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

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Drive Unit Design and Layout

System Description

I'll describe the parts of the drive system starting at the prop and working to the motor.

JProp Feathering Propeller

I have an 18" JProp feathering propeller. I got this for the diesel in 2002. The pitch can be changed (not underway) so it was easily adapted to the electric drive installation. And it's relatively large diameter means it will be more efficient with a larger pitch running at a slower speed. With the diesel the prop RPM was 1200 max. My target with the electric drive was 800. That means a simple change of the pitch setting from 6 to 9. That translates to an estimated pitch of 13.5". The large prop is probably also beneficial for regen where the motion of the boat while sailing causes the prop to turn the motor and put current back into the battery. I didn't do a lot of experimenting with that but I did manage to generate 400 watts sailing at 6.5-7 knots.

More about JProp propeller...

Prop Shaft and Stuffing Box

Next is the prop shaft. The boat has a conventional prop shaft riding in a standard cutlass bearing in a strut and a standard stuffing box with GFO Gortex packing. Same setup as the diesel. The shaft is 1-1/8 diameter Auquiment 19 stainless steel. I got it when I installed the propeller in 2002.

More stuffing box...


The couplers are basic off-the-shelf units from Buck Algonquin. The prop shaft coupler has a boss that is meant to located in a bore on the coupler of the transmission on the diesel engine. I machined that boss off on the output shaft coupler and machined in a bore to match the boss on the prop shaft coupler. Both couplers were bored for a snug slip fit on the shafts. And both couplers were faced to their shafts. That is when the shaft is setup on the lathe with the coupler installed and the face of the coupler is machined to make it flat and truly perpendicular to the shaft.

Output Shaft

The output shaft was the most complex part I made for the drive unit. Its stainless steel with six different diameters, two keyway slots and a threaded section. The coupler mounts to the larger end and forward from that there is a shoulder that bears against the thrust bearing to transfer the forward loads from the prop. The thrust bearing is a slip fit just forward of the shoulder, then there is a threaded section where a threaded locking collar screws on to firmly clamp the shaft to the thrust bearing and to transfer reverse loads to the thrust bearing. The next section is where the drive gear hub mounts. Forward of that is a smaller section that fits into a pilot bearing at the front of the housing. It's a shame this shaft is buried in the unit. It's quite pretty. It took me a couple days to make this on the lathe.

There is a photo here of the bare shaft, and the shaft with the output coupler installed.

Thrust Bearing

I spent a lot of time fussing over the design of the thrust bearing. An early idea was a trailer hub mounted to a plate. Basically an upgrade of what Thunderstruck uses. They use a small 4-bolt hub for a 7/8" shaft. It's strong and the bearings are readily available. There are larger more robust units that can be had is stainless steel. The bearing spacing is large enough to also provide lateral support for the shaft to keep it running true. I planned to use this but it was too long for the limited space I had.

I looked at what was used for a thrust bearing in the Walter V-drive that came with the boat. It uses a double roller angular contact ball bearing (type 5304). The v-drive is rated for about 50HP at 2500 rpm. I found the same bearing but several sizes larger with rubber seals (5207-2RS). This would easily be strong enough and would not require an oil bath or any additional seals. And they are not very expensive. I made an aluminum housing with a slip fit for the bearing. The housing bolts to the back of the motor mounting plate. The bearing is rated for 6000 lbs and 7000RPM so I am operating well within its performance envelope (thrust load for a 25HP diesel is on the order of 500-800 lbs - reference?). I ran the motor for about 100 hrs in the first season with no signs of wear on the bearing.

First photo of Walter V-drive thrust bearing (left) and the 5207-2RS bearing (right) I used in the drive unit. Second photo is the thrust bearing housing I fabricated from aluminum.

Motor Mounting Plate

The motor mounting plate is a 3/4" aluminum about 20x12". I was going to use a piece of c-channel but I couldn't find one wide enough for the 60 tooth drive pulley. The 3/4 plate is very stiff and the thickness meant I could set the fastener heads flush with the surface for a tighter clearance where belt runs. It took some time to machine with some tricky setup to maintain registration but the aluminum is easy to cut. There is a large hole in front of where the thrust bearing mounts that the threaded shaft collar fits inside. There is a recess on the aft face of the motor mounting plate that the motor fits into.

