Question

With reference to Roll-on, Roll-off ferries:

(a) Describe the problem of free surface effect

(b) Explain how it is intended that water should be cleared from car or cargo decks

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

Answer 1

Part (a)

The problem of free-surface effect in Ro-Ro ferries.

Ro-Ro ferries have large, open car and cargo decks that extend over a large part of the ship. If water is shipped onto these decks (e.g. through the bow or stern doors, or in heavy weather), the water spreads over the large deck area, creating a very large free surface. The free-surface effect reduces the effective GM by rho x i/Delta, where i is the second moment of area of the free surface (i = L B^3/12 for a rectangular deck). Because the car deck is very wide and long, the free-surface effect is very large and can reduce the GM to a dangerously low value, causing the ship to lose stability and capsize. This is the principal stability problem of Ro-Ro ferries: the large open decks create a huge free-surface effect if flooded, and the ship can capsize rapidly.

Part (b)

How water should be cleared from car or cargo decks.

Water on the car deck should be cleared by:

Providing adequate freeing arrangements (scuppers, freeing ports, drain valves) at the deck edge and at the ends, so that water can drain overboard.
Providing a camber (transverse slope) on the deck so that water runs to the sides and drains through the freeing ports.
Providing a longitudinal slope (sheer) so that water runs to the ends and drains.
Using bilge pumps and drainage systems to remove water that cannot drain overboard.
Ensuring the freeing ports are of adequate size and are not blocked by cargo or lashings.

The freeing arrangements must be adequate to remove water quickly and prevent the build-up of a large free surface.

Part (c)

Methods for improving the stability and survivability of Ro-Ro ferries.

Lowering the centre of gravity by placing heavy weights low and ballast in the double bottom.
Increasing the GM by increasing the beam and the waterplane area, and by lowering KG.
Providing adequate freeboard and reserve buoyancy.
Subdividing the car deck with watertight bulkheads or providing a raised deck to limit the spread of water.
Providing adequate freeing ports and drainage to remove water quickly.
Fitting bilge keels and stabilisers to reduce roll.
Designing the ship to meet the damage stability criteria (survive flooding of a compartment).
Using a higher freeboard and a stronger, more watertight structure, and ensuring the bow and stern doors are watertight and properly secured.
Operating with adequate stability margins and following the loading manual.

✓ Verified Model Answer (Text Solution)

The problem of free-surface effect in Ro-Ro ferries.

How water should be cleared from car or cargo decks.

Water on the car deck should be cleared by:

Methods for improving the stability and survivability of Ro-Ro ferries.