ground effect problem

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Created by

decrepit > 9 months ago, 2 Oct 2014

decrepit

12802 posts

2 Oct 2014 6:31pm

I'm converting an ancient board to experiment with ground effect. I've cut the back off, so it's nice and wide, made it a single concave, with flats near the rails. Mounted 2 long US finboxes on the edge of the flats, about 70mm in from the rails.
After I'd mounted the boxes parallel, I had a thought they probably should be toed in. The displaced water, is going towards the rails, so the flow at the leading edge of the fin, is coming at whatever this angle is, +/- the boards angle of attack.

So my question is, does anybody have a clue how to work out what this flow angle is?

It's a bit late now to change the box angle, but as I haven't made the fins yet, I can make them assy, that should solve the problem.

And don't worry, my first experiments will be on light wind days only!
After Elmo's dire warnings, I'd hate to end up with a broken neck.

mathew

QLD

2142 posts

3 Oct 2014 10:20am

got a pic?

Haggar

QLD

1670 posts

3 Oct 2014 2:31pm

Some of the early speed boards had a single concave. Windaction made one and the concave was quite deep I think. Is this similar ?

decrepit

12802 posts

3 Oct 2014 1:19pm

OK I guess a pic is worth a thousand words, at the moment it's wrapped in black plastic getting a sun cure.

unwrapped just for you.

You can see how subtle the concave is.

Actually I already had a go at this several years ago, but then the board had twin concaves and the fins were right in the middle of the concaves.
It accelerated to 28kts then went sideways. So I've finally got around to changing the bottom to single concave in order to keep the fins in the water.
But now I'm thinking part of the problem may have been the sideways flow under the hull.

Yoyo has just phoned me to say his software predicts a 1 to 2 deg sideways flow, which shouldn't cause too much of a problem. He's printed the flow out so you may just see it here a bit later on.

I've used the long US boxes so I can also use long shallow fins, keeping the leading edge inside the box, so there's not that horrible weed trap you get with front overhang.

Haggar

QLD

1670 posts

3 Oct 2014 6:15pm

Decrepit do think that because the cutout is in the middle that there may be an action for a low pressure to be created and suck the board onto the water ?

decrepit

12802 posts

3 Oct 2014 8:35pm

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Haggar said..
Decrepit do think that because the cutout is in the middle that there may be an action for a low pressure to be created and suck the board onto the water ?

Hmmm, possibly haggar. but as it's behind the rider, it's going to unstick the board, not make it more sticky. But I think the water squirting oout the back will probably fill most of the gap.

Simon100

QLD

490 posts

5 Oct 2014 9:50am

Im lost as to what your doing here , do you mean ground effect as in you want the whole board to fly on a cushion of air ? Towing them in would only make it slower wouldnt it , the board would only influence the flow near the fin at the very top .

decrepit

12802 posts

5 Oct 2014 10:08am

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Simon100 said..
Im lost as to what your doing here , do you mean ground effect as in you want the whole board to fly on a cushion of air ? Towing them in would only make it slower wouldnt it , the board would only influence the flow near the fin at the very top .

NO, nothing to do with air. I'm talking about ground effect in shallow water. It's harder for water under the board to be displaced when it's shallow, so you get more lift, and need less wetted surface area. There is a relationship between board width and water depth, so a wider board makes it harder for the water to be displaced sideways.
I've reasoned that using 2 fins close to each rail will also make it harder for water to be displaced sideways, and 2 shorter fins will allow entry into shallow water.

So because the flow under the board, isn't parallel, any displaced water is going sideways, the fins should probably be inline with that flow.

Looking at Yoyo's printout, (and it isn't for shallow water) pronounced sideways flow only happens very close to the rails, it looks like I have the fin boxes in far enough to be in a sideways flow of a degree or so.
We'll see what happens when it hots the water.

Lessacher

89 posts

5 Oct 2014 3:58pm

In front you get vacuum.Thats not good. I took 1cm higher 2mm alu in. I stopped in front 1cm earlier. The vacuum takes air between the bottum and aluminium
to the front and reduce the vacuum. No pipes and so on. Wolfgang

N1GEL

NSW

861 posts

6 Oct 2014 10:19am

Toe-in is important for generating lift during a turn... if the fins are parallel to the rail, the board will be more inclined to bog and loose speed when on a rail/carving. I've played with angles (including the upright angle... fins should be leaning outwards) a lot with surfboards I've shaped - particularly twinnies, which have been my main go-to surfboards for well over a decade (unless it's massive). There's heaps of info out there on the web about toe-in wrt to surfboard shaping. I can only assume the same applies to a sailboard, although sailboards are on a rail far less than a surfboard.

decrepit

12802 posts

6 Oct 2014 10:48am

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2035 said..
Toe-in is important for generating lift during a turn... if the fins are parallel to the rail, the board will be more inclined to bog and loose speed when on a rail/carving. I've played with angles (including the upright angle... fins should be leaning outwards) a lot with surfboards I've shaped - particularly twinnies, which have been my main go-to surfboards for well over a decade (unless it's massive). There's heaps of info out there on the web about toe-in wrt to surfboard shaping. I can only assume the same applies to a sailboard, although sailboards are on a rail far less than a surfboard.

