Selecting the Correct Propeller for Torqeedo Electric Motors

 Ever wondered if you can get better performance from your Torqeedo electric outboard?  You aren't alone and it's a good question.  Follow the guide below to optimize your Torqeedo electric boat motor by choosing the correct propeller.

What Type of Hull?

First thing you need to understand is what type of hull the boat has where the electric motor will be installed.  There are two very basic types of boat hulls, planing and displacement.

Displacement

This type of boat hull is designed to move through the water with a typically fine edge at the bow to aid in slicing through the water and then push it aside.  Most sail boats, trawlers, and ships have displacement hulls and are efficiently propelled up to a theoretical maximum speed based on the hull length.  The hull speed (HS) in knots is calculated by multiplying the square root of the length at waterline (LWL) by 1.34 so HS=√LWL x 1.34. 

For example, if your boat is 16 feet in length overall and the length at the waterline is 13.5 feet then the hull speed is √13.5 x 1.34, which equals 4.92 knots.  If you are thinking, "hey wait a second my 16 footer goes much faster than 5 knots!"  Then, you probably don't have a displacement hull boat and it likely has a planing hull.

Adding more power than is required to reach hull speed does not result in an appreciable increase in speed through the water.  So if a 5 horsepower (hp) outboard allows you to reach hull speed at half-throttle, using a 10 hp outboard at half-throttle will only result in using more fuel (or electrical energy) without adding much if any additional speed. 

Note - Some modern displacement hull designs can exceed their theoretical hull speeds due to radical departures from traditional hull shapes.

Planing

This type of boat hull is designed to climb its bow wave and glide over the surface of the water with only the aft most third or less of the boat in the water.  This requires more energy to get on plane but once on plane it becomes quite efficient and greater motor power results in greater forward speed.

A planing hull boat operates in one of three "modes"; displacement, plowing, or planing.  From a stop the boat starts in displacement mode then, as motor power is applied, transitions into plowing mode and, as more power is applied, transitions finally into planing mode.  If power is reduced or removed completely the process is reversed.

The plowing mode of operation is very inefficient and should be avoided (it also results in the largest wake, so other boaters will appreciate you minimizing the time you spend with the your boat plowing).

What Type of Use?

 Now that you understand the different types of boat hulls it time to understand the different uses for those hulls.

Slow Speed / Heavy Load

Typical slow speed boats have displacement hulls and operate at hull speed or slower.  Boats designed for heavy loads are also typically displacement hulls and the load can be either within the hull or exerted by towing/pushing.  Monohull sailboats and push/tug boats fit into the slow speed/heavy load category.

High Speed / Light Load

Typical high speed boats have planing hulls and operate above hull speed or faster.  Boats designed for light loads are also typically planing hulls.  Most recreational boats boats fit into the high speed/light load category.

What Type of Conditions?

Fair Weather

 The average recreational boater can be considered a fair weather boater as most of us avoid the extremes of tidal currents and wind conditions.  However, sudden changes in conditions can catch you off guard so it's always prudent to think about how your boat, motor, and equipment will perform when conditions are not as expected. 

All Weather

Some boaters by choice or by profession must endure most of what nature throws their way and must choose the right equipment for those conditions prevalent in their boating area.  

While expected weather conditions do not directly influence propeller choice they certainly influence the horsepower choice.  If you routinely experience conditions where you need to battle headwinds additional power might be necessary (even when this power would under calm conditions be considered excessive).  Also, under these headwind conditions a slower speed (higher thrust) propeller on the higher horsepower motor might be just the answer.  Read more below about propeller choices.

What Type of Propeller?

Torqeedo motors are shipped with a standard propeller that works for the average use case designed for that specific electric motor.  When the way you are using the motor is further from the expected average use then it is time to consider a different propeller that can optimize the motor and get the best efficiency.

Torqeedo Propeller Desginations

All Torqeedo propellers carry a designation like v35/p4000.  At first this may leave you scratching your head but no worries...it's easy to decode.  The number after the "v" indicates velocity (speed in kilometers/hour) and the number after the "p" indicates power (watts).  So a higher v number indicates the potential for higher speed and a higher p number indicates the potential to use more power.

For example, the Cruise 10.0 R comes standard with a v22/p10k propeller. (The k here indicates thousands, so read 10k as 10,000).  There are optional propellers available such as the v15/p10k and the v32/p10k.  The v15/p10k model produces more thrust at a slower speed but still uses all 10,000 watts of input power of the Cruise 10.0 R.  The v32/p10k produces less thrust at lower speeds but is designed to reach higher speeds and still use all 10,000 watts of input power (perfect for planing hulls).

Slow Speed

 If you are using your Torqeedo with a displacement hull boat moving at slow speeds and or carrying heavy loads it's possible you would benefit from a slower speed propeller that produces more thrust at lower revolutions per minute (rpm).  This will optimize your motor to turn all of its available power into forward thrust and reduce efficiency robbing aspects like propeller cavitation.  So if your were using a v22/p10k you might consider a v15/p10k instead.

High Speed

If you are using your Torqeedo with a planing hull boat designed to move at higher speeds, then it's possible you would benefit from a higher speed propeller that is able to reach a higher rpm.  This will optimize your motor to produce its greatest thrust at a higher rpm.  So if your were using a v22/p10k you might consider a v32/p10k instead.

Propeller Selection Table

The table below summarizes all of the propeller options for each of the Torqeedo models. The "Standard" propeller is the one that ships with the motor.  Notice that for most motors there are both slow speed and high speed options.

Note - the Cruise 2.4/4.0 models from 2017 onwards utilize a splined shaft like that of the Cruise 10.0.

If you need any help at all deciding what Torqeedo propeller is best for your application, please contact Wee Boats to discuss your needs.