Solar Cruising Part 4 (DIY?)
So maybe I should consider a DIY electric-propulsion project...
View the other articles in this Solar Cruising series here, here and here.
The best news for solar cruising is the wider availability of high-quality, high-power, lower-cost lithium-ion batteries and solar modules, a trend driven by the rapid growth of volume manufacturing for residential, RV, and marine “house battery” applications. But here’s the bad news: electric outboards are still quite expensive, and there is no reason to expect that their prices will drop. So maybe I should consider a DIY electric-propulsion project…
For an older inboard boat in need of an engine replacement or overhaul, converting from gas or diesel to an electric motor and battery bank seems like a reasonable choice, especially after the recent run up in fuel costs. It turns out that Thunderstruck Motors has been in the electric propulsion business – for both cars and boats – for more than 25 years, and there's a lot of useful technical information on their website. To replace an engine up to about 15 horsepower, they offer a 5-kW Sailboat Kit that includes a permanent magnet AC (PMAC) motor, a solid-state “golf cart” power controller, and almost all the necessary wiring and controller programming.
The Motenergy ME1718 motor in the kit is brushless and sealed, rated IP65, and weighs 24 pounds. It also has a temperature sensor to protect it from overheating, and it’s rated at 10.5 ft-lbs of torque at maximum power. While that might be enough to directly drive the prop, a gearbox or a toothed-belt speed-reducer rig from Thunderstruck Motors can reduce the shaft speed and multiply the torque. And as a general rule, a bigger prop turning at slower speeds increases overall efficiency. The cost of the Thunderstruck Motors kit with a 2:1 speed reducer is around $3,000. Ouch! But when you watch their wiring video and see all the wiring harnesses and controller programming required to do a complete job, it starts to look pretty reasonable.
With improving supply chains, the prices for lithium-iron-phosphate (“LFP”) batteries are hitting new lows, and this trend should continue as they displace lead-acid batteries. “Drop-in” LFP batteries are selling for under $300/kWh in mid-2023, and framed solar modules cost less than $1/watt. So it's feasible to assemble a complete electric propulsion system with 10 kWh of battery storage and 1 kW of solar charging for around $7,000. Pricey, yes, but since a 6-kW Torqeedo Cruise outboard – not including the battery or any solar equipment – lists for $4,900, a Thunderstruck Motors conversion still looks like a fairly economical solution for powering a solar cruiser if you’re willing to invest some sweat equity.
Although inboard-powered electric conversions have been the norm, converting a gas outboard to electric power is a tempting DIY project for a smaller boat, especially if you can procure an outboard in good shape that is in need of a new powerhead (go to any Antique Outboard Motor Club meet to find a cheap donor). If you want “proof-of-concept'' evidence that it can be done well, Elco Motor Yachts sells electric outboards that are factory conversions built with the same drive legs used for gas outboards. The 6-kW EP-14 Elco outboard – without the battery, controls, or cables – lists for $4,950. Recently Mercury Marine introduced a small electric outboard, the Avator 7.5e (“equivalent to a 3.5-hp Mercury outboard”), that sells for around $3,500. It has a 750W motor, uses a plug-in 1-kWh battery and looks like a mini version of any other outboard in their regular lineup; and Mercury has promised that there will be bigger electrics in the near future.
Considering the prices of these off-the-shelf electric outboards, converting a mass-produced, reliable outboard from gas to electric might be a good way to save a few bucks. And for some of us, an electric outboard that looks like an outboard is very appealing (not that I don’t appreciate the performance of the Torqeedo or ePropulsion outboards). Therefore, it’s surprising that there is very little information about DIY electric outboards on the internet, in spite of plenty of “good old” donors. Charles Mathys described his conversion of a small Evinrude outboard in My Electric Boats. He chose a 5-hp Johnson/Evinrude outboard leg based on its gearbox transmission ratio; and since it was 25 years ago, he used an old golf cart motor that weighed a hefty 48 pounds. With today’s lighter, high-torque motors, I think that building a bigger, higher-power electric outboard now has a lot of potential.
However, for the slippery hulls that make good solar cruisers, there’s really no need for much power. Roughly speaking, a 3-kW electric motor could easily repower a 6-hp gas outboard. And a 20-hp gas outboard might be a good candidate for a 10-kW electric conversion. Although the outboard’s original prop can be reused, a bigger than stock prop is even better for slower, power-efficient cruising. Torqeedo’s small “Travel” motors, for example, have propellers that are 10” in diameter; and ePropulsion’s “Spirit” outboards have 11” props. But finding bigger props that will mate with a vintage outboard’s prop shaft can be a hassle, especially for smallest outboards. As a result, converting a higher-power outboard – and using its larger diameter, higher pitch prop running at lower speeds – might be a better strategy. And it turns out that there are usually more propeller choices for the higher-power motors.
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