Fastest board quad or tri?

[Eric Carson]

My F2K quad feels faster than my tri, both shaped by Blast. All winter I rode my fish, but for the past two weeks I've been on the tri, having some serious fun, making quick adjustments and riding high in the pocket. Now I'm not so sure which board is faster.
Any input on board design would be appreciated.

[DrStrange]

So many variables, so little time. 3 vs 4 fins only small part of issue. Rocker counts for a lot. Outline template, rail shape, bottom contours (concaves, v, etc) thickness and thickness distribution. Probably a number of other variables all contribute to speed. So really can only compare one specific board to another specific board or at least very similar boards.

Good question if all other variables same still depends on fin placement, angles etc slow quad, fast tri, fast quad, slow tri. Depending...

[Beeline2.0]

..

[Bryan Jackson]

"...the less fins in the water the higher the potential, less resistance to overcome."

Actually this is a rather gross oversimplification of a much more subtle and sophisticated subject .

Considering just the dynamics of the fins themselves (ignoring the rail, the tail, the outline, the amount of rocker, and how it all works together with the fin set-up), with multi-fin set-ups the most important consideration is not necessarily the number of fins, but the amount of toe-in they possess (toe-in being the degree they are angled in towards the nose) .

The greater the toe-in the greater the drag they generate, at least when traveling in a straight line (how they behave when the board is turned is a whole other consideration), whether tri or quad .

Tri-fin designs (Thrusterlike set-up) tend to have a greater degree of toe-in than do quads, which tend to be more similar to a twin fin in this regard (in a way quads can be thought of as a doubled twin fin! ). Pure twin fins often have their fins set completely parallel to each other (Lis Fish, for example) in which case drag due to toe-in is minimised .

Thus all other things being equal (which, of course, they never are! ) a quad with minimal toe-in will be faster than a tri with maximal toe-in.

If you're looking for pure down-the-line speed then a single fin is, without question, the fastest set-up of all.

BTW, fins with one foiled side and one flat side are actually slower than fins foiled on both sides . A flat surface on the fin creates drag due to turbulence and cavitation . The water flow around a properly foiled fin is a smooth laminar flow . Everyone seems to have bought into the misconception (sales pitch?) that flat sided fins are faster cuz' they're skinnier and perhaps look they should be faster, but they really aren't .

[MTBarrels]

"...The greater the toe-in the greater the drag they generate, at least when traveling in a straight line...

BTW, fins with one foiled side and one flat side are actually slower than fins foiled on both sides . A flat surface on the fin creates drag due to turbulence and cavitation . The water flow around a properly foiled fin is a smooth laminar flow . Everyone seems to have bought into the misconception (sales pitch?) that flat sided fins are faster cuz' they're skinnier and perhaps look they should be faster, but they really aren't .

Actually, a small amount of toe-in can have a beneficial effect as, if properly designed, they act like poor man's Whitcomb winglets (e.g. check out the tips on a Gulfstream 4 corporate jet). The effect is to redirect some of the crossflow (some water would rather go around the hull than under it) associated with the finite aspect ratio (wetted width/wetted length) to more parallel to the longitudinal axis (increasing the lift per unit angle of attack, and hence reducing the induced drag).

Unfortunately, the proper amount of toe-in depends on the aspect ratio of the wetted area and the angle-of-attack of the bottom relative to the mean flow...and, in contrast with an airplane (where the winglets can be designed for an optimal cruise speed), this is highly variable for a kneeboard.

Symmetrical fins do not necessarily have less drag than a fin with different curvature on each sides (flat on one side being the limiting case). In fact the opposite is more typically the case if comparing fins with the same thickness/chord ratio, and identical planform and area.

Foils with asymetrical curvature are commonly symmetrical foils to which camber has been added. The purpose of the camber is to shift the angle-of-attack where the minimum profile drag occurs to where the foil is generating lift. As with the winglets, this occurs at a specific angle of attack and thus is more applicable to an airplane operated at design cruise speed than to a kneeboard operating over a wide range of speeds.

Check out the foil sections on highly engineered (but sub-sonic) aircraft, or look at the lift/drag polars for NACA sections.

Another effect of camber is that the angle-of-attack for zero lift is not along a line connecting the leading edge to the trailing edge. Rather the zero lift condition occurs with a small negative angle (i.e. at a small toe-in, if the fins are mounted on a kneeboard or surfboard with flat sided fins with the curved side out).

