Author Topic: Getting ejected out the front door...  (Read 5293 times)

Solent Foiler

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Getting ejected out the front door...
« on: October 25, 2021, 04:47:58 AM »
Would like to understand why I get ejected out the front door.
Usually when I'm cranking upwind, loaded up in the harness, and I then feel something ventilate and usually ends up in a roll to windward, (so not a sudden, catastrophic loss of lift, / nose dive as you would over foiling)

Weirdly happens on flat'ish water as well as choppy.

I thought it might be something to do with the Gong Fluid foils, as it didn't happen so much with the Veloce, but it happened a few times yesterday on my brand new, perfect condition ART 999 and 380 tail, so I wonder if it's the mounting of the tail on top of the fuse rather that the middle, as the Veloce tails are. But the Veloce tail was a bit bigger too...

Clearly I'm the rider so will take responsibility for the crashing, but I feel a bit cheated as I don't feel like I'm making an obvious mistake. I wonder if it's a chop thing, because it does feel like a white cap, hits the mast and then it happens, but could be my imagination? Also, on a 1m wide wing can air get all the way down to the tail to cause ventilation without also causing a main foil tip breach? Is air going down the mast, down the fuse, to the tail? Feels unlikely... Am I loading up the tail too much and causing it to 'spin out' (upwards/negatively)?

I don't think it's a main foil issue for various reasons but could be that too?

Thoughts, comments, suggestions welcome...
I'm 5'10", 65kg riding:
Gong Lethal 4'6 65L
Gong Catch 5'3 34L
Gong Hipe v2 4'11 70L
Axis ART 999
Gong Fluid L-S, XXL-S on 85cm mast
Duotone Slick 3m, 4m, 5m

Funsup

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Re: Getting ejected out the front door...
« Reply #1 on: October 25, 2021, 05:55:41 AM »
Iíve experienced the same issue with other black fuselage foils on a carbon mast. Different lengths of masts or fuselages did not alleviate the problem.  Charging upwind and chop slaps the side of the mast, then the foil momentarily loses lift on the up wind side causing me to lose balance and start falling upwind.  Sometimes I can save it and sometimes I just explode.  I would like to know how to eliminate this problem as well but have not found the solution yet.  I donít ever recall having the issue on any of the red fuselage foils.

PonoBill

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Re: Getting ejected out the front door...
« Reply #2 on: October 25, 2021, 06:23:59 AM »
Sounds like ventilation. We're ALL going to have this problem as we start going faster. It may be that the 19mm Axis mast is a bit more prone to it than skinnier masts but it's a phenomenon hanging out there waiting to plague anyone that pushes hard and goes fast. That doesn't generally include me, though I've had a few unpleasant moments with the 1300 wing.

Presently I don't know of any mast that combats this. Tapered carbon masts should be less prone to it. Thick aluminum masts will probably be the worst. I made a little fence to try adding on to my mast. I haven't installed it yet, and I expect it's too big, but high-performance masts will probably be going that way. The pic shows a stub of a mast with the fence temporarily hot glued to it.

It's a huge problem for really fast foiling sailboats. Some people think it represents a barrier to going faster than 50 kts. Outboard motors add ventilation plates to the masts, some high-performance ones have multiple plates.

Something new to screw with.
« Last Edit: October 25, 2021, 06:41:38 AM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

Dwight (DW)

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Re: Getting ejected out the front door...
« Reply #3 on: October 25, 2021, 06:42:24 AM »
Is your foil kit perfectly balanced at any speed? Meaning, your foot pressure balance is the same at low speed and high speed?

Mine is, and this never happens to me. Itís the only wild idea I can come up with.

Iím real particular about getting this balance/low drag setup spot on. I never have back foot or front pressure at any speed.


