What Size Electric Motor Do I Need For My Boat?

Electric Motor

Electric motors are taking the fishing and boating world by storm. What started as a niche product for bream and freshwater fisherman has quickly become a masthead of the boating community, and it’s easy to see why. Electric motor technology has taken off in recent years—both on the water and on the road—and these developments have improved the reliability, price, and features of electric boat motors. 

Understandably, they’re hot property, so we’re often asked “what size electric motor do I need for my boat?” In fact, we’re asked so often that we decided to put together this list of tips. Despite improvements in price, electric boat motors are still a big investment and it’s important that the one you choose is the right fit for your boat.

Electric boat motor sizes

Petrol and diesel boat motors are measured in horsepower and kilograms, which is their first difference from electric motors. Unlike their fuel-powered counterparts, electric boat motors are measured in pounds (lb) of thrust, which is an unusual measurement by Australian standards. 

Luckily, there’s an easy trick to convert pounds of thrust into the right size electric motor for your boat: simply apply a minimum of 2lb thrust to every 100lb of weight in your fully-laden boat. To put that into perspective, one pound is equal to roughly 0.45kg. So, if you have a fully loaded boat that weighs 600kg, you can work out the right size electric motor with this formula: (600 x 2.20462) ÷ 100 = 13.23 x 2 = 26.45lb of thrust. Of course, that’s only a minimum figure – there may be situations that call for a little more power.

Think about purpose 

Purpose is an important consideration when you’re working out what size electric motor to get. If you’re using your electric boat motor to take the dinghy out on the dam and set some yabby pots, then you won’t need a lot of power at all. You’ve got a light boat, and you don’t need to take it far, so why not save some money and opt for the smaller options? On the other hand, if you’re using your electric motor to hold your large offshore boat over a deepwater mark, then you’ll need to look at the top end of the scale. 

Offshore conditions come with fast currents, big waves, and strong wind, which is a lot for many standard electric boat motors to contend with. After all, they were developed with much less demanding conditions in mind. So, if you’re planning to use your electric motor offshore, then you should choose one with as much power as possible, and also with spot-lock compatibility. Spot lock will use GPS technology to propel the electric motor and keep your boat stationary without an anchor. Choosing the right amount of thrust will ensure that the power is there to back up the GPS technology.

Saltwater and freshwater applications 

Aside from currents, wind, and towering waves, the ocean has another danger: corrosion. Saltwater is punishing on almost all forms of metals, and that includes the ones you’ll find in your battery connection and electric motor body. Of course, it’s all perfectly designed to withstand those conditions, but they still require frequent maintenance, and it’s important to keep them as dry as possible. Choosing the right size electric motor will help. If yours is too small then the housing may end up a bit too close to the water, which will leave it susceptible to splashing and even inundation if a larger wave comes along. 

Choosing a larger motor, with an appropriate mount, will give it that extra height and hopefully keep it clear of unnecessary contact with saltwater.

Battery power 

Getting a powerful electric motor for your boat is going to be a waste of money if you don’t have the right battery to power it. Electric motors draw a lot of power to operate, and it goes without saying that the more powerful models require the most power. So, before you choose, think about whether your boat’s battery setup can accommodate the electric motor you want to get. If not, you’ll need to make some changes. All electric boat motors are configured to operate off battery power, which means that any models that draw more than 12V of power will require more than one battery. 

As a general rule, 12v will accommodate 55lb of thrust, which is enough for most boats. However, if you exceed 55lb of thrust then your motor will be capable of drawing up to 24v under load and it will need two batteries to run. Anything over 101lb of thrust is likely to require three batteries, which equates to a respectable 36v of power. 

Weight considerations

Now that there’s a chance you’ll be carrying three batteries to power your new electric motor, you need to think about weight. Batteries are heavy, and they can have serious effects on the balance of your boat if they’re placed incorrectly. It’s also important that they are stored well away from your fuel, to avoid potentially explosive situations. As a result, your possible storage space is limited and it’s always best to keep the batteries close to one another. So, does your boat have a reinforced space for up to 90kg of battery? If not, you may need to think about choosing a smaller electric motor for your boat, or having a reinforced storage space fabricate.

Manoeuvrability 

Manoeuvrability is another key factor in choosing the right size electric motor. Larger ones can require a lot of clearance, both above and below, which means that they’re less suitable for close quarters fishing among snags, pylons, and mangroves. They can also be heavier, which means that they can weigh the front of your boat down enough to cause trouble if you are fishing in skinny tidal waters. That means inshore lure fisherman often tend to choose smaller electric motors and smaller boats. On the other hand, smaller motors are less suitable for long distance applications, which means that trollers and spin fishermen usually prefer larger, more powerful engines that can maintain slightly higher speeds over longer distances.

Bow height and mounting 

Finally, let’s look at mounting options. The most popular is a bow-mount, which is suitable for electric boat motors of almost all sizes. However, it can restrict the size of your electric motor depending on what sort of boat you have, and the space you have available for a proper mount to be installed. It’s rarely a matter of bolting a plate straight on – for aluminium boats there’s usually some welding involved, and fibreglass boats require moulding work. 

If your boat doesn’t have the space for a secure and sturdy mount to be installed, then you’ll need to limit the weight of your new electric motor, to avoid damaging the bow of your boat. 

Bow mounting also requires you to think carefully about the length of your electric motor: will it be able to submerge the propellers enough to work without cavitation occurring? 

Remember, bows sit much higher than sterns so there’s far more distance between the water and a bow-mounted electric, when compared to a standard transom-mounted outboard. 

Enjoy this article? Check out our Outboard Motor Buying Guide for 2021.

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