Sub-cavitating propeller design for high powered small craft

9 FEB. 1931i

ARCHEF

Paulsen, J F Doom, J P van Fransen, H P

SUBCAVITATING PROPELLER DESIGN FOR HIGH POWERED SMALL CRAFT

(Fjellstrand Aluminium Yachts) (Lips Pröpe1ler Works)

Lab.

_{y. Scheepsbouwkund}

Technische Hogeschoól

Deift

The design procedure as weil as analyses procedures fOr subcavitating wake adapted propellers for high powered small craft for speeds over 20 knots, will be explained. Problems normally met with these type of vessels, such as cavitation and vibration will be discussed As a source of vibration, the propeller is considered where a connection is made to the propeller generated pressure pulses on the shiphull Comparisons in this respect are made between different propeller designs with variation of diameter, tip clearance and shaft inclination, for a given design condition. Furthermore .the influence of power increase., resulting in higher speed, on cavitation and vibration will be discussed.

1. PROPELLER DESIGN PHILOSOPHY

From the point of view of the propeller designer the main characteristics for propel-lers in general is that the highest possible level of propeller efficiency must be aimed at while maintaining vibrations and noise and hence cavitation at the lowest possible level. This leads to conflicting bóundary conditions, e.g. postponing cavitation incep-tion gives rise, to moderately to very large tip chord lengths and/or strông tip off-loading while trying to obtain a high propel-ler efficiency requires the adverse.

Therefore such a propeller must keep the subtle balance between several extremes resulting in' a compromise according to the experience öf the propeller designer and the correct use of the design tools at his dis-position.

The propeller design system as used at, and developed by Lips, consists of a seies'of interactive design and analyses modules (fig. 1). Before any propeller design can be intitiated, the design criteria should _be selected.

In the following an impression of the propel-1er design system computer Programs, the line of thought in the use of it'adapted to the design'of a propeller and sorne results of calculatioñs are given.

1.1 Design criteria

The design criteria as used in ur propeller design system consists out of the condensed information with respect to shiptype., _mission and mission profile óf the ship and possible

5/1

limitations regarding propeller diameter, cavitation behaviour and propeller generated. pressure pulses on the ship hull. These design criteria are to be considered as the boundary conditions or constraints for any propeller design and have normally their greatest inpact. in the off-design behaviour of the propeller. Especially the designs of controllable pitch. propellers intended to use with constant RPM, due to 'presence of a shaft-generator, are sensitive for the design criteria, since those types of cavitaton which tend to erosion under prolongued opera-tion should be avoided as much as possible.

In this case the desin criteria will influ-ence the design condition as well.

Before we continue with our discussion, we will first define the concepts of design- and off-design condition.

With design condition is ment that combination of shipspeed, number of propeller revolutions, power absorbed or thrust to be delivered and shaft submergence, where the pitch is selected for. Normally the propeller will have its highest efficiency in this condition. Any other combination of above mentioned variables, then those in the design condition, is called an off-design' condition.

It is self-evident thàt' during the propeller design stages only realistic off-design con-ditions will be cönsidered. '

1.2 Design data . .

In order to be able to make a propeller design the following basic data are necessary of which some of it can be assumed based _on statistics and available information, other

data has to be supplied by shipowner, shipyard or other parties.

- Wakefield as measured on modelscäle or

assu-med.

- Power, RPM, shipspeed and mean wake fraction for the design condition, which determines the mean pitch.

- Power, RPM, shipspeed and mean wake fraction for the strength condition.

- A variety of geometrical parameters amongst which:

- propeller diameter - number of blades

- radial pitch or circulation distribution

- amount of skew

type of propeller material.

- Definition of maximum allowable amount of suction side sheet cavitation at each radius

in the design condition (fig. 2).

- Margin against occurence of pressure side cavitation expressed in an extra angle of attack in the design condition (fig. 3). The amount of back sheet cavitation and the margin against pressure side cavitation as prescribed in the design condition is a func-tion of the cavitafunc-tion behaviour in the off-design conditions as well, since in those conditions the performance of the propeller should be satisfactory and should fulfil the design criteria.

1.3 Design

The propeller design module executes in an iterative sequence a number of calculations with the stability of blade area as a cri-terium of. convergence.

The module can be roughly divided into five segments (fig. 4).

