Application of Air Cavities on High-Speed Ships in Russia

INTERNATIONAL CONFERENCE

ON SHIP DRAG REDUCTION

(SMOOTH-Ships)

ISTANBUL TECHNICAL UNIVERSITY 20-21 May 2010

Macka Campus, Istanbul, Turkey

Editors Mustafa Insel

Ismail Hakki Helvacioglu Sebnem Helvacioglu

© Copyright 2010, SMOOTH Consortium

Paper No: 11

Application of Air Cavities on High-Speed Ships in Russia

A.V. SVERCHKOV

Krylov Shipbuilding Research Institute

INTERNATIONAL CONFERENCE ON SHIP DRAG REDUCTION

(SMOOTH-Ships) 20-21 May 2010

Istanbul Technical University

Faculty of Naval Architecture and Ocean Engineering ISTANBUL-TURKEY

International Conference on Ship Drag Reduction SMOOTH-SHIPS, Istanbul, Turkey, 20-21 May 2010

Application of air cavities on high-speed ships in Russia

A.V. SVERCHKOV Krylov Shipbuilding Research Institute, St. Petersburg, Russia

ABSTRACT: The paper briefly reviews state-of-the-art research studies in Russia on application of artifi-cial bottom cavities for reduction of hydrodynamic resistance of high-speed ships. The paper also considers specific features of different types of ship propulsors used in air cavity ships and provides information about Russian air-cavity ships, which have been built and successfully operated. Advantages of the air cavity prin-ciple are demonstrated for a new design of fast motor yacht.

1 INTRODUCTION

The development of ships with artificially inflated air cavity was initiated in Russia in 1961 at the Kry-lov Shipbuilding Research Institute. Initial investiga-tions were focused on the application of the concept to slow river vessels and barges. These investiga-tions included theoretical research based on the lin-earized 2D theory of cavitation flow, numerous model tests and finally full-scale trials of three river ships.

Later, starting from 1965, a series of research projects have been undertaken aimed at reducing the drag of planing hullforms like patrol and utility boats, fast passenger ships. These research efforts were started with extension of the linearized 2D cavitation flow theory to planing hullforms. Then a considerable amount of model tests were carried out resulting in the design and series construction of river-going high-speed passenger ships and patrol boats with air cavities.

In 1985 similar investigations were performed for fast displacement vessels. These studies revealed that the 2-D theory was not applicable for this type of ships, and the 3-D linearized theory and software were developed to cover these cases. The methods have been validated by extensive design studies and at present these techniques are successfully used in practical design of the air-cavity ships.

In 1993 investigations were expanded to cover planing and semi-planing catamarans. The next stage started in 1995 with research extended to application of artificial cavities to monohulls operating under transient modes like fast marine passenger or

car/passenger ferries, and high-speed sea-going mo-tor yachts. In 2000 model tests of fast containerships were started.

For about 50 years researchers have been devel-oping applicable computation methods and resolving challenges associated with various types of propul-sors, sea-keeping performance and specific issues relevant for ships designed to have several different operation speeds. Along with the monohulls, cata-marans and ships with outriggers were examined.

The model tests carried out at the Krylov Institute have provided the evidence that artificial cavities could be efficient for the following types of ships: - River cargo vessels and barges (Butusov et al

1999a); - Supertankers; - Bulk-carriers; - River-sea cargo vessels (Sverchkov 2002); - Fast conventional monohulls (Butusov et al.

1999b); - Monohulls operating under transient conditions

(passenger and car/passenger ferries, sea-going motor yachts, rescue ships) (Butusov et al. 1999, Sverchkov 2001);

- Fast landing craft (Jane’s 2001-2002); - Planing craft (passenger, service, utility and patrol

boats, sea-going motor yachts) (Sverchkov & Poustoshny 2003);

- Planing catamarans with asymmetric demihulls (Sverchkov 2005);

- Catamarans operating under transient conditions (passenger and car/passenger ferries) (Butusov et al. 1999);

- Container ships with outriggers (Anosov et al. 2003). Full-scale trials have been conducted to confirm

the performance of river cargo ships and barges, fast passenger boats, fast landing craft, and various util-ity and patrol boats fitted with systems for producing artificial cavities.

Other countries followed Russia in the race for economically more efficient ships. Among the for-eign companies most actively working on the im-plementation of air cavity technology we should mention DK Group, Netherlands, which is focusing on cargo ships, high-speed containerships and car/passenger ferries.

Note that the air cavity is quite a sensitive tool and, as it is seen from the worldwide experience, if air-cavity ships are tried to be designed without proper research support failures are very likely.

