paper

International Conference on Ship Drag Reduction

SMOOTH-SHIPS, Istanbul, Turkey, 20-21 May 2010

The efficacy of air-bubble lubrication for decreasing friction resistance

E.J. FOETH & R. EGGERS & F.H.H.A QUADVLIEG Maritime Research Institute of the Netherlands, Wageningen

ABSTRACT

Reducing the frictional resistance by air injection below the ship in combination with special coatings is an

active area of research; anecdotally, performance gains are usually large. The paper gives an overview of

some model scale and full scale measurements results of ships with one type of air lubricationair bubble lubricationperformed by MARIN. The experiments were performed for the EU-SMOOTH project. The first series of experiments focused on an inland shipping vessel that was tested both on model scale and on full

scale, with and without air lubrication. A second series of tests consisted of maneuvering and seakeeping

tests with a model painted with different coatings and with and without air lubrication. No appreciable

effects of air bubble lubrication were found during the resistance and propulsion tests at either model or full

scale and no significant effects of air bubble lubrication on maneuvering and seakeeping model tests could

be determined.

1 INTRODUCTION

For a number of years, air lubrication has been

under investigation at MARIN as a means of

reducing the frictional resistance of ships. Three

general approaches are identified

Injection of air bubbles along the hull

Air films under the hull

Air cavities in the bottom of the hull

Several projects were started up in the

Netherlands in 1999. The PELS project has studied

the capabilities on theoretical and numerical grounds

and by extensive model tests (Thill et al., 2005). The

positive conclusion spurred two follow-up projects:

PELS 2, focusing on air cavity ships and the EU-

funded SMOOTH project, focusing on air-bubbles

and air-film lubrication. Both projects are focused

on inland ships and coastal ships and both projects

include a full-scale test with a demonstrator ship.

This paper presents the results of model scale and

full scale tests within the SMOOTH project. The

effect of air lubrication by bubble injection on

resistance and propulsion, seakeeping and

maneuverability using both model scale and full

scale experiments is discussed.

2 BACKGROUND

The frictional resistance is the dominant resistance

component for low-Froude-number ships. Pressure

drag (i.e., form resistance) and wave resistance are

frequently optimized using Computational Fluid

Dynamics (CFD) but the total wetted surface

remains a given. Reducing this frictional resistance

by air lubrication is attractive. The power needed to

compress air and inject it under the vessel should be

less than the alleged power reduction due to the air

lubrication.

For displcament ships, any reduction of the local

skin friction leads to decreases of the resistance and

commensurately fuel savings. As the Froude number

increases and the wave resistance becomes

progressively larger, the effect of air lubrication on

the total resistance expectedly decreases. The

injection of air requires constant pumping power and

if the ship sails too slowly it represents a significant

part of the propulsive power. Therefore air injection

is expected to be suited for moderately fast ships

with a target speed range of Froude numbers

between 0.05 and 0.15.

This paper focuses on air-bubble lubrication only. If

bubble lubrication is effective, it requires only a

small change to the hull compared to an air-cavity

ship.

Laboratory results of micro-bubble injection by

Madavan et al. (1983) showed reductions of the

frictional drag up to 80%. These micro-bubbles are

very difficult to create on a ship scale. As the bubble

increases in size, so does its tendency to deform in

the shear and turbulent fluctuations of the flow and it

is no longer a spherical micro-bubble. Bubbles are

on a millimeter scale for current ship applications;

the term micro-bubble is no longer applicable. As

the term micro-bubble is used ambiguously, a

distinction between (mini)-bubble drag reduction

and micro-bubble drag reduction is required.

