Model tests on planing hull models with systematic change of form

ARCH

Model T

Lab.

v

Scheepst,ouw!und

ests on Planing Hull Models

with systematic Change

Technische Hogeschoo

Prof. Ata Nutku(t.T.U.)

Deift

It islmowri that the resistance of a planing hull is defined

by the maguitude and forni of the pressure configuration producedf under her bottom and, at the respective dynamic trim angle at which

it is occurring. By the aid of model tests on simple geometrical

planes and theoretical analysis, the form of longitudinal and, transversal pressure distribution can be assese and the specific

resistance of the form evaluated. The first input data into the problem will the be the static values as'.

The elements of form and proportions of the bottom at rest, and

The position of C.. in length and height(KG).

Following the above, the dynamically ballanced state of the model

in planing condition(i.e. dynamic trim angle ,the rise of C.G.

and the draft aft dA',at the advance speed of the plane); will

acquire their dynamic values according to the abovementioned input

data, an addItional parameter for the dynamic state being:

The angle of shaft inclination and the point of applica-. tion of the propeller thrust with respect to L.C.G.

For an overall review of the factors effecting the behaviour and performance of a planing hull, it is requested to refer Fig.l

resulting in the total resistance, R.t

For this purpose, model tests were carried out in Turkish Tank(t,T.Uiwìth systematically varying hull forms,to study the

effect of each variable on behav-iour,performance and resistance and to link them with previous research, correlate with. full-scale

trial results and interpret them under the light shed by theory. However, due to limitations here, we shall only deal with test

results and observations on them. Descrit,tion of I1odels:

Family of models were evolved from a mother model ofsimple rectangular cross section having flat bottom with rounded off

circular nose in profile, which has been marked as form U.Suffixes as U1, U2, U3, U4 indicating aspect ratios of 1, 1/2, 1/3, 1/4

respectively. The evolution to other forms are as indicated in Fig.2 . The second generation ,hown in the middle refers to a model marked V with an angle of rise of bilge

¡3

₌

_{12,5° for} which similarly four models with varying beam _{were built and.}

tested. At the right of V, models with rectangular cross-section with sides tapering in plan form marked _{as TU and UT are shown,} which coupled with V form type of _{cross section there appears}

models marked. TV and VT

Further down a warped model marked. UIT having _{a bottom}

form starting with rectangular cross section at transom and going into V form at bow( No 93)with an angle of inclination

at chine has opened the way to the birth of a more

sophisti-cated form of conventional boat, a model tested _{in DT which} is related from the other colmn with the tapered family (Noes). Further down, a semi. circular model with _{pointed bow (cut out}

of a body of revolution marked. U- _{is presented.. Model Z (No95)}

is dented in longitudinal _sense.

Other models on the left hand colu _{are designed for natural}

and partly artificial wall sided air cushioning as to be accepted

out of the scope of thIs report.

The models were generally of 0,50 _{m. in length but models}

having twice L,0.A.(l,0O Meters )were also _{built and. tested. for}

simple U and V forms, to study the scale and _{temperature effects.} U models geosims wIth22m. 0,44m.,0,55 m.,0,66 _{m.in breadth were} built and tested for study of scale effect _{and. dynamic trim}

angle.

Purpose of the tests:

The aim of the test programme constituted _{for enlightenment} on the effects of different variables _{on resistance and}

pro-pulsion, as stated belo'-ï:

1)- Effect of Proportions and. form elements:

Aspect ratio B/L

Trim angieZo , a ¡L

Swept channel section _{BSdA /}

a). Bilge or angle of rise of floor/S ,

Variation of angle of bilge with lerth(warp)E,

Entry, nose curvature r

). Taper in plan form(beam),angle

0e'0<r

Curvature of transverse sections(convex or concave)

Lon3itudinal dentine ( clinker,steppin)

i). Auxiliary ateps,wedes,flaps( at stern and at thebow)

Spray battens at chine,

Muitihull confiSurations (catamarans ,trimarans, Quadre.

and. Quintamarans)

2)- Elements effecting incipient lanin afid dynamic trimminR bailan ce:

Position of c.G.(LCG) HeiSht of C.G.(KG)

Inclination of shaft an3le and. hei.ht between shaft center line and. C.G-.

a). Proportions of appendage resistance In total e) Propulsive elements w, t, eq

3) - Elements effectIn. the correlation of models of different sizes and. boats:

a) Atmospheric pressure

effect,-b)=. Image wave observation,evaluation of hydraulic jump, conversing and tail waves In relation to performance,

c). Pressure measurements on the bottom, its correlation with sizeinteraction with aove(3b)and form elements(l)

Temperature effects,

e). Surface tension and. air content on bow wave and. effect on spray resistance.

