Seakeeping characteristics of the trehantiri type boat in regular waves

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NATIONAL TECHNICAL UNIVERSITY OF ATHENS

DEPARTMENT OF NAVAL ARCHITECTURE AND MARINE ENGINEERING

ATHENS 10682 - GREECE

NAVAL ARCHITECTURE LABORATORY

SEAKEEP ING CUARACTERISTICS OF THE TREHANTIRI TYPE BOAT

IN REGULAR IIAVES

G. Garios

T. Loukakis

1. INTRODUCTION

Three trehantiri type boat models., which fOrm a mini

series with L/B 2.5, 3.0 and 3.6, were tested for

seakeeping in head regular waves and at various speeds

A sample of the results with some comments are presented here.

.2. MODELS DESCRIPTION

The trehantiri type is the most frequently used fishing boat in Greek seas. The same huilform is also widely used as a pleasure boat and to transport passengers or cargo for short distances The length of the vessels varies between 5 and 20 meters.

To contribute to the proper design of these. boats a research program has been initiated at the Naval Architecture Laboratory of the National Technical University of Athens with main goals:

to create a computerized procedure for the generation of the hull lines,

to create a systematic resistance series,

to establish experimentally the values of the

propeller-hull interaction parameters,

to examine the agreement between theoretical

predictions and experimental data for the seakeeping

behaviour of these especially seaworthy vessels

The resistance characteristics of the initial three model mini-series have been already published [1]

In this paper, some seakeeping initial results in

regular head waves are 'presented.

In the following Table 1 are shown the main particulars of the three models at the tested displacement. By r* is represented the longitudinal beam of inertia of the model

Table

No1L (m) L/B1L/V" BIT: Cm Cb : LCB% I LCF% Trim r*IL

I__I ---I

. I.

I____I_-

I I I . I -.- I

I I I I I

'I---- I

I ' I I I I

111 6302 551 3 80 13 821 7351 615 l3fwd l3fwdll 5aaftl 311

1211.93613.061 4.29 :3.82:.735:.615:.l3fwd:.l3fwd:1.sDaft: .251

1

32.3473.67

4.85 3.82.735.615.l3fwd.l3fwd:i.5°aft: .251

I _! I I I I I I I I I I

In Fig. 1 are shown the waterlines and the body plan of model No2.

3. MODEL TESTS

Each model was tested in head regular waves at Fn=.00,

.15, .23 and .3O

The experiments were conducted at the towing tank of NTUA with dimensions LxBxT : 90x4.6x3.5rn.

The model was attached to the carriage via a heave rod-pitch bearing assembly, which can also measure resistance

electronically. Resistance, pitch arid heave were

automatically digitally recorded.

The time history of incident wave were also digitally recorded using a resistance wave probe attached to the carriage at some known distance before the model.

The model were fitted at the bow section 3 with two resistance wave probes in order to measure model relati've motion. One, very near to the model surface and the other at. a distance of 1.05m from the model centerplane.

Total resistance in waves of the model was obtained by intergrating the resistance time history over an integer number of wave periods At each speed a test with zero wve

period (i.e. without waves) was conducted in order to obtin

the calm water resistance and also the running pitch, heve and relative water level at the wave probes attached to the model.

After each run in waves all the oscillating time

histories, corrected in order to refer to the same point of

the model longitudinally, were analysed by the technique of

reference [2] So the phase angle, the amplitude and the mean

level of each signal were obtained. Phases and amplitudes

results were non-dimensiorialized using the corresponding

measured incident wave characteristics. WaVe resistance as

obtained by substracting the calm water resistance from the

total model resistance in waves at the same speed .nd

non-dimensjonaljzed using the square of the incident wave amplitude. By substacting the calm water running pitch, heave

and _{relative motion3 measured by the wave probes attached to}

the model., from the corresponding mean levels in waves at the

same model speed, the mean values of the heave, pitch and

relative motion due to the waves (mean shift due to the waves) were calculated.

In addition the model absolute motion at the bow station 3 was obtained as a combination of the measured model pich and heave results.

The aforementioned results were plotted, as a function of /L and compared to the theoretical, predictions. All the theoretical predictions were obtained using the new strip

theory [3] with extended Lewis-form representation for the

cross-sections [4].

For each speed a number of experiments were conduOtéd at different i9/L values. The incident wave amplitude were keeped

constant arount /L=.O15'. For the model Nol and at Fn=. 23 a

number of tests were conducted with C/L..0075, .015 and .025

in order to determine possible non linearities due to the

4. RESULTS

A sample of the results with some related comments are presented here.. Experimental results. are shown. by asterisk

(*). Theoretical results _{are shown by solid line. or 0.} Non-linearities.

In general non-linearities due to the wave amplitude _are not very important. In Figs 2 (a),(b) and (c) the heave

response of model Nol at Fn=.23 for _{/L=.0075, .015 and .025}

are shown respectively. Wall-effects.

Considerable wall-effects were remarked at zero or .l5Fn ship speed. In Figs 3 (a) and (b) the heave response of model No2 at Fri=.15 and .23 are shown respectively.

a. Mean shifts..

In most cases a mean shift due to the waves with a

maximum at 9/L around 1.5 was recorded. In Fig 4 the

corresponding results f or the pitch of model No2 at Fn= 30 are shown.

Relative motion..

In Figs 5 (a) and (b) the relative motion amplitude at

Fn=.23 of the model Nol measured near the model surface and

at a distance of 1.05m from _{the model centerplane are shown} respectively. Figs 6 (a), (b) and Figs 7 (a), (b) refer to the same quantities but for models No2 and No3 respectively.

Clearly the relative motion amplitude near the model surface is greater than the motion measured at the same

station but at a distance from the model sideways. The

theoretical predictions of _the relative motion are much

closer to the measured values away frOm the model than close

to the model surface, where the measured _values are much

greater than the predictions. The differences between

measured relative motion close and away

_{from the}

model increase with the decrease of L/B value.

Pitch. heave and added resistanoe

In Figs 8 (a), (b), (c) and (d) heave and pitch

amplitude responses and heave and pitch phases, for the model Nol _{at Fn=.23, are shown respectively. Figs 9 (a), (b),, (c),}

(d) and Figs 10 (a), (b)., (c). Cd) refer to the same

quantities but for models .No2 and No3 respectively.

It can be seen _that the theoretical. results _are in

general _{satisfactory.} The greater discrepancies between

theory _{and experiments lies in resonance areamainly for the.} pitch amplitude responses, which are underpredjeted by theory, and heave phases.

In Figs 11 (a), (b) and (c) the added resistance

coefficient in waves, at Fn..23, is plotted for models Nol, No2 and No3 respectively. As it can be seen the greater discrepancies between experimental _{and theoretical results}

lies in low /L area where the theory clearly underpredicts

REFERENCES

1-.. Ganos, G. and Loukakis, T. "Resistance

characteristics of the trehantiri type boat", 3rd

International Congress on Marine Technology, Athens 1984.

Ganos, G. A new technique for the analysis of model

motion in a simple harmonic wave", Conference on Computer

Technique and Advanced Scientific Instrumentation in Ship

Hydrodynamics, BSHC, Varna 1984. I

Saivesen, N., Tuck, E. and Faltinsen, 0. "Ship

motions and ship loads', SNA14E Transactions Vol 78, 1970. Athanassoulis, G. and Loukakis, T. "An extended-Lewis

form family of ship sections and its applications to

seakeeping calculations", ISP Vol 32 ,February 1985.

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