Two segments to perform the input operations on the one hand, such as reading, checking and possibly correcting and if necessary supplementing the input or design data, on thé other hand to present the calculated

results in a printed and graphical form. The other three segments are related to power absorption, strength and, cavitation properties

in design condition.

In the power absorption segment the mean pitch of the propeller is determined to accomodate

the design condition and taking into account the specified radial pitch distribution. The hydrodynamical model used is a modified lifting line theory method with some aspects of lifting surface theory incorporated, together with emperical correction factors based on regression analysis from results of previous designs, t wing tank results and full scale results.

The strength calculation segment is based on

beam theory [.1].

5/2

The radial thickness distribution is determined taking into account the fatigue load due to varying stÑss levels during a propeller revOlution and the fatigue properties of the propeller material to be used. When requested

the resulting thickness distribution will also meet the classification requirements.

The cavitation segment determines at each radial station a combination of minimum chord-length and camber, such that the requirements as stipulated by the designer as to suction side and pressure side cavitation together with a built in criterium against the occurence

of bubble cavitation are met. During these calculations the pressure distributions of the blade sections are developed to Theodorson's method [2] for all camber-chordlength combi-nations considered. When convergence is ob-tained the results of the hydrodynamiÇ design calculations are presented on a graphic display, and if requested also on plotter and/or printer. A sample of presentatiOn by the output segment is shown in figures 5 and 6. The following data are given in fig. 5:

- TOp left:

Radial hydrodynamic pitch distribution in the design condition. This pitch distribution will be modified in a later stage with vortex surface corrections as given by Cumrning et all [3,4] necessary in order to take into account the curvature of the fluid flow. The resulting pitch. distribution being the

so called geometrical pitch distribution. As mentioned before the hydrodynamical model

used is a mòdified lifting line theory, with some aspects of lifting surface theory in-corporated, hence not thé full liftina surface corrections are applied.

- Top middle:

The main circumferential radial circulation distribution resulting from the propeller working in its given or assumed wakefield at the design point of operation is presented. The circulation is given in a non-dimensional

form.

- Top right:

Radial thickness distributions.

The curves have for this particular sample the following meaning:

Left curve: thickness needed due to the design condition.

Middle curve: thickness needed due to the strength condition.

The above mentioned thickness distributions are based on maximum allowable stresses, which are derived from the stress varia-tiôns during one propeller revolution for the conditions under consideration. Right curve: ultimate thickness distribu-tion taking into acçount all thickness distributions calculated and if required, the classification regulations.

The symbols plotted, in this diagram are the requirements of the classification society under consideration.

- Bottom left:

Variation of loading of the section at 0.8 radius during one propeller revolution due to the speed variation in the wakefield and th,e variationof the total, pressure. Cavitation inception line for the profile section under cönsideration.

Line of average cavitation number.

As can be seen from this diagram, this par-titular section is free of cavitation in the design condition.

- Bottom middle:

Radial chordlength distribution (faired. curve).

Radial distribution of minimal chordlength per section for the selected load distri-bution in order to avoid bubble cavitation and to satisfy the suction side and

pressure. side cavitation behaviour as se-lected by the designer (knuckled curve). - Bottom right:

-Radial camber distribution.

The following data are given in fig. 6: -Left:

Summary of the cavitation characteristics of the propeller in design condition. On each section considered the minimal distances. between the corresponding loading and cavitation inception curves are searched for, for the suction side as well as the pressure side cavitation. For. all sections the determined values are plotted in the diagram. In other words, the difference between the two bucket curves gives infor-mation about the maximum variation of

loading each section. can take without cavi-tation during one propeller revolution. The difference between the two loading curves is the variation of loading which is requested for by the propeller during one propeller revolution, due to the velocity in the wakefield of the ship. Thedifference between the lower bucket line, or cavitation inception line and the lower loading line gives the. minimal margin against pressure

side cavitation available in the design condition. The difference between the upper

bucket line and the upper loading line gives infOrmation about the available minimum mar-gin against suction side cavitation in.the design condition.

It-is evident that the cavitation behaviour in off-design condition will control the margins against cavitation in the design condition and, as a result, will influence

the propeller blade geometry during various design stages.