At very low speeds, around 1 m/s, bubbles with a

diameter of only a few Kolmogorov length scales of

the flow can generate a 10% decrease in resistance at

only 1 volume percent of air in the boundary layer

(Park & Sung, 2005). At more realistic flow speeds

of 5 to 15 m/s, this viscous length scale drops

rapidly, enforcing a small bubble that is difficult to

produce in large quantities. Moriguchi & Kato

(2002) used bubbles between 0.5 and 2.5 mm and

reported up to a 40% decrease in resistance for air

contents over 10%. Shen et al. (2005), using smaller

bubbles between 0.03 and 0.5 mm, found a 20%

drag reduction at an air content of 20%. No

appreciable influence of bubble size was found here,

but Kawamura (2004), using bubbles from 0.3 to 1.3

mm scale, found that larger bubbles persisted

downstream longer and were more effective at

reducing the resistance. As larger bubbles showed

less dispersion this may have been an effect of

concentration only (Harleman et al., 2009).

The mechanisms by which mini-bubbles reduce

friction are as yet unclear. Mini-bubbles affect the

density and viscosity of the flow; viscosity actually

increases for small amounts of air, but at high

Reynolds number the turbulent stress is more

important than viscous stress. A decrease of the

density outside the viscous sublayer may be more

important. Kitagawa et al. (2005) found that bubbles

deformed with a favorable orientation with respect

to the flow, reducing turbulent stress as the flow

field around the bubble is more isotropic, although

other mechanisms have been proposed, such as

compression (Lo et al., 2006) or bubble splitting

(Meng & Uhlman, 1998).

Watanabe & Shirose (1998) tested a 40 m plate at 7

m/s to test the persistence of air lubrication. Skin

friction sensors indicated that the skin friction

reduction diminished from the injection point

onward; after 20 m, the effect of lubrication was

nearly gone. Sanders et al. (2006) performed

experiments with a large flat plate of 13 m length for

speeds of up to 18 m/s. This experiment allowed for

tests with bubbles ranging from 0.1 to 1.0 mm at

Reynolds numbers that were previously not tested.

The experiments showed that the bubbles were

pushed out of the boundary layer a few meters

behind the air injectors, against the direction of

buoyancy. An near bubble-free liquid layer was

formed near the wall and the effect of air lubrication

almost vanished. It is hypothesized that the lift force

experienced by a bubble in the boundary layer is

more than sufficient to overcome the buoyancy of

the bubble. The experiments by Watanabe and

Sanders indicate that air lubrication will not persist

over long length or time scales. This indicates that

for model testing with bubble injection a strong

Reynolds-scale effect is present and that tests using

full-scale ships will not yield the expected resistance

reductions as found during model tests.

For example, the full-scale demonstrator vessel

Seiun Maru showed a 2% decrease at only a limited

speed range, notwithstanding resistance decreases

measured at model scale (reported in Kodama et al.

2002), with an increase in required power over most

of its speed range. The Filia Ariea has been fitted

with so-called Wing-Air Induction Devices, i.e. a

slot in the hull fitted with a small protruding wing

over the slot inducing a natural low-pressure region;

tests with the Filia Ariea fitted with these air

injection devices did not show a change in shaft

power after the air supply was switched on

(Belkoned, 2008).

Although air lubrication by mini-bubbles can show a

decrease of frictional resistance for ships, the results

are not always convincing. In order to gain more

experience with air lubrication, a consortium of

industrial companies and research institutes initiated

the EU-funded project SMOOTH. The project

focuses on air lubrication and its effects on

resistance, propulsion, maneuvering, and sea

keeping of ships. This paper presents the results of

the resistance and propulsion tests on model scale

and full scale, and the results of maneuvering and

seakeeping at model scale.

Figure 1 Top and side view of the Till Deymann. The accented areas are the location for air injection, consisting of one array of

strips in the bottom plating and one area at the downstream end of the forward tunnel thruster opening in the bow. The bottom

right photograph shows this area prior to fitting the porous medium. The locations of the forward and aft azimuthing thrusters are

indicated.

3 RESISTANCE AND PROPULSION OF THE TILL DEYMANN,

The test vessel is the 109.8 m inland-shipping

vessel Till Deymann see Figure 1. The vessel has a

semi-twin-hull bow with two openings in the sides

fitted with two 1050 mm azimuthing thrusters

rotated 15 degrees outboard and two nozzled

azimuthing 1300 mm thrusters are fitted aft. The air

is injected at the far wall of the recesses in the bow

and a strip in the bottom through a porous medium

with a 20 m pore size. The four thrusters were all

powered and instrumented during the tests. The aft

thrusters were fitted on a 6-DOF measurement

frame with both propeller thrust and torque

measured in the propeller hub, while the forward

thrusters were rigidly mounted and only propeller

torque was measured.

