UNIT 7. SHIP STABILITY

I. Words and word combinations to be remembered:

calculation – вычисление

rule – of – a thumb – эмпирический метод

measurement – измерение

equation – уравнение, равенство

computational fluid dynamics – вычисляемая гидродинамика

wind gust – порывы ветра

stability system – система устойчивости

electric actuator – электрический привод

straightforward – движущийся прямо, вперед

buoyancy – плавучесть

finit element analysis – анализ с использованием метода конечных элементов

tedious – громоздкий, трудоемкий

to drain – дренировать, стекать, истощать, фильтровать

flooding – затопление, заводнение

II. Read and translate the following text:

Ship stability is an area of Naval Architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves.

Fig.5. A model yacht being tested in the towing tank of Newcastle University

Ship stability is a complicated aspect of naval architecture which has existed in some form or another for hundreds of years. Historically, ship stability calculations for ships relied on rule-of-thumb calculations, often tied to a specific system of measurement. Some of these very old equations continue to be used in naval architecture books today, however the advent of the ship model basin allows much more complex analysis.

Master shipbuilders of the past used a system of adaptive and variant design. Ships were often copied from one generation to the next with only minor changes being made, and by doing this, serious problems were not often encountered. Ships today still use the process of adaptation and variation that has been used for hundreds of years, however computational fluid dynamics, ship model testing and a better overall understanding of fluid and ship motions has allowed much more in-depth analysis.

A. Add-on Stability Systems

Add-on Stability Systems are designed to reduce the effects of waves or wind gusts. They do not increase the stability of the vessel in a calm sea. The IMO International Convention on Load Lines does not mention active stability systems as a method of ensuring stability. The hull must be stable without active systems.

B. Active Stability Systems

Many vessels are fitted with active stability systems. Active stability systems are defined by the need to input energy to the system in the form of a pump, hydraulic piston, or electric actuator. These systems include stabilizer fins attached to the side of the vessel, or tanks in which fluid is pumped around to counteract the motion of the vessel.

C. Intact Stability

Intact stability calculations are relatively straightforward and involve taking all the centers of mass of objects on the vessel and the center of buoyancy of the hull. Cargo arrangements and loadings, crane operations, and the design sea states are usually taken into account.

D. Damaged stability

Damaged stability calculations are much more complicated than intact stability. Finite element analysis is often employed because the areas and volumes can quickly become tedious and long to compute using other methods.

The loss of stability from flooding may be due in part to the free surface effect. Water accumulating in the hull usually drains to the bilges, lowering the centre of gravity and actually increasing the metacentric height. This assumes the ship remains completely stationary and upright. However, once the ship is inclined to any degree (a wave strikes it for example), the fluid in the bilge moves to the low side.

III. Answer the following questions:

1. What is ship stability?

2. What did calculations rely on?

3. What system did master shipbuilders of the past do?

4. Were ships often copied from one generation to the next with only minor changes being made?

5. These systems are designed to reduce the effects of waves and wind gusts, aren’t they?

6. What are many vessels fitted with?

7. How are active stability systems defined?

8. What do active stability systems include?

9. What are intact stability calculations?

IV. Translate the text: “Stabilizer Fins.”

Active fin stabilizers are normally used to reduce the roll that a vessel experiences while under way. The fins extend beyond the hull of the vessel below the waterline, and alter their angle of attack depending upon heel angle of the vessel. They operate in a very similar way to airplane wings. Cruise ship frequently use this type of stabilizer system because the high cost of incorporating it into the vessel can be justified. Pleasure yachts down to 15M LOA will increasingly choose active fin stabilization as the cost/benefit ratios are perceived to improve. This system may have any of the following disadvantages:

· Because the fins may be retractable, they may take up valuable space in the engine compartment.

· When fins are not retractable, they constitute fixed appendages to the hull, possibly extending the beam or draft envelope; at a minimum, requiring attention for additional hull clearances.

· Altering the angle of attack requires the vessel to use fuel in supplying the power required to do so. However the power expended for fin motion may be offset by power recovered through more stable tracking on course. Power saved by following a more accurate course may be difficult to quantify.

· The fin and actuator mechanism is expensive to manufacture and fit into the vessel, especially when compared to a bilge keel.

Поиск по сайту: