On the bollard pull of marine propulsion devices

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4 PROPELLER i operate4 a1 zero speed of sdvance the usual concept of effiçieiicy as the ratio f power output to power, input becomes, meaning-. ess, since the power output vanishes, Under these

ircumstances the thrust itself is the only suitable rieasure of the return for the input power inyested. In order to compare the sevèral kinds of

propul-ion devices at the dead pull conditpropul-ion, and to elimi-iate size as a factor, it is desirable to use

non-dimen-ional (or pseud9 non-dimensnon-dimen-ional) parameters to press the thrusting or pulling performance. In iddition, emphasis has been placed on the ratió of hrus to shaft hore.spower, TIP.

A non-dimensional coefficient which has been ised in the treatment of aircraft taxiing performance.

nd ni ore recently of helicopters and ground-effect iachines is the "Figure of Merit," "C"* defined by

ON THE BUI 141W PULL OF

MAR/NE PROPULSION

PET'7

Edgar D. Hoyt is Chfef engineer of Robert _{Taggart Incorporated, a firm}

doing contract research and development work in naval architecture, hydro-çlyizainics, acoustics, propulsion, anti control. He rece ft,ed his Bachelor of Science degree from Webb Institute of Naval Architecture fri_{193e and spent} p year of graduate sttdy in naval prchitecture and engineering mechanics

pt the University of 27ichigan. Ile has serùed with Fairbanks 'Morse and

Çompany, the Puget S1pund Naval .Siipyard, the David Taylor Model Basin, Voss Engineering and Construction, ¡nc., and Reed Research, Inc.

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_1JT_ 1fKa/a

pVU'1YK0i

T is the thrust,

P is the delivered shaft power,

A is the swept area of the devIce,

pY is the mass density of the fluid, alt In

con-- sistent units.

1n terms _{f this coefficient the thrust-power ratio}

vor, Mises. R.. "Theory of Fllgh$," Dover, 1959. p. 354. Sec also Schoenherr, K. E., in "Principles qf Naval Architecture." SNAI

1939, and Chaplin, H. R., A Preliminary Deslgr, Technique for Annular-Jet Ground Effect Machines." David Taylor _{Model Basin}

"C..

Report 1371, September 1959. von Mises uses and Chaplin

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Technische Hogeschool

Dem

EDGAR D. HOYT

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T -_:

P -.

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The moméntum theory of propeller action can be used to show that for n ideal open water propeller in dead pull operation the theoretical value of the

figure of merit is "C"( Foi' a Kort nozzle the

theoretical limit Is "C' These relations are showt in Figure 1 which is a plot of TIP against P/A with contours of constant "t." Contours have also been drawn on this figure of constant valUes of 'l'lA. 1h this plot P has been ecpFessed In horsepower, T 1h

pounds and A in. squar feet.

Practically, of course, the theoretIcal v1ues of "C" cannot be attained: As an indication f what can be achieved, some representative full scale trial

results have been plotted in Figure 1. F'rom these the

marked superiority of the Kort hozzl Is quite ev-dent. This is usually attributed to suppressioti of th contraction of the propeller discharge jet. Theoret-cally the discharge jet of an open propller cotitracts

tu ½ the area of the propeller so that the potentil

gain with the Kort nozzle is a factor of 'VT at

the same value of AIT. The actual gain is somewhat more than this, indicating that the propeller with

nozzle comes closer to achieving its ideal results. There is quite a spread in the values of "C" re-ported for the nozzle installations. This is ptobablP due to the fact that some of them are quite old and may not have been properly shaped to achieve full suppression of the discharge jet contraction. In fact several of these are tunnel stern towboats on which only partial nozzles could be accommodated. But

also the "C" value for a propeller in a nozzle depends

on the pitch ratio which in turn is governed by the

rate of revolutions. Quite probably some of the

poorer Kort nozzle installations resulted from an in-ability to utilize optimum RPM for the power and diameter chosen. With these thoughts in mind it seems safe to predict a "C" value of 1.3 for a well-designed, complete nozzle if suitable RPM can be

used.

For open screw propellers the value of "C" ob-tàinable is more sensitive to the pitch ratio; low pitch ratios being more desirable whereas with the nozzle propellers a higher pitch ratio is preferable. The lowest value in Figure 1 is believed to be a tnel stern boat where probably the diameter was

un-duly restricted and the flow perhaps not good.

Among the others there is little spread and an indi-cation that a "C" value of 0.85 can be attained with careful design.

670 NdvaI Engneers JouTnal, Nov.mber 1962

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IB 2. 3 50 10 72 IO

Pigifre i. Propellers at DestI Pull.

Two sets of results are shown for cycloidal pro.. 'pellers. The point at P/A of 15.5 is for the Voith

Schneider twin screw "water tractor"

This ship is speciall,r designed for towing and has a

plate below the blades for Which some contzolof 'the discharge jet contraction is dlaimed. The other points at P/A vaKte.s of 19.5 to 86 ate for the German

mine-sweeper liberated a'ter World War II and tested by

the Navy under th

clear why the ltte' results are

however, the blade thotion is not so good as the later

tsign. 'to judge from these results, and from the

publications of the Voith company, a "C" value of 0.8 can probably be obtained with modern units and perhaps a little more with the end plate below the

blades.

There are also shown some data for bow thrusters installed in transverse tubes. The KaMeWa units, which have controllable and reversible propellers, re claimed to provide a "C" value of just over 0.8. A custom-designed unit with reversible, fixed-pitch propeller installed on the new P&O liner Canberra is

supposed to give a "C" value of 0.72.

The' information included in Figure 1 is not suf. ficient, of course, to indicate the detailed design of any propulsion device. it is intended, rather, to show.

what performance can be obtained from different

de-vices in terms of the power and space available and assuming satisfactory rates of revolution for the propeller. .01111 s sfl ¡cliv :

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