Along with successful stories of implementing air-cavity technology reported in Russia it should be noted that some foreign companies failed to build efficient air-cavity ships on their own without con-ducting appropriate scope of R&D. Here we could mention an Australian company Oceanfast Ferries PTY LTD with its 45-meter catamaran and a Ukrainian shipyard that started to build a series of patrol boats. The image of air cavity technology has been marred by reluctance of these companies to obtain solid R&D support.

2 SOME PHYSICAL ASPECTS OF USING AR-TIFICIAL AIR CAVITIES FOR RESISTANCE REDUCTION

Artificially inflated cavities have a positive effect on friction and roughness resistance by isolating a sig-nificant part of the wetted hull surface from water. In high-speed and semi-planing craft the air cavities may decrease the gradient of hydrodynamic pres-sures and thus somewhat reduce the residual resis-tance. In this connection, this technology provides the highest benefit for the ships with a large friction resistance component. It primarily refers to slow-speed merchant ships, in particular river-going ves-sels with a large beam-to-draft ratio. Another type of vessels with a large friction resistance component is planing craft. The cavities give maximum effect for these vessels reaching up to 25~30% reduction. Note that the air cavity efficiency is growing as ships are aging because of hull fouling.

For attaining significant reduction in towing re-sistance the cavity should have optimum characteris-tics, i.e. have a large plan area, high positive pres-sure and smooth closure. A large cavity area is re-

quired for isolating the maximum possible wetted hull surface. Desirably the isolated hull surface area should be not less than 40-50% of the total wetted surface. High positive pressure allows the cavity to carry a large part of the hull weight and ensure the highest lift to the ship hull. Smooth cavity closure is required for reducing the air supply under the bot-tom and save the power consumed by air fans.

Artificial cavities can be applied to reduce full-speed power or to raise the full speed without in-creasing the output of the power plant. It should be noted that in an effort to resolve each of these prob-lems for the same vessel one may come to somewhat different technical solutions which has to do with the fact that the air cavity will be designed for different ship speeds.

Power reduction is more advisable for displace-ment vessels and sometimes for planning and semi-displacement ships since it enhances their economic performance. This may entail some benefits due to the opportunity of installing less powerful engines and saving fuel. Thus, both ship construction and operation costs can be reduced. Additionally, some reduction in ship displacement can be expected with a possibility to increase speed or cargo-carrying ca-pacity.

Application of air cavities to increase the full speed is advantageous for planing and semi-planing craft. In this case it is possible to make the ship more competitive and increase her range limited by time considerations.

3 APPLICATION OF ARTIFICIAL AIR CAVI-TIES TO PLANING CRAFT

The planing craft with artificial cavity look like con-ventional vessels of similar hull form, especially in the bow. The main distinction is the bottom recess where the cavity is to be formed (Fig. 1).

1 – bottom in front of step 3 – step 2 – bottom behind step 4 – sideboards

Figure 1. Scheme of the Planing Boat on Artificial Cavity

1 32

4

On the ship bottom, forward and aft of the cavity, there are permanently wetted areas. In the forward the cavity is limited by a transverse bottom step of an arrow-like shape. On the sides the cavity is lim-ited by skegs. In the aft the bottom surface between skegs is specially shaped to ensure smooth cavity closure. Bottom profile is unique for each design and depends on hull lines, displacement, trim, operating speed and type of propulsor. If in the process of ship series construction it is decided to increase the full speed by raising the output of main engines then it is also required to change the bottom profile accord-ingly.

In the case of planning craft the pressure inside the cavity is usually not higher than 300-500 mm of water column (30~50 mbar). With such a low pres-sure, one can use fans rather than compressors for air supply, and on some smaller boats (under 40 t) it is possible to do without fans making use of exhaust gas from main engines.

The use of air cavities on planing craft makes it possible to reduce resistance by about 20~35% as compared to conventional vessels of the same hull form. This translates into 10~15% increase in full speed and better economic performance at cruising speed. Reduction in resistance at intermediate speeds leads to noticeable improvement of acceleration characteristics.

The power consumed by air supply into the cavity accounts for no more than 2~3 % of the main engine output.

1 2 3 4 5 60,0

0,1

0,2

Dra

g- d

ispl

acem

ent r

atio

Volumetric Froude number (speed-displacement coefficient)

with cavity

without cavity

Fn∇

0

10

20

30

ΔR

ΔR, %

Figure 2. Relative Resistance of Two Options of the Patrol Boat Tornado versus volumetric Froude Number Fn∇.

Figure 2 compares drag-displacement ratios of a traditional hull boat and an air-cavity boat of identi-cal principal dimensions. Figure 2 also shows the resistance reduction related to the towing resistance of the traditional boat. In the volumetric Froude number range of 4.5~5.5 the air cavity gives a 20~25% reduction in towing resistance. The air pressure in the cavity does not exceed 30~35 mbar, so it is possible to use main diesel exhaust gas to inflate the cavity without any loss of power.

