“How is it that a board with a motor costs 1,000,000 rubles?
I’ll do it with my dad in the garage for 100k”
If, after this thought, you started looking for information on the Internet and came across this article, open a beer/tip a glass/pour a glass of white, and let’s go.
What does an electric surf consist of:

Propeller / Water Jet

electric motor

Power Controller

battery

corps
1. Propeller/Water Jet
The principle of their operation is completely different: the propeller, like a screw, is screwed into the water and thereby pushes the ship forward, and the water cannon is essentially a jet engine (creates a jet, the counterforce from which sets the ship in motion).
In the case of surfing, the use of a propeller is possible, but this design will look extremely ridiculous, so only a water cannon remains.
The question of buying a water cannon leads us to Aliexpress, where caring Chinese engineers have laid out many samples of their handicrafts.
“Which one to choose, you ask,” you ask, finishing a bottle of beer. This is a very good question, because the water cannon is the main point, and the other components along the chain will depend on it.
It so happened (in practice) that a water cannon with a diameter of about 90120 mm is enough to push you through space. If there is a choice by price, then it is like with the choice of wine – you do not need to take the cheapest, it is better to take something in the middle.
After paying for the order, reward yourself with the opening of a second bottle of beer. And today it is better not to buy anything, but just read the following paragraphs and return to the question tomorrow.
To go to planing (sliding on the surface of the water, with minimal resistance) you will need 50 kg of traction.
“Dude, what are the cravings? We’re not talking about shoes here,” you might reasonably say, but the force of the water jet is measured in mooring mode, i.e. literally resting the board with its nose on the scales and measuring the force. For a water jet with a diameter of 90 mm, about 6400 rpm is needed to create such thrust.
Now we need to understand the torque characteristic of the purchased water cannon (because in 99% of cases the manufacturer will not provide it).
In the simple case: a water jet with a diameter of 90 mm at a rotational speed of 6400 rpm will require about 10 Nm.
If you want to delve into the topic: Open the calculator According to the calculation of water cannons and the method of selection, we try to select the geometric parameters of the purchased water cannon. As soon as the parameters are selected, the graph of the water cannon will also be known.
Then you definitely can’t figure it out without half a liter – so prepare the 3rd bottle.
Water cannon price: 20 000 rubles – 70 000 rubles.
2. Electric motor
To rotate the water jet at 6400 rpm, you need an electric motor. We will only talk about brushless direct current (BLDC) motors, as others are not used in this area. They have such a characteristic as KV (revolutions per volt).
In other words, If the motor has 200 KV, then when 1 volt of voltage is applied to it, it will rotate at a speed of 200 rpm.
For water transport, you should not exceed 60 volts (lifethreatening and almost all European manufacturers fit into this limit), so a motor with a characteristic of 200 KV will produce about 12,000 rpm at 60 volts without load.
Under load, the RPM will definitely be less – on average, the drawdown will be 1530%.
It is also worth considering that a discharged battery has a much lower voltage (45 volts), and you should proceed from the worst case:
200*45*0.7 = 6300 rpm That is, +/ in the worst case, the engine will give the desired revs in any case.
There is also such a characteristic as the moment – and it is better to request it from the manufacturer. In any case, it became clear that when using a 4560 volt battery, we A 200 KV motor will suffice.
Okay, well, the motor has many other characteristics, what’s next?
Next, you need to find out the diameter of the electric motor and its type (inrunner and outrunner).
Inrunner – classic type of electric motor, the shaft (rotor) rotates inside the housing (stator).
Outrunner – The central part (stator) stands still and the outer part (rotor) rotates. This design is more complicated, but the torque of the engine is also greater.
To drive the water cannon, it is preferable to use an outrunner. The outer diameter of the electric motor should be 20% larger than the diameter of the water jet.
That is, you have a diameter of 90 mm, so the diameter of the electric motor will be 90 * 1.2 = 108 mm.
