Dr. Strangefoil or: How I Learned to Stop Worrying and Love Hydrofoils

Bob Abelin - I14 USA 1090

The milled cores from Phil

When I decided that I needed a lifting t-foil rudder for my International 14, I knew I had my work cut out for me.  While there are off-the-shelf t-foil rudders available, I decided to save a few bucks and design my own and find someone to build it.  Wanting to support my SA friends, I began talking with Phil of Phil’s Foils.  Phil had done some work on lifting foils for other 14’s, so he was familiar with the designs and knew what kinds of forces the foils would have to deal with.  He gave me some good design tips and some ballpark figures of the costs, which were very reasonable. 

The first step in the design process was to select a shape for the main rudder.  If you look around the boatyard it’s apparent that almost every rudder shape is unique, especially in development classes such as the I14.  It’s not difficult to come up with a reasonable rudder shape based on what other boats and other fleets are using.  You may not win a world championship with your own design, but if you put some time into it, it will likely perform well.

Another important aspect of design is the structure of the foils.  Rather than taking the time to figure out how much carbon and/or glass of what weight to lay up on the foils and what kind of wood to use as a core, I simply told Phil to make it very strong, figuring he’d have much better idea of what to use than I would.  I also let Phil select the contour shape for the main rudder from his standard designs.

Using the Internet I assembled the data to select a hydrofoil shape for the lifting foils.  I selected a thin asymmetric foil section that should have acceptable performance characteristics and (hopefully) be strong and durable.  I was also able to glean enough operational data to deal with the intricacies of the design.  When I was satisfied with the shapes I sent Phil my hand drawn designs with the contour data for the lifting foils.

From the beginning I was a bit concerned about the core milling process for the lifting foils.  Generally construction of very thin foils is done using molds.  While molds can makes construction easier, building a mold is time consuming and too expensive for a one-off project such as this. 

The Finished Product

The cross section I had selected had a finished thickness of only 1.0 cm at its thickest point.  This made the carbon skin a significant proportion of the overall thickness of the foil and required milling the core to only a few millimeters thickness in some spots.  The cores would be tricky to produce, but Phil was confident he could do it.

I was quite pleased when Phil emailed me with pictures of the finished cores.  The milling had gone very well and he was soon starting the process of lying up carbon fiber matt around the cores and putting on the clear epoxy finish.   The whole process was very timely and I soon had a box from Canada at my doorstep.  The results are very impressive and show considerable skill in workmanship. 

Now I have to construct all of the control mechanisms, the mounts, and get everything adjusted properly.  Hopefully the design will perform as anticipated.  And why did I go with the clear epoxy finish as opposed to the simpler and cheaper paint finish?  Because it just looks freaking cool, that’s why!  Thanks Phil!