Apart from fuel savings the improvement of hy-drodynamic performance leads to considerable re-duction of harmful emissions like carbon dioxide (СO2), sulfur oxide (SOX) and nitrogen oxides (NOX) with a positive environmental effect.

Apart from reducing resistance, the artificial cav-ity improves the sea-keeping performance of fast boats; in particular, it reduces the heave and pitch amplitudes as well as vertical accelerations. It is known that even with significant power reserve heave accelerations and slamming cause speed re-duction in high sea states.

0 1 2 3 4 50

1

2

3

4

5

6

Volumetric Froude number

with cavity

without cavityn+

H

Fn∇

Rel

ativ

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atio

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Figure 3. Results of comparative model tests of a 32-ton plan-ing craft in head waves in sea state 4.

2 3 40

1

2

3

4

5

Volumetric Froude number

with cavity

without cavity

Rel

ativ

e ve

rtica

l acc

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atio

ns

n+H

Fn∇

Figure 4. Results of comparative model tests of a 32-ton plan-ing craft in follow waves in sea state 4.

Figures 3 and 4 show the results of comparative

model tests of a 32-ton planing craft in head and following waves in sea state 4 (significant wave height 2 m, wave length 35 m). Two options were tested: model with initial (smooth) hull lines and model with air cavity. The test data show that the air cavity provides reduction in heave accelerations by 10 ~ 50%.

The sea-keeping performance of air-cavity boats is comparable with the fixed-hydrofoil ships. It is not surprising that the cavity improves sea-keeping performance because a significant part of the rigid bottom surface subject to wave effects is covered with easily compressed air preventing the waves to impart their energy to hull.

4 SPECIFIC FEATURES ASSOCIATED WITH THE APPLICATION OF DIFFERENT PROPUL-SORS ON AIR CAVITY SHIPS

The air cavity concept allows fitting same propulsors like on conventional ships, viz.: conventional pro-pellers with inclined shafts, surface-piercing propel-lers (SPP) with shafts passed through the transom, conventional or vented waterjets.

In case of conventional screw propeller with an inclined shaft, there is no need to consider interac-tion between the cavity and propeller because pro-pulsors are located below the cavity, but it is re-quired to put shafts in special fairings in way of its intersection with the cavity in order to protect the cavity from disruption.

The most simple and at the same time beneficial approach is to use surface-piercing propellers. A SPP placed behind the transom allows designing special air cavity bottom lines virtually without any need to adapt them to the propulsor. Surface-piercing propellers enable the best use of the bottom cavity effect in terms of resistance reduction, espe-cially if combined with an Arnison-type drive.

A conventional waterjet with its intake arranged as hole in the bottom requires special attention to protect the intake against the air escaping from the cavity. It may be protected by a fairing and some minor modifications in the adjacent portion of the bottom. For this purpose the Krylov Shipbuilding Research Institute has developed special shapes of fairings for the intake. However, it should be re-membered that (1) the intake reduces the area which otherwise could be covered by the cavity and (2) the extra fairing itself causes some additional resistance. That is why the cavity effect on ships with waterjet propulsion is some 20% lower than when choosing the SPP option (i.e., the required power reduction at the same speed is 20% instead of 25% with SPP). Nevertheless, since fast ship waterjets are highly efficient, sometimes markedly surpassing other pro-pulsors, this particular drawback may be compen-sated. At any rate, ship designers should carefully look into these aspects. The vented waterjet (ventjet) developed at KSRI (Ibragimova et al. 1995) are much less sensitive to air penetration into the propulsor than conventional waterjets but have somewhat lower efficiency. The ventjet should be installed at the transom edge. Like with the SPP option, this enables the best use of the bottom cavity effect. KSRI is at the moment carry-ing out a dedicated research program on ventjets for SAC ships.

5 HIGH-SPEED VESSELS ON AIR CAVITY DESIGNED IN RUSSIA

More than 40 designs of various air-cavity ships have been developed by today and seven of these designs have been used to build over 80 high-speed ships and boats with a displacement ranging from 14 to 105 tons and speed ranging from 30 to 52 knots. New developments in these applications are in pro-gress. The outlook and main particulars of the high-speed vessels are shown below in Table 1.

