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Daggerboard - use of?

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G.R.F. View Drop Down
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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 20 Sep 08 at 9:05pm
Originally posted by JimC

   These folk claim that multihulls which can't handle the
shedloads of wind but can do three times wind speed as opposed to not
much more than one times are far more efficient than boards. Frankly
that's ****, and I suspect you would agree.

Now extrapolate the logic...



Well as it happens, I've got a part built mini cat in the shed right now, I'm
hoping to improve on the efficiency ratio a bit further up the wind scale.

Now you're entering a whole world of theory of how to go fast(er)

Windsurfers can get twice windspeed, they have their moments..

But having come quite close to Cats in that round island race, I have to
accept they're fast, it's down to the wide footprint to hold the (big)rig up
there, and the well reduced wetted area particularly when they fly a hull.

Now this is going right off topic, I'm in the closing stages of my speed cat
board thing, I'll bung some pics up when it's finished, I'm also trying to
get a special paint surface made from a powdered hydrophillic compound
to coat the under hull surface with.

So we could do the Cat discussion on that thread as and when I start it.
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redback View Drop Down
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 22 Sep 08 at 10:56pm

Just getting back to the part of the topic about moving the board back.  An alternative is to move the rig forward!  The 4000 as with most skiffs do this by putting up another sail - often at least as big again and all forward of the mast.

 

With the 4000 its easy to get lee-helm so you wouldn't want to bring the board back any further.

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Post Options Post Options   Quote Granite Quote  Post ReplyReply Direct Link To This Post Posted: 22 Sep 08 at 11:13pm

Originally posted by G.R.F.

So then we're all agreed now are we?

That when the bottom of the foil goes ahead of the top, in 'filled in' wind,
it acts in such a way that to continue efficiently it needs to be moved
aft..? 

From my (basic) understanding of aerodynamics I cant think how sweeping the foil forward by 5 degrees would be significantly different to sweeping it aft by 5 degrees.

With the aft swept foil there is likely to be slightly more spanwise flow but this would make the foil less efficient. The spanwise flow contribution from the sweep is likely to be small compared to that from the pressure differential at the tip of the foil.

Sweep on aircraft is usually about reducing the drag at transonic speeds.

Forward swept wings on aircraft would be equally good at reducing the the drag at transonic speeds but due to flexing can cause all sorts of control problems, partiularly as the ailerons are at the tips and can cause the tips to flex even more leading to control reversal.

I think the problem seen is more to do with the forward raked foil twisting, increasing angle of attack and stalling. If you could build it stiff enough I do not think this would happen.

What were your windurf fins made from?

 

If it doesn't break it's too heavy; if it does it wasn't built right
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G.R.F. View Drop Down
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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 22 Sep 08 at 11:26pm
Windsurf fins, at least the sort that would make a difference were
generally carbon, or milled G10, a hi modulus super dimensionally stable
compound.

The sort of fin that has the effect I'm speaking of, were called blade fins, tall
hi aspect foils, generally fast profiles, although there were experiments
with slower profiles merging into negative almost reverse foil to induce lateral
resistance, but maximising the leverage for and aft, windward performance
and attack angles over and extremely narrow operating shaft.

Difficult to describe, I need pics but it was a while ago and i can't find any
on tinternet.

Anyway these fins were used to actually describe the planing angle the
board would attain.

Imagine the foil of the boat being designed in such a way that it would
resist stern drag, or nose dives by controlling the boats planing attitude.
I guess in the way those foilers must work, but without the horizontal
axis they use.

Probably unlikely it could be achieved given the greater weight of a boat
over a board and thus greater forces.

Edited by G.R.F.
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 22 Sep 08 at 11:47pm

I guess you've seen the "T" foil rudders used on some I14 and Cherubs. 

As for rake angle - I have experimented with this myself.  Its not so good with a forward raked foil.  I have no evidence for this but it didn't feel good if it stalled.  I surmise that when the foil stalls the turbulence starts at the tip and works up the foil.  I imagined the turbulence was "washed" up the foil if the rake was forward.  If however the foil was raked aft the turbulence was "washed" downwards and so it didn't stall so suddenly and the stall seemd to be released quickly if the angle of attack was quickly reduced.

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