Bulk Cargo Trimming
Bulk Cargo Trimming is levelling off cargo in a ship’s holds.
Bulk Cargo Trimming is shortened as trimming, (not to be confused with trimming in connection with the ship’s draft).
Bulk Cargo Trimming can be accomplished in many ways:
Most costly and time-consuming method is for a gang of men using shovels to physically shift the cargo in the top of the cone to the hold sides. Physically shifting top of the cone is not always possible on economic and/or safety grounds.
Some cargoes like grain can be spout trimmed, whereby the loading spout or chute is moved across a hold during loading so as to distribute the cargo more evenly. If the ship concerned has a large deck overhang preventing the spout reaching the hold extremities, spout trimmed may not be particularly successful.
Mechanical cargo trimming equipment exists which can be used to level off cargo peak heights. Mechanical cargo trimming may be successful only in the area of the hatchway opening, not beneath deck overhangs.
Modern bulk carriers are self-trimmers. Under weather decks of modern bulk carriers have an angled upper wing tanks can be used to trim cargo as it fills the holds and in such a way most commodities can be spread to fill a ship’s cargo compartments without the need for manual assistance. Generally, ship’s upper wing tanks are utilized solely for water ballast when a ship is otherwise empty at sea, but sometimes upper wing tanks have the added role of carrying cargo. When upper wing tanks are dried and cleaned at a loading port. Upper wing tanks may be filled with a free-flowing commodity such as grain. Access to the upper wing tanks are being gained through openings on the weather deck immediately above. Trimming of cargo in upper wing tanks is difficult if not impossible, and so it is unlikely that all the space in an upper wing tanks can be utilized. At a discharge port the cargo in the wing tanks can either be sucked out through the same main deck openings or bled through apertures into the main hold beneath. This activity is known as bleeding wing tanks. Not all commodities are suitable for carriage in bleeding wing tanks. It is crucial when describing a ship for chartering purposes to state clearly whether the ship’s cubic capacity is in main holds only or includes bleeding wing tank volumes.
Dry bulk cargoes with an angle of repose greater than 35 degrees are less prone to surface shift, but these cargoes need trimming sufficiently to cover the entire tank top area out to the ship’s side. Thus, reducing the pile height and distributing the cargo weight evenly over the hold bottoms.
In chartering market, there are a few tweendeck ships with self-trimming facilities. Generally, tweendeck ships are usually impossible to load using all available space, a fact to be borne in mind when calculating the amount of bulk cargoes tweendeck ships can carry. It is unrealistic to simply divide a tweendeck ships’ total grain cubic capacity by the approximate stowage factor (SF) of the bulk cargo involved. Due to deck overhangs in tweendeck ships, allowance must be made for loss of space.
Cargo carried in upper deck of tweendeck ships can seriously affect a ship’s stability. If the bulk cargo has a low stowage factor (SF), it may be possible to load the entire bulk cargo in main lower holds, avoiding tweendeck ships stowage altogether. Many modern tweendeckers (tweendeck ships) or multi-purpose ships (MPP) have feeders (feed cargo to the desired location, directly down to the lower holds) in order to facilitate the loading of the lower holds. Shifting boards are constructed in the tweendecks to reduce the overall area and prevent cargo shifting.