For example, the main foil on my HYPO (hydrofoil paipo) board is a Clark Y section (flat sided on the bottom) and its zero lift angle is a -4 degrees. Hence the leading edge is actually lower than the trailing edge since I want the zero lift angle to be parallel to the bottom of the hull. Visually, it gives the appearance that it would want to pull the board down, rather than support it.

FWIW, a common defect in some commercial molded surfboard fins is a sharp leading edge. For a foil to work to it's maximum potential the leading edge should be slightly rounded. The reason is that for a foil generating lift, the desired stagnation point (where the water splits with the "upper" half going over the "top" (more curved side) of the foil, and the remainder flowing "under") is "below" the leading edge. If the leading edge is sharp, this point will be shifted closer to the leading edge (reducing the lift created over the upper side) and separation of the flow may occur just aft of the leading edge on the "upper" surface--further reducing the lift generated.

MT

[albert]

I honestly think it has nothing to do with the board you are riding, but more with how well you ride it, as well as what wave you ride it on...

Thanks
Albert

[K-man]

Between bryant and mtb there's enough info to build 1great kb.2Americas cub racer.3 a small solar powered submarine.I'm siding with albert.Considering the fact most competent KBers have a board suited to their style, and the waves they surf.HEY,nobody mentioned hull speed!

[Doc you surf today?]

[MTBarrels]

I honestly think it has nothing to do with the board you are riding, but more with how well you ride it, as well as what wave you ride it on...

Thanks
Albert

That's a hypothesis that is capable of being tested.

[DarcyM]

That's a hypothesis that is capable of being tested.

Maybe ... in a wave pool or at a perfect point break.

Today I got this great wave, a slow peak that rolled onto a low tide section of the reef, and suddenly jacked what seemed like a thirty foot section in front of me that I had no choice but to backdoor through this massive slightly warped bowl, squeaking out the end just as it collapsed. I tried all day to find another one like it . ... but it was one of a kind.

I think that's gotta be a big part of what makes me keep surfing day after day.

[hart]

So many differing thoughts here..makes for such interesting reading.

If we look to our big brother for a moment, lets see what they are (and have been ) doing.

Pete Cabrinha has just surfed the biggest wave ever recorded in history. I wonder what he decided to risk his life with..even though I know the answer?

Once Pipeline was tamed by Mr Lopez..a whole new range of Pipe Masters has evolved. Carroll, Slater, Irons..just to name a few..lets figure what they were riding.

Over the past year, I have had the privelige to look at some of Irons' and Slater's boards.

They are now so comfortable with the speed that they are generating that Shapers like Merrick (Mainland) and Bushman (North Shore) are intentionally INCREASING curve forward of the front foot to allow Surfers to 'pocket' surf Pipe..they are, in effect, slowing the Surfer down.

My two main points here are:

1 Bottom curve does have enormous influence on speed (read more than fin number)

And

2 Surfers who risk their lives and do so professionally, all ride Thrusters.

Now, to our own kind..the fastest Kneeboarders that I have ever witnessed are (and in no particular order) Greenough, Crawford, Farrer, Novakov and Kyle Bryant..lightning quick down the line and throughout manoevres.

And none of these guys would ride (or have ridden) a quad.

Maybe it's just an Australian perspective..but I'm not sure that's a hinderance either.

Regards,

hart

[Bryan Jackson]

I never said that "Symmetrical fins...have less drag than a fin with different curvature on each side (flat on one side being the limiting case)..."

What I said was that fins with one foiled and one FLAT side create more drag than fins that are PROPERLY foiled on BOTH SIDES given that all other aspects of the fin design ("thickness/chord ratio, identical planform and area") are equal .

There is no doubt whatsover that this is true unless the fins are less than about 1 1/2 " across at the their widest point! Why? Because that is about as far as water can travel along a fin's surface before it starts to break into turbulent flow, and flows an even shorter distance along the flat side of a fin before it does the same thing (and turbulent flow equals increased drag) .

A flat sided fin is also more susceptible to cavitation, which means it will lose traction (i.e., the board will 'spin out') easier .

Fins with assymetrical foils, that is, each side is foiled differently (but each side is still foiled) and the fin is cambered (think of a fish fin or bird wing in a permanently flexed state) are already being developed for use on surfboards (and thus kneeboards) . In fact, that design is already commonly used for the blades on boat propellers (which, in a strange way, perfom a very similar function to that of surfboards fins!) due to the increased lift and power they provide .

