Hydrofoil Design Tool


Here is a numerical design tool which is really helpful for designing hydrofoils. This Excel spreadsheet was created by Zach Hoisington, a senior aircraft/aerodynamics engineer.

With this tool you can adjust speed, weight, wing size, motor pod size etc. Not only does this tool calculate total drag, it gives you a break down of where the drag is coming from to help you know where to focus your drag reduction efforts.

You can even estimate the stall speed of your design. The design tool calculates the required Cl for a particular design/weight and speed. By gradually reducing speed you will see a point Where Cl is above 1.00 and begins to climb very fast…the approximate stall speed.

The example below is my current design which I optimized for minimum drag at 15 mph. For optimized (minimum drag) at higher speeds, shorter wing spans are better.

Once we have more data from actual foils, it would be great to compare real world numbers to the predictions given by this spread sheet.

Here is the dropbox link for this spread sheet: https://www.dropbox.com/s/pf32dhyy7i1yovz/Hydrofoil%20medium%20AR.xls?dl=0

Example of my current design at 15 mph.

Home Made Flume

I made another model with a lower aspect ratio foil shown below. All else is the same as the example shown in the first post. Then I ran the spreadsheet for the low and medium aspect ratio foils to compare the predicted drag from stall speed up to about 30 mph. The medium aspect ratio wing is the clear winner up to about 27 mph as shown in the chart below:

Design tool with low aspect ratio wing

Comparative performance of the two foils


I used the design tool to see the effect of motor pod diameter as a function of speed. A smaller diameter motor pod always has less drag…as expected. But at the lower speeds the extra drag of a larger pod is pretty small. This analysis helped me decide that for my lower speed build, a large direct drive out-runner could be a good choice.

At my design cruise speed of 15 mph the larger pod only adds 1 lb. of drag. This is greatly offset by the thrust efficiency of the large out-runner with a well matched propeller. In the flume testing, the Alien c80100 with a well matched prop drew less than 10 amps to make 20 lbs. of thrust. If the numbers are correct, a 20 Ah battery should give 2 hours of flight time with my design.

If I were designing for higher speeds a smaller motor with reduction gear and a smaller pod would be more attractive if the drive system was efficient enough overall.



That is a great tool. Thank you for that. Is anybody already converting this into metric units? If so, please upload it.


I’ve contacted Zach to see if he is willing to create a metric version of this design tool. I’ll post when I hear back.


Zach kindly said yes to a metric version of the design tool. Should have this within a week.