4)- Pasic research on:

a)- Eoìrnary layer , flow regime with size,turbulance stimulation methods effectivity,

Wave resistance investIat1on as in (2b)

Similutud.e of planing with shallow water effect and sluice gate flow,

cl). ITagriitude of circulation, Lift,

Mode and arameters for presentation of results:

Results of Tests:

It, should be appreciated that only some of test results observations and discussions on them could be presented. in the limited. boundaries of the present paper as its scope is devoted.

ri.ncipally to remain as a test report.

parameters, some of the results are given as pure,obtained. from tests and. a few will be transformed into simple

nondimen-sional presentation of the nature not to prejudice the

compa-'ative evaluation of performance of different models.

As enhanced above, for prediction of resistance of the planing hull,('', AR',and dA') in dynamIcal state,as second

input values ought to be lmown,and. as these are the functions

of initial,statical(first input data), we shall,here,omit the

intervening output data of pressure configuration, circulation, etc. shown in Fig.l.

The results of tests are as given in Fig.5 to Fig.l?.

In Fig.5,the effect of change in loading and of the aspect ratio may be observed,where, the beamy model U2 shows herself superior to others in incipient planing region and also in resistance-minimum zone,distinctly. On the nondimensional basis of F to R/in Fig.6, the beamlést model U1 gives lowest peak for

inci-pient planing and earlier minimum, but at higher speed.s,the

optimum value of Lift/Drag being passed la resistance increase is accompanied by surplus of Lift, optimum AR being represented

by model U3.

Three models V1, V2, V3 in Fig.? confirm the effect of higher aspect ratio's resistance reducing property,whilst the pressure distribution along the span (width) become near to elliptical( see Fig.8) and the losses due to spill over from

chine(edge of profile)'s share in total lift becomes reduced.

In comapring V2 and U2, at light loading the lift being less due to edge losses,she becones only better off in highly loaded

condition. In Fig.? narrower V3 attains the required lift for planing at only higher speeds,consequently he rpeak resistance

position retards and requires higher spoed.s (stagnation pressure

v2/2g). In comparing the narrow models V3U3 the spray compo-nent of U in light loadings is noted by the apparance of peak

fluctuations of resistance curve.

It may be noted that the sinkage and squattirg at early stages of incipient planing, the only dimension controlling the aame,proves itself to be the beam. Therefore, the fluctuations appearing at the region of resistance peaks in narrow modele, disappear in wider models,like U2 . Narrow models require higher

- 5.

In Fig.? , a crossing over of specific resistance curves and total resistances occur.Deeply loaded model V2(2 KS.) performE better after F ., 3,5. Identical intersections of

curves occur with L,00 and 1,50 Kg.loadings of V2 at about

Fv, 2,25,and incidentally all on the same 0,20 value. The pressure distribution at the bottom with change of rise of floor and warp is sketched in Fig.8.

The model with warped bottomE. l2,500is shown in Fig.9 as TN2. TW2 show higher resistance peaks but deeper,sharper

resistance troughs and on higher speeds( above

₄

3) which rise steeper upwards compared with untwisted bottom model V2.

This might be attributed to abrupt change in pressure distribu-ton curves along the span(beam)of the bottom, where, instead o parabo1c distribution,curves with humps and hollows and

in some cases negative pressures occur,about the chine.

An evenly distributed(pressure mountain all along the bottom) usually maintains unimpeded,unifor'm flow accompanied with

least resistance to motion. Therefore, the selection of proper amount of warp(rate of change of bilge angle)ls of importance

in desi.

