Skiing in Current

[twhisper]

With Moomba fast approaching I was curious about who undersatands currents and how they affect the ski. I’ve skied Moomba several times, but have never managed to stay out on the water very long. The river level fluctuates quite a bit, and the current can be moving right along some times.

 

Which direction is easier to ski? Is a head current like a head wind, or is it the opposite with the water moving faster relative to the ski?

 

 

[vtmecheng]

I have no answers but want to say that this is a great question. My club skies in a river with current but I'm not good enough to really know.

[S1Pitts]

For me I ski better into the current. Going with the current the ski feels loose in the turns. I also seem to generate more speed into the current on crossing.

Not a pro by any standard but my main ski site is on the river which is a controlled body of water and varies from 0.6 mph right up to 4 or 5 mph which is a challenge

My 2 cents.

[DavidN]

We skied over a decade on a river with quite the current before moving to Florida.

 

There are actually two effects to the skiing.

 

First it’s the actual speed through the water.

Boat speed is set by gps, so fixed or over ground as you will.

 

Let’s make it easy and say, you have a current of 2mph. Boat travels at exactly 34mph (over ground).

Skiing downstream, the skier will ski @ 34mph above ground, but only 32mph through the water.

The opposite is true when skiing upstream. Boat - again pulls @ 34mph above ground - skier though will get pulled through the water @ 36mph. Feels quite weird and very different from the last pass!

 

Secondly, due to the current flowing in one direction through the course, the distance between the buoys feels totally different going upstream vs going downstream. Not side to side of course, but almost like the whole course is shorter (clenched?) downstream vs longer (stretched) upstream. That greatly effects your timing!

 

I always found it quite challenging and always skied my best going against the current (upstream).

[Gar]

Seems the same as a head and tail wind but smooth surface?

[skibrain]

@DavidN I am not a Zero Off skier but I understand since 2013 there is a River Mode option added for use to compensate for any water currents.

 

In theory if the entire volume of water was moving uniformily the only difference would be apparent wind speed +\- speed of current. I suspect reality is that across the width of the course, the flow in the middle of the river could be different than edges, or there is complicating turbulence. Cue the experts...

[RichardDoane]

remember back to the PP days of correction factors, with current being one of them - @DavidN hit it right on the head, it's the difference in "overland speed" which makes for quite a challenge in your timing adjustments, good luck at Moomba !

[Horton]

Two people riding on a train from New York to San Francisco play a game of ping-pong in the sport compartment of the train. Lets say, the train moves at 100 km per hour (= 27.8 m/s) and the two players hit the ball at a speed of two meters per second. In the reference frame of the players, the ball moves back and forth at this particular speed. For a stationary observer standing beside the railroad, however, things look quite different. In his reference frame the ball moves at 29.8 m/s when it is played forward in the direction where the train is heading, while it moves at 25.8 m/s in the same direction when it is played backwards. Thus he doesn't see the ball moving backward at all, but always moving towards San Francisco. For an observer in outer space, things look again totally different because of the Earth's rotation, which is opposite to the train's movement; therefore the outer space observer always sees the ball moving East.

 

...

[ALPJr]

I free skied tidal rivers and creeks in the Great Egg Harbor estuary for 15 years before discovering the course on freshwater lakes. We often found the glassy side of the river and skied for miles along the marsh grass banks which served as natural wind protection and wake eaters. My recollection is that the currents themselves didn't have much effect on skiing. How the current would interact with wind direction and areas prone to backwash could be a problem, such as the tide coming in with the wind blowing out.

[Horton]

Seriously my brain hurts thinking about this. I'm going to have to really sit down tomorrow and wrap my head around it.

[Horton]

@gar the more I think about it the more I think it is not like Headwind Tailwind.

 

[BraceMaker]

@skibrain river mode does not apply to slalom. Despite the ability to modify speeds slalom exists in reference to fixed objects and speeds are not actual velocity but time between fixed points. As such up river or down river the boat speed must match the time between balls. PP needed extra revs upstream, so does ZO. But how that correlates to engine effective power??

 

I would suppose only practicing on a river can solve the problem. What of a head wind downstream vs a tail wind downstream and vice versa

[skibrain]

so Theoretically the zero off boat, up-current OR down-current is moving relative to, and through the course at a consistent 36 mph. The boat might just be working a little harder to accomplish that one way than the other?

 

We need a Moomba skier to tell us if the water flowing under the ski feels faster /harder one way than the other.

[Rednucleus]

If boat maintains constant ground speed both directions, I would think while going against the current could help me generate better angle across the course because the water is pushing harder against my ski?

