Unifoil P170 vibration then nose diving

I’ve been SUP surf foiling for about a year now. I’m riding a naish 112L with a P170, 14" tail + .5 shim, Katana 7500 mast.

Has anyone experienced their foil vibrating when it gets close the to the surface, but hasn’t breached yet, then pitching you forward as if you did breach? My friend watched this happen and the foil didn’t come out of the water, but the board nose dived and I went flying.

I assume this is because I don’t have enough pressure front foot pressure, but once I feel the vibration I can’t fix it with pressure. I actually don’t feel like I’m too high, so I thought I would ask here to see if anyone has had anything similar.

Sounds very much like your tail is losing lift (it pushes down, keeping the nose up). Where is this happening? Bottom turn? If nothing is coming out, could possibly be mast ventilation causing this, but that would require some angle of attack on the mast or rewlly crazy high speeds.

As far as the nose diving, mine kind of does that too. It’s not a wing that likes to be pushed to fast. Throwing a big cut back generates a lot of speed really quickly so things almost always get dicey in that maneuver. For any wing a heavier rider is always going to be going faster. If you’re bigger, like I am, you will be in that part of the speed range frequently. I haven’t found a good way to tune that behavior out of a foil without sacrificing efficiency, which I really needed for pumping around.

With the vibration, it might be the rear fuselage connection. Mine arrived with quite a bit of play there. Ended up putting a few pieces of packing tape on where it connects and it tighten things up. Maybe feel for some play and add some tape. That connection should be snug before the fasteners are tightened.

@silas It’s happening when I’m at full speed going straight. My app says might top speed is about 20mph, but normally it’s a little over 17mph.

@TooMuchEpoxy I’ll double check everything/nothing is moving around. I’m on the lighter side, 160lbs. (72kg). I could probably be riding the P140 most of the time, but that’s gonna be faster. I should probably also try the 13" tail.

The P170 tends to get a nose down attitude at faster speeds. I just shift my back foot another 2” back in anticipation

@Washout Interesting! I would have thought more front foot pressure was the solve. I’ll give that a try.

I should’ve mentioned I’m running a -.5 degree shim on it.

If it’s happening while riding the foil near the surface and at speed it’s most likely ventilation causing that vibration and then sudden loss of lift. All foils will do that. Not a forward pitching moment issue.
The mast base or top of the foil creates a wave that breaks behind it and suck air towards the wing. That air trapped on the wing surface causes instability thats feels like vibration and the sudden loss of lift will cause forward pitching of the whole system.
Here’s 3 vids of two different foils ventilating. These were all at the tips but are not breaches and same principles apply.

Watch the wave forming, breaking behind the front wing and the vortex sucking air forward towards the wing. The 3 photos that follow the video are in sequence:

Same here on a DIY wing: