Hull shapes

In general, there are a few types of hull geometry:

• Flat-bottom hulls: These are characteristically very stable, great for fishing and other uses on calm, small bodies of water, but are the roughest riding in choppy waters.
• Round-bottom hulls: These are characteristically “displacement-style” hulls designed to move smoothly through the water with little effort (ex: canoe, kayak). Round-bottomed hulls are less stable and can capsize more easily.
• V-shaped hulls: These are characteristically “planing hulls” and are most commonly used for powerboats and PWC. V-shaped hulls are designed to plane on top of the water at higher speeds and provide a smoother ride through choppy water. V-hulls have a greater surface area (ie. resistance) thereby requiring more power to move.

When it comes to Jetskis, they often combine multiple hull shapes for the desired balance.

Flat-bottomed hulls are prone to “slap” or pound itself over waves – particularly at speed – and due to the minimal surface area, are typically “looser” or harder to control; requiring constant steering correction by the rider. GTI’s are marketed as being “loose” and “playful,” allowing the rider to freely “spin-out” on demand and generally goof-off far more than the Ultra.

The solution to maintaining the balance of a flat-bottomed hull while retaining the tracking and handling of a V-shaped hull has been found by reducing the deadrise and adding strakes to provide compensating lift. While chines can generate some lift, they are primarily used to manipulate how the hull will behave in a turn.

![[Displacement vs planing hulls]]

![[Hull speed]]

Propeller design

Propellers are defined by a variety of parameters: diameter, angle of attack, and many others. Each propeller blade is a foil shape, which adds to complexity. Whole courses are dedicated to this subject.

Theoretical foundation

Definitions:

• Pitch: how far the prop would move through a solid material if it was a drill.
• Slip: how different in practice moving this prop through water is vs. solid (see this calculator)
• Smooth vs. splined shaft: refers to this mechanical splining as a way for better attachment to the shaft. There are a shocking number of ways you can attach a prop to a shaft.
• Weedless design: keeps vegetation in water, off the propeller. For weed infested waters.

Basic equation: speed = rev_per_time * pitch * (1-slip)

For example, with a 3000 RPM shaft (my motor), what pitch do we need to go at 10 mph (= 16 km/h)?

pitch = speed / (rph * (1-slip))

pitch (in) = 10 mph / ((3000 * 60) * (1-0.3)) * 63360 (in/mi) = 7.5"

Practical attempts and notes

There is a trade-off between top speed and acceleration.

My theory (untested), based on torqeedo’s lineup(spreadsheet):

• The more power the motor has, the higher the diameter should be.
• The faster you want to go, the higher the pitch should be.
• the lighter the boat, the higher the prop pitch, and the smaller the diameter.

In other words, for my specific case:

• Pitch should be high, because we want to have high speed
• Diameter should be low because boat is light.

Torqeedo

Torqeedo props are designed for planing: prop is rated to propel a 200kg boat at 30 km/h under 4000W of power, with 16.3” pitch, 11.8” diameter.

Torqeedo spin at 1500 RPM, which is pretty slow, and why their props are so large. My motor should spin at about twice that, so can have a smaller prop.

Good summary of Torqueedo’s propeller offerings. Breakdown of properties:

• vXX/pYYYY means XX km/h at YYYY W, for example v10/p350 means reaching 10 km/h at 350W.

Prop ducts for safety

Ducted propellers are sometimes used in ships for efficiency reasons:

Advantages are increased efficiency at lower speeds (<10 knots), better course stability and less vulnerability to debris. Downsides are reduced efficiency at higher speeds (>10 knots), course stability when sailing astern, and increase of cavitation

But for efoils, having a duct is a safety feature. Unfortunately it also introduces a lot of drag generally speaking. The real solution is to build a well tested kill switch.

Resources I found useful

• Illustrated guide to Blade Element Theory from Cambridge
• Marine propellers from MIT 2.016
• Simple flat propeller in Onshape
• Sketch & loft propeller in Onshape, with airfoil profile.
• Succinct explanation about prop pitch and top speed vs. acceleration.
• How to measure prop pitch.
• Props 101
• Understanding a Boat's Propeller Pitch

Brushless DC motors

Brushless DC motors (BLDC) were invented in the 1960s, rely on an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. Sort of like a mini circular railgun.

There are many advantages of a brushless motor over brushed motors:

• high power-to-weight ratio,
• high speed,
• nearly instantaneous control of speed (rpm) and torque,
• high efficiency,
• low maintenance.

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.

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 because of the water resistance. It will just overheat and draw too much current.

In summary, there are two options:

• Direct drive with low KV motor (advantage: no gearbox!)
• High KV motor with a gearbox for reduction.

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.

References:

• Elco Motor Yachts FAQ, basic physics explainer
• Brushless motor buyer guide (mostly air drones)
• Understanding Kv ratings

eFoil motors

• BLDC FS65161: POWER=6kW, KV=195KV, CURRENT=200A, TORQUE=9Nm, VOLTAGE=20S (fully waterproof)
• Flipsky 80100: 7kW, 148A, 17Nm, 14S (not waterproof)
• Reacher Tech D65L161-270: 9kW, 150A, 80 kg*cm=7.8Nm, 60V=16S

I ended up with FS65161.

LiPo batteries and wiring

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.

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

Notation

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 when fully charged, 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.

Remember: wiring in parallel increases capacity (measured in Ah). Wiring in series increases 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 SUP drives) original kit used 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. A lot of the stuff you find online is specific to electrical wiring in houses.

In general, the thicker the wire, the more current it can handle, the less it will heat up, etc. Insulation also matters in this department.

As a rule of thumb, it seems like 10AWG is working well on my current setup which is running at 12S, and not really exceeding 100A.

Useful links:

• A Guide to Understanding LiPo Batteries
• Battery Bank Tutorial: Joining Batteries Via Series or Parallel for Increased Power
• LiPo Batteries and Safety for Beginners
• https://www.powerstream.com/Wire_Size.htm