A Current's Affect on Drag

Discussion in 'Main Morgan 38 Sailboat Forum' started by royaltern, Mar 4, 2019.

  1. royaltern

    royaltern Bert Willett

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    Moderator's Note (from Mark): This is pruned from another thread wherein Terry was asking about Feathering Props. I made a post about how our new feathering prop will improve our performance by reducing drag especially in light winds and opposing currents.


    from Bert ...
    Why would an opposing current make any difference? Should be measuring speed thru water, not over ground.
     
    Last edited by a moderator: Mar 7, 2019
  2. mpearson

    mpearson Mark Pearson Staff Member

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    Bert - because the opposing current creates more drag on our old fixed propeller.
    Picture dragging an empty 5 gallon bucket behind the boat. There is more drag force if there is an opposing current. Especially significant in light winds where we are going less than hull speed. If it was blowing 20 knots it wouldn’t make a lot of difference because we’d be at hull speed anyway.

    I did some calculations and the feathering prop was between 40-80 lbs for drag force reduction, which is significant.
     
  3. royaltern

    royaltern Bert Willett

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    Mark, I respectfully disagree.Drag is only present when moving thru the water, whether sailing , tied to the dock, or at anchor. If you were drifting in the Columbia River with the current, there would be no drag on your prop. Once you start sailing thru the water, drag will occur. If you were sitting in a tub of water going down the highway, there would be no drag.
     
  4. mpearson

    mpearson Mark Pearson Staff Member

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    Bert - the things you said in the last post are true. And we hope to be moving through the water, sailing against the current. The more opposing current there is, the more drag there will be.
     
  5. royaltern

    royaltern Bert Willett

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    Only relative to the speed thru the water, not over ground
     
  6. mpearson

    mpearson Mark Pearson Staff Member

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    True that. I’ve only been talking about speed thru the water, and not speed over ground.
     
  7. royaltern

    royaltern Bert Willett

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    I should stop this, but can't. Current has no effect on drag. It is onlyspeed thru water that has effect. Zero current, one knot current, two knots current, all the same. Zero.
    One knot speed, two knot speed, thru water, greater drag with two knot spped thru water.
     
  8. mpearson

    mpearson Mark Pearson Staff Member

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    Bert, respectfully, current does have an effect on drag. Because an opposing current increases the apparent water speed on the boat, and (as you said) increased water speed increases drag.

    Do you agree the boat in scenario A would be going faster than the boat in scenario B? It is because there is more drag due to the 3 kt current.

    I'm thinking we must be using different terms for something and meaning the same thing.

    upload_2019-3-6_21-33-36.png

    Terry, I'm sorry we've hijacked your thread. ;)
     

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    Last edited: Mar 7, 2019
  9. bluesbyrd

    bluesbyrd Chris Langton

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    I’m going to agree and disagree with both of you...

    In the diagrams above:

    boat A will be going faster than boat B over the ground.
    But they will both be going the same speed relative to the water.

    On this basis alone, the drag would be the same for both boats.

    However, since wind speed is measured relative to the ground:
    Boat B will experience a net wind of 8 knots
    while boat A will experience a net wind of 5 knots.

    Since there is more force pushing boat B through the water,
    there will be more force - drag - opposing its passage.

    But that quibble is mostly irrelevant to the discussion at hand,

    For all practical purposes, current should have no effect on the drag
    on a boat. Only speed relative to the water matters.

    C
     
  10. mpearson

    mpearson Mark Pearson Staff Member

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    Okay, I think I might understand why we are disagreeing.

    What you described Chris, and I think what Bert was saying, would be true at 'Time Zero'. That's when both boats are dead in the water (zero speed thru water). The wind has not been turned on and/or their sails have not been raised. I totally agree, there is no difference in the drag because there is zero boat speed through the water.

    I'm going to try a different drawing to illustrate what I'm talking about. I'll be back.
     
  11. mpearson

    mpearson Mark Pearson Staff Member

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    Fellow sailing nerds ... how's this? Making it a little extreme for discussion. We drag an open ended 55 gallon barrel behind Boat 1 with no current in scenario C. In scenario D we add a 3 knot opposing current. Important: we are not talking about the instant they start, we are talking about after it starts.

    In both scenarios Boat 2 will be ahead of Boat 1, but will the distance they are ahead be the same?

    In my opinion Boat 2 in scenario D will be significantly further ahead of Boat 1 compared to scenario C.

