Does anyone have a handle on the aerodynamic effect of the depression in foredecks of all the boards these days?

Would it be insignificant or not?

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NotWal said..

Does anyone have a handle on the aerodynamic effect of the depression in foredecks of all the boards these days?

Would it be insignificant or not?

Well your sail is in similar wind, it's a 7 square metre, double-sided foil designed to maximise lift. It's developing say 25 kg of lift. The area of the depression can't be more than 0.5 of a square metre, it's not all that aerodynamic. A fraction of a kilo difference at best, insignificant.

If windsurfer designers wanted to generate vertical lift for any reason why would they go through a whole new lift to drag exercise with an extra "hull airfoil"? An airfoil you can't get clean airflow over? If designers wanted more lift in that area just do it with the hull, it's already in the water after all. Or just rake the sail to windward.

A fraction of a kilo lift right out at the nose would have good leverage on board trim, so maybe a depression lessens any unwanted lift you get if the nose unintentionally rises. But it may also increase the drag a little when you're under control.

I think, just a guess, that a hollow nose allows the nose to be thinner, lighter and less stiff. That's all good for reducing nasty jarring resonances as you hit a bit of untidy chop. Doesn't make tacking easy though.

Yes I guess aerodynamic effects are secondary and probably considered a nuisance. The dip in the hull has the obvious effect of lowering the centre of lift of the sail a bit and making the deck more flexible. I dare say no one designed the thing for aerodynamics. Probably they just built it tested it and it worked well enough.

I was wondering if the depression would increase or decrease nose lift.
In an apparent wind of say 30 kt aerodynamic effects are considerable.
On your fastest point of sail the apparent wind I think would be somewhat forward of beam say 60 deg off the nose (guess).
That's when you want optimum control with minimum hull area in the water.
If wind lifts the nose that contributes to less wetted are but more wind drag and possibly more control difficulty. It would apply a roll moment counter to your ballast (sailor weight) that would tend to exacerbate fin lift at speed.
Maybe the dip disrupts that tendency. Maybe it doesn't matter.

Under the assumption that less aerodynamic lift is better, you'd think the hollow makes sense as the nose looks less like a foil. I'd think though there's not much aerodynamic lift going on either way, concave or convex deck. In both cases lift would just be bluff body sort of stuff and a hollow deck wouldn't make much difference.

My reasons for thinking there's not much in the way of efficient aerodynamics going on are

1. that the shape of the trailing edge is all wrong and

2. that for ground effect you still need an exit slot that is not too much narrower than the leading edge slot, otherwise the air cannot get through, will just dam up, causing disproportionate drag.

Look at the possible streamlines with an apparent wind 60 degree off the nose, there's not much room for much aerodynamic flow to happen. A lot of air will just dam up at the hull\water interface or be faced with a flow path that is just too squeezy. Check out the black browed albatross in ground effect. The angle of attack is not extreme and it's not that close to the water so the squeeze ratio of air flowing under the wing is not high.

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NotWal said..

Does anyone have a handle on the aerodynamic effect of the depression in foredecks of all the boards these days?

Would it be insignificant or not?

It's called Dazzle Factor.
Many designers are looking for it.

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Ian K said..

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Going for the win without firing a shot Slowboat? ........ it's only 10.30.

Worked for Russia

i thought it sales gimmick, mistral pushed it a bit in 90s but not quite as extreme. Since changing to boards with it- sometimes you have to bail the water out i run my mastfoot further back (to where i used to run it with old longer slalom boards) & boom used higher on cutout which puts you bit more in touch with upper sail but is pain when you forget & set it to where you have done for last 20 years. Also hate gybing some of them until you get used to it.