Questions for Quad Riders ........

[KenM]

Zorro, yes I have a couple of them and I love 'em. Have a new one on order right now.

[zorro]

Hi Kenm

thanks for the quick response.

In your boards do you feel this "quad boost?"
If so, how do you reckon it works?
And when do you feel it?

And out of interest: have you ridden a tri in the past?


Anyone riding a 'blast' got anything to say on this subject???

regards

I see I have two votes for "el zorro está de sus tuercas!!!!"

[W.G. Facenda]

Zorro! I'm feeling the boost Baby!! [/i]

[zorro]

[zorro]

few ideas to be transposed from aerodynamics

idea: americas cup etc spends big dough on research in flow under and above the water:

Traditional view of "slot effect" in sailing.

http://www.catalina-capri-25s.org/racin ... g_tips.asp

As a sailboat moves forward, it gulps huge quantities of air in the large opening between the bow and the mast, and the jib funnels that air into the narrow slot between the mainsail and the jib. When the slot narrows, it creates a venturi effect. The venturi causes the air pressure to increase as it passes through the narrow slot. After passing through the venturi, the wind generates more power than it would have, had there not been a venturi. This principle is important, because it helps us to understand the importance of precisely trimming the mainsail and the jib in relation to each other, so that the slot between the mainsail and the jib is neither too wide, nor too narrow. When the leading edge of the mainsail begins to lift and flutter, that tells you that you are trying to stuff more air into the slot than the slot is capable of accepting.

Versus the antithesis

http://www.arvelgentry.com/magaz/Anothe ... Effect.pdf

(this guy is the equivalent of mtbarrels)

Thus we see that the primary effect of a jib is to causereduced velocities over the forward-lee part of the main, rather than increased velocities. The slower velocities in turn give reduced pressure gradients that help prevent separation and stall rather than some higher speed "revitalization."The velocities in the slot are determined by the total effects of the circulation fields around the two sails necessary to give smooth flow off the leeches (the Kutta condition).
"The flow about any lifting airfoil can be thought of as the addition of a circulation flow and a non-circulating flow... The direction of the circulation flow goes forward over the windward surface, around the luff and then toward the rear on the lee side of the airfoil... Note that the circulation flows for the jib/genoa and the main oppose and tend to cancel each other out in the slot between the jib and the main. This fact tells us that we will not get all the increased air speed in the slot that is claimed by the old ("Venturi Effect") theories... Some of the air that we would think might go through the slot is actually diverted by the combined circulation flows so that it goes on the leeward side of the genoa instead."
.snip
"The high velocities created by the mainsail in the region of the genoa leech in effect end up causing increased velocities and reduced pressures all along the lee side of the genoa... So, the main increases the upwash (a lifting wind shift) for the jib. This gives the genoa its great drive and also helps keep the lee-side genoa flow from separating. Proper mainsail trim is very important in getting the most out of the genoa."

or the economic driven:

http://www.profurl.com/uk/produits/bome ... rmance.htm

The slot effect (patented)

Extensive tunnel tests carried out by PROFURL at the Centre of Research for Architecture and Nautical Industry (CRAIN) have clearly shown the advantage of creating a gap between the mainsail and the mast, to create a slot effect commonly used in the aeronautical field: when aircraft prepare for landing they open a foil at the front end of the wings to increase the lift and avoid airflow detachment.

The PROFURL in-boom system is a direct application of this feature.

With the luff profiles located behind the mast the airflow is literally sucked into the gap between the mast and the sail with a sudden acceleration, which dramatically reduces the area of the sail affected by turbulence.

The "working" area of the sail and the power are increased by 5 to 15%. On the other hand the "slot effect" makes the airflow more stable in spite of minor wind or course changes, making the power of the mainsail more constant.

more reading

http://www.onemetre.net/Design/Slot/slot.htm

http://www.esotec.co.nz/condorkeel/Html/hydro3D.html

http://boatdesign.net/forums/showthread.php?t=457

Q: could it be that quads "help prevent separation and stall rather than some higher speed "revitalization."" ?

Q2: can aerodynamics be used to model hydrodynamics?


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Genoa def:

large front sail: a very large triangular front sail on a sailboat, especially a racing yacht

anyone know what a geno jib looks like?

[willli]

my reference point for quad is a Blast, but your initial treatise seemed to rule them out as the trailers are closer to the stringer (no Headwax effect?).

But I will say this: my Blast fish delivers huge projection out of turns (more burn than boost). I attributed that to its pedigree (Hawaii and all the great Hawaiian waves that influenced Bud + Lis) The question about raised skegs and water deflection, or a feeling of "boost" may in the latter be a simple dramatic reduction of friction (skeg area suddenly half, rail engaged) or in the former a photo illusion.

