Naval Architecture 101


Q: Min speed for planing? TODO: read thread

Looks like about 10 knot wind. But what is the board speed? About same, possibly even doable at 7 knots.

Propellers, impellers, and trade-offs

TODO: Diameter Speed Angle of attack Torque? Thrust? Drag. Given propeller, convert RPMs to speed (roughly)

Propeller pitch - the distance the prop would move forward in one rotation if it were moving through a soft solid, like a screw being turned into wood.

Propeller pitch determines the final “gear ratio” between the engine and the water.

A boat should be “propped” to operate within its wide-open throttle (WOT). Ideally a motor with a WOT range of 5000-5800 RPM (as noted on the boat tachometer) will reach 5400 rpm with the boat running wide open and trimmed out for optimal performance.


Brushless motors

Jet skis can produce an average of 100-150 HP, within the 1,000-1,800 cc non-supercharged engines

Typical Jet Ski: 75 - 115 kW. That's a

eFoil motors

Living human tissue can be protected from damage by the insulating characteristics of dry skin up to around 50 volts. => Stay at or below 40V. That’s about 10S. With a 100A max ESC, I should be good.

Gearboxes vs direct drive

Keeping power fixed, there is a trade off between speed and torque (what is the relationship exactly?)

Motor KV describes how quickly the motor spins unloaded given 1V of input. RPM = V * KV

A lot of hobby motors have very high KV since they are designed for a small propeller which doesn't have too much drag on it (drag proportional to area — TODO: quantify).

To push a heavier boat (with a human on it) you need a larger propeller which will incur a lot more drag. A high KV motor will not have enough torque and require a gearbox for reduction (speed to torque). A motor that spins too quickly but doesn't have enough torque will fail to actuate the propeller and will just overheat and draw too much current.

In summary, there are two options:

Motor naming convention

Fortunately, most manufacturers in our hobby have settled on a standard naming scheme for their motors. It will normally be a 4-digit number that looks something like “2205”. The first two digits of this number is the diameter (in mm) of the stator, and the second two numbers is it’s height (also in mm)

For larger motors, this is often a 5-digit number. eFoil motors are often called 70120, which means 70mm diameter x 120mm length.


LiPo Batteries

Lithium Polymer batteries are the revolution that makes portable e-vehicles possible.

In general, P = I V. So the same power can be achieved with high voltage or high current or medium both. Current incurs heat. Voltage incurs risk of electric shock.


Some batteries are designated as 3S, 10S, …, kS. The k here is the number of cells connected in series. Each cell is by convention 3.7V, so 3S = 3.7 * 3 = 11.1V, and a 12S is equivalent to 44V.

Some batteries are designated 4S2P, 12s2p, …, iSjP. This means that there are a matrix of cells: j sets of i cells in series wired in parallel. Wiring in parallel just increases capacity (measured in Ah), not voltage. This notation is used less now since LiPo cells are greatly improving and you get more life from a single cell.

Other battery types

Original Solowheel had LiFePO batteries.

Dan (from Firefly) original kit uses LiNCM batteries.


High current wiring

There’s a thing called American Wire Gauge (AWG), which determines wire diameter and many other parameters of the wire. For example, breaking forces, maximum amps, ohm density, etc.

8 0.1285 3.2639 8.37 0.6282 2.060496 73 24 1650 Hz 480 lbs


Foiling and planing on my board

I bought a $300 board from Frank.

Board dimensions: 240 cm L x 95 cm W Measured Tuttle inset depth: about 80mm, 90mm on the other. (Standard deep Tuttle: 51mm, 67mm) Measured Tuttle fin: 155 x 32 x 144 x 51 mm (Standard Tuttle box: 156 x 36 x 140 x 50 mm)


Mast Length

Seems like beginners start with a short mast. Kits come with a 55cm mast. Short masts are easier to start with but harder to maneuver.



It’d be nice to know how much current my system is drawing in real-time. A: apparently VESC does this for you.

Also, how quickly is the prop actually turning? A: Probably will need a rotary encoder on there.

Water jets

For now, ignore this.