[gapullin]

I skied on a river for 10 years, current was anywhere from 0.2 to 0.8 mph. Zero off wasn't created then, we adjusted our Perfectpass speed by adding wind. In terms of boat speed, this worked well.

As far as the skier goes, I found it to be like skiing wind, but with glass water. I personally find it easier to ski upwind, and also found it easier to ski up current. Against the current, generating the same lateral vector, which is what is required to reach the line of the turn balls before actually reaching the turn ball, will generate increased lateral velocity compared to with the current. This gets us to the width of the ball earlier, with more set up time for the turn, on the up current pass. Like skiing with wind, one does need to pull longer going up current than down. I found that time pulling and slack issues were similar to skiing in wind.

The other issue is the ideal speed for which the ski is designed. I've always used a 36 mph ski and skied at 34.2. Naturally, the ski performs ideally at a faster speed, relative to the water. So, on the upstream pass, I felt the ski to be more responsive. That said, the smoothest, easiest short line pass I ever completed on the river was a downstream 32 off pass. Can't really explain that.

[MISkier]

@CaleBurdick, can you answer the question?

[6balls]

We had a river course in college, and one of our two swamp courses had some current...tho lots less. Up-current was always easier for me.

 

With higher relative speed through the water cross course is quicker, but decel in pre-turn is more dramatic and angle better off of the ball.

 

Downstream goes more downcourse thru the pull and less converted to cross course, less decel in pre-turn like a tailwind in that respect, and harder to finish as well.

 

Sorry not very technical, and not backed with data...so I guess these are opinions (everyone's got one) from experience.

[buski]

We ski on a river. We're all very far from pros of course but it's unanimously easier to ski up river. If it's not exactly like a head/tail it's pretty close to the same feeling. We have the wind blowing in the same direction as downstream pretty often and then it becomes pretty difficult to run anything in that direction.

[Horton]

I confess I went to bed last night thinking about this.

 

Up to 28 off I am think that skiing against the current is likely better.

 

For skiers at 35 off and shorter it seems like it could be a bit of a puzzle.

 

At 36 mph, if there is a 2 mph current and you are skiing against the current then the boat is moving at 38 mph across the water. If you were to free ski at 38 that would be the feel and I bet it does not feel comfortable for most skiers. On the other hand the course would feel long as the current would help you make speed into the wakes and help you slow down. At the ball things are going to be frantic as the boat is going to pick you up fast.

 

In the other direction the boat is going across the water at 34 mph. The feel of the water might be awesome but you would make less speed into the wakes and you are going to carry what speed you have better than normal. Slowing down might be an issue. The balls are going to come up fast but it is going to be easier to ski technical.

 

My guess is you want a tail current for your hard pass but this is all a guess.

[6balls]

Never ran 38 off down current...35 off lots both ways.

[thager]

@Horton Think about what happens when you get a strong 90 degree crosswind tonight!!

[dbressel]

Speed of boat relative to the distance to buoys will be the same in either direction. I imagine the feel of the water would be more like hard water soft water. You could probably have a tailwind pass into the current and even feel slower? Crazy; much respect to all the competitors on the River.

[BraceMaker]

@dbressel speed is - I think there are some profound aspects to what ZO does - it obviously is using more power against a current than with a current, I've been in boats that struggled pulling footers over 40mph - 2-4mph of current could deplete your available engine power one direction - but will certainly generate a delta up/down river.

[Horton]

@Bruce_Butterfield I see your thumbs down. I am listening....

[Bruce_Butterfield]

@horton, first the disclaimer that I have never skied in current, so this is my educated opinion fwiw. My main reason for the disagree is that you are invoking the boat speed relative to the water in each direction, which is completely irrelevant. Making the comparison that in a 2mph head current is like the boat going 38 is flat wrong. The key is the effect of the skier’s speed relative to the water and course in each direction is what causes the change in difficulty.

 

If there is a 2mph head current, there will be more resistance (call it drag) on the ski that will make it more difficult to accelerate out of the buoy and slow the ski down quicker coming into the next buoy and potentially making the skier more narrow. In this regard, it would be very similar to a headwind.

 

Similarly in a 2mph tail current, the water will be “pushing” you downcourse in the exact way a tailwind would push you downcourse out of the buoy and have the skier hauling ass into the next buoy if control is not maintained.

 

Now whether a 2mph current is comparable to a 5mph wind or 30mph wind, IDK, but the effect is very similar – the difference is that in a current the drag/push is exerted on the ski whereas in a wind the drag/push is exerted on the skier’s body.

 

JMO, and also based on the posts of the several skiers who actually have skied in current, you have it bass-ackwards. Again.