5/3

It appears in this,case, that suction side cavitation is to be expected in the design condition from about .81 radius upto the tip. The cávitation analyses procedures, as mentioned later, will give, information about the extend and angular position where, as in this case, the suction s,ide cavitation appears.

- Right:

Thè expended and developed blade outline is presented..

Arrived at this point the propeller designer has to consider if the designed propeller has the properties he expected the propeller to have in order to fulfil ali, design requirements. Whether the propeller design is acceptable or not depends on the calculated results and more or less on the. quantity and reliability of the available information. With the advance of a particular propeller design the time needed for the various iterations is rapidly de-creasing to the minimal turn around time which depends strongly on the workload of the com-puter system used.

1.4 Cavitation performance

With the available blade geometry the cavita-tion properties of the propeller working in its wakefield are determined by the cavitation analyses module for any given combination of shipspeed, number of propeller revolutions, absorbed power or thrust to be delivered and shaft submergence. The method used is based on a qúasi static, or quasi dynamic., theory. At fixed angular blade positions the loading on each blade is calculated with the modified lifting line model and Theodorson's [2] method for determination of pressure distri-butions. Since the design system, as described thus far, is used, for small up to very large propellers, monobloc as well as controllable pitch, whether or not working in combination with a nozzle, the following options were developed and bùilt into this module in order to solve frequently occuring problems in connection with propeller design:

- Calculation of blade stress levels during one propeïler revolution for the blade sections under consideration.

- Propeller generated shaft forces and moments. - Cavitation inception diagrams for each

section under consideration together with the variation of loading of the particular section due to the speed variation in the wakefield and the variation of total pressure. - Calculation of cavitation inception'diagram

'for the propeller.

- Data management in case a finite element strength calculation should be performed for a particular propeller design, working in the condition under consideration.

Last two options are however very time consu-ming and especially for small propellers

proportionally very expensivé and, only

meaningfull when ali.design data are reliable. Since primarily the purpose of this module is

the prediction of cavitation behaviour in any operationál condition only sample results of cavitation calculation are shown. In fig. 7 are the axis definitions given as used in fig. 8, where information about suction side

cavitation and pressure side cavitation is given for all blade sections and all angular

positions considered. An other way of repre-sentation and commonly used by the model basins, is shown in fig. 9 through 11. As the way in which the cavitation phenomenom is changing during a propeller revolution on the

blades is very important to the designer, the cavitation analyses modùle gives him the option to show him thé results by way of animation.

1.5 Propeller generated pressure fluctuations

When the cav.itation performanàe of the prò-peller fulfils the requirements as stated in the design criteria for their respectiVe operational conditions, then the propeller generated pressure fluctuations will be

calculated by using the method of Tamborsky [5]. A sample of the resùits of calculations is given in table I, where the huilpoints used are defined in fig. 12. When all calculated levels of propeller generated pressure fluc-tuations are below those as stated in the design criteria, then the design is accepted as final as appears in fig. 1.

A final remark for this section: the quality of the propeller design and the performance of the designed propeller on full scale is strongly dependent on the quality of the information

supplied to the designers.

2. SHAFT INCLINATION. AND TIP CLEARANCE

For our purpose the Fjellstrand 31.5 Catamaran, designed for fast transportation of passengers and light cargo is used.

The basic construction of this vessel is made such that any special requirement, for example offshore crew transportation, can be met. The maximum passenger capacity is about 300

persons The hull is of asymmetric catamaran type with a round bilge and hard chine at aft. The vessel has the following main dimensions:

Although the hull is designed to have an optimum speed of 27 knots, there will be a reasonable resistance up to 32 knots. The main aim when developing this vessel was tó be able to maintain a speed of at least 25 knots in seawaters with a significant wave height of 3 meters (H1/3). The twin-hull construction

5/4

was sélectèd due to the very good experiences with this type of high speed- passenger vessel along the Norwegian cast. The vessels'- good reputation among travellers is mainly based on their excellent stability. It seemS that passengers being unfamiliar with travelling at sea prefer the small rolling amplitudes of the catamaran instead of the considerable larger rolling angles of the normal single hull. Another important advantage of the catamaran is that all accomodation can be arranged on one deck, which makes it more easy to allow for changing of lay out according to the requirements of the customers. Since the construction of all high speed vessèls is very weight sensitive, with respect to shipspeed, the structural arrangement is based on low weight and the usage of seawater resistant aluminium. A combination of deep transversé frames and small longitudinals at short di's-tances is used.

