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physics question: surf and river current


augie09

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Trying to stop thinking about new boat purchase, but real work is not gonna happen, so had this random brain fart...

surf in an ocean is just wave of energy going through otherwise static water. lake water is sitting still, then a boat plows through it and it moves forwards. Not much difference than ocean, but not as deep of a wave either. Now, if in a river with current and boat heads down stream, does it's wave have more push? Upstream seems like it would have less push as all the other water is moving opposite direction, not just sitting still like a lake. Let's assume paddlewheel cruise so gps and motion relative to land isn't a factor. Is surfing downstream in a river better than lake? /brainfart

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There was recently a thread on a similar topic. Many believe there is a significant different between running up v down stream. I don't think so.

Consider your starting point - you are just floating with the current in the stream. The boat's water speed is zero. 'Ground' speed (relative the the river bank) is whatever the current speed of the stream. The river I use is usually about walking pace. But, the boat is static in the water.

Suppose you want to drive at 30 mph - you run the boat up to that speed and that is the water speed of the boat. You are pushing the boat through the water at 30 mph. The boat doesn't know if you are going up, down or across stream. It's just sluicing through the water at 30 mph and making 30 mph wakes.

Ground speed will vary +/- by the current velocity of the river. If the river flows at 3 mph, your speed relative to the land will be 33 mph downstream and 27 mph upstream but the water speed remains the same.

The above assumes we are using a paddle wheel or similar water driven speed reference. GPS (Star Gazer?) based units with no other compensation will run the boat relative to the land. In the river context, a set point of 30 mph will result in a water speed of 33 mph up stream and 27 mph downstream - 6 mph water speed difference is significant. At surfing speeds (8 mph?) the difference would be overwhelming - GPS based units would result in respective water speeds of 11 and 5 mph!

Imagine you are in the 3 mph stream, facing upstream and running with an indicated water speed of 3 mph. You will be making a 3 mph wake but going nowhere. Your water speed equals the opposite current and, relative to the land, you are in a static position. No do nothing else but turn around and run downstream. You still have a 3 mph water speed, still making the 3 mph wake but now you are travelling at 6 mph relative to the land (3 mph current plus 3 mph boat speed).

Edited by GreenMan
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I don't know the hydrodynamic reason why, but I've spent enough time on a surf board in the river and lake to tell you current certainly makes a difference. I can also tell you if there is a significant amount of wind on our lake that running with the wind will produce a better wave for us. My boat speed is controlled by GPS on the lake so yes, into and with the wind speeds are consistent.

YMMV but that's my experience.

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I don't know the hydrodynamic reason why, but I've spent enough time on a surf board in the river and lake to tell you current certainly makes a difference. I can also tell you if there is a significant amount of wind on our lake that running with the wind will produce a better wave for us. My boat speed is controlled by GPS on the lake so yes, into and with the wind speeds are consistent.

YMMV but that's my experience.

Martin, were you using GPS speed indication and control on there river?

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Yes we were. With the river we have to change up and down stream speeds to compensate for the river current. I will typically stop the boat after we're out and note the flow speed of the river on my GPS surf controller display and use that as a bias for setting our up/down river speeds.

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We surf a river too. Ours is narrow. I believe the difference is the water flows at different speeds at different depths. Ie faster at surface slower at bottom. After curves there are eddies etc. we only use the river when kids go back to school because it's closer. Depends on day and location in river sometimes upstream is better sometimes downstream. Not a huge difference though.

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What I've found, this relates to wakeboarding not surfing, is that the wake is the best, most consistent, when the river isn't flowing (controlled by TVA dams). If the river is flowing, the wake is better going downstream.

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I'd like to hear your wife's explanation.

the wave is simply a transfer of energy from the boat. adding the energy of the boat with the energy of the river it will make a better wave (wind as well?) If you are going against the flow of the river it will deconstruct the wave....slightly.

Wifey

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the wave is simply a transfer of energy from the boat. adding the energy of the boat with the energy of the river it will make a better wave (wind as well?) If you are going against the flow of the river it will deconstruct the wave....slightly.

Wifey

Hi, Mrs. REW! Sorry, but I remain unconvinced. If you are penetrating the water at 30 mph then that is what you are doing. The fact the body of water is flowing down a groove is all but irrelevant for the boat and the wave immediately behind it.

There may well be some infinitesimal influences and I suspect there would be a Doppler effect of some sort but al of these would, I believe, be minuscule, inconsequential and surely not readily detectable.

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Just thinking about this, I can see how the surf wave can be influenced by if you are going up or down stream. Here's my thoughts:

Consider a river that is 11 feet wide. Let's divide that river into 11 equal sections parallel to the shore, each 1' wide. You run your boat up/down the center, section 6.

