How do you design an electric boat? Here is the lesson of the “guru”


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“The numbers speak for themselves,” Japec Jakopin tells me at METS in Amsterdam (the world’s leading nautical accessories trade show), “those who use boats for pleasure today do not do so with long sailings but for short day trips. This explains the American success of ‘pontoon boats,’ or Malibu’s 22- to 26-foot models (which sold $684 million worth of boats in 2019, ed.). And that is why full-electric propulsion, which is ideal for day boaters, is not the future, it is the present.”

electric boat
Japec Jakopin (source

With Jakopin, who is the CEO of the renowned Slovenian firm J&J Design and who designed, among other things, the Greenline line of hybrid boats, we tried to understand what it means to think from scratch about an all-electric boat.

“If you retrofit an electric motor on a boat, instead of a traditional endothermic one, you will never achieve maximum efficiency. A boat that wants to mount a full-electric propulsion needs to be designed, right from the start, with the idea that it will be precisely electric: there is no escaping from this principle.”

It starts with the classic “design brief” where the designer must ask “What will be the owner’s use of his boat? Will it have a displacement or planing hull? How long will it be, what will be the weight to set as a ‘goal,’ what construction technology should be relied on?” And again, “What will be the sailing speed, the maximum performance required, the range of motor use? What kind of electric motor to install, with what batteries and what charging system? What other features will be required?”

Most importantly, “How much will the boat cost? You can design the best electric boat in the world, but if the initial or usage cost is too high, you won’t go anywhere.” You will make a nice prototype, and what has been seen has been seen.

Let’s take a 10-meter long boat with a sailing displacement of about 4 tons: “Energy storage and management is the main obstacle and to achieve an acceptable range (ideal for daily use of the boat, in sea conditions with waves of less than two meters: with rough seas and strong headwinds the range obviously decreases) we will need to limit the glide speed to around 20 knots“.

The decision to set the glide point at 20 knots goes through the choice of hull. Which form will provide the best performance? “The options are many: a V-shaped hull, which in its ‘deep’ configuration is used for fast boats, optimized for planing at lower speeds, thus with a wider hull; or a ‘stepped’ hull, which in glide offers less surface area in contact with the water; the one with a hybrid geometry which provides a variable waterline beam.

The various forms considered:

Or we could opt for a catamaran, perhaps equipped with submerged foils that increase the lifting effect by decreasing friction on the water but without making the boat fly. the most extreme solution is the full-foiling boat, which sails detached from the water.”

If you look at the graph above, the power, expressed in kilowatts, required for a 4-ton boat to reach 20 knots varies, even by a lot, depending on the shape of the hull: range from just under 50 kW for a flying monohull to over 110 for a stepped hull.

In the chart below, however, we see the range: sailing at 20 knots (80 kilowatt hours of power) the very low friction full foiling hull wins hands down, with over 30 miles: traditional planing boats have a range of about half that.

electric boat

Does this mean that an electric boat with foils will be far better than one with a traditional hull? That’s not quite right: there is the so-called “load tolerance” to keep in mind: “differently shaped hulls react differently, in terms of friction, as the load increases,” Jakopin explains.

“If a boat with a deep-V hull copes well with loads, increasing friction only slightly as the weight on board increases, the same cannot be said of the full foiling hull, which could vary its performance drastically-and for the worse we add-when weight increases. This is why prior calculation of weights is essential, depending on the type of hull chosen.” For the record, the hulls most tolerant to load increase are deep-V hulls, followed by hybrid-V hulls, V hulls with steps, catamarans, cat hulls with submerged foils, and finally the very sensitive full-foiling hulls.

Now we come to the thorny issue of weight: the weight savings/cost ratio changes between electric and endothermic-powered boats. Some considerations are necessary here: “Weight savings improve efficiency, and this is much more important in a full-electric boat. If in a diesel-powered hull, implementing horsepower to compensate for higher displacement is not cost prohibitive, the same cannot be said for an electric boat: more weight to move means more lithium batteries to have on board and much higher costs. If they are too high, the project, as mentioned, becomes impossible to sustain.”

According to Jakopin’s calculations, “Reducing the weight by one kilogram, on a boat with an endothermic engine, translates into an economic saving of about 10 euros: one kilogram less, on board a full-electric, can correspond to a saving of even 400 euros! Understand well that making a lightweight boat is crucial when you are talking about electric!”

This opens the chapter of materials: on the one hand there is the classic hand-laminated polyester fiberglass, which is inexpensive but heavy. Infusion vinylester fiberglass is more expensive, but it is lighter. Then there are composites (epoxy-carbon), currently the lightest and strongest materials available on the market. “The latter technological solution is the one to choose when it comes to building a planing electric boat: less weight equals less friction. Less friction means fewer batteries and lower costs.”

We anticipate your final question. Is it worthwhile, in a nutshell, to choose to rely on electric motorization? “The recovery of expenses from an initial investment that is certainly higher is there: it is not huge, but it is there. Between the cost of fuel and electricity, there is a ‘delta’ of 2.6 euros per mile traveled in favor of full-electric propulsion. Considering also the additional maintenance costs involved in an endothermic engine, the savings are around, at least, three euros per mile. This translates into 3,000 euros recovered for every 1,000 miles driven and 5,400 kg less carbon dioxide released. Then you will forget about maintenance, noise and bad smells on board. But the main benefit of the full-electric system goes, however, to the environment.”

Eugene Ruocco

A (dutiful) postscript. A separate chapter would merit choosing the right propulsion: what system to rely on (inboard, outboard, hydrojet), what batteries to choose and where to put them, deciding whether or not to use other renewable energy sources (solar panels) or “betray” the cause of full-electric with a back-up generator to increase range. But we promise to tell you about that shortly.



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