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