NSW
2360 posts
I have read on another forum the benefit of breaking the coanda effect.
It seems that some custom board guys already build their boards to achieve this. Others are adding a modification to achieve the same.
I am surprised that as this is a well know by experienced designers that this is not incorporated in current designs.
The closest I have observed is the Jimmy Lewis Flying V as it has a very sharp square release but no mention of the benefit.
Am I missing something.
3939 posts
You mean if you can't fair it, you chop it?
Applied to racing cars since 1950.
NSW
1351 posts
Any examples ?
This technical discussion has my interest!
3939 posts
Look at the tail of a slalom board.
Look at the tail of a wave board.
See any difference above the waterline?
143 posts
JP Australia SUP foil boards have a step tail, as do Kalama boards ( Turbotail ). That being said I have a Jimmy Lewis foilboard with a step tail and a ONE foilboard without the step tail ( just a bevel) and they both seem to pump onto the foil just fine in marginal conditions. Maybe the sharp edge works best for planing hills ( eg windsurfer) as less so for foiling hulls ? I also had a 5'3 inflatable foilboard with very rounded edges on the bottom and it pumped onto foil just fine - 3 pumps and off she went.
So it's an interesting hydrodynamic argument for sure, but my gut feel is that if you have good pumping technique and appropriately sized gear for the conditions ( wing , foil, board) then most things will work. FWIW
WA
527 posts
I have a wing board without a step (ONE 6'5") and one with a step (fanatic 6'). The fanatic seems to be sticky from rest and needs a couple of pumps to break out onto the plane and then onto the foil. The ONE seems to glide along and pick up speed from rest fairly easily, but then seems to get a bit sticky.
My guess is that the stepped tail needs some pumps to ventilate the area behind the step, which causes a fair bit of drag before that happens, whereas the ONE tail suffers from the Coanda effect, with the flow trying to stay attached from the bottom around onto the tail bevel.
Maybe the answer is to use a very small step, just 2-4mm to give minimal drag at very low speeds, but allow the flow to break away cleanly from the bevel once the board is moving. Maybe we are just overestimating the size of the step required. I might try adding this to the ONE board to see if it makes a difference.
