In this chapter, we’ll look at the different types of boats. At first, you can distinguish between motorboats and sailboats. The motorboats usually have a hull without a keel. The sailboats are divided into dinghies without a keel and yachts with a keel.
The deeper the hull of the ship goes down below the waterline, and the larger the keel is, the more resistance the boat will generate against lateral displacement by wind or when drifting. See the figure below.
1.1 Propulsion and rudder
Motorboats and sailboats can be powered by an outboard or inboard motor.
1.1.1 Outboard motor
The outboard motor is one unit consisting of engine and propeller. To steer the ship the entire unit and thus the propeller is turned left or right. The boat is steered easily and quickly with an outboard motor, as long as the propeller rotates and generates thrust. In neutral gear, the underwater part of the outboard motor acts as a rudder, however, the steering effect is very low in neutral.
The boat with outboard motor can be steered easily also at low speeds. In forward gear, if the unit is turned to the right, the stern will be pushed to the left and so the ship will turn right. In reverse gear and with the unit turned to the right, the stern is towed to the right and the ship will turn left. In neutral, the unit behaves like a rudder and the steering is depending on the speed through the water.
1.1.2 Inboard engine
The inboard engine is connected to the propeller by a shaft. In most ships, the shaft and propeller are fixed and the ship is steered by a rudder. The effect of the rudder depends on the speed of the ship through the water. The more the water flows around the rudder, the higher the rudder effect.
The rudder is usually located behind the propeller. So if the propeller generates forward thrust the water pushed backwards will flow around the rudder and the ship can be steered. In reverse, the propeller will generate little flow around the propeller and there is practically only a rudder effect when the ship is moving through the water.
If the ship is driving forward and the rudder is turned right, the stern will go left and the ship turns right. If the ship is driving backwards and the rudder is turned right, stern will go right, ship will turn left.
1.1.3 Prop walk
The propeller is usually located under the hull at stern and it is optimized for forward propulsion. However, additionally, the propeller generates a lateral force that moves the stern to the side. This effect is called prop walk. It is usually unwanted and the effect is highest when the propeller is going reverse. The prop walk can have multiple causes but can be described in a simplified way that can be easily memorized: The propeller rotates and moves along the ground like a wheel and moves the stern to the side accordingly.
The direction in which the stern is moved, depends on the direction of rotation of the propeller. A right handed propeller turns clockwise when in forward gear and as seen from behind. In reverse it turns anti-clockwise and moves the stern to the left. The left handed propeller does it the other way around: it moves the stern to the right.
The prop walk is practically only relevant with inboard motors with a fixed shaft and propeller. The magnitude of the prop walk is different from ship to ship.
All two engine ships have one right-handed and one left-handed propeller. This way, the prop walk of each propeller is compensated by the other if both engines go forward or reverse together.
1.2 Ships within the Hafenskipper-App
Within the Hafenskipper-App there are six ships available.
1.2.1 Motorboat with outboard motor
The motor boat with outboard motor is easy to steer and has no prop walk.
1.2.2 Motor boat with inboard engine and rudder
The motor boat with inboard engine, fixed propeller and a rudder has a strong prop walk and is difficult to steer at slow speeds.
1.2.3 Dinghy with outboard engine
The dinghy is very maneuverable and easy to steer. It has no prop walk.
1.2.4 Sailing yacht with engine and rudder (right-handed propeller)
The sailing yacht has an inboard motor and exhibits some prop walk effect. She is heavy with high inertial. The great keel lets her move like driving on rails. The sailing yacht does hardly drift in curves.
1.2.5 Sailing yacht with engine and rudder (left-handed propeller)
This yacht has a left-handed propeller. In reverse, the prop walk moves the stern to the right. Everything else is equal to the yacht described before.
1.2.6 Motor yacht with twin engine
The motor yacht (From Version 1.2) has two propellers and motors that can be controlled individually. This makes the ship very maneuverable at low speeds. If one motor is going forward and the other motor backwards with the same thrust, the ship can turn around its center of gravity.
1.2.7 Sailing catamaran with twin engine
The catamaran (From Version 1.3) has two propellers and motors like the motor yacht making it also very maneuverable at low speeds. Having two hulls with keels, the catamaran does hardly drift in curves.
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