The journey from horses to watts is racing at an electrifying gallop. Jamie Marley, an expert in the field of marine electrification, discusses the evolution of power measurement and propulsion in the boating world.

The evolution of transport in general is a testament to human ingenuity. From horses to steam, and then to the internal combustion engine, each method of propulsion has had its moment in the spotlight before being replaced by its successor. However, one form of power has stood the test of time: electrification.

The first mass adoption of electric passenger vehicles occurred in the early 20th century, thanks in part to Henry Ford. Despite the initial popularity of electricity, the discovery of the potential of oil led to its temporary eclipse. Ford, who built the first production electric cars, eventually shifted his focus to the benefits of oil. The internal combustion engine allowed Ford to produce cars at a lower cost, enabling their mass adoption.

Portrait of James Watt (1736-1819) © iStock-Luisa Vallon Fumi

Cornish jargon

Interestingly, despite the rarity of horses as a power source today, we still compare the power of modern engines and motors to that of horses. This brings us to the terms ‘watt’ and ‘horsepower.’ Most associate ‘watt’ with light bulbs and electricity, and ‘horsepower’ with the internal combustion engine. These terms, originating in 1760, were coined by Scottish inventor James Watt. He used a horsepower rating to help Cornish miners understand that the power of one of his newly invented steam engines was equivalent to 10 carthorses. In essence, ‘horsepower’ is Cornish marketing jargon from the 1760s!

The terms ‘wattage’ and ‘horsepower’ have been used side by side since the age of steam. Wattage is an exact metric measurement of the rate of energy transfer, while horsepower is a standard value at which work is done. Marketing, tradition and familiarisation are the only reasons it is still used to this day.

Modern internal combustion engine outboards are always referred to by their horsepower rating, which is the peak horsepower generated at a near top-end RPM. As we increase RPM, we generate rotational torque, which is great for turning larger propellers, giving us thrust and efficiency. Nowadays, with electric vehicles, e-bikes and e-scooters all around us, watts (W) and kilowatts (kW) can be commonly seen in their specifications. This enables us to make a fair comparison, especially in the case of electric propulsion for boats. Electric propulsion manufacturers help us understand the amount of power their product can deliver by giving an equivalent horsepower rating beside the kW. Torqeedo have gone one step further by rebadging their Deep Blue 80 outboard, which describes it as being an 80-horsepower equivalent to the newly named Deep Blue 50, which now refers to the 50kW continuous drive motor used. Similarly, Mercury badge their Avator range in kW not HP, the 75e being 7.5kW and so on. Evoy display HP on their cowl, with the 300+hp being 225 kW (450kW peak). Going forward, buyers will likely be unsure as to whether what is displayed on the cowl, means HP or kW!

2024 Yamaha F350NSA ICE outboard engine

2024 Yamaha F350NSA ICE outboard engine

Reading the signs

When diving into the specification, it is worth reading the fine print, as motor wattage and kilowatt ratings might be listed as ‘continuous’ and ‘peak’: ‘continuous’ means power can be delivered all day long, and peak refers to seconds or minutes. An electric motor power rating should be publicised at its continuous rating. For example, a 50kW continuous rating should be followed by its peak output rating, which would be 61.8kW in the case of Torqeedo’s Deep Blue outboard. The best part is that electric drive motors can generate torque at low RPM, so you now have the luxury of swinging a larger propeller to efficiently move your boat through the water.

Electric motors also have sweets spots in their operational RPM/torque ranges, and which in the ICE [Internal Combustion Engine] might have been referred to as a ‘power band’ in the past. The drive motor will be specified to enable the optimum efficiency at the desired output. This means the motor is running at its optimal torque and RPM to give you, the user, the power at your boat’s working speed.

Suzuki Kuro fast revving ICE outboard engine

Suzuki Kuro fast revving ICE outboard engine

The specified RPM has a big effect on the motor’s size and weight. A 100kW 900rpm motor weighs in at around 465kg, while its 2500rpm equivalent weighs over 50% less at just 200kg. The reason for this is that the lower the motor’s rated RPM, the heavier the materials used in the design of the motor. Electric motors do give a small weight saving: comparing 80hp versions, the Mercury 80 4-stroke is 17kg heavier than its electric equivalent. Most outboard propellers are rated to rotate at between 2400 and 3400 rpm converted from a lower unit gear ratio from an ICE engine that typically maxes out at around 6000rpm.

Today, electric outboard manufacturers inherit their lower gearcase units from the ICE world, which for the most part only serves to facilitate access to the COTS propellers available on the market today. One of the reasons you will not find any low-RPM outboard motors today is that electric outboards typically use ICE lower gearcase units, which reduces the possibility of fitting oversized propellers.

Avator 75e electric motor, 7.5kW

Avator 75e electric motor, 7.5kW

More torque

Since torque is a large, efficient propeller’s best friend, it’s worth remembering that if our motor can develop the necessary torque to swing a large propeller, we’re winning – as long as you don’t need to go fast! Speed requires the propeller to rotate faster, so we need RPM. For example, a 100kW motor designed to rotate at 900rpm will develop over 1000Nm of torque, and a 100kW motor rated at 2400rpm will produce approximately 400Nm of torque, with the latter being great for big props and slow displacement vessels, and the former being better suited to faster watercraft with smaller-diameter props.

Torqeedo Deep Blue electric motor

Torqeedo Deep Blue electric motor

Although physically most electric outboards resemble the historic norm, as the transition progresses, we are starting to see manufacturers becoming bolder with their designs. Steerable gearcases are currently only seen in the 600hp ICE world, but RAD propulsion has a 40kW steerable gearcase drive, giving the user 180 degrees of steerage. Its low-profile design enables a novel approach to transom design, allowing the trend to help naval architects to embrace the benefits this new product offers.

Finally, it’s pretty much a foregone conclusion that the ongoing focus on efficiency will result in yet more solutions emerging to specifically aid the use of propellers to suit the torque, power and RPM of electric outboards. And as technology continues to advance and evolve, while we are unlikely to forget the horses, we are certainly going to be embracing the watts.