    Thoughts? And thank you for indulging me ;)

    upload_2019-3-7_7-47-39.png
     
    Last edited: Mar 7, 2019
  12. mpearson

    mpearson Mark Pearson Staff Member

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    After thinking about this some more and having a cup of coffee ... I agree with Bert and Chris that the drag is the same (zero) at Time Zero. I also agree that after enough time has elapsed, acceleration is complete and things are in equilibrium - there would be roughly the same drag force with or without the current.

    Where I disagree is where I have really noticed the drag: when we are racing and round the upwind mark (which is downstream) and start heading downwind (which is upstream). This is the situation shown in the previous drawing. Going downwind with an opposing current.

    All of the other boats pull right away from us. This is because we are starting at near zero water speed and with our added drag force (F), our acceleration (a) is much worse than our competitors. F=ma, where m is mass, which doesn't change.

    I'm convinced that it takes at least a couple of minutes for the boat to reach equilibrium (finish accelerating). And all during that time, the other boats are pulling away from us at a higher rate because of the current. Hence my bulldog-ish opinion that opposing current does make a difference in the drag.

    Whew! Maybe I should switch to decaffeinated coffee?

    Please correct me if I'm wrong ... it wouldn't be the first time.
     
    Last edited: Mar 7, 2019
  13. bluesbyrd

    bluesbyrd Chris Langton

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    Mark ...

    What I want to know is what you are using to make those cool diagrams!

    C
     
  14. mpearson

    mpearson Mark Pearson Staff Member

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    Hi Chris - I use Adobe Illustrator, which is supreme overkill for simple little diagrams like that. But I have it installed & I know it already. ;)
     
  15. stnick

    stnick lee nicholas

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    Just my two cents. feathering props are just great engenering concepts. And you will get better speed when they colopse. But if and when one blade jams, its a nightmare.
    And you must go over the side to correct it !
     
  16. stephen

    stephen New Member

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    Sorry, I’m an engineer so I had to jump in. The drag on a body (boat) moving through a liquid is directly related to its speed relative to the water (measured by hull speed sensor, not gps).

    Assume a boat is moving at 5 knots hull speed in still water as the basis. If the boat then:

    - sails down stream with a 3 knot tail current the drag will be the same, but speed over ground (SOG) will go to 8 knots as measured by gps.

    - sails upstream against a 3 knot head current the drag remains the same, but SOG will drop to 2 knots.

    The folding prop would have a smaller & smoother cross section, so less drag than a fixed blade prop (regardless if it is locked or free spinning). The less drag, the faster the boat will accelerate.

    I hope this helps
     
  17. mpearson

    mpearson Mark Pearson Staff Member

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    Hi Stephen ... thanks for the input & I agree with you, with a caveat. I appreciate this discussion - I hadn't really thought a lot about it previously.

    Not that it means I should have any credibility but I’m also an engineer (Civil PE), an ASA sailing instructor, and a Coast Guard Masters license holder. We have all seen people with those papers be wrong, however, I did sleep at a Holiday Inn Express last night ;).

    This whole discussion started because I said a feathering prop would make a big difference especially in light winds & with opposing current. Bert said current has no effect on drag.

    I’ve been flailing around trying to describe why I thought current did make a difference. Not thinking about the physics of it, when we are racing & keeping up with the pack, we round the downstream mark, head upstream & all the other boats (with feathering or folding props) pull right away from us.

    It's very noticable and it is at the point in the race where we turn upstream.

    I agree with Bert, Chris and you that the amount of drag is generally the same with or without the current. However this is in a steady state condition (after acceleration). I've been trying to think of why there is a noticable difference when we are racing in the current. I've settled on the difference being because of the acceleration.

    If we were racing in still water, we would get the boat up to speed, cross the starting line and it would generally stay up to speed with smaller reductions for tacking, rounding marks, etc. When there is a strong current, I think there is more acceleration and deceleration.

    When we round the downstream mark and head upstream we are going from a water speed of 3 knots downstream to essentially a stop, then accelerating to 3 knots in the opposite direction. Because of the added drag, we accelerate more slowly and that's why the other boats pull away.

    At least that's my current theory. No pun intended.;)

    Opposing theories are welcome.
    Cheers,
    -Mark
     
    Last edited: Mar 10, 2019
  18. stephen

    stephen New Member

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    Mark,

    While the boat “stops” in reference to the marker as you round it the water/current does not. As a result the boat is actually slipping sideways down stream with the current as you are “side to” the current. By time your sails start to draw the boat has some dowstream momentum and drag forces that must be overcome before it can move upstream (going from a negative to positive velocity).

    Any extra drag will make the down current momentum and drag harder to overcome. Momentum of course will be weight, not drag dependent.

    I think that’s pretty close with out dusting off 30 yr old books from college.
     

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