Holes in your fin

[DanE]

"Anyone who has added too much length knows the feeling of the ski sucking behind them just before reaching the ball line. The result is often a feeling like hitting a roller or a curb " - that could also be "Stealth Rollers "

[eleeski]

@SkiJay I will stand by the content of my observations - not the tone. I apologize if I was harsh on you. Cell phone typing is hard to nuance.

 

Looking at your photos, I see minimal slipping. Obviously if the radius of the turn is tighter than the tangent physical length of the ski, something must slip. Perhaps the fin is the anchor point - not the trick ski's underfoot anchor point. The photos are not conclusive (I see a pre turn front slide/tail anchor, a turn finish tail slide/foot anchor). I see no slippage at all in the pull position photo (nice narrow track left in the water). Spray is coming from both sides of the ski in all photos (minimal from the down edge, but there none the less).

 

@Horton said "At the apex is when the fin slides the most. At this point fin depth is super critical. I call it grip (but I guess it is lift) Too much grip and the tail of the ski has too much leverage (resistance) so the tip comes up. Not enough depth and the tail of the ski is over powered by the skier and slides out." Spot on! (I note that when the tip comes up, the ski gets harder to turn).

 

Again, we try to apply too much theory to an incredibly complex situation. Empirical testing is best to get the real world setup that will improve buoy count. I really want to see @wileycoyote try some radical fins. While his explanations may be curious, his take on the feel should be priceless. And if he weeds out the toxic versions before I put my ankles at risk - even better!

 

Eric

[SkiJay]

@eleeski No need to apologize, no offence taken. Without question, tone is the most challenging aspect of written communication. I respect your experience through experimentation, Eric, and I remain fascinated by how differently we interpret these photos, so I hope you don’t mind if I go one more round with you on this fin-drift thing.

 

If the tail is anchored and the front of the ski is sliding in the pre-turn as you say, how can the ski be turning? The tail has to be sliding more than the tip, or the ski would be rotating in the opposite direction to the turn. I see tip sliding lots and tail sliding even more in both turn photos, implying that both fins are loaded and working.

 

In the pull position photo, true the track is narrow, but the ski is pointed about 15 degrees left of that track. Wouldn’t this only be possible if the ski was sliding sideways to some degree? More specifically, if the centerline of the ski is 15 degrees left of the relative water flow (the path travelled through the water) then the ski’s angle of attack has to have a 15 degree slip component. It’s geometry. The path is straight because the skier has engaged enough of the front of the ski that the drift at the front equals the drift at the tail. This suggests that here too, the fin is loaded and working so its design and adjustment is still in play.

 

I think it's important to understand that the fin is in play at all phases of the pass because it means the fin's profile, flex, hole pattern, position and wing, affect the whole pass. Making an adjustment to change the ski's behavior at the toe-side ball will affect everything else, so a clear understanding of the fin's dynamics throughout the pass is a prerequisite to productive fin fiddling. This angle of attack phenomenon is the foundation to that understanding.

[Horton]

@SkiJay No! No! No! This is the wrong thing to say to @eleeski "I respect your experience through experimentation" Never encourage him.

[thager]

That is as misdirected as asking Horton for ski instruction!!!

[Horton]

@thager Keep your handle high and your head down. Try to watch the tip of the ski as you round the ball. Always look at your spray as you finish the turn.

[thager]

You mean I'm not supposed to do that?

[Horton]

@thager No. That is exactly what you should do. Please take video when you try it.

[thager]

@ Horton I already have video. That's my normal style. If I fix those things I should get past -35 next year!

[Horton]

@thager do it my way an you will be looking at 75 off

[thager]

HMMMMMM Now you sound just like MS! Maybe I'll just ask Eric.

[wilecoyote]

Wow where to start?

 

First, @Horton, most of what I've stated is solid physics, it doesn't mater whether it's a slalom ski, a surfboard, a wakeboard an ironing board. It is fact that right around 5 degrees angle of attack (AOA)is the highest lift to drag ratio (L/D) a planing surface can have. Anything higher or lower than that will add proportionally more drag. Slalom skis, are usually operating well above 5 degrees AOA, so whenever you lower the tip, the drag should go down, that's a fact. However, if you look at the edge change photo above, the ski is riding right around 5 degrees. If you lower the tip at that point, the ski will fall below the magic 5 degrees and drag rises. Another major factor, is rocker. One thing that rocker does is stabilize the trim angle (AOA) The thing with rocker is, if there is enough of it, and you try to push the nose down, you end up creating suction at the tail and that creates a lot of drag as well, (also a scientifically proven source of "phantom rollers")

 

It is a fact that the higher the AOA of the ski the tighter it can turn. (to a point) I have no argument with your statement;

The sign of a skilled slalom skier is a more constant and tip down ski attitude. Big fast turns ending with the tip up equal loss of water speed and are basically a stall. If you stall or wheelie, you then have to re-create your water speed from scratch. The boat does not wait for you.

