Tuesday, May 13, 2008

Rudders and Propulsion Systems

While the mariner has an interest in the characteristics of all the various types of rudder and propulsion systems used on ships today, the shiphandler must first be concerned with how the ship responds to the system with which she is fitted. He can't change that rudder or engine, whether or not the ship handles poorly, but must live with the ship as she is. A practiced shiphandler will know in a very short time how effective the rudder is and how the ship will respond to varying amounts of rudder angle. The tendency towards smaller rudders on larger ships has made the shiphandler's task more difficult.

Plan ahead so the rudder and propulsion system can be used as effectively as possible. Proceed at a moderate speed so greater engine revolutions can be used as needed to increase the flow of water past the rudder and increase the rudder's effectiveness without creating other problems. By using the engine in this manner, for only as long as needed to obtain the desired results, the shiphandler can usually over­come any inherent deficiencies in the ship's design. This tactic is effective with a diesel ship since immediate, large changes in revolutions can be obtained, a steam turbine ship increases revolutions more slowly and more care is necessary to keep the ship's speed down while still controlling steering. Obviously then, speed is important when steering a ship-sufficient headway is needed to make steering possible, yet speed must be kept down to minimize unwanted hydrodynamic effects and to have sufficient engine speed in reserve to increase rudder effectiveness when required.

Ships fitted with balanced spade rudders often will not steer when large rudder angles are used. A turbulent flow develops over the rud­der's surface so its lifting effect is lost and it stalls. Occasionally a ship is still designed with such a rudder and all those involved in her design and construction seem surprised when she goes aground early in her life while trying to negotiate a normal turn. If a ship's profile drawings show a spade rudder, beware of turns requiring the use of more than 5 to 10 degrees of rudder angle. The rudder may stall, and the loss of effectiveness means that the ship will just continue along her course as if she had no rudder until she fetches up at some point where you never intended to be.
Propeller design has a significant effect on the handling character­istics of a ship. The direction of rotation affects the ship's behavior, the direction in which the vessel tends to twist when the engine is put astern, and the diameter of the turning circle for right and left turns. The size of the propeller affects the stopping and steering ability of the ship since higher revolutions are required to get a good flow over the rudder at reduced speeds when the ship is fitted with a smaller diameter propeller.

The handling characteristics of a ship fitted with a variable pitch propeller are significantly different from those of a ship with a fixed blade propeller. Variable pitch propulsion systems have advantages since a diesel engine so fitted does not have to be stopped and restarted to go astern, and an almost infinite choice of speeds is available. You can go astern indefinitely, which isn't possible with a turbine-driven ship, and, unlike a conventional motor ship, you can change the direction of propulsion repeatedly without being concerned about exhausting the supply of starting air. These advantages have at times been used as a basis for recommending that VLCCs be fitted with this type of propul­sion. The drawbacks should also be considered, before deciding whether variable pitch propulsion is as desirable as claimed.

1. When reducing the speed of a ship fitted with a variable pitch propeller, the flow of water past the rudder is significantly dis­rupted unless the pitch is reduced very gradually. The adverse effect on steering is significant. The propeller cannot safely be put at zero pitch to reduce the ship's headway since this com­pletely disrupts the needed flow of water to the rudder.

2. A variable pitch propeller going astern is less effective than a conventional propeller. This compounds the steering problems since, because it is more difficult to drift the way off a ship fitted with a variable pitch propeller, it is often necessary to use the engine astern for longer periods of time to stop her.

When approaching a berth, lock, or pilot station it is neces­sary to start slowing a ship equipped with this type of propeller sooner than would be required with a conventional propeller, and to then use minimum pitch to steer at slow speed once headway is sufficiently reduced.

Since a variable pitch propeller is normally turning at high RPM, even when in position and stopped at the berth at zero pitch, be sure to keep stern lines clear of the water when docking. Inform linehandlers on the dock that this is necessary, as a line can become fouled in this rapidly turning propeller in a short period of time. A tug working at the stern must also be cautioned.