I went for a sail on Saturday and my boat was still flying out. At one point I was sitting at the front wing bar, with full CB flap up, and full rudder flap down, and she still keeps on coming out. Not good.
So what is the problem? My current theory is it is being caused by a combination of two things: 1) too much AOA, and 2) flap twist.
When I put my foil in the boat initially, I had run out of adjustment in the case fore and aft and I felt I was running a bit bow-up, so I thought I should build some more AOA into the foil itself, to get the bow down level, and if it didn’t work I can always drill a new hole and reduce the angle and / or move forwards in the boat whilst sailing.
I measured the foil tonight and it looks like I am running 3.8 degrees on the foil … hmmm that seems a bit excessive so I ran some some numbers to check.
For this model I am assuming that my all up weight needing to be lifted is 125kg. I am also assuming a 30% flap, and that I am effectively loosing 50% of that through flap twist.
| Boatspeed | Flap Angle | Lift generated | Flap twist Adjusted | Delta | Boat direction |
|---|---|---|---|---|---|
| 15 kt | 5 deg up | 93 kg | 133 kg | 8 kg extra | Up |
| 15 kt | 0 | 173 kg | 173 kg | 48 kg extra | Up |
So at 15 knots, there is already no situation where the wand can bring the boat down, and even if my flap didn’t twist, I would only have 35kg to play with at 15 knots of boat speed. So what happens if the boat was at 25 knots.
| Boatspeed | Flap Angle | Lift generated | Flap twist Adjusted | Delta | Boat direction |
|---|---|---|---|---|---|
| 25 kt | 5 deg up | 258 kg | 321 kg | 196 kg extra | Up |
| 25 kt | 0 | 484 kg | 484 kg | 359 kg extra | Up, A lot |
So the numbers have confirmed that I am running too much AOA, so what are the numbers if I reduce it to say 2 degrees.
| Boatspeed | Flap Angle | Lift generated | Delta | Boat direction |
|---|---|---|---|---|
| 15kt | 5 deg up | 44 kg | 81 kg too little | Down |
| 15kt | 0 | 125 kg | 0 kg extra | Level flight |
All this number crunching explains a number of actual things I have seen on the boat.
1. Since I have foiled with the latest AOA, the boat can’t stay in the water, regardless what the wand does and this is confirmed by the numbers.
2. When I was foiling before (at the nationals and after) the boat was OK most of the time, but sometimes the wand would just float in the air and do nothing … This has also been confirmed by the numbers, as when the speed was over ~18 knots the flap starts to get a bit marginal on paper.
So the solutions appear to be to reduce the AOA to +2 degrees and stiffen the flap and make it a bit bigger, so it doesn’t have a 50% twisting loss.
Then at 15 knots of boat speed, with a flap up level the total lift is 120 kg. Then at 25 knots, with the flap 5 degrees up, the lift is 125 kg, the same number as my assumed all up sailing weight.
I think that co-incidence is telling me something. It also shows that there is definitely an AOA sweet spot, and 3.8 degrees isn’t it 
BRUUUCE!
The important thing is the delta to the rudder AOA! Of course you cannot go so positive on the rudder that it leaves the water…so there is a limit to how far you can go with the mainfoil! But then you knew that! And why is your flap so flimsy? 50%? What kind of wet noodle are you running down there?
Good Sailing,
Karl,
Of course I screwed up the rudder as well
, but I don’t want the bow to be down either, so I will reduce the AOA of the main foil and then tune the rudder to suit.
The flap is soft because there aren’t enough fibres in the 45 degree orientation to stop the twist.
It would be great to have a record of the AOA ( relative to sea level) while sailing. Can you write an iPhone app that uses the internal level and logs it once a second? With that, and a measurement to some hull datum, we would know what the AOA relative to the velocity vector really is, assuming the velocity is horizontal.