I think the river current is going to be less than 5 MPH in all sections other than 6, and decrease to near zero in sections 1 and 11 (those sections at the shoreline). Therefore, in sections 5 and 7, where you are surfing, the river current relative to the boat will be different going up river or down river.

For simplicity, say the current in sections 5 and 7 is 4 MPH....and 5 MPH in the center section 6. If you are going 10 MPH upstream, then the boat is moving at 11 MPH relative to sections 5 and 7. 10 MPH downstream, and the boat is moving at 9 MPH relative to sections 5 and 7.

I can see how this would affect the wave in the off center sections, but only in theory, and results would be smaller with larger river widths.

This assumes the river current is maximum at the dead center of the river and minimum at the shoreline.

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Just thinking about this, I can see how the surf wave can be influenced by if you are going up or down stream. Here's my thoughts:

Consider a river that is 11 feet wide. Let's divide that river into 11 equal sections parallel to the shore, each 1' wide. You run your boat up/down the center, section 6.

I think the river current is going to be less than 5 MPH in all sections other than 6, and decrease to near zero in sections 1 and 11 (those sections at the shoreline). Therefore, in sections 5 and 7, where you are surfing, the river current relative to the boat will be different going up river or down river.

For simplicity, say the current in sections 5 and 7 is 4 MPH....and 5 MPH in the center section 6. If you are going 10 MPH upstream, then the boat is moving at 11 MPH relative to sections 5 and 7. 10 MPH downstream, and the boat is moving at 9 MPH relative to sections 5 and 7.

I can see how this would affect the wave in the off center sections, but only in theory, and results would be smaller with larger river widths.

This assumes the river current is maximum at the dead center of the river and minimum at the shoreline.

I was thinking the same thing, but referenced to the bottom of the river. It's about 20ft where we go.
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There might be slight differences caused upstream versus downstream, wind or many other factors. However, the primary factor moving your board is just gravity. Surfing is kind of like snowboarding downhill, except the hill is endless because the wave is constantly forming. But was is causing you to move forward isn't the wave itself, but rather you riding down it. Gravity works the same whether you are going upstream or downstream a river.

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Going upstream, the waves are closer together due to doppler. Thus, they are steeper than they are when going downstream, where they are farther apart. If steeper is better, great. If not, great. But it will be different.

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Going upstream, the waves are closer together due to doppler. Thus, they are steeper than they are when going downstream, where they are farther apart. If steeper is better, great. If not, great. But it will be different.

Yup, I'm getting a taller wave upstream and the waves have different amounts of push too. We prefer upstream when we have a small crew, but with a larger crew, it doesn't matter as much.

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This cracks me up....

Debate the physics of it all you like. There IS absolutely a difference. The wave is firmer and faster going into the current (up river) ....slower and softer going with the current (down river). The faster the current, the bigger the effect.

Those saying there is no difference (Nyryan) either don't ride in a river much and shouldn't be responding to the question OR just dont know what they're talking about. This isn't an 'opinion' type of thread. The answer is "yes, there is a difference...... the more current, the more difference"

Agreed. Maybe the confusion comes from the effects on the boat vs the rider. They are to completely separate entities when discussing how the river effects them. They boat may be going the correct speed regardless of current but the current will effect the wave/rider. Just like skiing in a river. The boat will be dialed in to the course and put out perfect times all day long. But skiing down current is like skiing in a tailwind and skiing up current is like skiing in a head wind.

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Remember, the wave shape and speed is also affected by the river water depth. If the river depth is running close to half wave length then it will be slowed by the river bottom. This will change the wave shape due to the relative bottom speed and velocity gradient in the wave water column. End result, the river needs to be either very slow, very deep, or small waves not to see the effect from the bottom. As you would guess, the wave is taller, slower (relative to land and water), and steeper as it goes up-stream.

Edited by Teleman
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If the river was flowing 12+mph, do you think you could "jog" the boat into the current and surf behind the boat without the boat having any land speed?

Or would the speed of the current need to be faster? Like double the speed?

Haha.

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I don't have any idea why, but riding a foil on a river, you get way more air going upriver, than down. Something about the foil speed in relation to the water? The Colorado river in Parker is like 10 or 11 mph.

Edited by Bill_AirJunky
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I don't have any idea why, but riding a foil on a river, you get way more air going upriver, than down. Something about the foil speed in relation to the water? The Colorado river in Parker is like 10 or 11 mph.

When we are over in Entiat, we pretty much only see the Foil guys riding upstream. Never down.

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