REPORT ON RESISTANCE TESTS OF A FLAT PLATE

A flat laminated aluminum alloy plate, having the size shown in figure 1, on which a U.S. new

patented Anti Fouling coating was applied, has been tested in the towing tank of the University of

Trieste (Italy).

1. Introduction

On request of MarineEcoCoating Inc., Marina del Rey, CA., U.S.A. through his Mediterranean

licensee EcoCoatingMed srl, Castelrotto, Italy, a series of towing tests have been made, by using a

vertical flat plate on which a new A.F. coating was applied. The plate transportation to the

University and the mounting of the plate on the towing carriage of the University tank was made by

a technical staff composed by an university technician and one or two technicians of the Shipyard

“Cantiere Alto Adriatico”.

The plate (figure 2) has been placed in a vertical position, supported on two suspension points by

two movable carriages, which could move along two guide lines. By so doing, the flat plate could

move along the guide lines and could be connected to a load cell, to measure its resistance during

the tests. A series of centering screws allowed to place the plate in the central position, between the

guide lines so as to be run parallel to the advancing direction. Small water disturbances can indeed

create instabilities to the plate advance during the motion and, for this reason, the plate itself was

constrained to remain vertical to the water surface during the runs at different speeds. The

constraints, which bind the flat plate to the centre plane of the towing tank, do not prevent from

Figure 1 : Geometry of the flat plate tested in the towing tank (data in millimeters). The submerged

part is rectangular 2500 x 384 mm – Thickness of the bare plate after the coating with the A.F.

Seashell : 4 mm.

measuring the resistance during the tests. The vertical position of the flat plate is assured by two

extreme projections which are useful to bind the plate and to prevent side motions. It was decided to

limit also the maximum carriage speed to 5 m/s to reduce risk of unstable motions (fluttering).

The main characteristics of the flat plate are shown on TABLE A.

TABLE A : Main characteristics of the tested plate (also identified as “reference plate”).

Flat plate material : Aluminum Alloy (smooth laminated)

Plate length : 2.5 m; Submerged part: 384 mm Plate thickness : 1/10 in = 2.54 mm

Initial Wetted surface : 1.92 m2 The extreme sides of the flat plate have been rounded.

Some images of the flat plate in preparation and placed into the water are shown in figures 2 and 3.

Figure 2 : The coated plate prepared for preliminary tests. It should be noted that the submerged

part is still rectangular.

Figure 3 : The flat plate during the tests.

To assure an adequate plate stiffness, two longitudinal aluminum L sections have been placed on

the plate (see figures 2 and 3), outside of the water. External chains supporters were used to better

distribute the plate weight on the longitudinal guides, so as to have a perfect horizontal sliding way.

The towing of the plate is made by a load cell, having an extra charge up to 500 N and a linear

registration field up to 100 N. For safety reasons two mechanical stops have been placed on the

guide ends. The original plate shown in figure 1, was then tested in the towing tank, to determine

the maximum towing speed; no drag measurements were taken during these preliminary tests. A

maximum speed of 5 m/s before reaching instability was measured, so that the plate was slightly

modified truncating the lower fore end and rounding the fore and aft ends (see figure 4).

Figure 4 : Extreme truncation of the plate end after the cut.

Then the towing tank tests were resumed and no instability inconvenience was noticed up to a speed

of 5.5 m/s. Anyhow it was then decided not to exceed the velocity of 5 m/s during the drag tests.

2. Bench mark tests

The bare smooth-laminated aluminum rectangular plate (also called “reference plate” - see figure

1) was previously tested firstly to set a bench mark for comparing with the same plate after SeaShell

coating and, secondly for comparing the experimental results with Computational Fluid Dynamics

(CFD) calculations. The CFD program used for this comparison was the program STAR – CCM+

(CD- ADAPCO) in its version 7.06. The consistency of measured data and CFD results is shown in

figure 5.

Fig. 5 : Experimental and Numerical result comparison for the reference aluminum bare plate.

A comparison was also made among the experimental results and ITTC ‘57, ATTC ’47 (ATTC :

American Towing Tank Conference) and Hughes evaluated friction coefficients . Figure 7 shows

the comparison between the different friction curves and the experimental data obtained for the

reference plate. The dimensionless coefficient of the flat plate is obtained using the classical

expression : CF = R *2 /(ρρρρ WS V2 ), where ρρρρ is the fresh water density, V is the plate velocity

(m/s), R is the plate resistance (Drag) and WS is the wetted surface of the plate. The Total

Resistance (Drag) Coefficient calculated for the reference plate is subdivided into its traditional

components, that are the Frictional Resistance Coefficient CF and the Residual Resistance

Coefficient CR ; CT = CF + CR.