From full-scale trials was known that each

thruster received equal power, but the forward

thrusters could not deliver that power over the

entire speed range without overloading. The

forward thrusters were set to 60% of the power of

the aft thrusters during all tests.

3.1 MODEL SCALE RESULTS

An 11.8 m model of the Till Deymann at a scale

ratio of 1:9.286 has been constructed. Note that

even for such a large model, the propeller diameters

were small, making the test less suitable for

extrapolation to full scale. The model was well

suited for comparative tests between fully-wetted

and air-lubricated conditions.

The porous medium is not scaled and has a 20 m

pore size as on the real ship. In order to properly

scale the effect of the atmospheric pressure, the

model was tested in the MARIN depressurized

towing tank. Nitrogen instead of air was injected

from a pressurized gas canister where the nitrogen

was allowed to expand and was subsequently

heated before being injected. The gas-volume flow

was measured and controlled by Bronkhorst

EL_FLOW mass-flow controllers calibrated for

nitrogen gas. The model was set at the correct

draught and trim with chambers filled with nitrogen

gas.

The model was tested at a ship speed range from 5

to 10 kts with the air-volume flow rate set at 0

(reference), 3, and 6 L/min. Figure 2 shows the

results of the bare-hull resistance test. A 1%

increase of the resistance was measured (although

this falls well within the measurement accuracy)

while most model tests show decreases. The

amount of air may have been insufficient to have

any effect while the air injection may disturb the

boundary layer too much. It was observed that the

air from the openings for the forward thrusters

immediately rose to the surface. The exact cause of

the resistance increase is unclear and further tests

are planned with an increase in the air volume flow

rate.

Figure 2 Measured resistance of the model. A 1% increase is

measured (except for one outlier).

3.2 FULL SCALE RESULTS

The full-scale trials with the Till Deyman were

performed with the same draught as for the model

tests. These tests were performed in the

Netherlands, and in both fresh and salt water as the

coalescent behavior of bubbles is known to depend

on salinity. The ship was fitted with an anemometer

(horizontal plane), a six-DOF accelerometer, shaft

torque and rpm sensors (strain gauge and optical

sensor respectively), a boroscope placed aft of the

air injection array and fitted with an image

intensifier capable of a frame rate of 200 Hz, and

two GPS antennae to determine the course with 0.5

accuracy.

The tests consisted of sailing in 10-minute intervals

in each direction (track length permitting) and

taking the average of six of these runs per measured

point. Several 11 kW compressors were used for air

injection. The weather conditions were very good

with wind condition mostly at Bft 1 and

occasionally up to Bft 3. The repeatability of the

testswithin 2%is considered to be high. Although the measured trend is constant and

consistent, the effect of air lubrication is not

significant. The power required for air injection was

measured. It can be concluded that for the current

setup the power required for air injection exceeds

the power reduction by air lubrication. From the

model tests it was hypothesized that the wake of the

forward thrusters aimed partially at the rear wall of

the hull openings may benefitat least locallyfrom air lubrication. The full scale trials did not show any effect when the air lubrication at

that location was activated.

Figure 3 Measured shaft power onboard the Till Deymann

with and without air lubrication both for salt water and fresh

water conditions.

4 SEAKEEPING AND MANEUVERING AT MODEL SCALE

The influence of air lubrication and coating type on

seakeeping and maneuvering characteristics was

investigated using two wooden models of a tanker.

Different coatings were applied to the bottom of the

model by means of interchangeable pre-coated

plates. The first model was previously segmented

and had the plates bolted to the model, hence the

later need for a second model where the plates were

mounted flush with the bottom using recesses. The

models were instrumented with electrical drives,

thrusters, and an air supply system. Three types of

plates were used: uncoated, coated with Intersleek

900 and coated with an experimental highly-

hydrophobic coating designed by AkzoNobel.

