Electrical deck auxiliaries on ships

These auxiliaries, in the main, comprise cargo winches (which may include warping as a subsidiary duty), cranes, capstans, warping winches, windlasses and hatch-cover winches. Except for cranes, each of these may sometimes be used for duties other than those for which they are primarily intended. The systems of control as between these various applications bear a similarity but with variations to suit the operating conditions. It will be convenient to deal with them under their different headings, but there are divergencies between the methods favoured by different makers and descriptions will therefore be confined to representative schemes.

Electro-hydraulic winches do not call for special mention as they use a continuous running motor, which can be either a.c. or d.c. They can be operated either singly or in groups from one pump. Many electrical deck auxiliary schemes make use of contactors for control purposes and where these are of such size and numbers as to warrant it they can be accommodated in a separate contactor deckhouse instead of in the winch assembly. This increases the amount of cabling but on the other hand it economises deck space in the vicinity of the winch, making for cleaner lines and unobstructed viewing by the operator. It also facilitates maintenance work which in any case is not always opportune to carry out when the ship is in port and when the winches are in use. While at sea maintenance can be carried out under protection from the elements. In every winch, etc., in which the load is lowered while the motor is mechanically coupled such as in systems employing power lowering it is essential to prevent the load taking charge and lowering at a speed which will damage the motor armature. To safeguard against this contingency centrifugal brakes are provided in some cases and they are so set as to enable heavy loads to be lowered with an assurance that the safe speed cannot be exceeded. Provision must also be made to stop the load running back if the power supply should fail or the overload relay operate and in this event the winch, etc., must not restart when power is restored unless the controller has been returned to the starting point, usually the "off " position.

Variable Voltage Control. Ward-Leonard control

Where fine control of both hoisting and lowering speed is required either booster control or a modified form of Ward-Leonard control is suitable and footbrakes are not essential. 

A magnetic brake provides against power failure or when returning the controller to off. Generally speaking for straightforward Ward-Leonard schemes the motor for the generator set can be either a.c. or d.c. and as the set runs continuously and in one direction only it is started in the conventional manner. If the supply is a.c. the exciter would be replaced by a static rectifier.

Under these Regulations vessels of 200 gross tons or less must have two lines of hawsers, one at the bow and the other at the stern quarter, each leading through a closed chock. Larger vessels must have at least four lines so arranged that they can be used on either side of the vessel. Two must lead from the bow and two from the stern quarters and not from the extreme bow or stern. For vessels between 200 and 300 gross tons the windlass forward and the capstan aft may be used for the two lines ahead but those leading aft must run from the main drum of power-driven winches and not from capstans. For all larger vessels all four lines must be power-operated and run from the main drum of power-driven winches and not from capstans.

Electric Drive of Boat Winches

Boat winches are intended to handle rescue boats carried by sea-going and river ships. These do not include the winches used to regularly hoist and lower boats and launches in the performance of routine work and finishing operations of the ship.

Under the International Convention for Safety of Life at Sea all ships must be equipped with rescue boats. The same Convention specifies a number of requirements to davit on which the operating conditions of the electric drive depend to some extent.

The boat-handling gear must provide the means for:

• swinging-out of boat booms or davit with a fully fitted-out boat and lowering of the boat with the full number of people and with the ship rolling by 15° port or starboard;
• lifting of a fully fitted-out boat with the full number of people at a list of up to 8° at such an antirol at which the boat moves clear of the ship, and the swinging in of the davit with a fully fitted-out boat and a minimum number of people;
• swinging-out of booms or davits without a boat with the ship stopped even-keel.

Boats are lowered without the electric drive at a maximum speed of 0.5 m/s, obtained with the help of a mechanical brake.

The suspenders can be lowered either by unwinding the cable from the drum manually or by means of an electric drive.

The principle mode of operation of a boat winch drive consists of lifting the boat, once at a time, a limited time period not longer than 5 min. The boat lifting speed is 0.1 to 0.15 m/s. The choice of the motor must be made so as to ensure starting of the motor at the maximum design load. As the boat lifting time must not exceed 5 min the motors may have to sustain heavy overloads as compared with the catalogue data which represent 60 min ratings.

The control systems for the motors of the boat winches are quite simple. Starting and stopping of a-c motors are effected by means of magnetic starters with a local push button control which may be reversible or non-reversible, depending on the construction of the winches.

Electric Drives of Towing Winches

Flexible-link towing is a physical process involving unsteady motion of inertia masses interconnected by a flexible element. Reliability of towing depends on selection of an optimum length of the towline and on minimizing the towline, overloads under various conditions. Non-automatic towing winches with a pull of 120 kN and above may be used with three-speed a-c cage motors controlled by means of magnetic controllers.

Taking account of the optimum speed ratio of the winches for towlines 37.5 to 48.5 mm in diameter use may be made of three-speed motors with a speed ratio of 1:2:4, developing a rated power and a rated pull at the medium speed; the motors are controllers.

Winches with a 56 to 65 mm diameter towline may be driven by cage motors with a speed ratio of 1:3, producing the rated pull at the medium speed. To obtain low rotation speeds in order to limit the load on the towline these motors must be connected in a dual supply circuit.

The motors must be supplied from the supply mains when running at the rated or high speed and from a directly-coupled thyristor converter, when running at low speeds. To ensure the required power two motors may be connected to one shaft.

The maximum power rating of such systems is 140 kW, which enables them to be used for the largest winches with a pull of up to 650 kN.

The power rating of the motor to be used in the winches is selected to suit the pull required to heave in the towline at the rated speed with a medium towline diameter: P = (Qr*Vr)/n, where P — power rating, kW; Qr — rated towline pull, kN; Vr — rated value of towline heaving speed, m/s; n — efficiency of the driven gear.

The design power must be equal to or below the 30-min motor power rating for non-automatic winches and 60-min power rating for automatic winches operating in the impulse mode. The above power must be developed by the motor at the rated speed.