How does trim angle affect steering torque?
When the propeller is run fully submerged and with the propeller shaft approximately horizontal (parallel to the surface of the water) as shown in Figure 7-22, there should be little, if any, steering load. Although this also applies to stem drives, there are other complications due to a stem drives tilted steering axis, which can independently cause some steering torque. However, with the engine or drive unit (right-hand rotation propeller) trimmed in (Figure 7-23), because of the propeller shaft tilt, the downward-moving blade on the right side of the propeller shaft has effectively more pitch, while the opposite is true of the upward-swinging blade on the left side (relative to the surface of the water). This right/left imbalance pulls the engine or drive unit to the right, and makes the boat want to go into a right-hand turn. Naturally, the driver must resist this force if the boat is to continue in a straight line.