The following model data were collected:

1. Position, rotation, accelerations 2. Actual mass flow in air supply system 3. Rotation rate, torque and thrust of propellers 4. Steering angle of thrusters 5. Undisturbed wave height (clear of the model)

As with the resistance and propulsion tests, nitrogen

gas was used to produce the bubbles and the same

mass flow controllers were used to measure and

control the mass flow rate through small-pore

sintered steel inserts and then through 1 mm holes

in the steel plate, resulting in large bubbles (0.52 mm). Air injection points are placed from the bow

and every 20% of the model length, except at

100%, numbered 1 to 5. The air injection point at

the bow is only a third of the beam of the ship. The

following test were performed:

1. Powering runs to determine propeller rate

versus speed

2. Zig-zag tests

3. Combined Turning-Circle/Pull-Out

4. Roll-decay tests

5. Seakeeping runs in irregular bow-quartering and

stern-quartering seas

Underwater video observation was used to verify

the performance of the air layer. Experimentation

with the volume flow rate of nitrogen was

performed and the proper flow amount was

determined based on the general appearance of the

bubbles. The performance was highly sensitive to

the exact fitting of the plates; even minor alignment

problems of the plates caused nitrogen not

distribute evenly and to collect in certain locations..

A constant flow of bubbles along the bottom of the

plates was observed. A representative view is given

in Figure 4

4.1 Tested configurations

The tested configurations and associated names are

indicated in Table 1

Table 1 Tested air configurations

Mo

del

Flow [/min]

Air config per location Coating

1 2 3 4 5

1

- - - - - - -

Air A 1 3 3 3 3 -

- - - - - - Intersleek 900

Air A 1 3 3 3 3 Intersleek 900

2

- - - - - - -

- - - - - - Intersleek 900

Air B 3 3 3 3 3 Intersleek 900

- - - - - - Hydrophobic

Air B 3 3 3 3 3 Hydrophobic

Max Air 3 6 15 15 15 Hydrophobic

Stern Max 0 0 0 15 15 Hydrophobic

4.2 Maneuvering Behavior

Zig-zag tests and combined turning circle/pull-out

tests showed small differences in the results. Most

of these differences were erratic and fall within the

confidence interval of maneuvering tests. All

observed differences have little or no influence on

ship operation. The difference between coatings

was observed to be larger than the difference

between fully-wetted and air lubrication conditions.

4.3 Influence of air configuration

The influence of air lubrication was limited to the

following aspects:

1. In the turning circle test, the stable turning diameter increased by approximately 5% with

an increasing air flow rate (Figure 5, top).

2. In the turning circle test, the pivot point moved 5% forward from midships with an increasing

air flow rate (Figure 5, center).

3. In the turning circle test, the maintained speed increased by about 5% with an increasing air

flow rate (Figure 5, bottom).

4.4 Influence of coating type

As the different coatings were applied to

interchangeable plates, the measured changes were

influenced by small geometric differences between

the plates (size, paint thickness, accuracy of fitting

them to the model, etc.). Only minor differences

between plates were measured and as a difference

of a few percent can be expected from one set of

plates to the next, no significant influence on the

maneuvering performance could be attributed to the

coating type itself. Additionally, no consistent

trends could be observed.

5 SEAKEEPING BEHAVIOR

The model was tested in the irregular wave

conditions in bow and stern quatering wavesi.e. 315 and 135 headingwith a signigficant wave height of 1.5 m and wave peak period of 9 s for all

configurations as presented in Table 1.

These conditions were repeated from the PELS

project and are considered representative for what

an inland ship can encounter. It should be noted that

these conditions do not excite the ship in its natural

frequencies of motion and therefore the measured

motions are small. Measurements with a small

magnitude are more susceptible to disturbances and

therefore reduce the accuracy of these results. The

results show the root mean square (RMS) of the

motion divided by the RMS of the undisturbed

wave height. Roll and pitch are considered to be the

most relevant ship motions.

