1. Types of boats and their characteristics
This chapter deals with the basic ship and engine types. We can first subdivide the ships into motor boats and sailing ships.
The motor boats and motor yachts usually have a hull without a keel. The sailing ships can be further divided into dinghies without a keel and keel yachts.
The deeper the hull goes below the waterline and the larger the keel, the more resistance the boat creates against lateral displacement caused by wind or when drifting. See the figure below.
1.1 Propulsion and rudder
Motorboats and sailboats are powered either by an outboard or inboard motor.
1.1.1 Outboard motor
The outboard motor combines the engine and propeller in one unit. 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, producing only a low steering effect.
A ship with outboard motor can be steered easily also at low vessel speeds. For instance, 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 neutral, the unit behaves like a rudder, with the magnitude of the rudder force 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 magnitude of the rudder effect depends on the speed of the ship through the water. The faster the water flows around the rudder, the greater is the rudder force.
The rudder is usually located behind the propeller. In that position, when the propeller generates forward thrust, the water will be washed around the rudder. This is called prop wash and it helps to steer the boat at low speeds.
In reverse, the propeller will generate little wash around the propeller and practically, the rudder effect is only caused by the ships movement through the water.
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 in 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, moving the stern to the side accordingly.
The direction in which the stern is moved depends on the direction of the propeller rotation. 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 for inboard motors with a fixed shaft and propeller. The magnitude of the prop walk is different from ship to ship.
How does the prop walk influence a ship with two engines? In the Hafenskipper 2 app you can find a motor yacht and a sailing catamaran with two engines. Both ships are equipped with one right-handed and one left-handed propeller by default. This way, the prop walk is cancelled out when both propellers produce forward (or reverse) thrust at the same time.
1.2 Ships within the Hafenskipper-2-App
You can find seven different ships in the Hafenskipper-2 app.
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
The sailing yacht has an inboard motor and exhibits some prop walk effect. The great keel lets her slide through the water like the ship is on a rail: The sailing yacht does hardly drift in curves. In the default setting, the ship has a left-handed propeller. This can be changed in the ship detailed settings.
1.2.5 Second sailing yacht with engine and rudder
The second sailing yacht is heavier and longer than the previous sailing yacht. The propeller is right-handed by default. In this setting, the stern is pulled left when the ship is in reverse.
1.2.6 Motor yacht with twin engine
The motor yacht 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 has two propellers and motors like the motor yacht making it also very maneuverable at low speeds. Having two hulls with keels, the catamaran hardly drifts in curves.