Based on these two data (200 KV and diameter 108 mm), we will look for an electric motor. When the desired option is found, it is necessary to request a torque characteristic (graph) from the manufacturer.
Next, you will need to superimpose the graph of the water jet (or the 10 Nm point) and the graph of the electric motor on top of each other.
At their point of intersection, the electric motor will operate at a stable temperature, everything further (higher in rpm) will heat the electric motor. Naturally, this is an ideal case, and in fact, this point will be closer to the origin.
If everything goes smoothly according to the schedule, you can order a motor.
Next, you need to take care of buying a power controller. Simple math (6400 rpm at 10 Nm) gives us 6700 watts of power.
Electric motor price: from 40,000 rubles.
3. Power Controller (ECS)
If we remember that our battery has a voltage of 4560 volts, then you can easily calculate the amperage in the worst case (when the battery is discharged).
6700W/45V = 148.88 This is how much the controller should keep in constant mode.
In order not to get fireworks from fieldeffect transistors at the time of the first start under load, as well as to comply with the cooling requirements, the controller should be taken with a margin of at least 1.3 times. That is, for 148.88 A you need to take a controller with a declared direct current of 200220 A.
My personal opinion: VESCbased controllers do not hold the declared load and cannot cope with reverse current emissions (there is no protection), so it is better not to use them, and look for industrial controllers (of course, they will be more expensive).
For the controller, you will need to either buy or invent a remote control.
Controller price: from 40,000 rubles.
4. Battery
The most dangerous element of the structure. It all depends on the budget and your wishes for capacity.
The main thing is that the battery holds the maximum current (148.88 A), has a BMS (cell balancing) and a connection system (power relay). And it is definitely better to entrust the assembly of the battery to professionals, because a defect can cost you your life and property.
LiIon 18650 (or 21700) cells have better performance, in terms of capacity/mass ratio, and they are also quite safe. In this case, to get a voltage of 4560 volts, you will need an assembly of 14 cells (in series).
Now we need to calculate the required capacitance: let’s take the average current of 100 A and assume that the power reserve will be 30 minutes (0.5 hours) – i.e. 100*0.5 = 50 Ah. Since the capacity of the battery is obtained from the parallel connection of the cells, the total capacity (50 Ah) must be divided by the capacity of each cell (e.g. 2.5 Ah) = 50/2.5 = 20.
It would be good to check that the cells will hold such a load and does not overheat, so the maximum current (148.88A) will be divided by the148.88 / 20 = 7.44 A per cell.
Samsung 30Q (15A discharge current and 3 Ah capacity) fits these parameters – i.e. the total battery capacity will be a little more than 20*3 = 60 Ah.
In total, we get the configuration: 14S20P – i.e. the number of elements will be equal to 280 pcs (14*20).
Battery price: from 100,000 rubles (assembly will cost 140,000 rubles only for cells without the cost of welding, BMS and relays)
5. Housing
Here you can give one piece of advice – you need to buy a donor, which will have to be worked out for a long time and hard. In any case, it will be much cheaper and easier than building it yourself from scratch.
Maximum attention should be paid to the modularity of the design – because small volumes are easier to insulate from water.
Insulate from water, you need everything and as reliably as possible, and wherever you can use compounds like Sielagerm 2104 or Vixint PC68. It is quite convenient to work with them, both during pouring and repair: after hardening, the compound has a slight hardness and crumbles when it is necessary to disassemble the structure.
Case price: from 50,000 rubles.
Resume: Minimum 250 000 RUB It will be necessary to spend only on the main components. Taking into account all the details, by the time you go out on the water, the board will cost 350 000 RUB
And this is just the beginning of the path called “test operation”: the batteries will be flooded with water, the motors will burn, and the money will be spent with such efficiency that, at best, it will be possible to move at least somehow stably on the 3rd assembled prototype.
In fact, this is how it turns out million rubles.
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