Electric Boat Propulsion 2024

We’ve all started to notice fellow boaters carrying portable electric dinghy outboards around the marina as more and more small-boat owners decide that these impressive little workhorses are perfect for their needs. Everyone I ask responds with enthusiasm about their switch to electric. That, plus my own research, and experience with gasoline outboards, has made me consider switching to electric for my boat’s inflatable dinghy. Here’s why.

The electric outboard has advanced rapidly with new innovations in battery and motor technologies. You now can find them on everything from tenders and dinghies, kayaks, and many other small boats. Even daysailors are finding electric outboards a viable option to traditional outboards, depending on how they use their boats.

Near-shore lake and freshwater fishing is, in particular, seeing more and more of the electric outboard as a primary propulsion source. Silent running is an advantage when traveling to a favorite fishing location, and silence afloat is certainly addictive. In an era of sustainability, carbon neutrality, and low emissions, for many boaters electric propulsion may offer a great option.

Battery power

One of the catalysts for marine electrical propulsion has been the trickle down of strides in battery technology. We’re familiar with the old lead-acids, gel-cell, and other earlier batteries. But as a rule they were deficient in one way or another for powering outboards. In the recent past we’ve been hearing about lithium-ion (Li-ion) batteries that seemed much better suited for this and other uses. Also we’ve seen the lithium-iron phosphate (LiFePO4) batteries that are more stable than the lithium-cobalt (LiCoO2) or lithium-manganese (LiCoMn2O4) varieties. And LiFePO4, like the LiCoO2 or LiCoMn2O4 varieties, has a high energy density, although not as high as these other chemistries.

Battery choice is seldom simple. Let’s take a look at the batteries and outboards, keeping in mind that we can’t give you guaranteed advice about which is right for you, as this technology is in a constant state of development, and there are many variables depending on your needs. But we can give you lots of general info to help if you’re thinking of taking the leap.

Different battery

types Lithium-cobalt or lithium-manganese batteries consist of different chemistries for the cathode: lithium manganese oxide or lithium cobalt dioxide. Both have similar characteristics of very high energy density and are susceptible to autoignition and instability at high temperatures. Both have a graphite anode. The high energy density is perfect for small electronics with high energy demands like smartphones and laptops.

Lithium iron phosphate (LiFePO4) has a cathode of iron phosphate and an anode of graphite. It is more stable, not susceptible to autoignition, and not as prone to thermal runaway at high temperatures.

The discharge rate for LiFePO4 is better than that of other Li-ion types. LiFePO4 batteries have voltage discharges that are excellent when at higher temperatures, and the discharge rate doesn’t significantly degrade the LiFePO4 battery as the capacity is reduced.

LiFePO4 has a life cycle of 1,000 to 10,000 cycles. These batteries can handle high temperatures with minimal degradation. They have a long life for applications that have embedded systems or need to run for long lengths of time before needing to be charged.

The higher energy density and weak oxygen bond make LiCoO2 and LiCoMn2O4 varieties more unstable, especially when dealing with higher operating temperature environments such as when doing high discharge and charge rates in an electric-propulsion application. Each type has a life cycle of 500 to 1,000 cycles as it can be negatively impacted based on the operating temperature of the electronics or working components.

With all of this, consider what the battery manufacturer advises about best use, and do your own homework to see if this technology is right for you.

Battery comparisons

Currently, lithium batteries are still on the pricey side when compared to older nickel-metal-hydride (NiMH) and nickel-cadmium (NiCad) batteries. Yet, the long life of lithium batteries may equal out the initial cost. LiFePO4 may also be the cheaper alternative because of shelf life and the fact that the chemistry is safer; this can mean manufacturers don’t have to spend more money to recycle and store the materials.

Boaters must consider the operating temperature, ambient temperatures, and other environmental factors including vibration and maybe even pounding, length of running time, number of times the battery can be recharged and still be good, ease and time of charging, and other issues. It’s generally the consensus that the chemistry of LiFePO4 is superior to that of other Li-ion types for small outboard motors, particularly when you look at stability. A table of simplified characteristics (below right) shows attributes of each type of battery.

Storage considerations

Both LiFePO4 and the LiCoO2 and LiCoMn2O4 varieties have good long-term storage benefits. LiFePO4 has a shelf life of approximately 350 days. The shelf life of the LiCoO2 and LiCoMn2O4 varieties is roughly around 300 days.

Both types of batteries are rechargeable and already have a relatively long lifespan. LiFePO4 batteries are even longer-lasting because they’re more stable under the conditions of overcharges or short circuits. The LiCoO2 and LiCoMn2O4 varieties have a cycle durability of between 400 and 1,200. The cycle durability is around 2,000 for LiFePO4 batteries.

Safety advantages of LiFePO4

We’ve all heard of battery combustion and even explosion. There have been improvements since then. LiFePO4 has good thermal and chemical stability. This battery stays cool in higher temperatures. Normally, it’s also incombustible if mishandled during rapid charges and discharges, or when there are short-circuit issues. LiFePO4 doesn’t normally have thermal runaway, and the phosphate cathode will normally not burn or explode during overcharging or overheating as the battery remains cool.