 

The fact remains, the ski turns tighter as you raise the tip. But that doesn't equate to a better way around the ball for exactly the reason you stated above. You will stall out because you don't have enough inertia to maintain the ski at such a high (inefficient) AOA. It's the same as when you turn the boat really tight, you have to add power big time to keep the boat at speed.

"If all your thinking is biased the idea that you want the less ski in the water at the apex - you need to start over. "

Dude, if you saw me ski, that's exactly what you'd say! In fact that's what my instructor said the first time he saw me ski. But I'm getting better. I know that keeping the tip low through the turn is important because you are coasting pretty much from the edge change to the exit of the turn, so you must balance carrying speed with turning tight.

 

But I wasn't saying that you should try to get as little ski in the water as possible at the apex,I was just stating the way the ski works. There are basic laws of physics that cannot be ignored. The ski turns because of the lift created. When the ski is not on edge, all the lift is simply holding the ski (and skier)up. Once it goes on edge the lift is both pushing the ski sideways and holding it up. More overall lift is required. The only way to get more lift, is to either increase the AOA, increase the surface area (more ski in the water), or increase the speed. Any time you alter one condition another must change to maintain equilibrium. My point about a flat ski being able to corner and accelerate better than a rockered one was prefaced by the statement "Assuming you had the skill to move your weight around correctly" you don't. (don't take that personally, nobody does) It may be possible to design a good rockerless ski, but I doubt it. As I said, rocker helps to stabilize the trim of the ski. I'm going to throw this one out there, I don't know, I'm just supposin', a low rocker, smaller than normal ski in the hands of the -41crowd can really perform. Stick the same ski in the hands of someone like me or even someone in the -28 group and it will hinder our progress. I'm just guessing, but would you guys agree?

 

@Skijay, I'm with Eric on the spray thing. Spray is not an indication that the ski is sliding, it is an indication that the ski is planing. The amount of spray is related to how heavy you are, and your AOA. At the tightest part of the turn we are at our heaviest because of the g's created at the turn, and we have a higher angle of attack Thus tonnes of spray. Low AOA = small spray. I'm not saying that the ski doesn't slide, just that spray is not an indication of it. But those photos are a wealth of information, thanks for putting them up!

 

What I'm really keen on is the apex photo, there is a small trail of turbulence coming off the fin. That little trail (often 2 spaced about a fin length apart) is in every single video and photo I've examined where the camera angle and resolution is right to see it.

 

@eleeski, how do you play with rocker?

 

whew! That took me three days to write.

 

Chris

 

[eleeski]

@wileycoyote Sorry, you are totally missing the fundamentals of making buoys. 1, Drag is pretty much irrelevant. There's hundreds of horsepower to draw on. 2, Turns are best done with all of the ski in the water. At slow speeds you can rear back on a ski and have it turn really tight. But as the speeds increase, this technique no longer works. You must find a ski edge to hold a turn - the more edge the better. Get on the front of the ski! 3, Carryout and preserving speed is only relevant at deep shortline. Never coast! Go edge to edge and time the turn. 4, Edge as much as you can but the ski will never go too far over. I do tune skis with edge thickness (or thinness) but that is a subtle feel. 5, The fin is a minor factor in the feel of a ski. All things fin related (shape, holes, settings, and wings) only amount to a few buoys. Those few buoys are relevant to standings but not really messing with the physics.

 

I adjust the mold when I build my skis. Minor changes can be made with Superfil. I have cut a ski in half (corewise) and remolded it to change rocker. Rocker generates most of the feel of a ski. But there are so many factors. The changes in Reynolds numbers over the speed range of a ski in water make the physics too variable. Ski design and tuning is an art - not a science.

 

Work on your style on a factory setup. Get to where style is not the only limiting factor and experiment from there. I am constantly amazed by how counterintuitive many of the things I try feel. On the water testing carries much more weight than any theoretical prediction.

 

Eric

[SkiJay]

@eleeski @wilecoyote I just realized something that might explain our differences of opinion on whether or not a ski has a sideways "sliding" component to its path through the water. This is really hard to describe clearly so fasten your seatbelts; it might rile up the “who cares about all this pointless crap, just go out and ski” crowd.

 

There are two main planes that a slalom skier is dealing with. One plane is the surface of the water. It’s the easy one. It’s always horizontal and all of our leaning is relative to this plane. Then there is the plane we are standing on—the top of the ski. The skier usually remains pretty close to perpendicular to the ski’s plane, but the ski’s plane moves through a wide range of angles to the water’s plane. In slalom discussions, some people tend to think in terms of what their bodies are doing relative to their ski, while others tend to think in terms of the angles of their bodies relative to the water’s surface. Neither way is right or wrong, it’s just a matter of perspective.

