physics question: surf and river current

[Lees23]

6 hours ago, Raimie said:

It's not the wind. It's just as noticeable to me on a glassy day.

Sorry for confusion.. I am just talking about the part of his post referring to with it against the wind, not current.  I have never surfed on anything that has significant current. 

[Falko]

One thing is how the wave fills. Going upstream, your wave is being pulled from the direction of the water flow, the water has to change directions almost to fill in the void or fill from the bottom. Going down stream, the water can accelerate slightly and get more fill from the back. This I assume will have an effect on your wave shape and form. 

Airplane on a treadmill, this question turned friends to foes, elevated the rate of divorce, and may have caused an underground civil war years ago. not going there....

[kerpluxal]

28 minutes ago, Falko said:

One thing is how the wave fills. Going upstream, your wave is being pulled from the direction of the water flow, the water has to change directions almost to fill in the void or fill from the bottom. Going down stream, the water can accelerate slightly and get more fill from the back. This I assume will have an effect on your wave shape and form. 

Airplane on a treadmill, this question turned friends to foes, elevated the rate of divorce, and may have caused an underground civil war years ago. not going there....

LOL... maybe that is why I am divorced

I will stop there  

[Rednucleus]

lol airplane on a treadmill was done in a  myth busters episode and it does not fly - because the plane is sitting still relative to the ground so no air is passing across the wings so no lift is generated.  Personal disclosure: damn it Crew, I'm a veterinarian, not a physicist! 

[wilster]

i have seen this a handful of times on the delta in CA so i have some experience.  In reading the thread, it seems like most everyone is ignoring the force vectors (college physics class, thank you) of the surf gate, wedge, boat in conjunction with general wave dynamics.  going downstream, it is harder/impossible to maintain a "good" wave--the boat has to be going faster and because the water is already in motion the wave simply has a harder time developing because of the lack of resistance.  when you think about it, same general theory holds true in the ocean, the best waves occur on shorelines that have good undertow, creating resistance from the oncoming wave.  definitely a fun thread.

[kerpluxal]

33 minutes ago, Rednucleus said:

lol airplane on a treadmill was done in a  myth busters episode and it does not fly - because the plane is sitting still relative to the ground so no air is passing across the wings so no lift is generated.  Personal disclosure: damn it Crew, I'm a veterinarian, not a physicist! 

Plane does fly.. plane is powered by propeller... wheels on a plane are free spinning... indication of speed is through the pitot tube... so as long as the wind speed over the wings is 60 mph.. it will fly... ground speed is irrelevant.. The wheel speed would be 120 mph ground speed.

This all relevant to our sport as well.. Current speed of water has an effect on the wave as well... If you are traversing upstream in a 5 mph current you would need only 6 mph ground speed to have the water moving over the hull at 11 mph. If you were going down stream with some conditions, you would need 17 mph gs to achieve same speed of water over the hull to form the same wave. 

BTW... mythbuster's plane does fly:

 

[Rednucleus]

OMG I have been officially BUSTED!! Must have been a 2-3 scotch night when I watched that episode  

(hopefully my veterinary skills are better than my physics - think I might have earned C's in that back in the day)

[MadMan]

6 hours ago, Jmcclain01 said:

 if you had a river flowing at say 11 MPH, or close to surf speed, you would be able to point the boat in the upstream direction, set the cruise at river current speed, 11 MPH, and not move the boat relative to land but be able to surf behind it. 

Instead of setting the cruise, just drop anchor.

[mswsetter00]

This thread is funny. I used to surf/wakeboard two different rivers in IL, the Mississippi and the Rock. I have GPS speed so I would have to adjust speed for going up and down. The wakeboard wake didn't change to much just the speed was an issue. The surf wave was different, had more push or less push depending on direction with same weight and same relative speed.  If you want to argue that it's the same in both directions and have not surfed or waterboarded on a river, I suggest you go ride a river.

[gibbs114]

I run the Tennessee River daily with my boats. I always use paddlewheel because GPS is worthless in a current paddlewheel works great. You can tell a definite difference in surf wave  much better surf wave against the current although we do like to skim with the current.