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redback View Drop Down
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Posts: 1502 		Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Topic: Daggerboard - use of?
    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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G.R.F. View Drop Down
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Posts: 4028 		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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Granite View Drop Down
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Posts: 476 		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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redback View Drop Down
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Posts: 1502 		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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G.R.F. View Drop Down
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Posts: 4028 		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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Posts: 3941 		Post Options Post Options   Quote Black no sugar Quote  Post ReplyReply Direct Link To This Post Posted: 20 Sep 08 at 8:46pm
Yeah but can you take your sandwiches on board?
Lancing SC
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G.R.F. View Drop Down
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Posts: 4028 		Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 20 Sep 08 at 8:45pm
50 knots has been broken this week or it was then they downgraded it
apparently
Kites

Edited by G.R.F.
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JimC View Drop Down
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Posts: 6661 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 20 Sep 08 at 8:44pm
Originally posted by G.R.F.

So then we're all agreed now are we?
Yes?

No. Its an over simplification which relates to certain specific circumstances, most especially an over flexible foil.

Originally posted by G.R.F.

why skiffs in their present
incarnation are inefficient


Oh good grief, here we go again... Skiffs are course racing boats. They're not intended to hit top speed on a limited number of directions only, so amazingly enough they're not as good at it as craft that are...

And the fastest course racing boat in the World are probably the solid wing sail C Class Cats, and according to the guys that sail them they struggle to get much past 20 knots... However they sail close to that *all the time* at amazing apparent wind angles... And in some repects, notably speed achieved as a multiples of wind strength, they are some of the most efficient water craft of all...

One of the dimmer witted statements you'll hear round the net is that boards aren't very efficient because whilst they can get damn close to 50knots, and will probably get there very soon (possibly beaten by the kiters), they can only do it is shedloads of wind. 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...

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alstorer View Drop Down
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Posts: 2899 		Post Options Post Options   Quote alstorer Quote  Post ReplyReply Direct Link To This Post Posted: 20 Sep 08 at 7:54pm
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..?

Yes?


So how do we now feel about planing skiffs with fixed foils when the nose
is lifted by the Spinnaker and or a wave and that action has the very same
effect?

Which is the point I was trying to make, and why skiffs in their present
incarnation are inefficient and will, like foiling moths, always have a
'speed ceiling' created by that inefficiency amongst others..(Like stern
drag or 'tail walking')

Interesting looking at things from a windsurfers perspective isn't it?


There's many trade offs to be had in boat design- that's why the 50knot mark is being chased by windsurfers, wierd solid wing one-tack specials and a massive foiling trimaran..

And whilst the Moth and Cherub types might be able to make use of this perspective, those of us on stricter classes just have to learn to live with what we've got.

But yes, interesting. As ever, no definitive answer.
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Posts: 68 		Post Options Post Options   Quote dopamine Quote  Post ReplyReply Direct Link To This Post Posted: 20 Sep 08 at 7:04pm
Originally posted by redback

You are right - ideally the board should sweep back and
even better reduce in area but at high speed we'd not
want all the turbulence caused by a centreboard case, so
we'll have to stick to a dagger. I really must remember
to get the board up a bit in a blow it might help reduce
that horrible twitchy feeling when we're doing over
15knots.


But, in my limited experience, the 4k really lifts its nose when the kite powers up. The RS800 sails much flatter and you can sense it wanting to pitchpole you.

Looks like the Alto pole is quite high, lifting the bow?
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