Application of Kempf maneuverability test to six naval vessles

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j

)

C

Experimentol Towing Tonic Stevens Institute of Technology

Hoboken, New Jersey

\Otp,NG

Lab. v. Scheepsbouwkunde

Technische Hogeschool Deiff

APPLICATION OF tTKEMP? M&NEUVERABILITT TEST"

TO S NAVAL VESSElS

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Experimental Tawing Tank

Stevens Institute of Techno1or

Hoboken, New Jersey

APPLICATION OF " KELIPF M UVRASILfl)Y TEST" TO S NAVAL VESSElS

by

Marviü Ginrich, Consultant and

Wix'nl fred R. Jacobs

Prepared under sponsorship of

U S. NavV.

Office of Naval Research

Project No, NR 062-057

-(E.T.T. Project No. CKU2O)

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IZi a psper by G. Kef (Ref. 1), the results of 133 f'ull.soale

and model mane3iverability tests of 7 large single.uccrew freigit vessels-are reported.. From these reaultg, a nnormu or average

nem-erabmt ispreecribed.

It is the puxose of this memorandum to co13are the behavior

of the six naval vesse]s model-tested at the ei'1mental Towing Tank

(Ref. 2) with. the resultá of Ken. In this way, light n

1e thrown

on Ke'Z's mametwering nora, as well ai on the relative behavior of

the six naval vessels and the 7 freighters.

Kenpf Procedure

First let us ex1le the Kenf method of testing. According

to Reference 1, a test procedure is set for the ship. The ship is started from straight course and the rudder is suddenly laid over

an angle

_'

(in Ref. 1, 6 100). The rudder is held at till the ship's heading reaches e (0

, 100), then the rudder is

f1iped to

The ship overshoots the 9 heading at. which the

rudder is flipped, btit is finally checked, and turns in response to

the rudder. VJhen its heading reaches .O, the rudder is flipped to again and this process repeats per'iodiciily. A conarison of

the periods for the 7 freighters subjected to this program with

- 100 and

e 100 showed that ths average period (maxtmum in. the distribution curve) was one whose time is that required for a ship to advance S ship lengths when on straight course; the periods for 60% of the ships fell betwwen 6 and 10 lengths. In other words,

the nozna1 period is

T

--the space frequency is

k=2Tr4.).).!

1;;_

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aed the "non' space frequency is

k _O.78.

n

T.

Arialytica]. lning of Kempf' a thod

In order to ooaxe the results of ndel tests of the six

nave]. vessels with Kempf's results, it is neeessax to xmthi fm'ther

the meaning of Kempf's method. _{Easent(ally, it is that the relative}

periods during which different ships repeat their path, when steered

by the same program, are determined almost completely by _{the ship's}

body parameters. The Kempf program is identical with that of a body

steered by an on-off (bang..bang) automatic steering mechrism with udder thrott 6 and adjusted so that the rudder remeins hard _over

ti]. the eading deviates lOu. A body oontro].].ed. by such a meohaniaiu

will oscil) té at one specific frequency which ziiay be calculated4

It is easily seen that the frequency of oscfli.ation is governed by the requireiient that

Ship body lag + lag due to finite actuation angle (eu) + lag due to

finite

_{ruddei' speed} s

Ship 3od ]j

This lag is given by

5k

--

[2(CcC)jj(

+Tfl2Ck)]

--The negative of the phase of this expression is the body].agand is

seen to be a function of the ship parametere and the space frequency Ic. La& due to !inite Actuation Angie 0c

This control does. not flip as eorni as a heading deviation

ats $ it 'does not act until this deviation attains the value 0.

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-iather than a square-wave.

If the transit tine (to traverse the

angie 2 6) is 2t, then the t

lag is half the transit t1si

or t.

The phase lag is then given by

2TVt!

t.k

the si naval vessels, where the rudder rate is about 2.7°/eec.,

t

3.7 sec. (b1 f the tir

necessary to vve - the rudder 20°).

Cpa.rison of Naval Vessels of E.T.T. with 1cef's Freihterø

If the

drodynadc parwiters for the ships tested were kno the period could have been calculated. 7nfortunate]y, dth

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It is easi].y.seen that this lag is given by

-I

sin-i

I_°c

where ₀ is the actuation angle (in this case, O - 10°) and

is the a1itude of the ship oscii.1tion.

0m is calculated from the

aJT)lttuda of (I s) above,

here one inserts for where 5

is the rudder throw (here, 6 100). This value for 6k

is the

a1itude of the lowest frequency ouz'ier conoimnt of the

square-T?Ve tion. This actuation lag depends on. the frequency through

thedeendenceof 9m on k.

