TYPES OF 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, the ship handler 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 ship handler 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 ship handler's task more difficult. Plan ahead so the rudder and propulsion system can be used. 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 ship handler can usually overcome any 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 moe slowly and more care is necessary to keep the ship's speed down while still controlling steering. Speed is important when steering a ship because sufficient headway is needed to make steering possible.
Ships fitted with balanced spade rudders often will not steer when large rudder angles are used. A turbulent flow develops over the rudders surface so its lifting effect is lost and it stalls
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 sterring 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.
1. When reducing the speed of a ship fitted with a variable pitch propeller, the flow of water past the rudder is disrupted unless the pitch is reduced. 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.
1. A variable pitch propeller going astern is less effective than a conventional propeller. This compounds the steering problems because it is more difficult to drift the way off a ship fitted with a variable pitch propeller, it is 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 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 line handlers on the dock that this is necessary, as a line can become fouled in this turning propeller in a short period of time. A tug working at the stern must also be cautioned.
