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post #41 of 45 Old 10-15-2007 Thread Starter
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Originally Posted by sailaway21 View Post
The drogue serves two purposes. The first is to keep the stern to the wind and sea. This can be quite important at sea or in surf. While being pooped or damaging one's rudder is a concern they pale compared to the broach. The second purpose of the drogue is to slow the vessel down and, when this is coupled with making turns on the propeller, there will be prop wash over the rudder allowing control of the vessel's heading.

Absent the drogue, when the following sea catches the stern the vessel is accellerated ahead. Since the entire sea around the vessel is accellerating ahead at the same speed this does not cause increased flow over the rudder. It may be a practical impossibility to speed the propeller up enough to continue to provide prop wash over the rudder and thus there will be a loss of steerage with the likely result being a broach at the trough of the wave.

For purposes of large wave generation due to shoaling, I would consider thrity feet to be very shallow. We could do the calculus, or just watch the opening credits of "Hawaii Five-O" again. (g)
Thanks, Sailaway - I can actually understand that!!

I do have one question though: Would a line towed out the stern as a bight with each end tied to a stern cleat work as well as a drogue?? I've heard "old salts" swear by it as being safer.

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post #42 of 45 Old 10-16-2007
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Sorry with all due respect that is not true that the entire sea is moving around the vessel. "In deep water, a wave is a forward motion of energy, not water. In fact, the water does not even move forward with a wave. If we followed a single drop of water during a passing wave, we would see it move in a vertical circle, returning to a point near its original position at the wave's end. These vertical circles are more obvious at the surface. As depth increases, their effects slowly decrease until completely disappearing about half a wavelength below the surface." See http://www.onr.navy.mil/Focus/ocean/motion/waves1.htm.

The wave is an energy passing along much as a waveform in a rope held at one end. The water in the wave simply moves in a circle so near the crest may be moving in the direction of the boat and faster, effecting steering.

Where the boat picks up acceleration is primarily from gravity as it moves down the waveface, which is itself moving.

I suggest that broaches generally begin at the top of the wave when the stern is not square on, and so one side is caught before the other starting a turn. Once started this is almost impossible to correct, and at the trough the bow also hits obliquely turning it further and slowing the boat sharply. At that point you need to be able to turn the boat rapidly which a drogue may tend to slow down. The drogue would also at that point be to one side requiring a greater boat movement to realign it.

While it has it uses in ocean settings and can help align the boat and prevent pitchpoling, I doubt that it is desirable in crossing a bar, or in making an inshore approach as outlined in other posts.
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post #43 of 45 Old 10-16-2007
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post #44 of 45 Old 03-24-2008
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That one looks horrible.
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post #45 of 45 Old 03-25-2008
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chris gee is correct on his wave theory but little else. In a seaway the drogue serves to keep the stern pointed towards the seas and, if not perfectly, does prevent the broach. Historically the most common use of the drogue has been for landing boats in surf. Boats become dynamically unstable when moving at the speed of the wave. They must either be significantly faster or slower than the wave speed to avoid a broach. to achieve significantly faster speed than the wave the boat may have to run at a perilous speed coupled with exhausting helming. To run at a speed significantly slower than wave speed some type of drag device must be used, commonly a drogue or lines/warps towed astern as referenced by Hartley.

“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.
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