Two years ago I bought a boat with the hope that someday be able to electrify it. It is a displacement boat of a type called snipa or snäcka which typically has low drag at low speeds. For that reason I chose this boat type, the boat is a bit bigger than what I planned for, and I think that is the theme for most of this build. It is a Sandvik 25 and currently, it has a Volvo Penta MD3B as the engine.

It has a displacement of 2400kg according to the old datasheet. But it is probably more with the equipment that has been added over the years. Before starting the conversion we exchanged the rubber gasket on all windows to keep the rain on the outside of the boat since that was a bit of a problem before. Because it does feel quite important to keep the water outside of the boat before installing a high-voltage battery in it. We also repainted the bottom as it was for lakes and not for the sea.

Initial testing

At the end of the first summer with the boat, we went out and made a small test logging the rpm of the motor, GPS speed as well as speed through water (not used). During the test we drove back and forth with one rpm setting before increasing it to 100rpm, writing down the speed after it had stabilized after each turn. After a total of 30 turns, we were done with the test, and the average results of the data look like this:

My idea is to use this data to estimate the maximum power used to propel the boat at different speeds. I say maximum because it assumes that the engine can produce the power it could when it is brand new, which it most probably can not do. As well as there are other losses on the way to the propeller, which also should make the estimated power a bit higher.

A simple generic equation for static thrust can be found in this paper, which describes the relationship between the rpm and thrust using this equation:

Where T_s is the static thrust produced (in newtons), K_t is the thrust coefficient, ρ is the air density (in kg/m^3), ω is the propeller speed (in rad/s), and D is the diameter of the propeller (in m). I have removed some constants and changed units since K_t will just be scaled accordingly.

It only holds for stationary propellers, so let's hope that the propeller moves somewhat slowly enough through the water. And that there is not too much growth on it making the equation break even more. But this should only make the estimated power used to propel the boat higher, resulting in a further range after the actual conversation.

First, we find the power produced by the engine at the specific rpm. In the datasheet for the engine, such a point can be found for full throttle.

Then the rpm of the engine needs to be converted to the propeller speed, which has a reduction of 1.91 and then I also convert it to rad/s.

Since the speed of the boat, Vmax (in m/s), and power produced by the vessel, Pmax (in kW), is known at the max rotational speed of the propeller (in rad/s). We can solve for the thrust produced by the propeller:

Fmax can then be plugged into our thrust equation, where it is called T_s, which we then can solve for the constant. This constant replaces all the other constants since they can be multiplied together. Resulting in this equation:

This new constant, K, can now be used to calculate the thrust produced at a certain rpm of the vessel, and since the velocity is already known the power used can be estimated.

I did also do the calculations with a linear version of the formula (removing the square), to make a worse estimate at lower speeds. The electric motor will also have some inefficiency which will mean that more power will be pulled from the battery than is required to propel it forward. The results from both can be seen below:

I have also done calculations on the range with a 10 kWh battery, which can be seen...