Pearson 10M Electric Drive
Pearson 10M Electric Drive Back to Projects

E Drive || Main Page | Drive Unit | Controller and Wiring | Battery | Cooling | Instrumentation | Performance | Season Review | Diesel Genset | Videos | Seacock ||
Conversion to Electric Drive

In the fall of 2020 my 43 yr old Volvo diesel blew a head gasket. I considered fixing it (gasket kits are available) but on inspection there was some erosion on the cylinder head. That could be dealt with but the cost of a refresh (rings, injector service, full gasket replace, bearings, etc) was probably going to be several thousand dollars. A new diesel would cost about $16K just for the motor (plus another 1500? for materials for new beds, couplers, etc) with me doing all the install work. I decided to go electric with a 12kW kit from Thunderstruck Motors.

Quick Summary
The conversion has been very successful. I have a lot more still to do but I sailed the whole 2021 season (79 days sailing) with the new electric drive and by the end I didn't even think about it. It just worked. It's only real limitation is range (25 miles at 5 kts) but for the way I currently use the boat it's more than adequate. In fact that's not really a fair way to put it. It is as good or better at driving the boat as the diesel was in all respects except range (and perhaps recharge time). I could get about 110 miles from a tank of diesel at 5 knots. I can get about 25 miles at 5 knots from the electric with the battery I have. The electric costs less than 1/10 of the diesel to operate. It is much quieter (not quite silent), just as powerful (if not more) and always ready at an instant. I only experimented with regen a little bit but I was able to get 300-400 Watts into the battery while sailing at 6-6.5 knots.

The electric drive is not silent but is much quieter than the diesel. I'll get some actual numbers on that next season to compare to what I have from the diesel. The drive is close to silent from the cockpit at harbor speeds and normal conversation is no longer difficult running at speed. The raw water pump makes more noise than the motor. Noise increases with speed though water and wind noise are more dominant.

The installed system including battery, motor, drive unit, controller and cooling system weighs in at a total of about 400 lbs. The diesel genset I have in mind will probably add 300 (with fuel) for a total of apr. 700lbs. The Volvo MD11 with v-drive and fuel system (with 18 gallons of fuel) weighed about 860 lbs. The Volvo was a relatively heavy 23HP diesel at 500 lbs. A replacement Beta Diesel engine would be more like 350 lbs. 50 sqft of solar would add another 100lbs? That's getting back to the original weight of the diesel setup.

Cost so far has been about $9,000 including the motor kit, charger, all the parts for the battery, the raw materials to fabricate the drive unit and the new motor beds, cables and wiring, etc. Biggest costs have been the motor kit and battery components (about 2700 each) but all the raw materials for fabrication of the drive unit add up. I'll put together a better breakdown of that. It hasn't been cheap but it has been quite a bit less than a new diesel. And its been quite an education.

I have a lot more details to share on the system and all the considerations but for now here are some drawings and photos of the installation.
|| Drive Unit | Controller and Wiring | Battery | Cooling | Instrumentation | Performance | Season Review | Diesel Genset | Videos | Seacock ||

When I started considering the electric conversion I came up with some basic performance criteria that I thought the system should meet.

* Capacity to motor 20 miles at 4 knots. This would define the minimum size of the battery bank.
* Capacity to reach 85 or 90% of hull speed. This would define the size of the motor.
* Capacity to run continuously at 5.5 to 6 knots (with a generator). This would define the size of the cooling system.
* The motor should be brushless.
* Add a diesel genset capable of driving the boat continuously at 5.5 to 6 knots. This is a phase 2 consideration to be added later.
* Add solar charging capacity of 3-500 watts if possible to be placed over the cockpit in place or on top of the bimini. That would be about 50 sqft. Also to be added later.
* The installation should have a reduction drive to get prop rotation at 800 rpm for the 18" feathering prop.


The next step was to look for data from other installations on similar sized boats to get some idea of how big and powerful the components needed to be to meet the base criteria.

I found several examples on-line of installations in similar boats and made a data table and graph. Someone had come up with a formula for power consumption at speed. I took that and adapted it a bit to better match the results from the closest boat to mine that I could find (Pearson 36 of which I found 3 examples with electric conversion).

From this research I came up with some sizing for the components.
* A 13kWh LiFePo4 battery would give me an estimated range of about 26 miles at 4 knots.
* A 10 or 12kW motor should get me to hull speed though at that speed the battery would only last 90 min. But for occasional use for something like punching through a choppy entrance that would be fine.
* To be able to sustain 5.5-6 knots (with a genset) the motor would run at an estimated 5400 to 7200 watts. I would need good cooling which led me to choose a water cooled motor. Air cooled can get to the power levels I wanted but only intermittently. To run continuously without over-taxing the system water cooling was a must. The cooling could be handled by a water/water heat exchanger with a sea water pump or by a air-water heat exchager with a small fan.


Through my research I found you run into limits in the different aspects of electric drive system design. For example, if you have a 10kW motor but you have a battery max output capacity of 5kW your useable motor capacity is limited to 5kW. I wrote up a seperate page to discuss the limits and how they need to be accounted for in the system design.

Limits in Design of System

Power Required

My Pearson 10M came with a 23HP Volvo MD11 2 cylinder diesel with a Walter V-drive and a 2:1 reduction gear. I had installed a JProp 18" 3 bladed feathering propeller which can have the pitch adjusted (though not under way). I had fuel and power graphs for the MD11 and data on the actual performance of the boat. The match between the motor, reduction gear, and propeller were very good. With the propeller well dialed-in the boat would reach hull speed (7 knots) right at max RPM (2400) just as desired. And the propeller is quite large for a 33' boat at 18". Since it has an adjustable pitch I could set it up for lower shaft RPM for better efficiency with the electric motor.

Drive Unit Design and Layout

More to come...

JProp Feathering Propeller

Engine Compartment

Old engine beds were removed. They were very well installed. It took a couple days work to cut them out.

Motor Beds

Here's the jig used to align the motor beds with the prop shaft, the beds taking shape and being fitted, and the motor beds after glassing in place. The layup was started with a large sheet of biax/mat as a base to give a large contact patch to the hull. Epoxy resin was used. Drawing shows configuration of the motor soft mounts. The final version used 3/8" screws rather than 1/2". Numbers on the parts are McMaster Carr part numbers

Throttle Linkage

Systems installed in compartment

These photos show most of the components in place and almost ready to run. The battery is on the aft side of the aft bulkhead in the compartment. The cooling system components are mounted on the port side, the controller on starboard. The battery terminals on the brass cylinders from the buss bars can be seen on the face of the bulkhead.