 

I suspect you are both thinking in terms of the ski’s plane, where force from the skier (weight, pull from the boat and cornering G’s) is loaded perpendicularly onto the top of the ski. In this case, it seems the ski is being driven forward through the water in the direction it is pointed with no perception of sideways slip at all, except maybe at the on-side ball. I like this perspective when working on skiing technique because it focuses on what I’m doing on the ski. But when it comes to ski tuning, I find this perspective misleading.

 

For ski tuning, I prefer to think in terms of the horizontal plane of the water because it is easier for me to conceptualize how the ski is interacting with the water. In the water’s horizontal plane, there is lots of sideways slip. The photos above do a great job of capturing all this drift, but the downside to this perspective is it lends itself to the false assumption that the drift is affecting the fin more than it actually is. I’ll even concede that in reality, actual side-loading on the fin is closer to your perspective than mine, but I’m guilty of this exaggeration by choice. Whenever I’m conceptualizing ski or fin behaviour, I find it helpful to exaggerate, and this perspective exaggerates the fin/water interaction that I want to tune for.

 

The actual truth lies somewhere between no drift at all and lots of sideways drift. For this, let's try a few more practical examples. During the pull, for example, the boat is trying to pull you straight down the lake with up to 600 lbs. of force on the line. While 90+% of this force is converted into acceleration in the direction the ski is pointed, the balance is lost to inefficiency and sideways drift in the direction of the boat’s pull. Similarly, the G’s while cornering have to cause some degree of sideways slide or drift. The ski isn’t on steel rails; it’s plowing through a liquid that allows some drift.

 

A snow ski on a hard edge on an icy race course makes very little spray. As soon as the edge breaks loose and there is even a small sideways component to the ski’s path, there is spray. A water ski’s substantial asymmetric spray provides the same evidence of this sideways sliding component.

 

Anyone who has gone a few thousands of an inch too shallow on the fin has spun out or blown out the tail in the turn. A deeper fin gives you more acceleration during the pull but makes it harder to turn the ski; this wouldn’t be the case if the fin wasn’t loaded while pulling and turning. What if rather than being a few thou shallow, the fin fell off in mid-pull or mid-turn? All of these examples suggest the fin is rarely just tracking.

 

In the pictures above, clearly there is a sideways sliding component to the ski’s path across the top of the water. If the ski was leaned over a perfect 90 degrees to the water, the fin would be tracking with no side load on it. But in reality, the ski is only leaned over 60 degrees or less, so the fin has to engage that sideways water flow and be under load. These photos prove the fin is nearly always in a side-loaded state.

 

I think we have both been close but not quite correct. The ski is nearly always sliding out a bit, but less than I was originally saying, and more than you are suggesting: certainly enough that the fin affects ski behaviour throughout the pass.

 

My head hurts...

 

P.S. @ wilecoyote That small trail of turbulence coming off the fin is more likely turbulence off of the wing.

[wilecoyote]

@eleeski

 

You have no idea how badly I'm missing the fundamentals of running bouys! I'm sorry too. I have a good instructor, and I do what he tells me to. Well I try to at least. I'm in no way trying to come up with a new radical way to run the course or advocating rearing up at the apex. And I'm not looking for some magic piece or gear that will turn me into Nate Smith.

 

My plan for the winter is to set my ski to factory, (if I can ever get the numbers), remove the wing, work on my pulling posture, put the new footbeds I bought from Wiley's in my bindings, and annoy everybody here with my hydrodynamic analysis until the ice melts.

 

Seriously though, often a design element corrects a flaw elsewhere in the design. When I see holes and a wing, I'm thinking there are some major design issues being corrected here that might be more elegantly dealt with. But to come up with a better fin, we need to figure out what the current one is doing. But those holes... those annoying holes.

 

So back to this tedious analysis of ski behaviour. I completely disagree that drag is unimportant simply because you have unlimited horsepower. Imagine a bag dragging in the water behind your ski sea anchor style. Same hp, same hydrodynamic lift as before, just a lot more drag. I don't think anyone would argue that this would be a bad thing. A more efficient ski will convert more of the side pull energy into cross course angle. Also if drag can be reduced, then you get to bring the tip a little higher at the apex without stalling out. The benefit, a slightly tighter turn. The "design is more art than science" argument is held by many of the board shaping community as well. I wholeheartedly disagree that it needs to be this way. Trial and error is a tedious path especially if the trial part is not informed by solid hypothesis. Virtually every aspect of ski design (rocker, aspect ratio, planform (outline) edge chamfer,) have been studied in controlled environments, and quantified. I've read some of this material but there's still tonnes that I haven't read. I'm not so naive to think that you can just read all this literature and build a kick ass ski, but if you approach the problem as an engineering challenge, you can conceivably make a major improvement almost overnight.

 

These guys did just that. http://www.sailrocket.com/. The hydrodynamics and aerodynamics required to get this thing going that fast is far and away more complicated than slalom ski design, but they applied the theory did the maths and said this thing should do more than 65 knots, and they were right.

 

@skijay. You may be right about the turbulance being from the wing. I'm not sure.