A vessel is any craft used on the water, including seaplanes and hovercraft.
Any vessel propelled by machinery is a power-driven vessel.
A sailing vessel is a vessel only propelled by sails, once the engine is engaged it becomes a power-driven vessel.
A vessel engaged in fishing is only one which is restricted by it's equipment, and does not include vessels fishing with lines. However, we may see a fishing vessel and be unable to decide if it is fishing, because they rarely lower the shapes which indicate that they are fishing. For this reason small craft are wise to assume all fishing vessels they see are "vessels engaged in fishing" for the sake of applying the rules, and a prudent skipper will give them a wide berth.
A vessel not under command is probably a vessel which has suffered a major equipment failure and has stopped to repair it. However, they may also be moving through the water or "making way". This definition covers many of the unexpected situations which may arise and cause a vessel to be uncontrollable.
The term vessel restricted in ability to manoeuvre, does not imply that the vessel is damaged in any way. It indicates that because of the special type of work it is performing that it can not manoeuvre as might otherwise be expected.
A vessel constrained by her draught, is only a power-driven vessel which has exceptional draught. An example may be a laden tanker passing through the Dover Straits. The tanker may have a draught of 20m, and the part of the channel which is suitable for a vessel of this draught is very narrow. Any other ships, with a lesser draught would not be restricted to the same part of the channel as the tanker.
Remembering that there may be local variations to the rules, in the Solent there is an Local Notice to Mariners in force regarding vessels Constrained by their Draught:
LNTM 02/04 09/01/2004 - 10:10:00
1. Mariners are advised that many warships and merchant vessels (including ferries) operating within the Dockyard Port of Portsmouth and the Port of Southampton are at various times "vessels constrained by their draught or vessels who can safely navigate only within a narrow channel or fairway."
2. Save for the exception at Paragraph 3 below these vessels, when so constrained, are all to show the signals prescribed in Rule 28 of the International Regulations for Preventing Collision at Sea , i.e. three all-round red lights in a vertical line, or a cylinder, in order to be afforded the privileges of Rules 8(f)(i) and (ii) (Actions to avoid collision), 9 (Narrow channels) and/or 18(d)(i) (Responsibilities between vessels when one is constrained by draught).
3. Submarines operating on the surface within the Dockyard Port of Portsmouth and the Port of Southampton are to be considered vessels who can safely navigate only within a narrow channel or fairway in accordance with Rule 9 (Narrow Channel) even when not showing the signals prescribed in Rule 28.
4. If the masters or skippers of vessels under 20m in length, sailing vessels or crossing vessels are in any doubt as to whether a particular vessel is indeed a vessel which can safely navigate only within a narrow channel or fairway or a vessel constrained by its draught, then they are always to give it the benefit of the doubt and treat it as such.
5. This Local Notice to Mariners is issued jointly by the Queen's Harbour Master Portsmouth and The Harbour Master ABP Southampton.
The term underway, means that the vessel is not attached to the bottom in any way, it does not imply that the vessel is proceeding through the water. It is possible to be underway but not making way, or under way and making way, forwards or in reverse.
Vessels are deemed to be to be in sight of one another when they can be see visually, if they are only detected on radar or by sound signals they are not in sight of one another.
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