Foil Trim - how to get the balance right

A friend pointed out when I borrowed their big foil that I was too far forward. I moved WAY further back, and suddenly it felt balanced, and much more efficient. I then removed the little bit of tail shim, and it felt very fast, very easy to control, and very easy to pump, and able to go much slower. This surprised me as I felt that moving to the bigger foil it would have been obvious, but because of the huge lift, I was too far forward to compensate when going fast on waves.

This reminded me of the process of trimming an RC plane. To trim a plane, you start with weight too far forward, and then edge it back until the plane becomes hard to control (at the intended typical speed), then nudge it a tiny bit forward again to settle. Ideally once settled, it should fly without any stabiliser input (ie “shim”, but this isn’t fixed on a plane). Any stabiliser input is just unnecessary drag and should be avoided where possible.

If badly trimmed, both the foil and stab are working harder to lift you, and input to overcome this requires more effort. You have less glide. I think probably 95% of foilers including myself regularly are way forward of where they should be for pumping, which explains why they cannot link, why they cannot stay high on mast… Continual foot adjustments are probably the most important skill to develop to enable this.

Keen to hear what others think of this, what tradeoffs, etc.

@KDW as I know this is 100% your domain, would be keen to hear your thoughts. You mentioned that you can calculate the correct shim theoretically (in a podcast), I wonder how that assumes you are trimmed? Does it matter.

**Background reading on trim. **

CG vs lift explained. Afaik you can ignore the mast and board when thinking about this, and just think about the foil and where your weight is in relation. The mast only comes into it when pumping.

RC planes and how to trim

Note - best L/D important for racing, sink rate for staying on foil for hours