Question

With reference to activated fin stabilisers give reasons why:

For large vessels such units are preferred to passive tanks;

These units are preferred for passenger and fast cargo ships;

Partial rather than maximum damping of ship movement in heavy weather is advisable for reasons other than overstressing of fin stocks and activating gear.

SOLUTION.

(a) Fin stabilisers require much less internal volume than tank stabilisers and the internal space taken up by fins is not usually required for cargo. Typically, the space Liken by a passive tank stabilising system is approximately 900m3 which equates to approximately 20 containers.

The mass of the fin stabiliser system is also very small compared to the deadweight, whereas passive tank stabilisers take up approximately 1.5% of the displacement.

Since fin stabilisers are also much more-effective than passive tanks, there is less chance of cargo movement/damage and crews are more likely to work at optimum efficiency.

(b) In passenger ships, comfort is of prime importance and this necessitates the best roll reduction system available. Activated fins are the most effective method of roll reduction throughout all periods of wave encounter, mainly due to their rapid response time. There are also considerations of financial income with regard consumption of food/drink and other purchases. It has been shown that excessive ship motions have a marked effect on income.

Fast cargo ships derive great benefit from fin stabilisers because the force produced by the fins is proportional to the square of the ship speed. Therefore, the anti-roll moments required can be achieved with smaller fin angles and thus the drag component is small, resulting in a minimum fuel penalty.

(c) In heavy weather, full damping may have similar effects on roll behaviour as a large metacentric height, i.e. Reduced angles of roll and shorter roll periods. This short cycle roll condition is termed 'stiff' and can be very uncomfortable for passengers and crew, with increased possibility of cargo/structure damage.