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Old 14-07-2008, 23:10   #1
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Loss of Steerage Waves and Swell

In my last sailing adventure a couple of weeks ago on Martin's Ingrid 38 there were two times when a breaking swell caught us on the starboard quarter, we lost steerage and nearly jibed to port before steerage was regained.
The scenario was sailing under 50% jib alone with swells and waves sometimes breaking at 12 foot plus heights from trough to top. Monty the Monitor was steering and our speed down the face of the waves was estimated at over 9knots. The Ingrid is a full keel with an outboard keel hung rudder.
My question is: Do you think the rudder could have been out of the water (it didn't look like it) or do you think the speed of the wave was equal to the speed of the boat and cancelling out the rudder action?
What action would have prevented this if we had wanted to maintain our course?
We eventually jibed and changed course and sailed more beam to the swell which gave us a few waves swamping the cockpit but more control.
I'd like to get a handle on this if it happens again.
Kind regards,
JohnL
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Old 15-07-2008, 02:14   #2
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pirate Stranger things happen at sea...

Quote:
Originally Posted by SkiprJohn View Post
My question is: Do you think the rudder could have been out of the water (it didn't look like it) or do you think the speed of the wave was equal to the speed of the boat and cancelling out the rudder action?
What action would have prevented this if we had wanted to maintain our course?
You have already answered your question in a way.

It is possible that the rudder was out of the water or indeed too small for the boat but what seems more likely is that the waves were simply too much.

What a lot of people do not appreciate is that the ship's underwater profile is what determines her course.
You can see this, if you are on a smallish boat - up to 36 ft - and motor gently in a calm sea. Get the rudder midships and rock the boat port to starboard and back again. You can do this by putting your moveable ballast (passengers) to do the rocking. You should find that the boat weaves back and forth.
When the weight is on the port side the boat steers to starboard and vice versa.

A wave such as you describe can easily alter your underwater profile even if only for a moment. It is enough.

Then you must also remember that the rudder is well aft. When you steer, it is the stern that moves to one side and alters the direction of your ship.
Whether the stern is moved by the rudder or by a powerful wave is indifferent to the result of your ship changing direction.

The ony way I can think you can get "a handle on this" is if you can see the wave coming, anticipate the unexpected turning, and turn in the opposite direction just early enough. Tricky but doable by an experienced helmsman.

Fair winds,

Aris
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Old 15-07-2008, 02:32   #3
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This is a fairly common phenomenon. The rudder is stalling for a short moment and has no capability of altering your direction. If your sails are out of balance, the ship will turn. A drogue when these conditions get worse will help to control direction as well as slowing you down. An alteration of course so that you are not steering downhill will also help.
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Old 15-07-2008, 05:14   #4
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SkiprJohn,

Did you try steering by hand with the quartering waves?

I don't have a wind vane, but my autopilot hasn't got the intelligence to anticipate quartering swells, and doesn't begin to react until the stern has been pushed over. Steering by hand, I put the wheel over as the swell just begins to lift the stern, and my course stays true. That's been my experience in breaking swells on the quarter, up to 24'.
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Old 15-07-2008, 13:19   #5
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Thanks Guys,
I think your answers were right on the money.
No, I didn't try steering by hand. That would have been my next attempt but just after these incidents the seas starting coming down a bit and I thought we'd make it through the night with no further incidents so didn't try anything further. I watched the windvane work the swell and it reacted just as I would have except I would have anticipated the swell a bit earlier and started a turn across the wave before the loss of steering. In that situation I certainly didn't want to be watching a quartering wave and throughout the night would not have been able to see them anyway.
What seemed a little strange was that the boat wanted to turn to go straight down the wave instead of rounding up at an angle. Does that seem odd?
Regards,
JohnL
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Old 15-07-2008, 13:26   #6
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Hi John,
Your last comment about your full keel keeping you on a course straight down the wave face leads me to suggest wave speed = hull speed = no steerage!
One easy way to establish if it is the profile or the rudder would be to fire up the engine and create some flow from the prop. If with some flow over the rudder from the prop you regain steerage way, then it was indeed the water speed equalling your boat speed that negated your rudders effect.
If the prop drive makes no difference, it is your underwater profile.
Good luck
JOHN
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Old 15-07-2008, 13:34   #7
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I drive a full keel boat and in a quartering sea, she will turn into the wave as it builds under and turn the other way as it passes under after surfing. This is normal behavior for an old boat, even in beam seas if they're following at all. An active helm is the only solution and depending on wave size and speed, even this can get sketchy. I can see where a drogue would help here.
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Old 15-07-2008, 18:44   #8
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Your loss of steering was a consequence of the loss of water flow across your rudder coupled with a loss of stability due to heave. As opposed to common perception, water in a wave does not move forward with a wave (except in the event of a breaking wave where the crest tumbles “forward” over the face of a wave). In a non-breaking seaway (waves and swell), for all practical purposes, water simply moves up and down although within the crests, water molecules develop an orbital velocity that can be described by the equation: Uo (Water Velocity) = πHw/T where Uo is Feet per second, Hw is wave height in feet and T is wave period in seconds. This can be rewritten as Uo = πHw/(√Lw/2) or Uo = 2 πHw/√Lw where Lw is wave length in feet. Hence, the shorter the period T (=√Lw/2), Wave Length (Lw) or the greater the wave height (Hw), the faster the topical surface water flow.

Sailing in the same direction and at near the same speed as a wave train causes the flow over the under-water appendages (keel and rudder) to slow, or even reverse, at the crest of a wave but accelerate in the trough. Hence one’s rudder is of little use at the crest of a wave-leading to over-steering-but then quickly becomes “super effective” in the following trough. In conjunction with the foregoing is the fact that the centrifugal force arising from the acceleration of the water as it rotates tends to counter-act the force of gravity at the peak of a wave, hence the apparent water density is lessened and the yacht effectively has less support from the surrounding water (per Archimedes). With the foregoing combining, the result is “crash and burn” round-ups.

The physics of the foregoing was neatly described by Froud as a result of his experiments 1861 and later dissected by C.A. Marchaj in his study of stability in a seaway as described in his book of the mid-80’s “Seaworthiness—The Forgotten Factor”. Bernard Moitessier described his solution for the foregoing—run faster than the waves and bare-off at the peak of a wave crest—in his book, “The First Voyage of the Joshua”, but most are unwilling to experiment with Moitessier’s solution.

FWIW!

s/v HyLyte
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Old 15-07-2008, 19:22   #9
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Thanks HyLyte,
Although I didn't understand the formula I do understand your narrative and if placed in that situation again I will "be at one with Bernard."
The waves that got us were breaking so I would imagine they were moving at our hull speed or faster which made us round down when the moniotor was moving to keep us on course. In other words the rudder would have the reverse effect until we slid into the trough. I think I'm getting it now.
Kind regards,
JohnL
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