Solent Foiler

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Re: Getting ejected out the front door...
« Reply #4 on: October 25, 2021, 06:46:20 AM »
Just to pre-empt the conversation  :P (Copied from Google):

"Ventilation occurs when part of a hydrofoil pierces the surface of the water and air gets sucked down the lifting surface of the foil. Since air is much less dense than water, the foil generates much less lift and the boat crashes down. Ventilation can occur at any air-water interface."

"Cavitation occurs when the water pressure is lowered to the point where the water starts to boil. ... When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water."
I'm 5'10", 65kg riding:
Gong Lethal 4'6 65L
Gong Catch 5'3 34L
Gong Hipe v2 4'11 70L
Axis ART 999
Gong Fluid L-S, XXL-S on 85cm mast
Duotone Slick 3m, 4m, 5m

Hdip

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Re: Getting ejected out the front door...
« Reply #5 on: October 25, 2021, 08:28:17 AM »
Are you pushing sideways through the water when you fall? You say it's like you lose pressure on one side of the wing. The windward side? So if you're leaned over into the wind that sounds to me like you are talking about the side of the front wing that is deeper in the water. You also mention it happens on several different front wings.

That doesn't sound like front wing ventilation to me. It sounds like mast ventilation. The mast is a lifting surface too.

Is this what you're describing?

https://www.youtube.com/watch?v=BlCuVZK2aqE

seastudent

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Re: Getting ejected out the front door...
« Reply #6 on: October 25, 2021, 08:44:48 AM »
Is your foil kit perfectly balanced at any speed? Meaning, your foot pressure balance is the same at low speed and high speed?

Mine is, and this never happens to me. Itís the only wild idea I can come up with.

Iím real particular about getting this balance/low drag setup spot on. I never have back foot or front pressure at any speed.
Have you or could you post the process to achieve this? I used The KD balance method to start but my tuning seems to be pretty scattershot. Usually I try frowning and some incantations but that doesn't seem real effective.

Solent Foiler

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Re: Getting ejected out the front door...
« Reply #7 on: October 25, 2021, 09:18:44 AM »
Is your foil kit perfectly balanced at any speed? Meaning, your foot pressure balance is the same at low speed and high speed?

Mine is, and this never happens to me. Itís the only wild idea I can come up with.

Iím real particular about getting this balance/low drag setup spot on. I never have back foot or front pressure at any speed.

I didn't even think this was possible?! I always get more pressure on the front foot as speed increases. The thinner foils (Veloce, 999) less so, but how do you do it? Matching front and rear foils so they have the same lift profile over all speeds? Doesn't that limit stab choice?

To be absolutely explicit about the circumstances:

Flat water : hauling ass, fully loaded, canted over, going upwind. Seems a bit spontaneous when it happens, although I do get a bit of warning sometimes, and if I back off a little it seems to be better (but where's the fun in backing off?!?) Might be in conjunction with hitting a gust.

Moderate chop :  going fast upwind (but not as fast as flat water), canted over, feels like chop hits mast and down I go. Like Funsup says, sometimes the roll to windward is slow enough to catch and recover, often not...

I'm beginning to convince myself of the overloading tail hypothesis. In both circumstances, the tail is under rotational load, pivoting around the front foil with the force coming from the wing driving the whole system forward at a point about 2m above. I think the very thin profile of the 380 and Fluid tails mean they are less able to cope with an AOA that is not straight on, so when the force from the wing starts pulling forward and pushing the nose down down (rotating forward), the AOA changes so that it becomes more likely to stall and loses grip. Obviously the drag on the tail goes up rapidly as it gets pushed through the water flat face first as everything rotates around the front foil, so the crash is slower than might be expected as the whole thing doesn't nose dive like having the tail snap off, for example.

The Veloce tail actually has more volume and profile shape, which might explain why it doesn't suffer from this.

The choppy water could just be introducing turbulent water to the tail, again changing the AOA or introducing air so it tips it over and loses grip.

I'm fairly sure the whole thing is related to cant too, but can't figure it out. I've adopted a defence against this ventilation issue from happening by keeping the mast more vertical, but it's a compromise.