The twin-hull configuration doés lead to a twin-screw propulsion system. This gives the advantage of very good manoeuvrability and the safety of twin-engines. Due to the slender hulls the space available for engines and other equipment will be limited. As a result the propulsion engines must be of the high speed typé with the number of revolutions in the range of 1400-1800 RPM. The main engines are placed in an engine room positioned in the aft end of the vessel and connected through the cardan shafts to the reduction and rever-sing gearboxes and the propellers (fig. 13).

In order to minimise drag and weight, the shaft brackets have I-type profiles and the pÑpeller shaft is made out of high-tensilé stainless steel.

The vibration and noise on board ships have been accepted for many years as a necessary evil. 'However, during the last decade, social awareness of the énvironmental effects on health has led to the requirements of improved working conditions. Although there are no

statutory obligations in this respect in most countries, guide lines for acceptable noise levels, have been issued in many of them. At least in Germany and Skandinavia there are legal requirements for noise levels. From the construction of high-speed vessels with fast, runniñg engines good examples of easy and effective reduction of structural noise is shown by resiliant mounting of engines and gears. However, today, the worst noise problem is the noise'originating from the propellers. It. is obvious that from the limited space available, the configurations used in small ships lead normally to high shaft inclinations in order tó accOmodate the propeller with sufficient tip clearance. As is commonly known high shaft inclinations as well as small propeller tip clearances may lead to serious problems with respect to vibrations. In some' cases cavitation erosion will occur due to the

high loading and shaft inclination. Length over all :' 31.5 m

Maximum breadth : 9.4 m

Fr this discussion, however, we will consider the propeller as a source Of vibrations and noise and as a measure the calculated propel-ler generated pressure pulses in certain points on the ship hull will be used. Struc-turai vibrations with à hatural frequency close to the propeller blade frequency, or a multiple thereoff, should be avoided in any

case.

Propellers have been designed with optimal efficiency for the vessel, as described before, to absorb 1000 kW at 25 knots ship speed. These propellers have the following mai n characteristics:

diameter 1000 nun

mean pitch 1284 mm number of blades 4 blade area ratio 1.092 The general view is given in, fig. 14..

In the propeller aperture a propeller tip clearance is available of 20%, while the shaft incline corresponds to 9 degrees. From ship trials, and observations during normal service, it appeared that the vessel and the propeller aré operating good with respect to noise and vibrations. Although the propellers are operating good, investigations as to possible improvements with respect to pressure fluctuations were carried out.

The influence of shaft inclination on propeller generated pressurepulses was determined by analysing the. existing propeller in the wakefields corresponding to the various shaft angles. The wakefield used was measuréd on. model scale for an other high speed vessel with similar hull lines. The influence of the shaft inclination on the wakefield is deter-mined according to Gutsche [6]. For the given propeller and the wakefields the pro-peller generated pressure fluctuations were determined, according to Tamborsky [5] for a propeller tip clearance ranging from 10 to 30 percent. The horizontal component of the propeller thrust was kep.t constant in order to maintain a ship speed of 25 knots. The pressure fluctuation computer program used presents the results in Fourier coeffi-cients and corresponding phase angles, see table I. As.a selection criterium for the. results of the pressure fluctuations the double amplitude effective pressure variation was used, see Jenkins et ail [7] and table I. The calculated results are presented in fig. 15, from which the following conclusions could be drawn:

Shäft inclination up to 9 degrees will have no significant influence in propeller generated pressure fluctua-tions, while maintaining the same tip clearance.

- Tip clearance should be increased for shaft inclinations of 10 degrees and higher.

5/5.

For this particular propeller it appeared that the contribution :to the propeller generated pressure fluctuations, dUe to cavitation, dramatically increased for 13 degrees and higher. The cavitation analyses showed that the cavitation performance for this propeller became unacceptâble. For the purpose of this paper no efforts were made in order to optimize the propeller for operation in connection with these higher shaft inclinations. From previous designs we have the experience that only mar-ginal: improvements in cavitation behaviour could be obtained.

