Question       

With reference to Roll-on Roll-off ferries:

Describe the problem of free surface effect;

Explain how it is intended that the water should be cleared from car or cargo decks:

Describe possible methods for improving the stability and survivability of these vessels.

SOLUTION.

(a) RO-RO ferries allow vehicles to be driven through the ship from stem to stern. Entrance to the hull is provided by doors, either at bow and stern, or at the stern

Hence. RO-RO's are vulnerable because:

Low freeboard to vehicle deck

No transverse watertight sub-division on the vehick--decks

Large external openings (bow and stern doors) below freeboard deck

(b) If any slack water were to get onto the vehicle deck, the huge free surface effect could have catastrophic consequences (capsize can be very rapid).

Any such water must be drained as quickly as possible, therefore large capacity scuppers are fitted.

Large scuppers are less prone to blockage, and should cope with-any leakage through the door seals, and the large volumes of water from the drencher fire fighting system normally fined,

(c) At present there is a strong lobby to improve the survivability of these vessels by such design methods as:

Increasing buoyancy/reserve buoyancy by fitting sponsons

Fit only stern door with drive around load/unload

Fit transverse retractable sub-division doors/bulkheads

Fit inflatable buoyancy bags

Complying fully with present passenger ship regulations

Fit additional watertight door behind the forward ramp