 

BTW, your initial post in this thread was one of the better ones you have made!

 

[Hallpass]

So, I understand that the boat speed/timing from buoy to buoy is fixed, regardless of current. Question: If the current changes the boat's speed in relation to the water, does the size/shape of the wake vary with current?

[DUSkier]

A ski is designed to work with pressure on its bottom and fin, at an extreme imagine skiing at 55kph in a 58kph tail current, the ski is not going to respond how it is designed. I have skied in strong current and could not run a tail current pass yet head current was simple. I'll take a head current ANY DAY.

[BraceMaker]

@Hallpass it has to is your simple answer. A hull doesnt really know if its moving forwards or the water is moving past it, a paddlewheel speedo would see the movement as speed.

 

But there are also added factors with rivers. Depth changes are huge in slalom wakes. We have a stretch of water we ski where sandy flats go out then drop off. Going straight down the lake you'll have wake crossings in 5 feet and then in 50 feet. Huge difference in firmness.

 

Currents can eddy around things, pillars can cause turbulent zones behind them for hundreds of feet.

[twhisper]

I can imagine the idea of a head current feeling like it has some extra drag, but what about the extra speed of your ski relative to the water? Doesn’t that create some lift? Isn’t that similar to a surfer riding a standing wave in a river?

[motoskier]

I agree with bracemaker a paddlewheel is the only way to accurately tell the speed of a boat across moving water. I don't think a headwind speeds up the water it just creates drag (headwind) or speed (tailwind).

[Horton]

This really is fascinating stuff (for those of use to ski on still water). Thanks @twhisper for posting this thread. I have not thought about skiing in a abstract way this much in a long time.

 

@Bruce_Butterfield @twhisper

Do we agree that if we are free skiing in a 2 mph current going against the current - the boat "water speed" is 2 mph faster than the GPS speed? The inverse if downstream?

 

If the above is true then the mind bender the impact of balls being attached to anchors and not moving with the water.

 

 

[ScottScott]

I certianly have no experience to speak from, but i have seen mentioned in other threads what @ twhisper is suggesting. The relative speed of the ski over the current can effect how the ski responds. In 2mpg head current going 34mph the ski will sit more on top of the water and respond ss if you were going 36mph, which could help. Down current the ski would respond like you were skiing at 32mph and sit deeper in the water.....which could be a bad thing. Thats a 4mph difference from one pass to another which could make a big differnce in how the ski reacts.

[Sethro]

@Horton you are correct...if free skiing upstream in a 2 mph current gps speed control needs to be set at 32 mph. skiing downstream gps speed control needs to be set at 36 mph to maintain the 34 mph actual speed.

 

This gets terribly, terribly annoying when tricking on a river with a variable current. That is the main reason I have not upgraded to a gps speed control system. Around bend #1 the current is 1.2mph for several hundred yards. The narrow part just ahead is 1.7 mph for 3/4 of a mile. The wide area is 0.9 mph. It's windy around that bend so let's head back downstream. Let's see, I want to ski at 19.2 so set Zero Off at 20.9 please, but don't forget to adjust for the narrow part of the river if I get there before falling. It's easier to just fall and adjust accordingly.

 

Yikes, I think I have more aggression regarding this topic than I realized.

[GregHind]

At 34mph over ground with a 2mph head current, it is the same speed over water and same feeling as skiing at 36mph except that the boys are spaced out further.

 

At 34mph over ground with a 2mph tail current it is the same feeling as skiing at 32mph except the buoys are closer together.

 

Mapple was famous for opting up at Moomba and running difficult passes with tail currents so he could have the head current for the win

[twhisper]

@Horton Yes, I think so. I would think the ski would feel better in the water with less water speed. Everyone seems to think that skiing up-stream is easier.

 

Would be an interesting test to have a different ski set up for each direction. Wondering if you could ride a larger/smaller ski depending on current direction.

[motoskier]

If I were twhisper and my first pass was with the current I would go off the dock at my hardest pass

[Horton]

@ motoskier If @ twhisper went OTF on a 220 jump and he then drank an 84 pack of beer and then put his bindings on wrong foot forward, his opening pass would still be better than yours.

[h2onhk]

We ski on a river with occasional current. Reading this thread is really intriguing. I'm just a hack, but for you guys that are deep shortliners I'm trying to understand the fundamental differences between skiing down current and skiing down wind. Seems that they both would have similar effects on timing, position, deceleration, acceleration, etc. Do you guys change techniques when skiing upwind vs. downwind? If so why wouldn't those same techniques apply to up current vs. down current?

 

One thing I know for sure is that when the wind is blowing against the current the water sucks, unless you are extremely sheltered.