PROPELLER DIAMETER AND SHAFT CLEARANCE The shaft clearance is defined as the vertical. distance from shaft centerline to the hull in the propeller plane (fig. 16). In this section the influence of the propeller diameter on the propeller generated pressure fluctuations, is determined, for a shaft inclination of 9 degrees. As in the previous section, the thrust to be delivered is kept constant for each propeller diameter. Shaft clearances of 0.6 up to 0.8 rn were considered, corresponding

to a propellcr tip clearance of 10 to 30

percent for the original propeller (D = 1000 mm). The calculated results are presented in fig.17. As can be seen in this diagram,. for a given

shaft clearance the average pressure variation will increase for decreasing propeller diameter. This phenomenum is caused by the fact that the propeller with the smaller diameter is more heavily loaded and, on top of that, because of

its lower efficiency, even more power is needed. As a result cavitation will increase., and hence its contribution to the propeller generated pressure pulses, notwithstanding the bigger propeller tip clearance. Fig. 18 shows for a shaft clearance of .7 m the contributions of the cavitation effect and the loading and blade thickness effects as function of the propeller diameter. From f 1g. 17 the following conclusions could be drawn:

= For existing configurations propeller generated pressure fluctuations can be decreased by increasing the propeller diameter. However increasing diameter may lead to overoptimal propellers with possible heavy loss, in efficiency. - For new ships it is better to use

sufficient propeller tip clearance and optimal propellers.

INCREASE 0F POWER

It happens that due to demands for vessels with higher service speed a shipyard wishes to install higher propulsion power in an existing hull design without modifications on the shafting lay out. When no alteration of the number Of propeller shaft revolution is applied the power increase normally leads to larger propeller diameters, which may result in unallowable tip. clearances and extremely high pitch values.

In order to investigate the influence of the increase of power on propeller generated pressure fluctuations, the propeller diameter is kept constant by se.lecting the. optimal number of propeller shaft revolutions in each case considered. The calulated results are

plotted in fig. 19, for various tip clearances and ä propeller diameter of 1000 mm. It

appears from this diagram that the pressure fluctuations as generated by the prope.11er are almost constant up to 1200 to 1300 kW, and are rapidly increasing for the higher power levels The increase in the level of pressure

flUctu-ations for the higher power levels, in this case say for 1500 kW, is strongly dependent on the propeller geometry and on the cavitation behaviour. When the propeller is not carefully designed than still much higher pressure levels will be the result.

FINAL REMARKS

In all graphs as a measure of propeller gene-rated pressure fluctuations the double ampli-tude effective pressure variation is used, see table I.

During the investigations it was found that due to the variety of parameters and the predominant influence of the cavitation beha-viour it was impossible to make generai graphs applicable for all kinds of situations.

The computer program used for calculating the propeller generated pressure fluctuations, based on the method of Tamborsky [5], has shown its reliability for calculations with respect to large propellers [83. At the moment the available full scale data on

propeller generated pressure fluctuations for small propellers (from 400 mm up to about 3000 mm) was insufficient to determine the reliability

of

the calculated results. There-fore correlation with full scale data will be presented in a separate paper as soon as sufficient data are available.

CONCLUSIONS

Propeller shaft inclinatiòn up to 9 degrees will have no significant influence on the

propeller generáted pressure fluctuations and hence vibrations and noise levels.

For propeller shaft inclinations of about 10

degrees and higher, more than the normai applied propeller tip clearance should be

used.

For propeller shaft inclination of 13 degrees and higher, cavitation performance of the propeller may become unacceptable.

For existing ships, vibration levels can be

reduced by increasing the propeller diameter, however, overoptimal propellers should be

avoided.

5/6

For new ships sufficient propeller tip clearance and propeller shaft inclination of 9 degrees and lower are to be prefered.

Maximum continuous rating may not be increased, without carefull consideration of the existing ship configuration.

The quality of the propeller design, and the performance of the designed propeller on full scale is strongly dependent on the reliability. of the information necessary for design

purposes.

The computer aided propeller design as used at Lips with full computerization of the process facilities allows to iterate untill the best compromise is found.

In order to get the best. possible propeller, the propeller designer should be involved in an early stage of ship design.

REFERENCES

KEYSER, R and ARNOLDUS, W:

"Strength Calculation of Marine Propellers" International Shipbuilding Progress,

Vol. 6 no. 53, 1959.

