An allision is a maritime accident where one boat hits another or allides with a fixed object like a bridge. This term is different from a collision, which os where two moving objects strike each other. There may be special legal considerations with an allision, namely increased responsibility on the part of the person who was controlling the moving vessel, especially in the case of a large stationary object like a bridge that should have been easily avoided.
There are a number of reasons why an allision can occur. When one boat hits another, it may be the result of a boat at anchor failing to display proper signaling or not using lighting to make sure other boats can see it. This can be a particularly large problem with small boats and very large ships; a sailing boat can be practically invisible to people on a supertanker if it is not clearly illuminated. In other instances, boat operators comply with laws about making sure they are visible and the people in charge of another boat fail to see them or lose control of their vessel and have trouble avoiding the anchored or docked boat.
Very large vessels can be difficult to stop or turn. In harbor, they are usually controlled by a tugboat rather than running under their own power. While underway at sea, they need considerable warning time to take evasive maneuvers. In situations where a vessel is disabled and cannot move, there is a risk of an allision with another vessel if it is floating helpless in a shipping lane or other heavily trafficked area. Vessel operators are required to yield to disabled boats, but this can sometimes be difficult.
Objects like bridges, wrecks, submarine cables, shoals, and so forth can also be struck by moving vessels. Outdated charts are a common cause, as people may think waters are safe and navigable when they are not. Poor visibility can also be a factor, as can malfunctioning navigation systems. Boats may not realize they are close to a dangerous object in the water, or may not see it until it is too late. Drift, as may happen with a submarine cable is pushed out of position by currents, can also be an issue.
In an allision, people are usually required to report the incident and they are generally considered liable for damage unless negligence on the part of the person in control of the fixed object can be identified. One serious concern with allision is the risk of rupturing compartments filled with fuel oil and other materials, potentially leading to a hazardous situation caused by the uncontrolled release of chemicals.