All very true, if you're talking waves and/or manoeuvrability, (and wave sailing isn't much different to surfing), but I don't expect to be able to carve gybe this board, may be it'll flair gybe ok, I'm content to stop at the end of each run, get off and turn the board around.

What I'm after is minimum drag.

mathew

QLD

2142 posts

6 Oct 2014 1:17pm

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decrepit said..
What I'm after is minimum drag.

What you *will* get is interference of the flow-displacement of the water from one fin, impinging on the other fin.

If anyone out there has seen a Evil-twin with two different sized fins, that was me... I couldn't stand the effect of having one fin spin-out due to flow from the other fin... ( turns out a Big.little makes it behave like a thruster ).

Whether that is flow-interaction is a good thing... who is to say.... maybe there is a reason we dont see bi-planes nowadays!

fangman

1906 posts

6 Oct 2014 11:32am

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mathew said..

decrepit said..
What I'm after is minimum drag.

So how far apart do the fins need to be to stop them interacting, and I suppose the size of the fins makes a difference as well ..and then the toe in....and then the speed ...and then hydrodynamic ground effect/ shallowness...and then the board concave. Man, this is a pile of worms complicated!
its going to be interesting to see how the final result behaves and the effect of the different parameters.

mathew

QLD

2142 posts

6 Oct 2014 10:09pm

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fangman said..

So how far apart do the fins need to be to stop them interacting, and I suppose the size of the fins makes a difference as well ..

Depends... is water compressible?

fangman

1906 posts

6 Oct 2014 9:19pm

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mathew said..

fangman said..

So how far apart do the fins need to be to stop them interacting, and I suppose the size of the fins makes a difference as well ..

Depends... is water compressible?

I see what you are saying, but is it not reasonable that that compression wave will disperse in every direction and ultimately flow along the path of least resistance and as a corollary to that, will the fins act as gates and force that compression wave downward and therefore enhance the ground effect off the earth below in shallow water that I guess is partly what this board is all about? So does the downside of fin interaction outweigh the upside of potential ground effect? Hurry up and build this thing Decrepit so I can stop asking myself questions..I am getting more confused than normal.

decrepit

12802 posts

6 Oct 2014 10:30pm

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mathew said..
>>>

Depends... is water compressible?

No but there is a transmission speed, by the time it's got to the other fin, it's not there any more?

fangman

1906 posts

7 Oct 2014 8:59am

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decrepit said..

mathew said..
>>>

Depends... is water compressible?

No but there is a transmission speed, by the time it's got to the other fin, it's not there any more?

You are going to have to be going really fast! Transmission time in water is 1.5 km per second!

decrepit

12802 posts

7 Oct 2014 7:38pm

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fangman said..

decrepit said..

mathew said..
>>>

Depends... is water compressible?

No but there is a transmission speed, by the time it's got to the other fin, it's not there any more?

You are going to have to be going really fast! Transmission time in water is 1.5 km per second!

Ha you're talking sound. I don't think think the slight hum from one of my fins will disturb the other. I'm talking waves, more like 15kts

fangman

1906 posts

7 Oct 2014 8:37pm

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decrepit said..

fangman said..

decrepit said..

mathew said..
>>>

Depends... is water compressible?

No but there is a transmission speed, by the time it's got to the other fin, it's not there any more?

You are going to have to be going really fast! Transmission time in water is 1.5 km per second!

Ha you're talking sound. I don't think think the slight hum from one of my fins will disturb the other. I'm talking waves, more like 15kts

Decrep, I am going to have to beg to differ on this one. The fin foils will induce a pressure pulse by virtue of displacing the water. Tsunami waves ( a pressure pulse causes by water displacement on a massive scale) travel at over 700 km/h and even with the frictional losses the fin pulse will still be a very fast pulse : so as I reckon Matthew has a point. I reckon the fins will interact, it's just whether that can be done positively, will the pulse from each interact in the midline and cancel each other out or will they be out of phase and affect the each other.

stroppo

747 posts

7 Oct 2014 10:24pm

Hey I think the board and its weted out area will have more drag than the fin as the displacement of the fin on its first half than the suck back on the 2nd half of it is quite efficient overall the water pressure around the fin is more stable than the board on the surface even in smooth water does that make sense?

decrepit

12802 posts

7 Oct 2014 10:53pm

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fangman said..
>>>

Decrep, I am going to have to beg to differ on this one. The fin foils will induce a pressure pulse by virtue of displacing the water. Tsunami waves ( a pressure pulse causes by water displacement on a massive scale) travel at over 700 km/h and even with the frictional losses the fin pulse will still be a very fast pulse : so as I reckon Matthew has a point. I reckon the fins will interact, it's just whether that can be done positively, will the pulse from each interact in the midline and cancel each other out or will they be out of phase and affect the each other.

Hmm I guess, but is a change in pressure going to have all that much effect? And is there a square law distance thing happening?