Another safety advantage of LiFePO4 involves battery disposal after use or failure. A LiCoO2 battery is considered a hazardous material as it can cause allergic reactions to the eyes and skin when exposed. It can also cause severe medical issues if swallowed. Both LiCoO2 and LiCoMn2O4 varieties have materials than are harmful to the environment so special disposal considerations must be made. On the other hand, LiFePO4 is nontoxic and can be disposed of more easily by manufacturers and recycling facilities in the U.S.

The chemistry of LiCoO2 and LiCoMn2O4 batteries doesn’t have the same safety advantages as LiFePO2. Their high energy density and weak oxygen bond have the disadvantage of causing the battery to be unstable. They heat up faster during charging and at high temperatures can experience thermal runaway.

Does this mean that LiFePO4 batteries are better than LiCoO2 or LiCoMn2O4 varieties? Not necessarily. LiCoO2 or LiCoMn2O4 batteries have the highest energy density among rechargeable battery types in the market, meaning charging is relatively easier and takes a shorter time. A LiFePO4 battery also has a good density but, generally speaking, has less energy density than LiCoO2 or LiCoMn2O4 batteries. Not all batteries are good for each use though; for some applications such as smartphones, laptops, watches, and other compact devices that need to emit power over long periods, LiCoO2 or LiCoMn2O4 types may be better. For applications where more space is available, the longer life and safer LiFePO4 will generally make more sense.

What is the trend?

Today’s trend with electric outboards is to often use LiFePO4 batteries due to their greater thermal stability. However, some manufacturers may still use batteries with nickel, manganese, or cobalt elements, which can be quite volatile when overheating and have environmental disposal issues. Make yourself knowledgeable about which technology you’re buying.

Many smaller dinghy outboards have removable portable battery packs. You can buy two or more and easily replace the used with the fresh one. You still have to keep them charged from a charging source, which may involve a special charger. If you need to connect two or more, you may also need a power center. The expense can add up.

Always exercise care when recharging any type of lithium battery pack and carefully follow instructions. This may seem like obvious advice, but it can’t be stressed enough, even for more stable batteries.

Larger outboards such as the Mercury Avator 35e also have removable battery modules; you can carry a spare for extra power and for range extension. You still often need a special charger. A 9.9-hp may cost around $5,000 and a battery around $2,000, depending on features and options. For example, if you have more than one battery and other users, you may need a power-center integrated hub that distributes power from the battery bank throughout the system. It can also simultaneously charge up to four connected batteries from a single charging port. (See mercurymarine.com/us/en/engines/electric/avator/avator-20-35e)

As boat size grows, so does your base power requirement. Battery-use monitoring is very important and an integral part of today’s electrical outboard propulsion systems. Many outboards incorporate easy-to-read displays that show throttle positions, current demands, and remaining battery capacity. Many also have phone apps providing the same valuable real-time data – important when optimizing both range and speed to minimize power consumption.

Battery recharging

Recharging batteries is always at the forefront of electric-boat considerations. There are many battery chargers available that allow fast charging of battery banks, and it’s advisable to use the designated charger that’s recommended by the battery manufacturer. Larger boats do have more options with renewable battery charging solutions when out on the water, including the incorporation of flexible solar panels integrated with the bimini and portable fold-up solar panels. If you use a portable generator or your outboard to charge the battery, this requires gasoline. If using on-board chargers on a larger boat, you may be able to charge your portable outboard’s batteries if voltages and charge profiles match manufacturers’ specifications.

Many users are placing larger capacity LiFePO4 battery sets in their dinghies, pontoons, or bass boats to power their electric outboards using third-party batteries. If using a lithium battery house bank in your boat or in your small boat/dinghy for electric propulsion, be aware that charging devices will need to have settings adjusted for the lithium battery manufacturer’s specifications, and if such adjustment is not available, you may need to replace the charger. And you should have a multifunction battery monitor installed, more than just a voltmeter, to properly monitor the status of your lithium battery bank. Also, if using an alternator to charge the lithium battery bank, note that special steps need to be taken to protect the lithium batteries and to protect the alternator. Consult an expert for these steps, or have the installation performed or inspected by an ABYC-certified electrical technician.

Range anxiety?

Autonomy refers to range – how far and how long can you go on a fully charged battery module. Worry about this is sometimes referred to as “range anxiety.” Simplistic ratings are based on the potential distance, factoring in electrical power consumption. While the manufacturer’s brochure or user manual can provide a baseline number, on-water realities can be quite different depending on conditions.

We know what speed we want and where we want to go, but once we factor in tidal flows and currents along with adverse wind and wave conditions, that manufacturer’s predicted range can quickly diminish. Fortunately, smart monitoring reduces some of those anxieties as we can watch real-time performance information as we go along and adjust our speed or current draw accordingly. For example, ePropulsion’s Spirit 1.0 Plus published data clarifies the relationship between speed, run time, and range:

Electric propulsion now makes sense for a larger category of boaters than ever before – anglers who enjoy local fishing, families and friends who like to go out for a fun day and return to the dock, trailer-boat owners who enjoy freshwater lakes. If, like me, you want improved reliability and near silence when out on the water, it’s worth considering an electric outboard. Today you’ll find an impressive range of viable options.