[willli]

penny for your thoughts?
and some of these pennies are being squeezed awful hard!
I didn't feel qualified to comment on kneeboard thrusters cos my first boards were Blast quads, but now I have a remarkable thruster from Hartdesign and guess what?
I'm still not qualified. My two fastest boards... meaning what? when I enter the pit I have a high probability of coming out smiling? What the wave gives the board taketh away? Or does the skill of the rider have more bearing?
This funny argument about flat sided skegs... probably has more to do with the cost of manufacture than intentional design which is a circular argument cos if dual foil side skegs cost $75 each, AND because of less material in crossection were prone to breaking, how long would the company retain its loyal base of drag reducers?
Now among standups there is a definite shift toward the fish. Its #1 on the wish list of summer mush chargers looking for a 'funboard".. oxymoron.. cos all surfboards are supposed to be fun.. unless of course you are a SERIOUS charger in which case the mysto life and death aspects of moving mountains consume you.. but.. Bud's interpretation of the fish specifically for the respectful few who kneel is an opposite concept to the standing world. I have one, and you can't understand the tisk tisk tut tut "you'll likely get yourself killed!" standing semi-gun hurricane swell crowd commentary as you prepare to paddle out to Long Island's best impression of a loooong feathering wall. And they can't get their heads around the SPEED these boards are capable of. Of course, made in Hawaii helps them understand, a little, but THEIR interpretation of the fish design makes ME look like I'm crazy, to them. I once told a kook that by kneeling my farts took on the proper thrust angle and all I needed to generate the insane speed I was getting on my fish was a large plate of beans two hours before surfing!
SPEED! you want the SPEED? YOU CAN'T HANDLE THE SPEED!
which brings me to the Flashie. my Mysto board. Bruce prob doesn't know I've owned a few Merricks. Al tries to create what Bruce has already succeeeded in doing. I knew it when I pulled the board from the box in a shipping bay at Kennedy Airport, checking for dings and such from half round the world travel. I got lost in the bottom contours, the rails, rocker, planshape.. this board felt like nothing I had ever owned.. no stock concepts here, every curve thoughtfully placed to enhance overall design..
Does it matter that its a thruster? YES, by design. It could be nothing else. It was designed for A frames but does MUCH much more.
So what of this SPEED thing. To use cars as an analogy.. could a dragster win at Le mans, or Indy? The problem is really one of matching capability to conditions. The "slowing the board down at pipe" cos I want to stay deep in the hook question. Some want to fly from danger, some want to dance with it. Speed is a relative question.

[MTBarrels]

I never said that "Symmetrical fins...have less drag than a fin with different curvature on each side (flat on one side being the limiting case)..."

What I said was that fins with one foiled and one FLAT side create more drag than fins that are PROPERLY foiled on BOTH SIDES given that all other aspects of the fin design ("thickness/chord ratio, identical planform and area") are equal .

I concede that I was in error when I stated that a foil with one convex face and one flat face is a limiting case.

What I should have said is: The two limiting cases are: (1) a symmetrical foil with convex surfaces (ignoring for the time being that some specialized symmetrical foils may have both convex and concave surfaces on each side), and (2) a foil that is convex on one face, and essentially equally concave on the other face (analogous to a leaf in a Venetian blind). A foil with one convex side and one flat side just represents one possibility in the continuum of shapes in transitioning from one limit to the other--i.e. it still "foiled". The question becomes, is it "properly foiled"?

The convex/concave foil can be most efficient foil section in some circumstances (typically at low Reynolds number and operated at a unique value of the AOA). The convex/flat foil can be the most efficient for a different set of conditions (it is very often used in "bush planes"), and a symmetrically convex foil for another condition (typically only used on aerobatic planes, stuts, rudders, or other similar situtations as it is the most "efficient" (in this case, meaning "desirable") foil only when operated at zero angle of attack--and hence not performing any useful function other than minimizing drag.

So it's really a tautology to insist now that by "properly foiled" you mean the most efficient foil...and inherently in error to also say that a 'fin properly foiled on both sides has less drag than a foil with a flat surface on one side' since for some circumstances, the foil with one flat side may be the "properly foiled" foil.

There is no doubt whatsover that this is true unless the fins are less than about 1 1/2 " across at the their widest point! Why? Because that is about as far as water can travel along a fin's surface before it starts to break into turbulent flow, and flows an even shorter distance along the flat side of a fin before it does the same thing (and turbulent flow equals increased drag) .

