A ship is nothing more than a water tight container or storage compartment with its own means of propulsion. Its purpose is to load and carry cargo, whether the cargo is passengers, fish, or a host of other commodities. Each type of ship is specialised for the trade in which it will operate. One of the most important factor of design is to ensure that the water in which your vessel floats, does not enter the hull and cause progressive flooding. We call this characteristic of a vessel its watertight integrity.
Watertight means :
a) In relation to a fitting above deck, that it is so constructed as to resist effectively the passage of water under pressure, except for slight seepage.
b) In relation to the structure of the vessel, capable of preventing the passage of water in any direction if the head of pressure were up to the freeboard deck, which in your case would mean the main deck.
Weathertight means that the structure or fitting will prevent the passage of water through the structure or fitting in any ordinary sea conditions.
Hull Watertight Integrity
The steel plating in a metal vessel, the planking in a wooden one, or the FRP laminate, have as their primary purpose, the task of keeping the interior of the vessel free from water. In all types of vessel construction, a structural framework is built first to provide the strength. This, when combined with the external covering, forms the hull. In steel and aluminium ships, the hull is made watertight by welding the steel plates together and to the framework. Often the frame is built upside down and the shell plating is welded onto the inverted frame. The hull is then righted and the internal welds are completed. This procedure allows for a better weld and hence improved water tightness since all welds are ‘downwelds’.
FRP and ferro cement hulls are continuous with no joints and are inherently watertight, as is their deck/hull connection.
Vessels constructed of timber are not normally totally watertight but rely on seepage of water to swell the planking and thus make them watertight.
Openings in Watertight Bulkheads
Openings may be necessary in watertight bulkheads to allow the passage of pipes or electrical cables, and special arrangements are made to ensure that the watertight integrity of the bulkhead is maintained. All pipes passing through a watertight bulkhead must be flanged to the bulkhead and do not pass directly through it. The pipe on the left has a valve incorporated in it for filling the tank on the other side of the bulkhead. There is a spindle running up to the main deck from where this valve can be operated. The siting of the valve outside of the tank it is servicing reduces corrosion and maintenance.
Doors may also be necessary, in watertight bulkheads, to allow the vessel to continue its normal operation whilst at sea. These doors can be of either a sliding or hinged type and must be capable of operation from both sides of the bulkhead.
Hull and Deck Openings
When a hatchway is cut into the deck of a vessel, the corners are rounded to reduce stresses.
Doors providing access from the main deck to lower compartments must have sills, which serve the same purpose as hatchway coamings. The sill heights are the same as for hatch coamings. Access doors can be hinged and should be marked “THIS DOOR IS TO BE KEPT CLOSED AT SEA”.
Ventilators and Ventilators must be a minimum height above the deck and must have some means of making them watertight. This may be metal flaps, or in smaller vessels, wooden plugs and canvas covers. Airpipes, where exposed, should be of substantial construction and if the diameter of the bore exceeds 30mm bore then the pipe should be provided with means of closing watertight.
Side Scuttles (portholes)
Airpipes– All portholes below the main deck should have hinged metal covers (deadlights) that can be closed watertight.
Access Openings in the Hull
The loading hatch in the side of the hull is bolted and secured while at sea. An alarm system is fitted which will sound on the bridge if the door is opened.
Scuppers, Inlets and Discharges
All sea inlets are to be fitted with valves of steel or material of equivalent strength attached direct to the hull or approved skin fittings (in case of non metal hulls).
Drainage Arrangements From Weather Decks
Weather decks are to be provided with freeing ports, open rails or scuppers capable of rapidly clearing the deck of all water under all weather conditions.
Maintenance of Watertight Integrity
Watertight integrity can be breached through any activity or happening that allows the ingress of water in unwanted areas or compartments of the vessel.
Typical examples include :
Lack of maintenance to seals, screw threads and other locking devices.
Damage caused by collision, grounding or heavy weather.
Leaving hatches, doors, vents etc open.
Blocked freeing ports or scuppers.
Cracks along welds in metal vessels or loss of caulking from planked seams in timber vessels.