blade strength2

Propeller blade strength

Submitted by Buvanewari .p 191 NA

1

SUBSTANCES

Need to learn the propeller blade strength???

What does propeller blade strength mean???

Forces acting on a propeller blade

Various methods of assessing the propeller blade

strength

1 cantilever beam method

2 Numerical blade stress computational methods

3 Detailed strength design considerations

Propeller backing stresses

Blade root fillet design

Residual blade stresses

Allowable design stresses

Full-scale blade strain measurement


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Tools and references

References:

1. Marine propellers and propulsion by

JOHN CARLTON

2. Paper presented on forces on a marine

propeller blade

Tools :

1. Auto cadd 2010

2. Ms equation creator

3 Propeller blade strength

Need to know the blade strength

The propeller has to with stand the maximum load acting on the blades with out leading to any breakage of the blades

In a simple term the load acting on the blade must be made more or less equal to the accorded strength

Propeller blade strength 4

What does a propeller blade strength mean???

In order to design a propeller we would 1st find few basic data's like blade dia, no of blades, its rpm these are all the data's which we have to know in the design part of a propeller

Knowing the forces acting on the propeller and bequeathing the equivalent strength makes the propeller sound strength wise.

Propeller blade strength means the ability of the propeller blade to with stand the forces acting on it


Forces acting on a propeller blade


Propeller thrust and torque are developed from local

lift and drag of the propeller blade foil. sections at their defined radial position. In other words, the total thrust of the propeller will be the integration of axial lift vectors for the sections from root to tip.

The influence of any particular section depends on its chord length and radial position.

The velocity of any rotating section, of course, is a function of its radial position .the tip is traveling faster that the root due to its larger radial arm.

However, as open propellers have a tip of nominally zero chord length, the tip section no longer has any effect on thrust or torque.

This is why propeller designers use a median radius, such as the 0.70 or 0.75 radius, as the nominal design figure. The combination of large chord and high radius gives it a predominant position.


NOW WE SHALL SEE THE MOMENTS DUE TO THE

THREE FORCES(THRUST,TORQUE,CENTRIFUGAL)

T=∫(dt/dr).dr

Mt=∫dt/dr).r.dr (THRUST)

MQ=∫dq/r.dr).r.dr (TORQUE)

C=mr²ω²

mc= 𝑚𝑅

𝑟𝑜r²ω².dr

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ro

R

ro

R

R

ro


𝑚 = 𝜌. 𝑎𝑜. 𝑑𝑟𝑟

𝑟𝑜

r = 𝜌.𝑎𝑜.𝑟.𝑑𝑟𝑅𝑟𝑜

𝜌.𝑎𝑜.𝑑𝑟𝑅𝑟𝑜

9

R r

dr

ro

FC


Mc = 𝑚𝑟²4п²𝑛². 𝑑𝑟𝑅

𝑟𝑜

Mc =4mп²n²[𝑅³

3−

𝑟³

3] (CENTRIFUGAL)

Thus we find the moment due to the centrifugal force

And by knowing the thrust and torque forces given above we find the moments due to thrust and torque

MT

MQ THESE THREE MOMENTS ARE CALCULATED

MC

LETS SEE THE OTHER MOMENTS


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RAKE MOMENT

MR=FC.Z

Z FC

Y

SKEW

MOMENT

MS=FC.Y

IN ADDITON TO THE OBTAINED THRUST MOMENT AND TORQUE MOMENT WE HAVE TO CONSIDER 1.RAKE MOMENT 2.SKEW MOMENT RAKE MOMENT: IT’S THE MOMENT CAUSED DUE TO THE RAKE GIVEN TO THE PROPELLER BLADE(MR) SKEW MOMENT: IT’S THE MOMENT CAUSED DUE TO THE SKEW GIVEN TO THE PROPELLER BLADE.(MS)

RAKE AND SKEW MOMENTS


Lets calculate the total moment

M𝑥𝑜 = − 𝑀𝑇 +𝑀𝑅 cos𝜙 + (−𝑀𝑄 +𝑀𝑆) sin𝜙

𝑀𝑦𝑜 = − 𝑀𝑇 +𝑀𝑅 sin𝜙 − −𝑀𝑄 +𝑀𝑆 cos𝜙

𝑆 =𝑀𝑥𝑜𝐼𝑥𝑜

𝑦𝑜 -

𝑀𝑦𝑜𝐼𝑦𝑜

𝑥𝑜 +

𝐹𝑐

𝑎𝑜 where Fc= force on the blade

ao = area of the blade

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ao

φ (xo,yo)

xo

yo

(-MQ+MS)

-(MT+MR)


Methods: cantilever beam method The cantilever beam method relies on being

able to represent the radial distribution of thrust and torque force loading as in fig


σ = σT + σQ + σCBM + σCF + σ⊥ (fig)

where σT is the stress component due to thrust action

σQ is the stress component due to torque action

σCBM is the stress component due to centrifugal bending

σCF is the stress component due to direct centrifugal force

σ⊥ is the stress component due to out of plane stress components.