The transition from laminar flow to turbulent flow for a flat plate parallel to the flow is at a Reynolds number of about 5 x 10^5. At 20 ft/sec (~15mph) this would correspond to a transition point about 3.2 inches from the leading edge. So are you assuming that the surfer is going > 30mph? (I think that's quite excessive for a surfer on the size of waves most commonly surfed).

But that's for a plate parallel to the flow. For a flat-bottomed foil at a positive angle of attack, there will be a positive pressure gradient along its surface and the transition to turbulence will be suppressed.

On the top side of either foil there will be a positive pressure gradient on the leading portion of a foiled surface--but this changes to a negative pressure gradient at some point across the foil. This change promotes the onset of turbulence somewhere in the region of the decreasing positive pressure gradient--and for sure by the point where a negative gradient has developed. This not only tends to "trip" the onset of turbulence (if it hasn't already occurred), but can lead to separated flow and greatly enhanced turbulence.

The situation on the bottom side of a foil in which the bottom side is convex represents an intermediate condition. Again the forward portion of the foil has a postive pressure gradient (and even greater than for the flat plate). But this gradient peaks close to the leading edge and decreases quickly with increasing distance away from the leading edge. Thus the positive gradient can (will) become less than on the bottom of the foil with the flat surface with increasing distance from the leading edge. Hence this foiled surface will become more prone to "trip" the onset of turbulence than will the flat surface--again, if it hasn't already occurred by this point.

Hence to determine which foil has the least drag (and what range of conditions it holds that advantage) requires some serious computational fluid dynamics boundary layer simulations.

A flat sided fin is also more susceptible to cavitation, which means it will lose traction (i.e., the board will 'spin out') easier .

I presume that you intended to use the term "ventilate" (suck in air from above the sea surface to the low pressure side of the foil) rather than "cavitate" (the pressure on the "suction" side of the foil becomes so low that the water boils, filling the resulting cavity with water vapor). The latter condition is virtually impossible to achieve with the relatively thin (thickness/chord ratio) foils used on surfboards and at the speeds they travel?

I am curious as to why you would come to the conclusion that a foil convex on one side and flat on on the other would be more prone to ventilation than one with convex surfaces on both sides? My guess would be that the opposite would be the case (for the same thickness/chord ratio). Since the effect is related to the minimum pressure created by the foil, we need a something like a copy of XFOIL or some other equivalent computational package to determine which foil (convex/flat vs convex/convex) has the greatest peak negative pressure. (Do you have one?)

Fins with assymetrical foils, that is, each side is foiled differently (but each side is still foiled) and the fin is cambered...

An asymmetrical foil, in which each side is foiled differently is, by definition, "cambered".

...(think of a fish fin or bird wing in a permanently flexed state)...

Another example of a foil with both convex and concave curvatures.

...are already being developed for use on surfboards (and thus kneeboards) . In fact, that design is already commonly used for the blades on boat propellers (which, in a strange way, perfom a very similar function to that of surfboards fins!) due to the increased lift and power they provide .

Such fins are also frequently used on hang gliders (i.e. at very slow speeds and low power situations). A boat propeller is not a good analogy as in this application the pressures on the foil are getting down to where cavitation of the foil (prop) is one of the most important design considerations. (Aside: The "cavitation" that most boat operators refer to when the prop suddenly looses thrust and generates noise is again, actually ventilation).

In fact, high speed propellors are typically designed to minimize the adverse effects of cavitation through the use of so-called "supercavitating foils". Supercavitating foils typically have a concave "thrust" face, and a convex "suction" face--as you describe. But in contrast to a conventional (subcavitating) foil the latter face contributes only a little to the total thrust (as it is operating in the cavity of water vapor). Supercavitating foils are typically MUCH less efficient (lift/drag ratio) than a subcavitating foil. However, a supercavitating foil section can be advanageous if ventilation occurs. In fact, the forward foil on my HYPO board has such a section as ventilation is virtually guaranteed by the design. I minimize the consequences of the poor efficiency of this type of foil by minimizing the load it must carry.

If you would like to discuss/debate this in more detail, perhaps it's more appropriate that we do it via PM's so we don't bore everyone else to death.

MT

[stemple]

And here I was thinking a modified breadboard would do the trick. Can anyone tell me how to program this dam! HP I think I have worked out a water proof housing. By my calculations if I can stuff the testorone poisoned 20 year old at the peak maybe I can get up enough speed to just barely prevent cavitation on the bottom turn and utilize my symmetrically convex foil shaped fins during the cutback