|Pearson 10M 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.
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.
* 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.
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.
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.