Am I making any sense? Tell me I'm wrong, because I'd love to understand what's going on. I've got a couple things in mind to try...

I'm 5'10", 65kg riding:
Gong Lethal 4'6 65L
Gong Catch 5'3 34L
Gong Hipe v2 4'11 70L
Axis ART 999
Gong Fluid L-S, XXL-S on 85cm mast
Duotone Slick 3m, 4m, 5m

Solent Foiler

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Re: Getting ejected out the front door...
« Reply #8 on: October 25, 2021, 09:25:51 AM »

Is this what you're describing?


Possibly in terms of effect, but that's a more extreme situation and I don't think I get nearly the same lift from the mast as they do, especially as I run an 85cm mast and in one instance the 999 so limited as to how far over I can go to get the mast lifting.

It's a good suggestion though. Hadn't considered it!

I agree that the front foil, especially the windward tip isn't the issue...
« Last Edit: October 25, 2021, 09:29:44 AM by Solent Foiler »
I'm 5'10", 65kg riding:
Gong Lethal 4'6 65L
Gong Catch 5'3 34L
Gong Hipe v2 4'11 70L
Axis ART 999
Gong Fluid L-S, XXL-S on 85cm mast
Duotone Slick 3m, 4m, 5m

jondrums

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Re: Getting ejected out the front door...
« Reply #9 on: October 25, 2021, 10:51:22 AM »
I have a theory -

When going upwind, you need side force to counteract the pull of the wing.  In a sailboat, the centerboard/daggerboard/keel react this load and keep the boat from going sideways.

On a windfoil setup, there are actually two ways you could counteract side force - 1) the mast in the water acts as a daggerboard 2) if the foil is tilted/heeled over, then the front wing lift has a side force component.

My theory is that you are riding the board too upright and using the mast to gain sideforce reaction.  Perhaps it would work better if you concentrate on heeling the board/foil over to windward and using the lift of the foil instead.  The mast can easily ventilate because it pierces the air/water boundary, but the foil won't if it stays underwater.  If you take a look at foiling sailboats, they all heel to windward in order to use the main foil for side-force.

Wingfoil2001

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Re: Getting ejected out the front door...
« Reply #10 on: October 25, 2021, 12:25:03 PM »
Itís a really common problem when kitefoiling due to the higher speeds. Yes, some masts are more prone to it than others. Att higher speeds the condition of the foil parts is critical, no nicks or damage that could interrupt smooth waterflow across the surfaces.

Clean your foil before going out if your planning to push some speed.  I use window cleaner to remove fingerprints etc to get it perfectly clean then not let anyone touch it. Watch kitefoil racers clean their kit before a race to help prevent ventilation.

bigmtn

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Re: Getting ejected out the front door...
« Reply #11 on: October 25, 2021, 01:23:42 PM »
I have what I think is the same issue, mostly while riding toe side and trying to go fast upwind while trying to drop into a wave. It feels like someone underwater just yanks my foil sideways out from under me.  It happens heal side, but I'm usually able to use my toes to push the board back down.  I figured that either the mast is far enough over that it turns into a wing of its own and the side chop causes it to lift, or the mast ventilates causing the same thing. I've gotten to the point where I can feel when I'm in a position/speed/condition where it's about to happen, and can bring the mast down more vertical for a bit to keep it under control. 

