Question-

(a) Describe the constructional arrangements for clearing decks of green seas or slack water.

(b) Sketch and describe a double bottom oil fuel tank air ventilator of the automatic float valve type.

Answer.

(a) A 'bulwark' is an extension of the side shell above deck level which acts as a safety barrier. 'Green seas' taken on board open deck areas would be retained by a bulwark for some time, reducing stability and endangering the ship due to the large load raising the centre of gravity and the large 'free surface effect’.

The problem of stability may be overcome by cutting 'freeing ports' in the bulwark, or by not connecting the bulwark directly to the side shell, leaving a gap of about 150mm to allow the water to rapidly be unshipped.

The bulwark is attached to the deck by 'stays', which could be brackets or offset bulb plates. Scuppers are fitted approximately 20m apart in the weather deck at the sides to ensure complete drainage of smaller quantities of water on the deck; Scuppers from the weather deck are led directly overboard.

The automatic float valve shown has a ball that is buoyant and floats up on any water entering the ventilator, sealing against ingress. Since the vent is from a fuel oil tank, there has to be a copper gauze flashback arrester as shown.

M 1171 relates to the failure of the main engines of a small motor ship by (it was assumed) the contamination of oil fuel by water ingress through an air pipe haying an automatic float valve. Tests showed that in very bad weather, green seas are taken aboard, and small quantities of water were entering the fuel oil tank via the air pipe, contaminating the fuel, causing machinery failure when fed directly into the engines. It was recommended that vent pipes for bunker tanks carrying oil for the ships engines should be such that the head of the vent is clear of any level of water likely to be retained on deck. Those areas where water would likely to be trapped were quoted as recesses in the sides and ends of superstructures, between hatch ends and behind high sections of bulwarks. It was also recommended that for all ships over 500 g.r.t., means should be provided of fitting water drains to daily service tanks, settling tanks and other fuel oil tanks, or water separators fitted in supply lines to propulsion machinery.