With the increasing popularity of electric drive trains coupled with the availability of new battery technologies and solid state control systems - we are seeing every increasing interest in using the unique features of the Kiwiprop™ as the propeller of choice for these types of installations.

Of particular interest to many is that by simply engaging reverse when sailing, the Kiwiprop™ effectively locks up and operates exactly as a fixed propeller at maximum pitch.

Thus it can be used in a regeneration mode to drive the motor - which can then be used to recharge the batteries with the appropriate control systems.

To disengage the unit - one needs only to engage the motor in Ahead, which disengages the reverse rollers, and then reduce rpm and stop the motor whereupon the unit will simply feather as it would do normally.

We are frequently asked for propeller sizing advice with owners correctly emphasizing the torque available at low rpm from electric motors.

Effectively the Power vs RPM curve is a straight line because the torque is constant.

However the fundamental equation:

     POWER = TORQUE x RPM

is applicable irrespective of the type of engine and thus we simply use the same Power vs Shaft RPM graphs that we do for any other installation.

While owners will typically seek a large diameter unit for supposed efficiency gains, the available power constraints and need to obtain a sufficient theoretical 'speed of advance' from the pitch of the unit x the shaft rpm - will always place a constraint on the maximum diameter propeller that can be fitted.

The power absorbed by a propeller approximates a Function of Diameter^5 so in simple terms very small changes in diameter have a very large impact on the power required.

Speed of Advance is the speed the propeller would move forward without any slip.

Slip is essential to generating thrust and the objective is not to minimize slip - but to optimize thrust at expected vessel speeds.

Hull speed will always be constrained by Power vs Displacement ratios and these will apply irrespective of the type of engine installed.

Electric drive installations must still recognize this variable in estimating future vessel speeds.

As a rule of thumb shaft speeds at cruise should always be less than 1000 rpm. This will require the installation of reduction ratios on most electric motors.

The following Power vs SHAFT RPM graphs give a good guide to the size of propeller that will be optimal for any particular installation.

Enter the Y axis with the motor power @ maximum - Enter the X axis with the shaft rpm at this same maximum power - ie after reduction ratio.

Where these two points intersect on the body of the graph will be over or near a coloured curve labeled with Diameter and Pitch and it is this sized propeller that will be most likely be optimal on a particular installation.

For lower shaft rpm's you can extrapolate the curves to the left.

Remember the pitch on a Kiwiprop™ is easily varied by the user from 18 - 24 deg and thus a particular installation can them be fine tuned to the drive train and vessel.

Kiwiprops™ are sensitive to pitch to the extent for example - on a 16.50" unit a pitch change of ~ 1 deg will equate to an engine max rpm change of ~ 300 rpm on a 30 hp engine @ 1450 shaft.

Power vs Shaft RPM curves for larger blade sizes are also available on this web site at Power Curves