Marine Kit

Page Last Modified On: October 18, 2021


We've long had an interest in electric propulsion for Marine applications and have been dabbling over the years with electric drive concepts for Kayaks, Paddleboards, Canoes, and Rowboats. After all, these mostly human powered vessels can see all the same benefits that electric assist brings about for cyclists and ebikes. And just like with ebikes, all it takes is a few hundred watts to make a world of difference in the experience when paddling against currents, waves, and winds.

But a personal project to overhaul a Yanmar diesel engine on our CAL25 sailboat convinced us that that we could play a more immediate and ecologically significant role making retrofit kits for the sailboat market. There are countless vessels already in the water with aging diesel engines, and owners who are keen experience sailing without the noisy rumble and stenchy fumes. Plus an electric sailboat doesn't need a massive battery with the perk of wind propulsion, so the economics is more viable than other large watercraft. Once we realized that our own All Axle hub motor had exactly the necessary torque output to spin typical sailboat propellers without any gear reduction, adapting it for Marine drives became a 2nd calling, and in late 2020 we decided to make this a full pursuit.


Unlike almost all other sailboat motor systems, the Grin Drive is an outrunner motor that requires no gear reduction to the propeller shaft. It is already optimized to give maximum efficiency as a directly coupled motor.

    • Super Light Weight: Our 5kW motor is just 4kg (9lb), while the 8kW model is 6kg (13lb).
    • No Gear Loss: The simplicity of not having timing belt, pulley, or gearbox means fewer possible wear points and better freewheeling efficiency.
    • Versatile Mounting: Our use of 80-20 aluminum extrusion tubes for the mount provides a flexible platform for securing and aligning the motor to the existing engine stringers in the boat. We supply a tube extrusion that clamps directly to 1" or 25mm propeller shafts.
    • Regeneration: The direct drive system is very well suited to capturing regen from the propeller, as there is less static loss to spin the shaft.
    • Open Standards: The system can be purchased as a motor only, or as a motor with an ASI controller but with no battery, or with a 48V or 72V LiFePO4 battery packs as well. You are free to use 3rd party batteries, and 3rd party controllers, and have infinite upgrade capabilities down the road.

Boat Compatibility

Our target for this project is sailboats from 24 to 36 feet in length that have an inboard diesel engine with a straight shaft drive going out a stern tube. The kit will not be very useful on sailboats that are designed exclusively for an outboard motor or which have a geared saildrive type of inboard engine. 

Motor Models

We currently have 3 versions of this Marine Motor, a 27mm wide 5kW model in either 20 rpm/V or 30 rpm/V winding constant, and a larger 45mm wide 8kW motor that is only available in a 21 rpm/V wind.

Grin High RPM Marine Motor Models
Motor Model Stator Width Nominal Power Winding Speed Weight
G2730 27mm 5kW 30 rpm/V 4kg
G2720 27mm 5kW 20 rpm/V 4kg
G4521 45mm 8kW 21 rpm/V 6kg


A faster wind motor (like 30 rpm/V) allows for high shaft RPM's at lower voltages, but it also requires a higher current motor controller to achieve the same torque output. In general we have seen that most marine diesel engines run at about 3000 rpm, and are supplied on boats with either a ~2:1 gear reduction or a ~3:1 gear reduction. Those with a 3:1 gear reduction have a ~1000 rpm shaft, while those with a 2:1 reduction have a faster 1500 rpm shaft and a correspondingly lower pitch propeller. The table below shows the recommended motor model, controller size, and battery voltage combination we'd suggest for a given boat setup. 

Recommended Part Combinations
Approx Shaft RPM Boat Size Battery Voltage
Motor Model Controller
1000 rpm 24'-27' 48V G2720 BAC2000
1500 rpm 24'-29' 48V G2730 BAC4000
1500 rpm 24'-29' 72V G2720 BAC2000
1000 rpm 28'-32' 48V G4521 BAC4000 / BAC8000
1500 rpm 30'-36' 72V G4521 BAC4000


When the motors are spinning at just 1000 rpm, their maximum power capability is proportionally reduced as well, so the 5kW models is more like 3.5kW, while the 8kW model will max out at more like 5kW. If this is a concern in any given installation, then it can be resolved by changing to a lower pitch propeller to run at higher shaft speeds.

48V or 72V?

The motor controllers are all 72V compatible and you will have less phase current and controller heating using the slower motors at 72V than the faster wind motors at 48V. So as engineers trying to optimize things we want to say that 72V is the way to go at these power levels.

However, 48V is a much more common voltage bus for electric marine systems and is also below the 60V max that ABYC specifies for low voltage DC electrical hookups without getting into high voltage territory. There are also many more marine chargers and accessories (inverters, lights etc.) designed to run off 48V than off 72V.

Batteries and Battery Capacity

One thing we have learned both with ebikes and with eboats is that you can never have too much battery capacity. The more energy on board, the more at ease you can be about having sufficient reserve for whatever planned or unplanned events come up. We feel that many electric boat projects skimp out in this regard and often have like 6-8 kWhr battery banks. That is enough for short trips and certainly gets you in and out of harbour with lots to spare, but it won't do much at all if you have a day to travel with unfavorable winds.

In our initial installation we had 18 kWhr of battery capacity, but had a few longer trips where this was barely sufficient. Upgrading to 24 kWhr plus a solar charging array finally hit the sweet spot for us.

Unfortunately large lithium batteries are expensive. LiFePO4 batteries for marine use normally retail for $600-$1000 per kWhr. In order to facilitate the battery sourcing we have brought in a supply of budget priced 1.7 kWhr LiFePO4 packs made from 33650 cylindrical cells, in both 48V 36Ah and 72V 24Ah nominal sizes. These have 100A BMS circuits and are terminated with dual Anderson SB50 ports for easy parallel connection. But they are not encased in a full waterproof plastic or metal enclosure like commercial batteries and that aspect of mechanical protection is left for the user.