Threaded Collar and Bushing on Output Shaft

Forward of the thrust bearing there is a bushing and a threaded locking collar that secures the output shaft to the thrust bearing. The threaded collar is made of steel but I will probably replace it with a stainless version to protect from corrosion. The threaded collar tightens down on a bushing between it and the thrust bearing. The bushing is needed because the thrust bearing sits back in a recess in it's housing which would block access to the collar clamping screw. So the thrust bearing is clamped between the shoulder and the bushing/threaded collar.

There's a photo here of these parts without the thrust bearing housing to show how they fit together.

Drive Pulley Bushing

Forward of the locking collar is the drive pulley bushing. The large drive pulley fits onto this bushing which in turn clamps it to the output shaft. The toothed belt drive pulley fits onto this bushing.

There's a photo here of these parts without the thrust bearing housing to show how they fit together.

Front Cover and Pilot Bearing


At the forward end the output shaft extends through the front cover (made from 3/8" aluminum) and fits into a pilot bushing mounted to the front cover. The pilot bushing is self aligning and there is some play in the mounting that allows it to be positioned concentric with the shaft. After the drive unit is assembled but before the drive belt is tensioned, the pilot bushing is tightened in place. I wasn't sure this pilot bushing would be needed but after installing it the run-out of the shaft was reduced from .005" to a bit better than .001".

The front cover is a 3/8" aluminum plate secured to the mounting plate with long bolts through stand-offs. The cover protects the belt and provides a mounting for the pilot bearing.

There's a photo here of shaft with all the parts on it.

Drive Pulleys and Belt

The drive pulleys mount to the output shaft (on the drive pulley bushing) and on the motor shaft(with another bushing). The drive pulley is 60 teeth, the motor pulley is 20 teeth for a final drive ratio of 3:1. The max motor RPM at 48V is 2400 RPM. The 3:1 reduction gives the prop rotation of 800 RPM. The pulleys and bushings are made of steel and need protection from corrosion just like the parts of an inboard diesel would.

The drive belt is 1" wide type H with a tooth pitch of 1/2". This is a pretty standard belt and pulleys. I sized the belt length using the center-to-center distance between the pulleys (10.73") and the pulley tooth counts (60, 20). There are on-line calculators for this from the belt and pulley manufacturers.
Belt Calculator


The motor is a Motenergy ME1616. Its 12kW at 48V, water cooled, totally enclosed (IP67), and brushless. It mounts in a recess on the aft face of the mounitng plate to get the motor pulley at the correct height over the front of the plate. I made a spacer for the front face of the motor so it sits flush on the bottom of the recess in the mounting plate. The motor mounting screws fit through slots in the mounting plate so it can slide to tension the belt. The motor weighs about 50lbs.

Photos of the face of the motor and the motor with some of the cooling components.

Motor info at

More to come...
Thrust Bearing Housing

The bearing housing was fabricated from aluminum with operations on the milling machine and the lathe. The bearing is a snug slip fit. The bearing is a 5208-2RS double roller angular contact ball bearing with two rubber seals. It is the same type of bearing used in the original Walter RV-10 V-drive used in the boat but several sizes larger. The bearing is mounted to the shaft between a flange on the aft side and a locking threaded collar on the the forward side.

5208-2RS bearing (right) next to the thrust bearing from the original Walter V-drive, thrust bearing in housing attached to back of the drive frame with couplers in place, bearing housing on back of drive unit in place on motor beds.

Motor face plate, drive belt, and assembled frame
Drive Bench Test Videos

Shaft Details

Boring one of the couplers to fit its shaft, facing the prop shaft coupler, and the completed output shaft of the drive unit. The output shaft coupler was machined with a bore to receive the boss on the prop shaft coupler.

Drive frame test fit on motor beds