I use a 90cm Axis 19mm alu mast, and when I mentioned it to friends on other gear, they hadn't experienced it.  Kind of just figured it was caused by the mast shape. And if my memory is correct, I think Kane mentioned something similar about it in his last progression podcast.

jondrums

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Re: Getting ejected out the front door...
« Reply #12 on: October 25, 2021, 05:46:48 PM »
Could well be the mast shape too.  If the leading edge is too sharp flow can separate pretty easily.  Masts foil section probably matters a good bit - maybe axis for something wrong on this respect

PonoBill

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Re: Getting ejected out the front door...
« Reply #13 on: October 26, 2021, 10:37:38 AM »
I thought my post was fairly clear that I'm talking about mast ventilation, not wingtips. Masts can actually ventilate in a vertical attitude, though the speeds need to be higher. As you lean them over to turn or go upwind, both the angle relative to the water and the angle of attack of the mast changes, and the mast foil characteristics become more important. Any surface moving through a medium has pressure differentials, airfoils are effective and efficient at creating pressure differentials--it's how they work to lift us. The pressure differentials are a function of surface area times a factor for foil shape times velocity squared. The foil shape factor is empirically measured, but it's got a lot to do with thickness--all other elements being equal, the thicker the foil, the higher the factor.

The Axis aluminum masts are thicker (19mm) and wider than most foil masts on the market. It's why they are so rigid. Every design consideration has consequences, and the designer chooses which characteristics are most important and which to sacrifice. The Axis aluminum mast is ideal for probably 95% of their customers. It's strong, stiff, and cheap. It's also cool looking which is more important than most of us would like to admit. I think it's probably axiomatic that a 19mm, strong, stiff mast is going to ventilate more easily than a 15mm, weak, flexy mast. It's probably feasible to push the 19mm Axis mast ventilation to higher speeds, but it won't be the nice, simple hundred buck mast that we currently have. As far as anyone knows, it's impossible to eliminate ventilation when speeds get high enough. It's the limiting factor on all kinds of high-speed watercraft. It's one reason why the fastest powerboats have weird surface-piercing propellers--the biggest reason is efficiency, but the secondary reason is that submerged drives will ventilate anyways--the designer might as well design for it.

There isn't any element of this situation that is unexpected or new. Most of the basic research on this stuff is 50 to 100 years old. Outboard motors from the 1920's have ventilation plates. Thicker, wider masts ventilate at lower speeds than thin, narrow ones. If you're having trouble with the issue, congratulations, you're pushing your gear to a higher level of performance than most folks. You'll probably get some relief from using the carbon mast, though even a duffer like me can feel some flex with a 999, and lots of flex with a 1150. I've never had a carbon Axis mast ventilate, but your mileage will vary.

Ventilation aside, I still prefer the Axis 19mm mast over anything else. I used a slingshot 15mm X 75cm mast for my boosted foil because the clamp on the motor was designed for that size. That is pretty much the standard aluminum mast for foils, probably everyone gets the extrusion from the same factory and then cuts it to length for their masts. I got my foil and drive system put together, turned the board over, and wiggled the wing around. I was fairly horrified. I can easily rotate the fuselage about 30 degrees and bend it sideways with just hand pressure to about 10 degrees. I know it's going to work anyway, my much heavier Takuma eFoil has the same mast, though it's super short. I now know why it's so short. Riding the Takuma is like driving a car with worn out, loose steering. It takes a while for any control input to actually do something. Not a great thing for precise control.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

Dontsink

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Re: Getting ejected out the front door...
« Reply #14 on: October 26, 2021, 11:28:47 AM »
KDmaui talks about waxing (with car wax) his setup because it gives him more speed.Roughly 1mph more.

If you ask race sailors they will tell you that a waxed smooth&clean hull is slower than a non waxed one(some surface tension thing)...but the key part is clean&smooth.
Waxed is faster than dirty and scratchy,it smooths the surface and prevents the dirt from sticking.
Serious racers sand and clean their boats like mad before a race, most recreational foilers do not.
We put our foils wet & scratched in a cover to dry out and all the crap that is in the seawater dries out and sticks to the foil.

I have noticed that my foil runs a bit "rough" at the beginning of a session.It feels a bit slower,more sluggish and then gets better.I think it takes a while for all the deposits to soften up and get washed away.
With wax it "feels" good from the beginning.

So maybe it will make your mast perform better,worth a try.

 


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