5.1 Influence of air configuration and coating type

Figure 6 shows the obtained results for roll and

pitch in bow- and stern quartering seas. Note

thatexcept for the extra large roll in bow-quartering waves for UNCOATED FULL AIR A, following from a single measurementall the measurements show little variation between fully-

wetted and air-lubricated conditions.

When considering the accuracy level in the tests

one must conclude that the differences between

these results of each air configuration fall within the

confidence interval of these tests for all coating

types. Also, no consistent trends can be seen.

Therefore, it is concluded that no influence of air

lubrication and coating type on seakeeping

performance could be found.

Figure 4 Air bubbles under ship model

Figure 5 Turning diameter, Pivot point , and maintained speed

fraction in a turning circle test, maneuvering tests.

Figure 6 Roll and pitch in both bow quartering and stern

quartering seas, seakeeping tests.

90% 92% 94% 96% 98% 100% 102% 104%

UNCOATED

INTERSLEEK 900

HYDROPHOBIC

DSTC [-]

Turning Diameter (2nd model; relative to UNCOATED NO AIR)

STERN MAX AIR MAX AIR FULL AIR B NO AIR

80% 85% 90% 95% 100% 105%

UNCOATED

INTERSLEEK 900

HYDROPHOBIC

Non-dim pivot point [-]

Non-dimensional pivot point (2nd model; relative to UNCOATED NO AIR)


75% 80% 85% 90% 95% 100% 105%

UNCOATED

INTERSLEEK 900

HYDROPHOBIC

Vstc/V0 [-]

Maintained speed fraction(2nd model; relative to UNCOATED NO AIR)


0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

UNCOATED (1st model)

INTERSLEEK 900 (1st model)

UNCOATED (2nd model)

HYDROPHOBIC (2nd model)

rms/rms [deg/m]

Roll; bow quartering seas

STERN MAX AIR MAX AIR FULL AIR B FULL AIR A NO AIR

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60





rms/rms [deg/m]

Pitch; bow quartering seas

STERN MAX AIR MAX AIR FULL AIR B FULL AIR A NO AIR

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00





rms/rms [deg/m]

Roll; stern quartering seas

STERN MAX AIR FULL AIR B FULL AIR A NO AIR

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90





rms/rms [deg/m]

Pitch; stern quartering seas

STERN MAX AIR FULL AIR B FULL AIR A NO AIR

CONCLUSIONS

Experiments have been performed with ships with

and without air-bubble injection at model scale and

full scale. The results of model scale experiments

showed a small increase in resistance and a small

increase in propulsion efficiency, both around 1-

2%. A trial with the ship with air lubrication at full

scale showedat besta 2% reduction in required propulsive power with air lubrication (roughly

2.6%). The net power reduction was consistently

measured at 0.6%, i.e., an increase, for both fresh water and salt water conditions. The behavior of the

bubbles in the boundary layer of the full-scale ship

(insofar they could be seen) showed that bubbles

did not attach to the hull.

Maneuvering and seakeeping tests showed very

small differences between air-lubricated and fully-

wetted hulls for maneuvering and no differences for

seakeeping. Therefore, any vessel with air bubble

lubrication does not need a specially-trained crew to

handle the ship.

In conclusion, no appreciable effect was found of

the injection of air bubbles on resistance,

propulsion, and maneuvering characteristics of a

ship. Barring unforeseen effects of special coatings

and other surface treatments, an ad hoc application

of bubble injection for ship hulls is not expected to

yield any significant results. It should be noted that

this conclusion does not apply to air lubrication by

either air films or air-cavity ships.

6 ACKNOWLEDGEMENTS

SMOOTH is supported with funding from the

European Commission's Sixth Framework

Programme with participation of MARIN,

AkzoNobel, Bureau Veritas, Damen Shipyards,

Istanbul Technical University, Atlas Copco Ketting

Marine Centre, New Logistics, SSPA, DST,

Thyssen Krupp Veerhaven, & Imtech.

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