All about Screws (Propellers)
by Roger Harper

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type, the best blade section for model propellers is undoubtedly a thin cambered section, with a quite sharp leading edge and trailing edges. Thicker sections are inevitably less efficient, although some increase of thickness toward the hub is at times need for strength reasons.  One of the main reasons a "home made" propeller can compare favorable in performance with a "proper" propeller is because it utilizes a thin, cambered section.

(Click on picture to enlarge)

Right or Left Hand

The "hand" of a propeller is determined by it's design direction of rotation when driving the craft ahead.  To determine this, look at your boat from the stern.  A counterclockwise rotation of the propeller is known as a "left hand" propeller, while a clockwise rotation is a "right Hand" propeller. 

The direction of rotation has no effect on propeller efficiency, because a right hand propeller is merely a mirror image of a left hand propeller of the same shape and size.  The choice is largely a matter of matching a propeller to the normal direction of rotation of it's driver.  Model engines turn their crankshafts in a counterclockwise direction. Since the crank shaft faces aft, this calls for a left hand propeller to be used.  On the other hand, most electric motors can drive in both directions of rotation, depending on the polarity of the battery connections.  In this case eighter a left or right hand propeller can be used, provided the battery the battery is connected to give corresponding motor spindle rotation.

Mounting

As a general rule, propellers are mounted underneath the hull in a position equal to about 2 times the diameter of the propeller in from the transom.  This will ensure optimum working conditions for the propeller.  Clearance from the hull bottom need not exceed one quarter of the propeller diameter, and can be a little less in order to reduce drive shaft angle.  The lower the drive shaft angel, the greater the forward component of the thrust (i.e. the more effective thrust the propeller will generate), and less the effect on trim.  The use or need for trim tabs  is only likely when used in high power boats.

Torque is another important factor   to consider when dealing in propellers.  Torque is simply the reaction generated by the propeller rotation, tending to roll the hull in the opposite direction to propeller rotation.

Torque can be compensated on a fast running hull by a wedge or transom flap; a slightly offset rudder, a tab on the rudder, or by offsetting the propeller shaft to impart a compensating "side thrust".   This offset can be at an angle to the center line of the hull, or lateral displacement.  The former provides direct compensation by side thrust, in the same manner to model air craft practice.  The ;later provides thrust off set generation a separate turning force opposing any turn induced by "torque roll".   Remember that these methods are used mostly on high speed hulls.

The running speed of the motor also affects the actual torque force present.  Torque decreases with the increases in motor speed.  Lower rotational speeds and large diameter propellers are inevitably associated with higher torque values, regardless of how the speed reduction is obtained.   Thus, with a straight drive, a larger propeller diameter will impose a greater load on the motor, slowing it's rotational speed and increasing it's torque.  If reduction gearing is used to allow a higher speed motor to be used, with a lower final speed, the reduction gearing will act as a torque multiplier to the propeller in the same ratio as the gear ratio.

A complete answer to torque problems is, of course twin opposite hand drives.  However, this has considerable limitations in model practice, notably in lack of synchronization of the two propeller speeds IF each shaft is driven by a different motor.