Twin Screw Boat Handling
Keep in mind that the smooth maneuvering that is possible with a twin-engine boat is not possible until after you master both the theory and the motor skills necessary to make your boat do what you want it to do.
Vessel Control Characteristics
Several characteristics influence the control of any vessel. A basic understanding of how each of these characteristics influences boat handling is necessary before you can attain competence in handling a twin-screw vessel. Also, knowing the natural conditions that will affect your boat’s operation and the ordinary problems that will arise is essential.
Small boat handling is 10 percent principles and 90 percent application of those principles. The motor skills used in the application come through repetitive use.
Once these simple principles have been memorized, practice will bring mastery of twin-screw boat handling skills -- and close-in boat maneuvering will become second nature. You will be able to dock a 50-foot cruiser just the way you ride a bicycle: without really thinking about it.
Design characteristics -- such as hull size and shape, draft, trim, weight and amount of superstructure -- will affect the boat’s handling characteristics.
For example, vessels with a shallow draft and short length are easy to handle and respond quickly to the helm, but are readily affected by winds. A vessel with a deeper draft and longer length is slower and more difficult to maneuver, and is more affected by currents than the smaller craft.
Power boaters often forget to take into account their power boat’s “sail area” when coming up to a dock. A flybridge cruiser with full canvas has a huge “sail area” -- almost equal to that of a sailboat of the same length. However, the sailboat skipper can take his or her boat’s sails down, but the power boat skipper cannot reduce sail area.
Every vessel has its own individual quirks, and no two vessels handle in exactly the same way.
The type and size of an individual boat’s engine also determines how that vessel handles. Each type of engine and each type of propulsion has special characteristics, which the boat operator must learn in order to determine the effect on the handling of his or her craft.
Diesels differ from gasoline engines and outdrives differ from inboards. The same engine connected to an outdrive handles differently than it does when connected to a traditional inboard shaft.
The important thing to know is, just because you have mastered one kind of powerplant, that does not automatically mean you’ve mastered the next one.
In this article, we will discuss principles for twin-screw inboard vessels with rudders. However, the skills presented here can also apply to boats with outdrives.
The propeller reaction, called thrust, is what drives a power boat through the water. It acts like a screw pump, drawing a stream of water from forward (when going forward) and passing it out astern in a rotary motion. The force of this stream moving astern reacts to the still water around it, forcing the vessel ahead.
The principle is the same going ahead or astern, but the prop wash, which allows the rudder to perform its steering function, only strikes the rudder (mounted behind the prop) when the engine is providing forward propulsion.
A propeller is either right-handed or left-handed, according to the way it rotates on its shaft. If, as you look forward from the stern, the propeller turns clockwise to go forward, it is right-handed. If it turns counter-clockwise, it is left-handed.
Almost all single-screw vessels have right-handed propellers. So, when we discuss single-screw boats, we will be talking about right-handed propellers.
On the majority of twin-screw vessels, the port propeller is left-handed and the starboard propeller is right-handed, with the result that the lateral forces set up by the two propellers cancel each other out. That keeps your stern from “walking” when the propellers first start to turn.
With single-screw maneuvering, during a turn, your vessel always pivots in the middle of the boat. This means that both the bow and the stern swing wide during any turn. So, if you forget to allow room for the bow and stern to swing during the turn, you will always bang into the next boat -- or the dock.
With a twin-screw boat, when one engine is going ahead and the other is going astern, there is a turning moment -- throwing the bow over toward the side of the backing engine. This occurs regardless of the rudder angle.
However, when both engines are turning forward, or both are turning astern at the same rpm levels, the rudder creates all the turning moment.
Experienced skippers learn to master the technique of steering the boat using only the engines during close-in maneuvers, while keeping the rudder amidships. In other words, you can forget about using the helm in these circumstances. Just place the wheel amidships -- and don’t touch it again until you’ve finished your maneuver.
Remember: When you are motoring at dead slow speed, you can shift in and out of gear regularly, to move at an even slower-than-dead-slow pace. Work the shift, not the throttle.
Never rev up your engines. Just proceed calmly, and everyone will know you are a pro.