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Sea trials are complete, took the board out into the surf with the home made rear wing today. Took some shims along just in case the zero angle of incidence needed adjusting. The lower angle worked to my favour, lifting later but at speed needing to be less forward on the board, so was more managable to control pitch at higher speeds. Gave a nice stable flight. There was a hum though, will need to investgate possible causes. And the rear wing did not break, ya hoo.

After the sea trials:



(No fairing added between wing and fuselage, might be the cause the the hum.


Counterbores applied to the wood were a neat flush fit, but when the glass was applied, the precision was lost, so this might also be the cause of the hum by way of turbulence. All final brush strokes of paint applied in the stream wise direction, to avoid roughness in the wrong direction.


Highly approximate, but I remembered the frequency of the hum, then when I came back to my car, by memory I sang the same hum and used the stroboscope on my mobile phone to measure the frequency. 146 Hz, rather low frequency.

Foil thickness distribution was "eye" balled, so maybe there are some parts of it causing too negative a pressure, leading to cavitation which also might be the reason for the hum. The wing feels reasonably stiff so would not have thought the hum was due to the wing being too flexible.


Oh and pumping the board works better now, used this once standing with kite to get the board quicker up to speed.

Edited by SoutherlyBuster
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Some googling, reading and applying some engineering common sense ....

There appears to be a lot of “wive’s tales” and snake oil surrounding the source of the humming, but also a lot of good information out there. From my reading it appears the shape of the trailing edge is the most important aspect to reduce or eliminate the humming. As the two flow streams from the top and bottom sides of the wing come together at the trailing edge, where the vorticies peel off is the important bit. If they peel off consistently on either the top, middle or bottom then no vibraton; if they peel off alternatively between the top and bottom surfaces, this causes a vibration and this is what the hum is. From a Yatching design book, there are some guideliness to reduce or even eliminate this vibration, see below for extracts and source of the book:




What I have is a rounded squared off trailing edge — not a favourable shape. So what I need is some thing like option 5 or 8. The fibre glass is wrapped around a shape like option 1, so as not to expose the wood underneath the fibre glass protective layer, I need to add some fairing to the trailing edge to make it one of the more favourable shapes, but also ensure I don’t make it too sharp to avoid cutting myself.


Thanks @Kamikuza for the suggestion, it lead me to find the above information. By the way your name appeared in some of the searches :) . Looks like a lot of other foilers experience the same problem. Also one suggestion was to apply some tape to one side of the trailing edge to stop the fluctuatin of where the vortices peel off, might be a quick way to see if this is where the vibration problems come from.


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That's interesting. I wonder if it matters that the article is talking about vertical surfaces rather than horizontal, and surfaces that are much further apart than kite foils are...

5 or 8 is going to lead to cuts when you kick the foil under water. I think I still have scars from kicking a stab last year.

I think my name pops up because I've asked questions and commented on my experiences, rather than offering anything useful :D Searching images for "axis kite foil" used to turn up my photo in first spot, but now for me it's the end of the second row :(

This topic is interesting to me because I've ridden two different foils of one brand, and one has whistled like a steam train, the other has been completely silent. We've got a guy with an Alpine foil that whistles so loudly you can hear him literally hundreds of meters away. He's swapped the wings and it still whistles. Another guy with the same foil is silent. Why?

Is the whistling related to vibrations, or is that something else? Both my Axis foils are smooth as silk through the water, but the kite foil would squeak softly if I loaded it up in a carve, and the S-series has a gentle whistle when running flat out. Neither are as bad as the Alpine, which is bloody loud.

So I think it comes mostly from the stab, or interaction between stab and water flow around mast and stab.


Is it vibration you're feeling too, or just the sound? With the tape residue on the other wing, it was feeling like I was driving over shingle rather than any noise...


