On the propeller power coefficient

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OF GEIVOA UNIVERSITY, ITALY

On The Propeller Power

oetlleiont

BY

Prof. Ing. ALFIO DI BELLA

EDITION IN ENGLISH.

BY PUBBLICAZIONI SCIENTIFICHE D'INGEGNERIA GENOA 1962

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ON THE PROPELLER POWER COEFFICIT

Within recent years in naval field has been introduced this propeller power coefficient (see list of symbols)

C - -P

D27LV3

2 3

2tQn

55OSHP

D

,tV

a.

See, for example, Tachznindji and W.B. Morgan

[iJ,

_van de Voorde and J. Esveldt. [2

_J

On the 1960 ITTC we presented a Note [3

_J

where we pro-posed the following propeller power coefficient :

K27tKQ

Since our Note was presented late and as the matter concerning the Note was not on the agenda, it was not possi ble to discuss our proposal.

The starting point of (2) is as follows. In &iglish units

550 SHP -

2 i

nQ1

In metric units

75 PCA - 2inQ

On division byp1 D _n3 _{equation (a) becomes}

550 SHP

Q1

p1D

-2

t

n

P1Dn2

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Our coefficient presents these qualities :

- it is non-dimensional;

- it is of easy comput&tiôn.. on the model propeller experi ments; in fact is equal to the usual KQ multiplied

by 2 it;

- it is of easy computation on trials analysis; in fact, it is substantially; equal to the power divided by the rivolutions to the cube;

- its numerical values on trials analysis vary from 0,10 to 0,20 about.

The Cp coefficient of formula (i), coming perhaps from other fields of study, seems not to be useful in Na-val Architecture, because it contains the speed of advance of the propeller _VA This speed is well known in open wa ter propeller, but is unknown on self-propelled test _and on trial.

Using coefficient the open water propeller dia-gramn4 appears as in fig. i

This diagramm is useful to determine directly the speed of advance, the efficiency and the t1rust of

propel-ler when on trial trip power and rivolutions _{are known.} In fact, whit power and rivolutions it is possible to calculate Kp ; and hence, from open water propeller diagramm, J and are known. Therefore it follows :

V

-

JND

T

- ICT p D4 n2

1-1

-J

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and these X values

p

Onprogre.ssive trial it is possible to find

eluden-cies not corresponding to the

points

A, B ...

of L line, but stayng on the little intervals A' A" , B' B" ,

(fig. 3).

However L

values corresponding to A'

and A" are

not substantially different froua the value

correspon-ding to A

5-

0,066

It is true that on designing propeller, if there is

not cavitation, the "envelope line" L of efficiency is

suitede So, on full power tri.l, the propeller

efficiency

is according with L line.

To the following pitcP P

- 0,8

ato veiue.

D

rr.spond U.se

i.

line points

A

0,8 1,0 1,2 1,4

B C D E

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REFERENCES

A.I TACHNINOJI and W.B. I&)RGAN - The Design and Estimated Performance

of a Series of Supercavitating

Pro-pellers.

Second Symposium on NAVAL HYDRODINAMICS Department of the Navy U.S.A (pag. 489)

1958

2 C.B. VAN de OE and J. ESVELDT - Tunnel Tests on Supercavitating

Pro-pellers.

Fourth Symposium on NAVAL HYDRODINAMICS Department of the Navy U.S.A (pag. 227)

1962

3 Prof. Ing. ALFIO DI BELLA - Proposal to adopt 2,t K instead øf

KQ on th. open water propeller

diagramm.

NINTH INTERNATIONAL TOWING TANK CONFERENCE (19m) PARIS.

4 Prof. Ing. L. TROOST - Open Water Test Series with Modern

Propeller Forms.

NORTH EAST COAST INSTITUTION

15th

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K p 1,4 -

(K)

p- S.- S..- .5-.0O66 ..C,093 .0, 12 FIg. 2

3 blades - - 0,50

F E' E _E" t D b" j A" i I '7 Fig. 3 J