Using the definitions of Figure the bending moment due to hydrodynamic action (MH) on a helical section of radius (r0) is given by MH = FTa cos θ + FQb sin θ(1) in which FT and FQ are the integrated means

of the thrust and torque force distributions and a and b define their respective centres of action.

The total bending moment (M) acting on the blade section due to the combined effects of hydrodynamic and centrifugal action is therefore given by

M = MH + MC (2) the centrifugal component (MC)


Hence from equations (1) and (2) the maximum tensile stress exerted by the blade on the section under consideration is given by

σ =M/Z +FC/A (3)

where FC is the centrifugal force exerted by the blade on the section. The term M/Z embraces the first three terms of equation (1), the term FC/A is the fourth term of equation (1), whilst the final term σ⊥ is considered negligible for most practical purpose


The appropriate values of the local section thickness (t) and the pressure face ordinate (yp) can then be interpolated and integrated numerically according to the following formulae:

𝑨 = 𝑻. 𝒅𝒄𝑪

𝟎 (4)

And then calculate the section modulus

Z=𝟐 𝟑𝒚𝒑 𝒚𝒑+𝒕 +𝒕𝟐 𝒕.𝒅𝒄 . 𝒕.𝒅𝒄

𝒄𝟎

𝒄𝟎

𝟑 𝟐𝒚𝒑+𝒕 𝒕.𝒅𝒄𝟑𝟎

−𝟏

𝟐 𝟐𝒚𝒑 + 𝒕 𝒕. 𝒅𝒄𝒄

𝟎(5)

It will be noted that the final form of the blade stress equation (3) ignores the components of stress resulting from bending in planes other than about the plane of minimum inertia


This simplification has been shown to be valid for all practical non-highly skewed propeller blade forms, and therefore is almost universally used by the propeller industry for conventional propeller blade stressing purposes.

Clearly the cantilever beam method provides a simple and readily applicable method of estimating the maximum tensile, or alternatively maximum compressive stress on any given blade section.


Although the cantilever beam method provides the basis for commercial propeller stressing, it does have certain disadvantages:

1. These become apparent when the calculation of the chordal stress distribution is attempted, since it has been found that the method tends to give erroneous results away from the maximum thickness location

2. This is partly due to assumptions made about the profile of the neutral axis in the helical sections since the method, as practically applied, assumes a neutral axis approximately parallel to the nose–tail line of the section


However, the behaviour of propeller blades tends to indicate that a curved line through the blade section would perhaps be more representative of the neutral axis when used in conjunction with this theory. Complementary reservations are also expressed since the analysis method is based on helical sections, whereas observations of blade failures tend to show that propellers break along ‘straight’ sections as typified by the failure shown in Figure


Numerical blade stress computational methods

In order to overcome these fairly fundamental problems, which manifest themselves when more advanced studies are attempted, intensive research efforts led in the first instance to the development of methods based upon shell theory.

finite element approach using plate elements initially and then more recently isoparametric and superparametric solid elements.


The principal advantage of these methods over cantilever beam methods is that they evaluate the stresses and strains over a much greater region of the blade than can the simpler methods, assuming of course, that it is possible to define the hydrodynamic blade loadings accurately


Furthermore, unlike cantilever beam methods, which essentially produce a criterion of stress, finite element techniques develop blade stress distributions which can be correlated more readily with model and full-scale measurement.


In order to evaluate blade stress distributions by finite element methods of the type referenced, the propeller blade geometry is discretized into some sixty or seventy thick-shell finite elements; in some approaches more elements can be required, depending upon the element type and their formulation. In each of the approaches the finite elements naturally require the normal considerations of aspect ratio and of near-orthogonality at the element corners that are normally associated with these types of element. Figure 19.5 shows some discretizations for a range of biased skew propellers: clearly in the extreme tip regions the conditions of near-orthogonality are sometimes difficult to satisfy completely and compromises have to be made.