The residual Resistance coefficient is mostly generated by the wave drag components and the

eddies generation. The values obtained for these components, calculated by using the CFD methods

are shown in figure 6.

Figure 6 : Resistance Coefficients obtained from the CFD calculation for the original plate.

In figure 7 a comparison between the friction resistance (Drag) coefficients obtained or calculated

for the flat plate is shown.

It is possible to see that the friction lines obtained from the experiments and classical theories agree

very well.

Both ATTC'47 and ITTC'47 friction lines include a small form factor and for this reason they are

higher that the Hughes friction curve.

The work done set the stage to resort to this mixed experimental-theoretical method to analyze the

towing test results and make the necessary corrections to the measured data to take into account the

difference of plate thickness and shape.

Figure 7 : Friction resistance(Drag) comparison between the experimental results and the classical

friction curves.

Smaller values can be noticed at low Reynolds Number, where some transition phenomena can

arise; transition generates laminar flows and reduces the friction coefficients. In general the

Reynolds Numbers are quite almost higher than 1*10 6, and this number signifies that we are

operating in turbulent regime.

3. The tests with the SeaShell coated plate

The lines of the plate are shown in figure 8. The depth during the tests was set to 380 mm and the

plate wetted surface was equal to

1.82 m2. A picture of the tests is

shown in figure 9.

The maximum speed of the

carriage was set to 5 m/s. The

Drag was measured by a load cell,

placed at the top of the plate.

Figure 8 : Test shape of the SeaShell coated plate.

(dimensions in millimeters)

Figure 9 : The SeaShell

coated plate assembled on the

carriage and ready to be

tested.

As made for the reference flat bare, a numerical CFD simulation of the Drag tests was made for this

configuration, and the Drag data were compared with the experimental ones. The results obtained

are shown in figure 10.

As it can be seen, the

experimental data are smaller

than the numerical ones,

although they are getting closer

as the speed increases. By

comparing the numerical

components of the total drag, it

is possible to notice that the

residual components are slightly

higher than the values obtained

with the reference plate. This

phenomenon is caused by the

increased thickness of this plate

(4 millimeters now, 2.54

millimeters for the reference

plate) only partially

compensated by the “bow

effect” due to the modification

Figure 10 : Experimental and Numerical results obtained during the tests and by numerical

computation.

of the submerged part of the plate (from rectangular to trapezoidal) ; by increasing the testing speed

the thickness the thickness effect tend to prevail.

The CR values of the new

plate are slightly higher

than the ones shown on figure

6.

Due to the change in plate

thickness and plate geometry

it was not possible to directly

determine and compare the

friction coefficient of the bare

laminated smooth aluminum

used testing the reference

plate (see paragraph 2) and

the friction coefficient of the

SeaShell coated plate used in

the last performed tests (see

above). As a consequence, as

said at the end of paragraph 2,

the following corrective

procedure was adopted :

Figure 11 : Resistance (Drag) coefficients obtained from the CFD investigation in the SeaShell

coated plate.

The total resistance coefficient measured on the coated plate (figure 10) and subtracted the residual

resistance coefficient, derived from CFD calculation (figure 11). Then we compared the friction

resistance (drag) curve with

the classical friction curves

evaluated for the smooth

laminated aluminum plate.

The results are shown in

figure 12.

Figure 12 : Friction

Resistance Coefficients

comparison between the

tested results and the

classical friction lines.

In figures 13 and 14 two

images showing the

numerical (CFD)

reproduction of the reference

plate and of the coated plate

tested at a speed of 5 m/s are

reported.

Figure 13 : The reference plate Figure 14 : The coated plate

4. Final considerations

The final results show that an appreciable friction resistance reduction has been obtained testing a

SeaShell coated plate at low Reynolds numbers.

From figure 12 it is possible to presume that at higher speeds the plate frictional resistance is

approaching to the friction resistance curve of the smoothed laminated aluminum plate. For this

reason it is strongly recommended to extend the investigation up to a Reynolds number equal to

1.6*107, whereas the Reynolds Number typical of full scale ships are in order of 109.

Trieste, 18th April 2014

Igor Zotti

Prof. Igor Zotti

University of Trieste – Department DIA

Via Alfonso Valerio, 10

34127 Trieste – ITALY