Stern Forces on a Powerboat

The rudder used with inboard drives or turning the pivot drives with outboards and stern drives can be a big help because it can change the direction of the thrust force at the stern of the boat.

Simply by turning the rudder or pivot drives, you can change the entire dynamics of the force and moment balance equations. Thus, you can turn the boat in the desired direction and also gain an advantage over the wind and current. For example, you can prevent your bow from being blown downwind. When springing, you can make your bow actually turn upwind.

In the image below, the rudder is turned and the stern of the boat feels forward and sideways forces aft, which collectively act to turn the boat.

Below, if the boat is tied to the dock, the forward thrust is counterbalanced by the spring line but the force on the rudder changes how the boat sits. Thus, if the rudder angle is increased the boat changes position.

In a side wind, working the rudder is particularly useful to “spring” your boat to windward or let it drift down to leeward. You are using your engine speed and rudder to achieve equilibrium.