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I applied some tape to the trailing edge starting on the top side level out extending beyond the trailing edge by about 2mm fold over and stick to the bottom surface of the foil. Humming gone (which I could hear and feel).Screenshot_20181117-201031.thumb.png.180c76eb21e4489a6653d3fc912ec9b6.png

The tape is very sticky, I use it for reinforcing my foam gliders, sticks like sh@#$/ $# on a blanket. Did not come off during the light wind session 6.6 to 9 knot wind with 18m FlySurfer Lotus.

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17 hours ago, Kamikuza said:

That's interesting. I wonder if it matters that the article is talking about vertical surfaces rather than horizontal, and surfaces that are much further apart than kite foils are...

5 or 8 is going to lead to cuts when you kick the foil under water. I think I still have scars from kicking a stab last year.

Is the whistling related to vibrations, or is that something else? Both my Axis foils are smooth as silk through the water, but the kite foil would squeak softly if I loaded it up in a carve, and the S-series has a gentle whistle when running flat out. Neither are as bad as the Alpine, which is bloody loud.


Vertical or horizontal makes no difference as gravity makes no difference here on the formation of the vorticies, and it is the vorticy formation alternating between either side of the foil that causes the vibration.

Surfaces being further apart, yes agree. I thought about this too, there must be some sizing effect going on. If you zoom in close enough there will always be a blunt end of the trailing edge. I suspect the characteristic length would be the boundary layer thickness. I expect the above article was an A to B comparison, same foil size, just alter the taper angle. So at some point a blunt end would make no noticable difference.

Agree some of these sharp edges look nasty for rider safety. Reckon the blunt hard trailing edge plus the relatively soft tape I used is a good solution, just a mater of find tape that lasts long enough. Helicopters use a special tape for the leading edge for abrasion resistence ... hmmm.

10 hours ago, Kamikuza said:

That's very interesting too. You stuck the tape to itself past the trailing edge? Basically just longer and thinner trailing edge, eh. So are you going to thin out your trailing out to a point? Or just slim it down some more?

Yes the tape was double backed onto itself to creat a thinner trailing edge. At the moment I am thinking, don’t mess with the foil, just use tape, main motovation is rider safety against cuts. The tape lasted well for one session, will see how long it lasts. Longer term solutions would be tapes with heat activated glue applied to the resin coating.

I suspect if the foil wing and mast were made of a very heavy highly damped material with zero vibration transmision you would not hear the wistling. Apart from the very low frequency vibrations that one would definitely feel through your feet, the high pitched frequnecies may be more noticable depending on the stiffness and mass of the mast and board, if the whistling frequency excites one of the natural frequencies of the mast and board, then it would be clearly audible, like a sounding board of a piano.

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I was thinking more differences in water pressure at different depths, and how they might travel.

My foil's TE are noticably squared, the front thicker than the rear.

Tape would be good for free ride but I doubt the racers would countenance it :D

I think the relative stiffness of the fuselage matters too -- wouldn't stiffer transmit more "sound"?

"Rumbles" are the turbulence in the water I think, and the trim difference between wing and stab feel similar but more rearward to me...

So where is the whistling actually coming from?

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14 hours ago, Kamikuza said:

I was thinking more differences in water pressure at different depths, and how they might travel.

So where is the whistling actually coming from?

Depth, ah right the p=density x depth x g. For the mast can be important but I doubt important for whistling.

Where is the whistling coming from? Well in my opinion, it is not just from one thing. Think of it like a guitar or violin, the sounds come from the vibrating string (our vortex sheet coming off the foilig trailing edge), the vibrations then get transfered to something which amplifies (the wing to mast to the board) the sound, then eventually to something that creates air pressure waves so we can hear it. If the vibrations were solely confined to the water, I doubt we would hear it. To amplify the vibrations, you need to tune the natural frequency of your strucutre to match the forced vibrations, hence why same foils with different mast/board some sing, some don’t. Ever pick a guitar and notice the sound rings nicely and others just sound dull? It is all about getting that resonnance right in the structure of the body and for guitars and violins the air space since it has it’s own set of natural frequencies.