Oh well, if it's too bad, may have to go with only one rail having a fin, then I guess the mast track has to be shifted to the same rail.
But which rail? the windward or leeward on the run. My guess is it will be better on the leeward rail, that way you can trim windward rail up and still have the fin in the water.

Remember the old vampire plywood jet that had twin fuselage and a rudder on each, admittedly you don't see them in the sky any more but they must have worked.
and according to wikepedia they did.

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The de Havilland DH.100 Vampire was a British jet fighter developed and manufactured by de Havilland. Having been developed during the Second World War to harness the newly developed jet engine, the Vampire entered service with the Royal Air Force (RAF) in 1945. It was the second jet fighter, after the Gloster Meteor, operated by the RAF and its first to be powered by a single jet engine.

The RAF used the Vampire as a front line fighter until 1953 before it assumed secondary roles such as pilot training. It was retired by the RAF in 1966, replaced by the Hawker Hunter and Gloster Javelin. It achieved several aviation firsts and records, including being the first jet aircraft to cross the Atlantic Ocean. The Vampire had many export sales and was operated by various air forces. It participated in subsequent conflicts such as the 1948 Arab?Israeli War, the Malayan emergency and the Rhodesian Bush War.

Almost 3,300 Vampires were manufactured, a quarter of them built under licence in other countries. The Royal Navy's first jet fighter was the Sea Vampire, a navalised variant which was operated from its aircraft carriers. The Vampire was developed into the DH.115 dual-seat trainer and the more advanced DH.112 Venom ground-attack and night fighter.

And the rudders looked very delterish.

decrepit

12802 posts

7 Oct 2014 10:54pm

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stroppo said..
Hey I think the board and its weted out area will have more drag than the fin as the displacement of the fin on its first half than the suck back on the 2nd half of it is quite efficient overall the water pressure around the fin is more stable than the board on the surface even in smooth water does that make sense?

Sorry Glen, not really

fangman

1906 posts

7 Oct 2014 11:31pm

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i thimk I get it Strop. I am a bit keen to see Decrepit's invention in action and see if we can put any of this conjecture to the test

mathew

QLD

2142 posts

8 Oct 2014 2:33pm

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decrepit said..

fangman said..

Decrep, I am going to have to beg to differ on this one. The fin foils will induce a pressure pulse by virtue of displacing the water. Tsunami waves ( a pressure pulse causes by water displacement on a massive scale) travel at over 700 km/h and even with the frictional losses the fin pulse will still be a very fast pulse : so as I reckon Matthew has a point. I reckon the fins will interact, it's just whether that can be done positively, will the pulse from each interact in the midline and cancel each other out or will they be out of phase and affect the each other.

Indeed we are talking about pressure waves... for the mathematically inclined, the "bulk modulus" is what explains this behaviour... simply put, speed-of-sound is correct. I'd still try two fins... either dont make them the same size and/or don't put them "inline" with each other... if done properly, you will actually get increased lift (think canard fins).

I'm not sure we can compare tail fins from planes, as air is significantly more compressible. Also (and I'm probably wrong about this, as my knowledge of planes is rather small), AFAICT most *modern* dual-tail planes don't have parallel foils.

decrepit

12802 posts

8 Oct 2014 7:26pm

Looks like I'd better get stuck in and get this beast on the water to see what actually happens.

choco

4177 posts

10 Oct 2014 10:05am

ground effect?

decrepit

12802 posts

10 Oct 2014 8:17pm

not sure about that, but very impressive!

decrepit

12802 posts

10 Nov 2014 7:14pm

Nothing very positive to report, haven't had a lot of luck so far. Tried it in light winds and got planning OK, but it was slow, sitting much too flat. So I reduced the rear planning area and that helped a bit, but found the lack of foot steering a bit frustrating, especially trying to bear off. So toed the fins in about 2deg and tried it in stronger conditions. Well I had some foot steering could even carve it into a gybe, but powered up there was lots of directional instability, possibly due to Mathews pressure wave fin interference. Took one fin out to test this idea but board was virtually unsailable.

I was talking to a kiter the other day with one of there formula boards, vaguely resembled my thing. Well it was wide and had 2 fins on the rails, (even though they were 40cm) but also had a 3rd rear fin. He didn't seem to have interference problems, so maybe my delta fins are too thin, and I'm having stalling problems with them, or the 3rd fin, makes the whole thing work.
So my next mod, is to put some of the rear of the board back with a tuttle box in it, so I can play around a bit more.

fangman

1906 posts

10 Nov 2014 9:21pm

Well if perseverance is worth anything, it will be a cracker! I am looking forward to seeing the next version on the water- have you got a sail to go with it?

decrepit

12802 posts

10 Nov 2014 10:04pm

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fangman said..
Well if perseverance is worth anything, it will be a cracker! I am looking forward to seeing the next version on the water- have you got a sail to go with it?

Well you did offer your 6.6!
Remember it's all for the good of science.

fangman

1906 posts

10 Nov 2014 11:41pm

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decrepit said..

Well you did offer your 6.6!
Remember it's all for the good of science.

"Research is what I'm doing when I don't know what I'm doing." Wernher von Braun

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