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I recently stumpled across this question about the ram-air parachutes for use in pulling ships.

In this answer it is stated that the parachute needs to travel perpendicularly to the ship's motion in order to work against the hull drag in the ocean.

This is something I do not quite understand, even after watching videos that confirm the chute actually going sideways.

I have consulted the usual lift-drag diagrams, but have not been able to apply them to this case her. Can someone briefly explain the forces at work with their respective vectors, preferably with a diagram?

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    $\begingroup$ This belongs on a sailing forum. $\endgroup$
    – John K
    Commented Jul 6, 2023 at 12:22
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    $\begingroup$ @JohnK: The initial question seemed to fit here, so I thought this one will too, since they are directly related. $\endgroup$
    – pat3d3r
    Commented Jul 6, 2023 at 12:56
  • $\begingroup$ @quietflyer: OK thanks. I am still having a hard time figuring it out. Edit: Thanks for the links, didn't see them at first. $\endgroup$
    – pat3d3r
    Commented Jul 6, 2023 at 13:00
  • $\begingroup$ Reposting to revise-- previous comments referred to these links --aviation.stackexchange.com/questions/99796/… -- key point is that you want the kite to pull using Lift rather than Drag, and you want the kite to be moving at a high airspeed, not stationary relative to the boat-- it's complex and the statement you quoted in the other answer is not really accurate. The lift vector can exert some pulling force even if kite is stationary relative to the boat and directly "ahead" of it-- $\endgroup$ Commented Jul 6, 2023 at 16:20
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    $\begingroup$ As what amounts to a sail, tethered to an object to pull it through water by generating lift at extremely low Reynolds Numbers, a fancy more efficient spinnaker on a reach in effect, I think the expertise on a sail boat design channel might be better suited. $\endgroup$
    – John K
    Commented Jul 6, 2023 at 19:20

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Two perpendiculars make 180 degrees, force opposite the ships drag.

It's the same with iceboats. Instead of using wind as propulsion, wind is used to move the sail. The lift from the sail (more efficiently) creates propulsion.

The sail is essentially a wing. Sailplanes use gravity as propulsion to move the wing, and the perpendicular wing lift more efficiently resists gravity in the vertical direction. This is why a sailplane has a lower rate of descent than a parachute.

Making the kite move perpendicular to the direction of travel enables the more efficient airfoil to lift opposite the ships drag.

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