La Due to Fin.itè Rudder Speed

ven when such an autonatic control calls for an angle 6,

the rudder is at

_-

5 and cannot reach 5 rudder nt,ion is of the foxn

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a given sbip,the use of on2y one pa bicular program does not yie34 sufficient data to infer the znagrrttudes of the

odia

parameters

or to specify vérr preciseLy the. value of the dynamic stability

indexp.

It appears that the best method available to compare the le-. suits of the aix ships tested at E.T.T. with Kevipf Is observed results

ia to calculate the periods for such program, by the method óut].ined

above. This was carried out, remembering, that

0,785

60% values: 0,589 k _0.981 _{60% values: 6 4-}

_{10. 4-,}

It is to be noticed that a monotonic relation between the

value of k and p1 does not exist. This arises from the fact that the body lag alone does not deternine the stability or Vice versa,

bu.t that the body amplitude response is also involved. Another

reason is that the period resulting from the phases involves the

rudder coe±Zioients which are not involved in the p1 calculations. Comparison shovrs that the periods for the six E.T.T. ships are all- Longer than those for KempZ's freighters. Three of the

ships are in the 60% range and 5 are in the 75% range, 75% rurming

up to a period of ii 4. A longer period generally means

TU-89

and the £ollavd.ng results wexe obtainedi

si

I

-.1.18 0.70 9.0.

II

-0.91 0.585 10.7

in

..o.1.$ 0.6i 9.8 -.0.35 0.(25 9,7 V -.0.25 0.357 17.6 VI -0.17 0.59 10.6

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damic stability (p1 less negative). That is, on the average, the

freighters are more stable than the E.T.T.. naval ships, although in

most cases the difference is not too great. Discussion of, Reslte

The re$ults are souevihat as would, be expected iniUmCh .s

it is clear that naval vessels mill have to maneuver imich better than. con nercia. vessels in waxare. Hence, a lower daid.c stability is inpliéd (as is actually the case). Freighters, on the other hand.

steer straight courses and are designed that way. Tints the fact, that

all of the six naval vessels have a longer pei'iod than the no (aixi probab].y have leSe dnamio stability) is not a matter for conoern.

In fact, from studies of automatic steering, it appears that the commaztcial freighters tested by Kef probably have more dynamic

stability than is necessary. Turning Circles

Kef also measured turning diameters on the 7 freighters.

It appears that on the average they circle in lL,.ii. ship lengths.

This means that the turning Mter is about 18 lengths with full

rudder. This value of the diameter is in itself not sufficient to

give the dynamic stability since the rudder power (rudder

lift and

moment coefficients) is also involved. This follows from the ex-pression f,or the turning diter.

,C +C

D £ m

- Cx6

CICk_mCm

where X Cp /C Q$. The dynamic stability index, p1, varies as

(Cz Ck - m Cm) but the factor C6 differs for the ships and. probably varies eatly for different types of vessels.. .ffover, it appears

that the. turning diameter, oi' the freighters is of the sane order of

magnitude as that of the naval vessels (and possibly slightly poorer on the average).

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-S.-TM-89

-6-RENCES

1 G.: ;.ttMafleuV$riIlg Standards for Ships", Deutsche Schiffahrts-Zeitschr±ft 'Hansa", No. 27/28, 8 'July l9Uh.

2. Davidson, Kenneth S.M. and Schiff, Leonard I.: '1Turning and

Course-Keeping Qu1ities." Transactions of the Society of

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Copies

15 Chief of Naval Research Navy Department

Washington 25, D.C.

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Chief, Bureau of Aeronautics Navy Department

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DISTIBUTION LIST

E .T P TECHNICAL MEORAN]]JM NO. 89

Project No. HR 062..OS7 Contract N6Om'-217

Copies .

1 Chief, Bureau of Ordnance

Navy Department Washington 25, D.C. 6 Chief, Bureau of Ships

Navy Department

Washington 25, D.C.

Director

David Taylor Model Basin

Navy Department.

Washington 7, .D.C.,

1 Underwater Ordnance Division

Naval Ordnance Test .5ation I 3202 E. Foothill Blvd.

Pasadena 8, Calif.

1

California Thst. fTm1

}rdrodrnandos Laboratory

Pasadena, Calif.

}rdrau1ic Laboratory-and Model

Testing Basin

Newport News Shipb]Ldg. & Dock Co.

Newport News, Virginia

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Research

Navy 100

fleet Post Office New York, N.Y.

Forward to: Admiralty

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