How to make electric propulsion possible
The key challenge when designing a boat with electric propulsion is to minimise the energy needed as much as possible. Normal boats are, compared to cars, extremely inefficient. They need lots of energy to move. Therefore, all electric boats on the market are either very slow (< 7 kn) or have an insufficient range at planing speed (< 15 NM). To achieve a good range at a good speed, every aspect of the energy losses needs to be worked on. To put it simply, the total energy consumption can placed in five main buckets:
- Propulsion losses
- Hydrodynamic losses – energy used to accelerate water in other directions than backwards. The wake build up is a simple indicator of a boat’s hydrodynamic resistance. When the boat is making big wakes, it consumes a lot of energy.
- Aerodynamic losses – same as water but in the air.
- Drive train losses - heat generation in inverter, motor, gears and bearings.
- Ancillary losses – electronics, 12V loads, lights etc.
Compared to a wheel, using a propeller is a fairly inefficient way to push a vehicle forward. But there is not much one can do about that. Propeller is basically the only method at hand. The most efficient propulsion is an outboard or a stern drive set-up. A shaft drive is less efficient. But still, you lose some 30-40% of the energy at the propeller axis when converting it to a water thrust. The easiest way to get as high efficiency as possible to to have a rather large but slow propeller. So at Candela we will reach around 30% losses.
In a conventional boat, hydrodynamic losses represent more than 80% of the total losses. So this is the key issue that needs to be solved when reducing the energy consumption. This is where hydrofoils come into play. Almost any hydrofoil design reduces the hydrodynamic losses more than 50%. The reason is that the upper side of a wing generates two times more lift than the lower side. On a normal hull you only have the lower side of the wing. See the force diagrams below. "D" for drag is the one to make short here.
There are two main types of hydrofoils: surface-piercing and fully submerged.
The surface piercing hydrofoil configuration has the advantage of being self-stabilising. When the boat flies high, the wing area is reduced through less of the V-shape being in water. Tipping to the side, roll, increases lift on that side thus tending to right the vessel.
The advantage with the fully submerged type is that it is more efficient and it does not get affected by waves, i.e. you get a smother ride. However, the fully submerge type needs some kind of control system to regulate height over water surface as well as the roll angle. If not, the boat will either fly out of the water, fall into it, or, due to lack of roll stabilisation, fall to one side. Candela is using a fully submerged hydrofoil type with a unique system for sensing and controlling height, pitch and roll.
Furthermore, the aft strut containing the drive shaft can be made thinner than a traditional combustion engine stern drive. We have no need to leave room for cooling water or exhaust pipes. It is all electric!
In a conventional boat moving at moderate speeds, the aerodynamic resistance is not adding very much to the overall energy consumption. However, in Candela, with an extremely low hydrodynamic resistance, aerodynamics become significant and account for some 15% of the overall losses. Therefore, we have gone to great lengths to minimise the air drag. Using design patterns from racing boats, the aerodynamic drag is minimised a lot. Letting air to flow in-between the hulls, a tilted wind shield and a rounded aft - all measures contribute to lower drag.
Drive train losses
The typical efficiency of a combustion engine is some 30-35%. The remaining 65-70% of the fuel’s energy becomes heat that needs to be cooled away. An electric motor typically has an efficiency of 90%. So with the same amount of initial energy, you get 2-3 times more energy on the engine's shaft with an electric motor. Next step is to get the remaining energy from the motor to the propeller.
In a traditional stern drive or outboard set-up, the rotating energy has to make two turns; first, from the engine's horizontal shaft to the stern drive’s vertical shaft and; secondly, it is turned again to get it horizontal at the propeller. The normal set up is a cardan shaft at the top and a pinion gear at the bottom. Both generate considerable losses in the form of heat. Added benefits are reduced weight and, not least, reduced part costs, failure points and maintenance.
At Candela we mount the motor vertically which means that we avoid one of the turns and by that half the transmission losses.
To sum it up, through several engineering measures we cut the energy losses (measured from the engine/motor shaft) by ~75%. A modern boat of the size of Candela uses 1-1,2 litre fuel per nautic mile. If Candela were to use a combustion engine (which she does not!) she would have a consumption of 0,25-0,30 l/NM at 25 knots. Add to that the 2-3 times higher efficiency of an electric motor and you get some 90% reduction in overall energy consumption. Then you get a smooth and silent ride as a bonus. Probably a revolution.