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Electric Drives


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

Of particular interest to many is that by simply engaging reverse when sailing, the KiwipropTM 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.

Because of the internal torsion spring which will restore the unit to normal feathered status, it is essential once reverse has been engaged to ensure an appropriate load is maintained sufficient to maintain the spring torsion and then allow the unit to rotate in a reverse direction whereupon the electrical load from re-generation will provide the torque to ensure the unit stays in the reverse situation.

To disengage the unit – one needs only to engage the motor in Ahead, which disengages the reverse rollers, and then throttle down 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 Fn 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. Refer to our web page  Speed - Power Required  where the power required is calculated for displacement hulls.

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  Power Curves - 3 Bladed 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 KiwipropTM is easily varied by the user from 18º – 24º  and thus a particular installation can then be fine tuned to the drive train and vessel.

KiwipropsTM are sensitive to pitch to the extent for example - on a 16.50” unit a pitch change of ~ 1.5º will equate to an engine max rpm change of ~ 400 rpm on a 30 hp engine @ 1450 shaft.

An issue that can arise in electric motor installations as a result of feedback from users about their installations  and is relevant to new K3 / K4 electric drive train installation owners - is that their controllers should not be set for soft start - Hence do not start the engine slowly in reverse and then increase the rpm.

Starting with "soft start" can leave the blades initially adopting a partial reverse engagement with pitch at ~ 45º - whereupon they generate reverse thrust and then stay in that position under reverse loadings rather than move progressively to full reverse at normal max pitch of ~ 23.5 - 24º.

Increasing the starting reverse load so as the blades promptly  " flick " into full reverse position as do normal engine drive trains in reverse will address this issue if it occurs.