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.