Table 1. Main particulars of the high-speed vessels designed in Russia

Fast patrol boat Saigak Delivery date of first ship Number of ships in series Displacement (t) Length, overall (m) Beam, overall (m) Draft, maximum (m) Engine power (kW) Maximum speed (kn) Propulsor

1981

more than 50 13.0 14.05 3.5 0.65 735 40 water-jet

Fast landing ship Serna

Delivery date of first ship Number of ships in series Displacement (t) Length, overall (m) Beam, overall (m) Draft, maximum (m) Tonnage (t) Engine power (kW) Maximum speed (kn) Propulsor

1992 5 105 25.65 5.85 1.52 45 2×2430 32

vented water-jet River boat for 70 passengers Linda

Delivery date of first ship Number of ships in series Displacement (t) Length, overall (m) Beam, overall (m) Draft, maximum (m) Engine power (kW) Maximum speed (kn) Propulsor

1992 11 24.6 24.1 4.6 0.95 660 38 SPP

Tug for hydroplanes Muflon


1992 1 13.2 15.5 3.56 0.7 1100 50 SPP

Fast patrol boat Merkury Delivery date of first ship Number of ships in series Displacement (t) Length, overall (m) Beam, overall (m) Draft, maximum (m) Engine power (kW) Maximum speed (kn) Propulsor

1995 4 99.0 35.4 8.3 2.0 2×3670 52 propellers

Frontier boat Sokzhoy


1996 2 99.7 35.2 8.0 2.1 2×3670 50 propellers

Fast patrol boat Tornado


2000 2 30.8 19.6 3.9 0.9 2×1220 50 SPP

6 NEW ADVANCED DESIGN

Currently the Krylov Shipbuilding Research Institute is engaged in the hull form optimization of an air-cavity 60 feet fast motor yacht Barracuda under contract with an Italian company Giemme S.p.A.

The principal difference of this yacht from the ear-lier tested craft is the application of a very light so-called sprint system material, which gives a consid-erable reduction in displacement. The yacht outlook and main data are shown below in Table 2.

Table 2. Main particulars of the 60 feet fast motor yacht Barracuda Displacement (t) Length, overall (m) Beam, overall (m) Draft, maximum (m) Engine power (kW) Maximum speed (kn) Cruise speed (kn) Propulsor Accommodations

20.8 18.4 5.0 0.72 2x895 55 45 SSP 8+1

A 3.5 m yacht model of was made to 1:6 scale. Two hull options were tested in the towing tests. The first option had smooth traditional hull lines. The second option was adapted for generation of bottom air cavity. The results of towing tests are shown in Figure 5.

In the operating speed range the air cavity has re-duced the towing resistance by 20-24%. Such resis-tance reductions have made it possible to raise the full speed by 5 knots (from 50 to 55 knots) and save some 20% of fuel at cruise speed. The cavity is gen-erated using main engine exhaust gases. Sea-keeping and maneuvering model tests are planned to be per-formed at further stages of this project.

0 1 2 3 4 5 6 7 8 9 10 11 12

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resi

stan

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Model speed VΜ

, m/s

RT

with cavitywithout cavity

0

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ΔR

ΔR, %

50kn

Figure 5. Results of towing tests of Barracuda model

REFERENCES Butusov, A., Sverchkov, A., Poustoshny, A., Chalov, S. 1999.

State of Art in Investigation and Development for the Ship on the Artificial Cavity. IWSH’99, China.

Sverchkov, A. 2002. Perspectives of artificial cavity applica-tion aimed on resistance reduction of ocean/river ships. Pro-ceedings of Third International Shipbuilding Conference. St.Peterburg, Russia : 95-100.

Butuzov, A., Sverchkov, A., Poustoshny, A., Chalov, S. 1999. High Speed Ships on the Cavity: Scientific Base, Design Pe-culiarities and Perspectives for the Mediterranean Sea. 5th Symposium on High Speed Marine Vehicles HSMV’99. Capri 24-26, March.

Sverchkov, A. 2001. New Type of Ocean Fast Speed Car/Passenger Ferries. Conference Abstracts of 6-th Interna-tional Conference for Shipbuilding, Shipping, Offshore Equipment & Support Vessels, Marine Engineering for the Continental Shelf and Ocean Developments (Neva 2001), St.Petersburg, Russia : 130-132.

Jane’s High Speed Marine Transportation, 2001-2002. Sverchkov. A.and Poustoshny, A. 2003. Air lifted catamarans

or air cavity ship – Which is better? Fast Ferry Interna-tional, April : 30-33.

Sverchkov, A. 2005. Prospects of artificial cavities in resis-tance reduction for planing catamarans with asymmetric demihulls. Proceedings of 8th International Conference on Fast Sea Transportation (FAST 2005). Saint-Petersburg, Russia, 27-30 June.

Anosov, V.N., Galoushina, M.V., Poustoshny, A.V., Prok-horov, S.D., Rozhdestvensky, S.O., Sverchkov, A.V. 2003. Prospects Of Unconventional Hydrodynamic Configurations For Large High-Speed Marine Ships. FAST 2003: 7th-10th October 2003, Ischia (Gulf of Naples), ITALY.

Ibragimova, T.B., Mavludov, M.A., Roussetsky, A.A. 1995. Basic Principles of Propulsor Efficiency Comparisons. Pro-ceedings of International Conference FAST 1995, Hamburg.

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Application of Air Cavities on High-Speed Ships in Russia