where it is
essential to the design.
7.5.3 The
mechanisms contained within the hubs of controllable pitch propellers
and their associated piping arrangements are to be designed to be
capable of operating within defined vibration, cyclic or other loads
during their service life. As such, a factor of safety of 1,5 is to
be demonstrated against all modes of failure for components in the
pitch control system at full power operating conditions. Similarly,
the sealing systems within the hub mechanism are to be selected to
provide integrity of operation within defined survey or inspection
intervals.
|