The finite element method is of particular importance for the stressing of highly skewed propellers

the presence of large amounts of skew influence the distribution of stress over the blades considerably.

Figure, shows the distributions of blade stress for a range of balanced and biased skew designs of the same blade in comparison to a non-skewed version.

In each case the blade thickness distribution remains unchanged


Detailed strength design considerations The detailed design of propeller thickness

distributions tends to be matter of individual choice between the propeller manufacturers.

based largely on a compromise between strength, hydrodynamic and manufacturing considerations.

Additionally, in the case of the majority of vessels, there is also a requirement for the propeller blade thickness to meet the requirements of one of the classification societies.


Some special type of propeller design consideration

1. Ducted propellers. As ducted propellers, in common with transverse propulsion unit propellers, tend to have rather more heavily loaded blade outer sections than conventional propellers, the effective centres of action of the hydrodynamic loading tend to act at slightly larger radii. However, since a proportion of the total thrust is taken by the duct, the appropriate adjustment must be made for this in the

stress calculation. Additionally, the duct can also have an attenuating influence over the wake field,

to some extent improves the fluctuating load acting on the blades.


2.Tip unloaded propellers Noise-reduced or tip unloaded propellers, which have largely evolved from naval practice and modern thinking on reducing hull pressures in merchant vessels, tend to concentrate the blade loading nearer the root sections. This feature, tends to reduce the effective centres of action of the hydrodynamic loading coupled with the slightly lower propulsive efficiency for these propellers.


3.Controllable pitch propellers Controllable pitch propellers tend to present a more difficult situation in contrast to fixed pitch propellers due to the problems of locating the blade onto the palm. The designers of hub mechanisms prefer to use the smallest diameter blade palms in order to maximize the hub strength, and, conversely, the hydrodynamicist prefers to use a larger palm in order to give the

flexibility to the blade root design. These conflicting requirements inevitably lead to a compromise, which frequently results in the root sections of the blade being allowed to ‘overhang’ the palm.


4.High-speed propellers High-speed propellers generally have better in-flow conditions than their larger and slower-running counterparts, although poorly designed shafting support brackets are sometimes troublesome. Consequently, high wake-induced cyclic loads are not usually a problem unless and shafting is highly inclined. Centrifugal stresses, as shown by Table 19.2, tend to take on a greater significance due to the higher rotational speeds, and therefore greater attention needs to be paid to the calculation of the mechanical loading components.


Comparison between tip unloaded and optimum efficiency radial loadings


Propeller backing stresses When a propeller undergoes a transient

manoeuvre considerable changes occur in blade stress levels and their distribution.

Experience with fixed pitch highly skewed propellers when undertaking emergency stopping manoeuvres has led to the bending of the blade tips in certain cases.

This bending which frequently occurs in the vicinity of a line drawn between about 0.8R on the leading edge to a point at about 0.60R on the trailing edge.


It’s due to two principal causes:

The first is due to simple mechanical overload of the blade tips from the quasi-steady hydrodynamic loads causing stresses leading to plastic deformation of the material.

The second is from the transient vibratory Stresses which occur during the manoeuvre. These stresses are not wholly predictable within the current state of theoretical technology but need to be estimated.


As part of the design process a fixed pitch, highly skewed propeller should always be checked for overload against the material proof stress capability based on the quasi-steady mean hydrodynamic stresses using a suitable hydrodynamic criterion.

In the case of a controllable pitch propeller the leading edge normally remains the leading edge during these types of manoeuvre and, therefore, the trailing edge is protected from high loading.


Blade root fillet design root fillets is the point where the blade

meets either the propeller boss or blade palm.

The root fillet geometry is complex, since it is required to change, for conventional propellers, in a continuous manner from a maximum cross-sectional area in the mid-chord regions of the blade to comparatively small values at the leading and trailing edges.


Notwithstanding the complexities of the geometry, the choice of root fillet radius is of extreme importance. For conventional propeller types, if a single radius configuration is to be deployed, it is considered that the fillet radius should not be less than the thickness of 0.25R.

The use of a single radius at the root of the blade always introduces a stress concentration

The introduction of a compound radiuses fillet reduces these concentrations considerably


The results of the blade surface stress distribution for symmetrical and balanced skew designs imply that the full size of the fillet should be maintained at least over the middle 50 per cent of the root chord.


Residual blade stresses The steady and fluctuating design stresses as

produced by the propeller absorbing power in a variable wake field represent only one aspect of the total blade stress distribution. Residual stresses, which are introduced during manufacture or during repair, represent the complementary considerations.

residual surface stresses measured in blades adjacent to the failed blade can attain significant magnitudes.