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Is a whistle the same as a hum though? We're using them interchangeably but I think they're unique.

The Alpine is a definite hum by the character of the sound, and the DA would just start humming at a certain speed, like a switch was thrown.

My whistle grows in volume with board speed, and probably speed into the wind, I'd have to concentrate. There are mounting holes in the plate, I wonder if they're the source. It's really almost non-existent, very quiet -- unlike the DA or the Alpine.

Now I think of it too, the Alpine is a Tuttle mount on a Spots board...

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  • 1 year later...

Adding the tape to the trailing edge of my stabilizer wing initially stopped the whistling and vibrations, but gradually started coming loose, which attracted grit and gave overall a rough surface finish. The result was that the hydrofoil slowed down and became prone to stalling. The stalling was evident by riding along nicely, as speed increased I needed to put my weight further forward, putting high loads on the stabilizer wing, then a sudden pitch forward without the wings exiting the water. So it was time to remove the tape and fix it permanently by filing in a sharper trailing edge. In the above example from "The Principles of Yacht Design" figure 6.3, I went for option 5, with the exception of instead a knife edge a 0.5mm radius. I figured the finite radius would be more wear resistant, more robust to the edge breaking off and friendlier to the rider (besides I do not push my feed on the wings to kick them out). I cut a bit into the underlying ply wood, so had to re-coat with West System resin, let it cure, then file, sound down for a smooth finish -- no humps. Then used wet and dry sand paper. Finally, used some cut & polish for buffing up car paint jobs. The front wing was getting a bit dagy as well, so mixed up some resin with filler, then got the same treatment as the rear stabilizer wing. The wings felt smooth to the touch.

Gave it a burl yesterday in the surf. Straight away noticed the nice swish feel, so parasitic drag was significantly reduced. No more rear stabilizer stalls and usual speed was restored.

Still finding trouble at the higher speed that I can not lean further forward to balance out the board to keep lift constant, hydrofoil exits water and I go for a swim. Will try some shims on the stabilizer to tilt the trailing edge up to get more down force -- currently at zero degrees.


Also have in mind to make a new front wing, same method as the rear wing (plywood, hand shaped, covered in fibre glass), making the front wing a higher aspect ratio than the one that came with the Naish hydrofoil.

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  • 2 weeks later...

Adding the shims made the ride go horrible, so took them off. They probably also added some nasty flow separation.

@Kamikuza, your comment of don’t try and lean forward rather move your COG forward, makes no sense. Perhaps we are saying the same thing, when I say lean forward, I mean shifting my body forward which then shifts the COG forward. Yes I know simply pushing down on the front foot does nothing to balance the board out because the COG has not changed.

At the moment the biggest helper for me was to reduce the kite size now that my skill level is better, that way I can depower more once the board speed goes up. Launches are a bit harder but I have the knack of that now. Also a kite with good upwind capability helps greatly to body drag out past the breakers in the surf.

By the way, those FlySurfer Peak 4’s have caught my attention, some of the local foilers here in Christchurch New Zealand are using them in the surf with rediculously small kite sizes. Anyone try the Born Race Stars for hydrofoiling?

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It may well be cross-talk...

If you leaning from the waist over your front foot, you're doing a lot less to shift your CoG, and are upsetting your balance.

The answer is Yoga lunges -- keep your torso upright and bend your front knee.

Yeah I quickly went from 13m tube to 12m Speed3, then 10m Crossbow. Had the 21 for super-light winds, but as soon as I was overpowered (10 knots maybe) I'd go straight to the 10m.

It's the same now. If I can't water start on the 10m, it wouldn't be much fun anyway. 

Key thing is a good power stroke, then being able to turn the power right down. When you get used to loading up the kite and foil like a racer, power is less important.

Honestly, the estuary is great. Getting out through the waves at Brighton is still a bit tricky :D

Peak 4 interests me too. Hopefully get a Debi this year...

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  • 1 month later...

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