Notch tugs are a relatively recent development, and despite their sometimes odd appearance they provide a very practical solution to many of the problems of marine towing. The notch tug operates with a matching tow that is fitted with a notch in the contour of the tug's bow. In cable retained towing rigs, the tug is fitted with additional fenders amidships that ride in extended portions of the tow's notch and retaining cables are run out to the tow. The tug is a tight fit into the notch and is allowed only limited degree of movement. It can take in its towing cables and back free if needed, but normally this would not be done except in the case of a towing emergency that requires the tug to pull clear of the tow, or in the most extreme weather conditions. Disengagement in rough weather carries the risk of damage to the tug and tow, so most tug captains prefer to stay in the notch if possible. The tug is however fully equipped with a towing winch and is capable of astern towing if the situation requires it. Operating in the notch shields the tug from sea conditions and is the most advantageous position for maneuvering the tow.

Above: The IOT (Interstate and Ocean Transport) tug Enterprise is shown underway with a large fuel barge. IOT became Maritrans and later was a part of Sonat Marine. Another IOT tug, Freedom, is shown in the image at the beginning of this section.

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Articulated Tug Barge is a relatively new technology that takes the concept of the notch tug one step further, fitting the tug and tow with an interconnecting coupling arrangement. The tug enters the notch and deploys a hydraulically operated pin assembly that engages a rack in each side of the notch. The rack is necessary to accomodate the changes in the load trim of the tow. Locked into the notch, the tug has only one degree of movement, its fore and aft pitch around the pin connection. Any side to side roll is done in unison with the tow. Operating in the lee afforded by the notch shields the tug from battering by sea conditions, while its ability to pitch fore and aft helps keep the propellers in water rather than having them draw air and cavitate in rough seas. The coupling of the tug and tow can be done in a wide range of sea conditions, and there are no cables or hawsers involved. Maneuverability and propulsive efficiency are improved by not having the tow trailing in the wash of the tug's propellers, and there is no risk of the tow tripping the tug as can occur with astern towing. Due to the great change in freeboard of the tow between loaded and unloaded condition, the notch tugs are usually fitted with an upper pilothouse.

Above: The Gulf Coast Towing tug Betty Wood is shown underway with the notch barge Marie Flood.

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