ABBOTT, I H and DOENHOFF, A von:

"Theorie of Wing Sections". MORGAN, W B and DENNY, SB:

"Propeller Li fti ng-Surface Corrections". Paper no. 10, S;N.A.M.E. Annual Meeting, November 1968.

CUMMING, R A, MORGAN, W B and BOSWELL, R J: "Highly Skewed Propellers".

Paper no. 3, S.N.A.M.E. Annual Meeting, November 1972.

TAMBORSKY, L:

"A Study on the Fluctuating Hull Surface Forces Induced by a Cavitating Propeller" 4th Lips Propeller Syiposium, October 1979. GUTSCHE, F:

"Untersuchung von Schiffsschrauben in schräger Anströmung".

Schiffbaufbrschung 3, 3/4/1964. JENKINS, G M and WATTS, D G:

"Special Analysis and its Applications". VERBEEK, R, WIEGANT, W W F L H and OIRSCHOT, P W C M van:

"Prediction of Hull Pressure Fluctuations for Propeller Design Purposes".

To be published in e.g. Norwegian Shipping News, no. 17. mid November 1982.

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SAMPLE OUTPUT 3F POPLLER DESIGN . TABLE I

HARMONIC ANALYSIS OF PRESSJRES (NIM2) P.A*SIN(N*PHI+PSI)

N -N-TN 8LADE HARMONIC.

A -AMPLITUDE

PHI -oLADE-POSITION.

PSI -PHASE ANGLE.

EFFECTIVE PRSSME VARIATION (KNÌM2)

(OOU6LE AMPLITUDE.)

2

EPV SQRT(Z*SUM(A ))

HULLP. NO. X R. .Ø. M. X. .000 M. THETA. .0 DEG. EPV- 30.0

TrfICKNESS LOADING CAVITATiON TOTAL N AMP. PSI AMP. PSI AMP. PSI AMP. PSI

i 26b0 -1.5 29U3 -73.4 6856 42.0 11513 40.3 2 94 -51.3 247 37.6 14061 34.5 14302 34.9 3 1 -4Ô.5 40 21.9 8604 -70.1 8607 -70.4 4 3 11.1 6 -75.4 5726 26.2 5724 26.3 5 5 -57.7 6 -21.8 2292 -71.8 2301 -71.7 b -88.9 9 -28.4 1426 5.5 1437 -25.6 7 J. -23.1. 4 5.0 427 55.0 430 . 54.6 8 1 63.9 5 -73.8 95 -44.5 90 -42.5

HULLP.. MD. 2 R! .700 N. X. .000 M. THETA. .0 DEG. EPV- 2.5.1

rIIICKNESS LOADING CAVITATION TOTAL AMP PSI AMP. PSI AMP. PSI AMP. PSI 1 1883 -8.0 1809 -79.4 ,897 -39.3 8692. -40.5 2 .43 -52.9 143 34.1 12233 36.0 12376 36.2 3 2 11.5 32 .4.1 7338 -69.6 7334 -6.9.9 4 3 35.1 81.4 4930 26.6 4930 26.6 5 4 -82.1 3 -1.7a5 1.942 -71.6 1949 -71.5 6 1 -85.0 b -29.3 1266 -25.9 1273 -26.0 7 1 -24.6 3 3.4

39

56.2 381 55.9 8 0 62.8 4 -59.4 82 -42.9 78 -41.6

HULLP. NO. 3 R. .750 M. Aa .000 M. THETAÒ .0 DEG. EPV. 21.6

THICKNESS LUADÌNG CAVITATION TDTÀL

h AMP. PSI AMP. PSI AMP.. PSI AMP. PSI

1 1286 -13.2 1176

-837

5166 -37.3 7159 39.9 2 27 -53.3 93 35.2 1803. 37.2 10897 37.3 3 4 -4.8 ¿4 11.0 6378 69J. 6372 -69.4 4 3 40.1 3 51.4 4322 ¿6.9 4322 26.9 5 3 -63.1 2 -7.7 1674 -71. 1679 -71.4 b A -83. 4 -29.3 1132 -26.3 1137. -26.4 7 1 -25.0 ¿ 6.4 339 57.2 340 57.0 6 0 62.0 3 -49.2 72 -41.9 69 -41.3