In Fig. 10,the comparison between two tapered models,

with decreasing or increasing beams) VT and TV are presented, where, contrary to common practice and belief the model with narrow transom shows herself worse than the one with wider

transom VT , however, in some theoretical works,this fact has taciturnly been proven. The amount of spray in VT being

re-duced, a better receptive entry into the plane helps to reduce the planing peaks in resistance curves and by causing an evener pressure dlstribution along the length , moving the effective

aspect ratio towards aft. As long as is kept within the limits of wave convergence, the reduction of pressure peak.

at fore end of the plane by filling in the rear,will be

ad-vantageous.

When four models of U2, V2, TN2 and VT are compared as in Fig.11, for same loading, VT shows herself with lowest planing peak,but is apar after Fv 3,2 with

In Fig. 12 flat bottomed tapered model TU compared

with V bottomed TV,the former shows a very high peak resistance

aft' by sharp chine of' TU. But in hig2ìer loadings ,the

t,ondi-tion is improved. The sharp fluctuat,ondi-tions of resistance curve may point to indication of ascillation, by change of trim

angle and AR around the res istance-optimuxn boundary.

Another explanation to low performance of of tapered-to-aft

TV' models may be based on the observation on (hydraulic jump)

at her wake, which remains closely attached to transom as

compared with her contemporaries. ( It is observed that a

surf-riding effect by the hydraulic jump is accompanied by resistance-minimum.) .The explanation to better performance

of VT over 112 may be attained by studying the relative

curves of (CL versus CD). Afetr 2 and 3,over-increase in

value of the bottom,the induced and total resistance pass

further away of the optimum point of CL/CD curve.

In Fig.12 an overall comparison of all models of forni significance are given. The best performing geometrical model VT compared with conventional planing hull model DTT is seen.

At the medium loading condition (1,5 Kg.)VT fairs better up

to 3,2 at higher speeds they run apar. It may be thought that model VT can further 'be improved.

In Fig. 13 some of the results with tests of stern

flaps are presented,where,addition of a stern step causing

c1ixnge in bottom pressure configuration, distorts the form

of resistance according to the angle of deviation. However,

addition of a flap would mean an increase of resistance, in

cases wharihe flap causes to even the pressure configuration a reduct3n in resistance can be obtained. This will depend

on speed, LCG ,incl.of Shaft,as well as B/L aspect ratio.

Position of C.G. (LC) effects the same way as a flap in

distorting the resistance curve's forni. It might be thought that,stern wedge and LCG are opposing elements. In Fig.13

position of LC are indicated on the curves,those in the

circle indicating the thickness of the wedge. The spray batten angle to bilge angle will perform the same way as the wedges

fitted under the bottom. A spray batten with changeable

angle of incidence to bie( on a model like VT) may regulate

the pressure configuration as a stern flap or an auxiliary

step(bottom flap) ,according to the designed speed,loading

condition

and LCG.

confi-confiurat1ons are presented,toether withsimple mother model U2 Peat resistances of multihulls are seen to remain

fluc-tuatin between the R/values of 0,25 to 0,30. The open

gaps between the hulls is though to be contributing to the

lift,by their interaction. The case of multthulls would require further testing.

In concluding the part concerning the results of tests

and cons±erations on themit may be pointed out that, by

the aid of theory, conditions for optimum liTt(and pressure distribuMon) on the bottom of a gliding plane should be

evolved, similar to that done for hydrofoils, the counterpart

of emerged plano. For this author humbly points out to the

need for correlation of theory, model experiments and sea

tr.als of the boats,before an attempt to rìydrodynamioal

correlation of models to ships.

For this,it might be emphasized that, gliding plane should be considered to be subjected to three different regimes ,amely,before planing, during incipient planing and fully gliding state. In Fig. 15 results of tests with

temperature difference,points out to a certain speed where the. resistance curve in the vicinity of planing peak is

indepen-dent of viscosity effects. Farther to this the comxnoily used.

parameters and, methods of presentations cannot 'be valid for

gliding planes, performing under different regime and law

of coinaprison. A mode of presentation which might be worth

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