Power prediction based on model tests for high speed craft

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POWER PREDICTION BASED ON MODEL TESTS FOR }HGH SPEED CRAFT

Rodrigo Azcueta Institut für Schifthau 22305 Hamburg, Germany Volker Bertram Institut für Schifthau 22305 Hamburg, Germany Gerhard Jensen Hamburg Ship Model Basin

22305 Hamburg, Germany

ABSTRACT

Experiments to determine the power requirements of high-speed marine vessels require

suitably large facilities with fast carriages. Test procedures must account for scaling errors due

to cavitation, spay, and other physical effects that usually are not important for conventional ships. Some guidelines are given for such procedures to raise awareness of the problem but

generally one must accept a lower level of accuracy in power predictions for fast ships.

INTRODUCTION

Model testing has a long tradition for the prediction of ship performance of conventional

ships. The scaling laws are well established and procedures have been developed to give

correlation between model and ship to high level of accuracy. The same scaling laws generally apply to high- speed craft but two fundamental, strongly connected problems may arise:

Experience for scaling effects cannot be extrapolated from classical model tests. Limitations of the test facility do not allow an optimum scale.

Important physical quantities influencing the scaling effects are:

- Surface tension (spray) - Viscous forces and moments

Aerodynamic forces

- Cavitation

-The most important limitations of the test facilities generally are: - Water depth

- Carriage speed

Special experimental and correlation procedures have been developed to handle these problems. Hybrid marine vessels, combining various forms of lift (air cushion/buoyancy or

foil/buoyancy) have increase the need for methods coping with these problems. Hybrid vessels

are a recent focus of attention as they are especially attractive for larger high-speed craft as demonstrated by the Japanese "Techno-Superliner" project, [1].

Subcavitating, cavitating, and ventilated propellers on shafts or gondolas as well as waterjets with flush or pitot-type inlets are used for the propulsion of high-speed marine vessels. Viscous effects and cavitation add to the problems in correlating to full-scale ships.

BASIC CORRELATION TECHNIQUES

The flow around ships is mainly governed by viscous, inertia, and gravitational forces. Tests with a geometrically similar model are generally performed in a towing tank so that the inertia and gravitational forces are properly scaled. These forces are responsible for the wave pattern and

the hydrodynaniic lift. This is possible if the Froude number Fn= V5/IL= V//gL , aspeed non-dinensionalized with the length and the acceleration of gravity, is equal for both model and ship. Indices s and m refer to full-scale ship resp. model.

The frictional forces would be similar if also the Reynolds number Rn=VL/v which

depends on speed, length, and the kinematic viscosity would be the same for model and ship.

This is not possible. Therefore the basic method to cope with this partial similarity is to compute the viscous forces from empirical formulas and scale the rest:

R5= (RmR)

Pm

Similar techniques are used for scaling of propeller torques. The validity of this procedure

requires:

Tank walls and bottom must not infiuence the flow.

Viscous effects are confined to the thin boundary layer near the body surface. The viscous resistance can be computed with sufficient accuracy.

Valid predictions from tank tests for the resistance of the full-scale ship in unrestricted water

are possible only if the tank is sufficiently large compared to the model to allow similarity in

flows. The ratio of the submerged cross section of the model to the tank cross section (blockage)

will generally be very low for high-speed ships. However, shallow-water effects depend mainly on the model speed and tank water-depth. A Froude number based on the model speed and the

tank water depth H Fn-v.1/fgH

is the major parameter for shallow water effects. It should

not be greater than 0.8 to be free of shallow-water effects. A limiting curve for the

non-dimensional rnodel speed for a tank water depth of 6 m is plotted in Figl. Of course, the

maximum Froude numbers are lower for smaller water depth.

If we assume a longitudinally stretched wetted underwater body, like for any planing or semi planing hull form, the frictional resistance is computed from frictional resistance of a flat plate

of similar length. It can be computed to a high degree of accuracy if we can ensure that the

boundary layer is turbulent. Even when turbulence stimulators are used, a minimum Reynolds

number has to be reached. We can be sure to have a turbulent boundary layer at Reynolds numbers higher than (5*106). This gives a lower limit to the speeds that can be investigated

depending on the used model length and is also shown in Fig.l. The diagram demonstrates that

only a certain speed range (vertical axis) can be safely investigated in a model tank of given

dimensions and that a proper model scale has to be chosen.

A practical limitation may be the maximum carriage speed.

2, 5 -o 0, 5 5 6 7 3 9 10 ,1odeI LgIh

Fig. 1: Possible speed range in a 6m deep towing tank with 15 C water temperature. .7

Mm sp

LI'llTATIONS OF MODEL TANKS

cx sced mrt rn onk

PLANING HULLS

In the planing condition, a significant share of the resistance is frictional and there is also

some aerodynamic resistance. At the design speed the residual resistance - i.e. the component which is determined from model tests - may account for only 25 to 30% of the total resistance.

This part is even smaller in model scale. Therefore accurate measurements of the model

resistance are required.

