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Reserve buoyancy, freeboard and trim

We’ve just looked at buoyancy and how it is due to the water displaced by the underwater volume of a floating vessel. Refer back to Figure 9. The shaded section is the underwater volume of the hull. The unshaded part is known as reserve buoyancy, and can be simply described as the volume of the vessel above the waterline.

Reserve buoyancy is important for the vessel’s safe operation at sea. Think about the following example. You have set off for a day’s fishing in a small fishing boat. You set off yourself with a small amount of gear from the moorings. Halfway down the harbour, you pick up a couple of mates, with their eskies of food and drink and a spare tank of fuel.

The boat is lower in the water now and doesn’t get up on the plane quite so easily. Nevertheless you head on out and have a good day’s fishing, so good that the boat is filled with fish. On the way back in, you have to travel very slowly because the boat is very low in the water now and you are worried about seas being shipped over the side.

What has happened is that you have reduced the reserve buoyancy of the boat. The more you load the boat, the greater is its displacement, the more it will sink in the water, the less reserve buoyancy it will have.

Every boat must have sufficient reserve buoyancy to operate safely: do not overload your vessel. Commercial vessels have a load line assigned to them and it is an offence to proceed to sea with this loadline submerged.
This line is sometimes know as the Plimsoll Line, after Samuel Plimsoll who campaigned heavily for the safe loading of ships in the 1870s.

Reserve buoyancy, as we have seen in the example above, is linked pretty closely to the freeboard of the vessel, the height above the waterline of the uppermost continuous deck, or in a small boat, to the gunwale.

Sufficient freeboard is required at all times to prevent the vessel being swamped and overwhelmed. For ocean going vessels, it is important to note that those structures above the waterline that are not watertight will not contribute to the reserve buoyancy of the vessel. Again, this makes sense – an open wheelhouse, or a cabin with the doors left open will not offer much protection if the vessel begins to ship water over the side.

Another factor affecting the freeboard to take into account is the trim of the vessel. Trim is defined as the difference between the draft forward and the draft aft. Draft is the depth of the hull below the water. See Figure 13. If the aft draft is greater, the vessel is described as being trimmed by the stern, if the forward draft is greater, she is trimmed by the bow. See below.

Note: Trimmed by the stern is called negative (–), trimmed by the bow positive (+).

Every vessel has a designed seagoing trim where a baseline is drawn parallel to the designed summer load waterline. See below.

On a merchant ship, the designed loaded sea going trim is even keel (drafts forward and aft are the same and the keel corresponds to the baseline). Smaller vessels are more likely to have rake in the keel (sloping from forward to aft) so that when floating at the designed sea going trim, the after draft will usually be greater than the forward draft.

An excessive trim, caused by excess weight forward or aft, will reduce the freeboard forward or aft, change the vessels condition from the designed seagoing trim and may affect the seakeeping characteristics of the vessel. For commercial vessels, the stability book or the survey book will set out the maximum trim that the vessel is allowed to operate at.