The answer may lie in a board that moves water very well (say a Hart design) on which you are always moving efficiently but you don't feel boost cos the transitions are so smooth, say the difference between a stick shift and automatic. Overall you KNOW you're surfing fast cos you can feel the wind on your face, but you don't feel a lurch of speed, actually more like the wave itself slows down cos on a relative basis you're gliding with it as you play with the bottom and lip and hook. But this digression doesn't apply, alas, Hart uses a mere three skegs (or five) so that makes him odd, this being an even discussion.

[willli]

oops

well its all fluid dynamics with one caveat, under the pressure/speed WE surfers work at most of the claimed effects are negligible. Air is a compressible fluid and all the discussions concerning "slot effect" ect. are applicable to trying to keep a completely flexible material (sailcloth) in a desired shape to maximize wind power usage. remember as the boat gains speed the real wind and apparent wind diverge. the slot is tuned so you don't "backwind" the main, ie have the air spilling off the trailing edge of the jib (or jenny) distort the shape of the main, at its most basic.

why this example came into usage to describe skeg pairs is beyond me, and why sailing texts are quoted is even more baffling. Be a giant, lift the sailboat and dip the sails in the water and see drag at work.

[zorro]

hi willli

I asked about the blast riders for that reason.
Did they feel this "boost"?
Havn't found out yet

And your right: air isn't water and visa versa

but.... I think it gives us an idea of how to think about what is happening.
(that's if we want to think about it ). Maybe a little understanding will lead to a refinement


AS far as flexable sails: from memory, this guy based his work on rigid foils:

http://www.arvelgentry.com/magaz/Anothe ... Effect.pdf

it's interesting reading I think. The slott effect is so well known in sailing and accepted. Why would that be?

Perhaps this fellow shows that it works (?) in another way to what people have traditionally felt....

cheers

[john -]

ah sorry lads Ive had to jump the phd stuff

wax, the identification off the effect becomes more focused with this conversation and I might change or add my "when" to also, when looking down the line after the bottom turn and seeking the best part of the wave to get some speed on.

at that point some engagement can be harnessed as I aim the board at the best part of the face to then piviot off for further speed

that movement, towards the mid point of the wave, is a boost point

critical in 2 foot surf

[crox]

Hi Zorro

In answer to your question ...when I put the link on to that board I wrote - My quad is fitted with the new Turbo Fins which are foiled on both sides and curved as in this shot of a stand-up fish - so the answer is no!!!!!

My board will fly in the face of all reason & be a pulled in rounded pin .......& I just don't care

However I will have a board very similar to that one soon....with spiral V

I am very interested in the idea of having the rear fins set fairly well back as the drivers [& I want DRIVE] & the fronts to set up the water flow but this is going to take some working out

[frankfqr]

Just food for thought, and it might be off base, but if the "boost" effect is felt primarily during an edge turn could it be that at that point we are disengaging any possible negative vortex off the tail? When looking at aerodynamics and hydrodynamics they both contend with this force. But not just the flow off both rails. Could it be that with the quad fin set up we are creating a more defined vortex area. And simply turning should break that. Just food for thought.

[willli]

yes, the aerodynamics of sails are a hot topic among those who depend upon wind to move, but use your head, do you see any of it (slot effect) repeated beneath the water line in yacht research? do you see any examples in nature among marine animals using it? Just cos skegs look like sails doesn't mean aerodynamics=hydrodynamics.

[crox]

So are you saying Willli that one fin in front of the other can't direct & enhance water flow?

[zorro]

Dorje wrote:

ah sorry lads Ive had to jump the phd stuff

my apologies, shouldn't have tried for an explanation but kept it to subjective observation...

http://www.arvelgentry.com/magaz/Anothe ... Effect.pdf

in summary: (from memory) this reference demonstrates, for certain conditions, the air between the sails actually slows down. But the air on the leeward side of the main sail speeds up.

What's that got to do with quads??

If (and only if ) you can use this as a "model" for thinking about
quads - the back fin is the equivalent of the main sail (by position not size). The lee side would be the inside edge of the fin (closest to the stringer).

That means that with certain quad setups you would find water travelling faster around the inside edge of the back fin because of the relationship of the two fins.

sure this speed up happens in tris and twinnies,

but the important point is that, in a quad, one fin is unfluencing another to speed up water - not in the "slot" but around the back fin.

Result a quicker, possibly more drivey turn.

Which is exactly what I feel.

willli said:

but use your head, do you see any of it (slot effect) repeated beneath the water line in yacht research?

quick pseudo references for Willli more later;

http://www.esotec.co.nz/condorkeel/Html/condor_F.html

sorry Crox, I realised it wasn't your board after I posted it

franfqr

Hi, could you explain more about this please?

But not just the flow off both rails. Could it be that with the quad fin set up we are creating a more defined vortex area.

BTY way Willli, its good to have the "devils'" advocate to keep us sensible

[zorro]

Crox wrote:

So are you saying Willli that one fin in front of the other can't direct & enhance water flow?

interested in the answer