Resistance of planing hulls strongly depends on the trim of the vessel. Only a careful set-up ensures that the towing force acts in the correct direction. The most important problem in model test evaluation for planing hulls is the accurate determination of the wetted surface and wetted

length which are needed to compute the frictional resistance for both model and ship. Side

photographs from the carriage are in many cases not adequate. Preferably under-water

photographs should be used. The accurate measurement of trim and sinkage may be adequate in many cases. Wetted surface and length are then taken from a hydrostatic calculation for this trim

and sinakge. The results based on this engineering approach are generally acceptable. As the flotation line of planing vessels strongly depends on speed, proper arrangement of turbulence

stimulation is needed as well.

Depending on the propulsion system, planing vessels will have appendages like rudders and

shafts. For a typical twin screw ship with shafts and one pair of I-brackets and one pair of

V-brackets, the appendage resistance could account for 10% of the total resistance of the ship.

As viscous resistance is a major part of the appendage resistance and the Reynolds number of the appendage will be in any case small for the model or the appendage may be within the

boundary layer of the vessel, only a crude correlation of the appendage resistance is possible. The

resistance of the appendage is determined in model scale by comparing the resistance of the

model with and without appendages. Then an empirical correction for transfering the appendage resistance to the ship is applied. In many cases it will be sufficient to perform accurate resistance measurements without any appendages on the model and then use an empirical estimate for the appendage resistance.

CRAFT WITH HYDROFOILS

Hydrofoils may be used to lift the hull out of the water to reduce the resistance. Besides the classical hydrofoil craft, which are fully supported by foils, catamarans and monohulls which are only partially supported by foils are also developed, [2], [3]. Resistance and propulsion tests for such vessels face these problems:

The Reynolds number of foils and struts will always be very low. Therefore the boundary

layer on the foil may become partially laminar. This will influence the lift and the

frictional resistance of foils in a way difficult to predict. The uncertainty level of

approximately5% is higher than for conventional crafts.

- Cavitation may occur on the full-scale hydrofoil. This may not only cause material

certainly occur if the foil loading is higher than i0 N/rn2. With configurations not fully

optimized for cavitation avoidance, significant cavitation is already expected for foil loadings higher than 6 l0 N/rn. Another important parameter is the vessel's speed.

Beyond 40 knots (1 knot = 0.5 144 mIs) cavitation has to be expected on joints to strut

flaps, foil tips and other critical parts. At speeds beyond 60 knots, cavitation on the largest part of the foil has to be expected. When model testing these configurations in

conventional towing tanks, cavitation will not occur. Therefore similarity of forces cannot

be expected. Resistance and propulsion tests in a free-surface cavitation tunnel would overcome this problem. Due to the limited cross sections of these tunnels, shallow-water effects will be unavoidable in this condition. Therefore we recommend:

Perform tests in the towing tank using non-cavitating foils from stock, vary angle of attack, measure the total resistance and the resistance at the foils.

Test the foils (including struts) in a cavitating tunnel, vary angle of attack, observe

cavitation and measure the forces. Tests in the cavitation tunnel should be

substituted by computer simulations of the flow during preliminary design.

Combine the results of both tests determining the angle of attack for similar lift

of foils and summing the resistance components.

We refer to recent work of Latorre and Ryan [4] for scaling of spray inplaning hulls

SURFACE EFFECT SHIPS (SES)

Fig.2 shows the resistance components versus speed for a SES craft. The magnitude of the

humps and hollows in the cushion wave-making resistance strongly

depends on ratio of

lengthlwidth of the cushion. Wave making from the submerged hulls and the cushion can be

scaled according to Froude's law (i.e. same Froude number for ship and model), as long as the

tank depth is sufficient to avoid shallow-water effects. Otherwise a correction based on the

potential flow due to a moving pressure patch is applied. This method has some disadvantages due to the significant influence of the trim. Observations with a video camera inside the cushion

are required to determine the wetted surface. The frictional resistance of the seals cannot be

separated from the total model resistance The pressure distribution between seals and cushion has to be determined by controlling the air flow. The flow rates will give some information about

the required fan powers. Also the model aerodynamic resistance in the condition under the carriage has to be determined and used for separating the wave-making resistance. GeneraLly

separate wind tunnel tests are recommended to determine the significant aerodynamic resistance of such ships.

CONCLUSIONS

Experiments to determine the power requirements of high speed vessels require suitably large facilities and a careful selection of tests procedures. Correlation techniques have to be adjusted for the individual problems. Besides friction, cavitation plays an important role in the correlation. Generally we cannot expect the same level of accuracy for power prediction as for conventional ships. The towing tank should provide an error estimate for each individual case.

0:10 0.08 0.06 0.04 0.02 RESIDUAL DRAG AERODYNAMIC DRAG FROUDE NUMBER

Fig.2: Components of drag of Surface Effect Ship

REFERENCES

[1] Various Authors, 1993, 2nd Conf. Fast Sea Transportation FAST'93, Yokohama [21 Meyer, J.R., 1992, 'Hybrid Hydrofoil Applications", Intersociety High Performance

Marine Vehicle ConL, Washington, D.C., pp.HF25-36

Azcueta, R., 1994, "Model tests for a HYS WAS" (in German), Studienarbeit (Independent

Thesis), Institut für Schifthau, Universität Hamburg

Latorre, R., Ryan, S. 1993, "Similitude of Planing Hull Spray", Ship Technology

Research, Vol. 40/4, pp.159-164