[JackQ]

This is more than academic question to me. I have skied in tides and currents for 15 of the last 17 years. At least with ZO it did not take multiple passes to sort out the net effect of the current and wind to get a accurate timed pass. I found a tail current much easier as your speed (over ground) is less and you have more time between buoys. With a 5mph current at 34mph, the boat is only going 29 mph "over the water "down course and you have more time to get to the buoy. Conversely, a head 5mph current and 39mph over water, at 38 off the buoys would seem to fly by. Good training, but hard on the psyche.

[Than_Bogan]

I may be under-thinking this, but shouldn't the into-2pmh current situation be exactly the same thing as running a longer course at 38mph? And the other way should be exactly the same as running a shorter course at 34mph?

 

In each case, the length of that imagined course has been set so that it takes 16.08 seconds to traverse.

 

(Or: What @GregHind said...)

[motoskier]

Nobody can drink that much beer

[Horton]

@motoskier never underestimate @twhisper

[h2onhk]

 

[JAS]

What speed control is used at Moomba? Hey here's an idea-- use a floating course that can be set free and allowed to float with the river during the pass. Maybe an underwater midline cable used to keep midline centered. 2mph river flows about 3fps.

[Jordan]

What if we think of the change in terms of increased or decreased pressure on the ski as opposed to speed across the water? I suspect the impact would be a smaller change than the speed difference.

I am sure that it there is a difference, certainly one you could feel, but maybe not as much as the difference between 34 and 38 mph.

[Bruce_Butterfield]

@Than_Bogan under-thinking something? OMG the world is ending and we are all going to DIE!!!!

[nleuth]

I have had some experience skiing in current and also have had many philosophical conversations about what happens when skiing in current. Here are a few of my thoughts:

 

1) It is easier to run your hardest pass upriver.

2) The current is much more pronounced than a wind, but harder to sense because you don't feel it like you do the wind. Though again, I feel it has more of any impact than the wind.

3) Upriver feels more stable, earlier, wider, and easier to maintain a tight line. Downriver it feels unstable since water speed under the ski is slower, thus you tend to river deeper but yet faster and it can be difficult to stay early and wide because of this.

4) I personally switch my ZO setting upriver and downriver. I like a C1 up and a C3 down. I would rather take throttle out of the ball rather than into the next buoy since it is much more difficult to initiate and finish the turn downriver.

 

I also agree with the description @6balls mentioned earlier.

[Tom351]

It is also worth mentioning that the when going upriver (and the boat is traveling faster across water surface) you will have a smaller wake (assuming higher speed=smaller wake- which seems to be true in most cases) and conversely skiing in 2mph downstream you would have a 32mph wake for a 34mph pass...

 

It sounds as if going upstream gives you all of the benefits of increased speed (ski control, smaller wake etc...) without reducing the time between buoys

 

and downstream gives you the bad things about skiing at slower speed (riding deep, larger wake etc...) without the one huge benefit of skiing slowly: more time between buoys.

[S1Pitts]

Everyone so far has been focused on the horizontal movement of the water and boat/ski speed.

Something nobody here has touched on yet is that when water moves it behaves much like air does in a light wind. It rises and falls as it moves over under water changes it terrain. It also swirls with back eddy's influenced by changes in course and from shoreline just to name a few. Basically it can be very active depending on the size, speed and path that it follows. This has a huge impact on the feeling of the ski under foot.

 

I find that after spending an extended period of time skiing on such conditions that when I ski on any lake let alone a man made tournament lake , the water seems lifeless, kinda feels like jello.

The ski becomes very predictable in all phases of the ride but most notably is the preturn and turn that seem to become a lot easier to execute with consistency.

In short it sharpens my senses and reactions and I ski better on a lake afterwards.

[Sethro]

Ok, so I'm watching the Moomba webcast and it's got me thinking about this again. I originally thought long ago that Zero Off maintained a speed input (say 36) and adjusted to the pull of the skier/wind/etc. to keep the speed at 36, thus resulting in a good time through the course. I was informed by those here on BOS that it actually maintains the speed by continually timing itself through the course and adjusts accordingly to result in a good time...still doesn't make complete sense to me as Zero Off also reacts to skier input in the course, but I accepted the explanation...further complicating my understanding is it maintains speed while free skiing with no timing input from it's position in a course, and only reacting to the skier/wind/etc variables.

 

So, my question now is how it maintains 36 in a current when it approaches the course? I understand it will provide the correct time through the course, but in a strong current the speed will be noticeably off on the pullout for the gates, and suddenly adjust up or down depending on the direction one is entering the course because it doesn't have the course timing inputs...no?