Three key factors determining energy efficiency

In my previous post, I touched on the challenges of electrifying planing vessels since they are inefficient and highly weight sensitive. To build cost-effective and longer-range electric boats, we need to make them more energy efficient. But - what makes one boat use less energy than another?

In this post, I highlight the three key factors determining the efficiency, and required power, for planing boats: 1) vehicle/hull resistance, 2) drivetrain efficiency, and 3) vehicle weight. Hull resistance and drivetrain efficiency are simply what is holding the vessel back and what is pushing it forward respectively – which are equal if we have no acceleration. I have deliberately added weight as a third factor, as it is so critically important for electric vehicles operating “not on solid ground”. You could write a PhD about each of these topics – here short.

1) Hull efficiency

Vehicle resistance is what is “pushing back” against the propulsion power pushing forward. This resistance is often called drag and can be decomposed into several sources – including air resistance and water resistance. For boats, water resistance is far more significant than air as water is roughly 1000x denser than air. There are three main classes of hull architectures: conventional hulls (monohull or multi-hull), hydrofoils using foils to lift the entire hull out of the water, and surface effect ships (SES) using air to separate the hull from the water. Hydrofoils and SES vessels have around 50% (plus minus) higher energy efficiency than conventional planing hulls, everything else equal. None of these technologies are new, as they have been around in various shapes and forms since the Second World War. Due to the increased need for zero emissions solutions, we do expect to see a “renaissance” of hydrofoil and SES solutions – which evidently is the case. Being the CEO of Pascal Technologies, I argue that our AirHull solution has clear benefits in efficiency, robustness, cost-effectiveness, and scalability across boat builders – which will make it a clear enabler and winner in the space.

2) Drivetrain efficiency

By drivetrain efficiency, I mean the ability of the vehicle to convert stored chemical energy into push-forward motion. Aside from the classic case that combustion motors inherently are inefficient compared to electromotors, several factors are important to end up with a “non-thirsty” drivetrain including motor/motor controller optimization and mechanical transfer. What sets boats aside from cars are propellers – which typically have efficiencies of only 0.5-0.7 (depending on the case) – significantly affecting the overall drivetrain efficiency. Additionally, the force often needs to be transferred from the generation source inside of the vehicle, through bends down underwater - with larger losses. There is even a large variety of propulsion types for boats – including inboard, outboard, shaft, pods, and water jet – with their pros and cons for different applications. Good old “engineering” is needed to make good and energy-efficient drivetrain solutions for the future needs of the industry.  As hulls become more efficient, drivetrain losses make out a larger share of total losses, making it increasingly relevant to improve this area.

3) Vehicle weight

Perhaps the sneakiest factor about vessel power and efficiency is vehicle weight. It is often said that the power needed for planing boats is roughly proportional to weight – that means that a 10% increase in weight equals a 10% increase in power. This is quite true in general – and good enough for many comparisons, but there are of course non-linearities and large variations for specific cases. What is clear is that weight is extremely important when talking about energy efficiency. There are many factors affecting weight – and important ones are using lightweight materials, properly sized propulsion systems (specifically batteries), and payload. What is clear is that proper light-weight engineering from the early design stage can have huge effects. Boat weight even has an impact on the seakeeping – where a heavier boat will feel more stable than a lightweight one. For these reasons, many fossil boat producers tactically place weights in the vessels to make them better in waves. Additionally, boat builders often are incentivized to over-engineer structures to reduce the risk of having failures and warranties to deal with. For these reasons, the fossil industry is typically not very cautious about minimizing vehicle weights – but in the future, this will be more important.

Conclusion

So, when it comes to building energy-efficient planing boats, it is all about their hull efficiency, drivetrain design, and how much they weigh. At Pascal Technologies, we are working with all three dimensions of the problem – resulting in cost-effective boats with longer ranges. We are on a journey to make boats that are not simply good for the sea but good for our planet too.

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