The technique used for these latter measurements is that of purpose-designed strain gauge rosettes to the surface of the blade and then incrementally milling a carefully aligned hole through the centre of the three rosette configuration.

At each increment of hole depth, a measurement of the relaxed strain recorded by each gauge of the rosette is made.

This method, used in association with a correctly designed milling guide, is relatively easy to apply and also has been proved to give reliable results in the laboratory on specially designed calibration test specimens.


An example of the results gained using this procedure is given in fig for a five-blade, nickel–aluminium bronze, forward raked propeller having an approximate finished weight of 14 tonnes


To extrapolate the results of a particular residual stress measurement to other propellers would clearly be unwise.

Nevertheless, since these stresses play a part in the fatigue assessment of the propeller, the designer should be aware that they can obtain high magnitudes, although full-scale experience in terms of the number of propeller failures would suggest that residual stresses are not normally this high.

The magnitudes of residual stress, although unclear in their precise origins, are strongly influenced by the thermal history of the casting, material of manufacture and the type or nature of the finishing operation.

Furthermore, it is also known from measurements that large variations can exist between measurements made at equivalent positions on consecutive blades of the same propeller


Allowable design stresses The strength design of a propeller in the ahead

condition must be based on a fatigue analysis, it is insufficient and inaccurate to base designs on simple tensile strength or yield stress criteria. In order to relate the blade stresses, both steady state and fluctuating, to a design criteria some form of fatigue analysis is essential.

Clearly, the most obvious choices are the modified Goodman and Soderberg approaches of classical fatigue analysis. In these approaches the mean stress is plotted on the abscissa and the fluctuating stress on the ordinate.


As is seen in Fig. the magnitude of the alternating stress σa is a single component dependent on the fluctuations in the wake field in which the propeller blade is working. The steady-state component is the sum of two components σMD and σR, where these relate respectively, to the mean design component, as determined from either cantilever beam or finite element studies, and the level of residual stress considered appropriate.


The comparison of the design stresses with fatigue characteristics of the propeller material is a complex procedure.

Casting quality has a profound influence on the life of a propeller in service. The defects found in copper alloy propellers are generally attributable to porosity in the form of small holes resulting from either the releasing of excess gases or shrinkage due to solidification. Alternatively, the defects can be oxide inclusions in the form of films of alumina, formed during the pouring stage of propeller manufacture, which have a tendency to collect near the skin of the casting. The location of a defect is obviously critical.


For conventional, low-skew propellers, defects in the centre of the blade section and on the suction face are of less concern than those located on or near the pressure face in the mid-chord region just above the run-out of the fillet radii. Alternatively, in the case of highly skewed propellers casting defects in the trailing edge region of the blade are of critical importance in view of the location of the stress concentrations within the blade


Full-scale blade strain measurement By comparison with the amount of theoretical

work undertaken on the subject of propeller stresses, there have been few full-scale measurement exercises. The reason for this comparative dearth of full-scale data has undoubtedly been due to the difficulties hitherto encountered in instrumenting the chosen ship.

Traditionally if propeller strain measurements were contemplated it has always been necessary to hollow bore the tail shaft of the vessel in order to conduct the signal wires from the strain gauges located on the propeller blades through to a system of slip rings inside the vessel.

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Figure shows in schematic form this arrangement. Despite the obvious disadvantages of this method some notable full-scale studies have been conducted

There are some references and these, together with others, have formed the nucleus of full-scale data in the publicly available literature.

In recent years the use of underwater telemetry techniques have been explored as an alternative form of measurement


The use of telemetry methods has obvious advantages in that the signal can be transmitted at radio frequencies across a suitable water gap and thus avoid the need to bore the tail shaft. The most usual procedure is to fix the transmitter to the forward face of the propeller boss, under the rope guard, and transmit the signals to a receiver located on the stern seal carrier,

Having bridged the rotating to static interface in this way the signal leads can be conducted over the hull surface, protected by conduit tacked to the hull skin, to a convenient location for the recording instruments.


Further reading Hecking, J. Strength of propellers. Mar.

Eng. &Shipping Age, October 1921.

Rosingh, W.H.C.E. Design and strength calculations for heavily loaded propellers. Schip en Werf,1937.

Romson, J.A. Propeller strength calculation. Mar.Eng. & Nav. Arch., 75, 1952.


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