Mampaey Offshore Industries

Controllability

Most tugs have the towing arrangement on their aft deck. This is the best position if the tug pulls in a straight line, because the towline force is then in line with the water resistance. If the tug is not pulling in a straight engine power is lost to control the tug because the tug wants to turn back in the straight position. The moment causing the tug to turn is the result of a lever between the water resistance and towing force. The turning moment is heavily influenced by the assistance speed of the tug. If the assistance speed is doubled the required engine power to control the tug should be at least 4 times as much.

A DOT-tug is easier to control because the system has the natural tendency to move to the equilibrium position where the towline force is aligned with the water resistance for all directions. The close match of the towline forces above the water and the hydrodynamic forces below enable an optimal maneuvering performance with full control during towing operations. Not only fuel is saved this way, but the speed of assistance is a far less limiting factor of the tugs controllability. The tug master can assist sea ships at increased assistance speeds without losing the control over his tug.

Flexibility

The oval shape offers the solution to connect the distance between the towing points for sailing ahead and astern. The shape of the oval rail positions the pull force in the forward towing point when sailing ahead and in the aft towing point when sailing astern. Towing operations can be freely hanged from any direction; either from bow to stern use or vice versa with one integrated towing system. The main advantage of this is that the tug operator can be flexible in deciding in what mode to assist the ship and maintain the connection to the tow at all times.

Stability

Traditional tug designs try to minimize the capsizing moment by installing the towing arrangement as low as possible. The DOT system reduces the capsizing moment differently by moving the towline attachment to the vessels’ side. The radial support of the towline attachment prevents the tug from capsizing due to towline forces, an effective safety measure. This increased towing safety can be extended to the overall safety of the tug now it is not necessary anymore to install the towing arrangement as low as possible. The seakeeping characteristics can be positively influenced since the freeboard and thus the reserve buoyancy can be increased.