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Old 19-09-2019, 01:56   #1
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Use of Sea Anchor

Hi

I was wondering about using a sea anchor in a wind over tide situation.

In the Mozambique channel you can get up to 6 knots of current, if you were to deploy a sea anchor at say 30 knots of wind from the opposite direction I am assuming that there may be some sort of issue. I suppose the stronger the wind the more effective it will be.

Thoughts?
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Old 19-09-2019, 04:21   #2
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Re: Use of Sea Anchor

A sea anchor is deployed off the bow, and is a (sort of) “brake”, used to keep a drifting (sails down) boat’s head to the wind and waves, so that it may ride rough seas more comfortably. The drag created by a sea anchor can keep a drifting boat in more or less one place, or at least prevent it from drifting too far too fast.
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Old 19-09-2019, 06:22   #3
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Re: Use of Sea Anchor

Smourt if I understand it correctly you want to deploy your sea anchor in this situation so you are “drifting” your boat against the prevailing wind and get Speed over Ground against the wind by using the 6 knots current?
So your distination is upwind but down the current.
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Old 19-09-2019, 11:13   #4
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Re: Use of Sea Anchor

Yes, that is correct.

I was wondering how a cat or mono would behave in these conditions.
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Old 20-09-2019, 08:50   #5
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Re: Use of Sea Anchor

Wrong application for tide but read the book by Capt John Voss
on the "Tlikum". He explains all uses for a sea anchor (A good read).
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Old 20-09-2019, 08:56   #6
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Re: Use of Sea Anchor

Interested in seeing responses to this one.
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Old 20-09-2019, 09:34   #7
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Re: Use of Sea Anchor

Interesting though, in theory you may get about 4kn up current and a corresponding 4kn increase in wind speed. May work better if you use a brace to to bare off some and effectively use the sea anchor as the 'motor' as in motor sailing. Never head of anyone doing this so may be a trial and effort job. Let us know what happens if you try it.
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Old 20-09-2019, 09:51   #8
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Re: Use of Sea Anchor

Strictly speaking, wouldn't it be that the sea anchor is pulled in the direction of the current at 6 knots, and the boat is downwind, the opposite direction, being blown downwind with 36 knots of force? In my experience though, a sea anchor often winds up holding the boat at an angle to both the wind, and the current, in a sort of cockeyed equilibrium that's not obvious to predict. Once you see how it develops, you can shift the sea anchor attachment point to effect how the boat is hitting the waves.
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Old 20-09-2019, 10:35   #9
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Re: Use of Sea Anchor

When we sailed around South Africa, from Richard's Bay to Capetown, there was a 35ft monohull, headed south and around to Capetown, who told us he got caught in a southerly gale on the Indian Ocean side of S.A. and deployed a sea anchor from the bow.
He told us the sea anchor in the current towed him upwind at a rate of about three knots.
I don't know the rate of current at the time, but it does flow strong when offshore.
The general rule, though is to go inshore, on soundings (600ft or less) and heave-to on the offshore tack if you get caught by a southerly buster on the Indian Ocean side of South Africa. The waves are less steep and not as high in the shallower water where there is less current.
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Old 20-09-2019, 10:47   #10
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Re: Use of Sea Anchor

Interesting. I read somewhere that sea water is 700 times denser than air, meaning that tides & currents normally have more impact than drag from windage. Of course the sea only affects the underwater area, which will be much less usually than the total area of the hull, masts & rigging exposed to the wind, so the 700 factor is much reduced - but not eliminated.
Ignoring the sea anchor for a moment, & assuming no sails set, the boat will drift at 6 knots with the current, slowed down by the drag of the 30 knots of true wind. So it might drift at (say) 4 knots into the wind, with 34 knots of apparent wind over the deck. Sea state is going to be rough with 36 knots of apparent surface wind, so deploying a sea anchor from the bow should IN THEORY hold the bow up into the waves, & reduce the drag from the wind, while increasing the surface area exposed to the current, thus possibly increasing the drift rate INTO the wind - maybe to 5 knots...??

What does the tide do in that area?
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Old 20-09-2019, 12:36   #11
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Re: Use of Sea Anchor

Wind against current can sometimes raise bigger seas than you would expect, depends on your relative windspeed vs true windspeed. But other than that you will only notice the current of 6 knots if you read your boatspeed with GPS. You could find yourself doing 2 knots to weather without using motor or sails!
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Old 20-09-2019, 14:46   #12
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Re: Use of Sea Anchor

Sea anchors are too big for your purpose--what you need is an inexpensive drogue which will keep your vessel heading into the current, or if the wind is really it will lie at an angle to directly downwind but with less moving about.
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Old 21-09-2019, 09:44   #13
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Re: Use of Sea Anchor

Boat and any drag device are in the same current, so it cannot align boat with current. Drag device only affects alignment of boat to forces of relative wind and seas. Any device steamed from bow will cause boat to try to head into wind and waces but boat will move backwards. So you want a device that will not allow much reverse speed to avoid stress on rudder. Sea anchor is better for this than a drogue.
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Old 21-09-2019, 14:57   #14
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Re: Use of Sea Anchor

Waterman--drogues are streamed from the stern. That kinda makes the rest of your comment a little odd.

The boat will lie to anchor regardless of wind and current. Streaming the drogue from the stern makes the vessel lie comfortably, either to a direction downwind and across current to a small degree if current is weak and wind is strong, or upwind from the anchor if wind is slight and current strong.

The original question was about streaming a sea anchor--presumably from the stern, when already at anchor in a windward tide situation. I would imagine this as being a way to drag an anchor if the ground is of medium holding, or to strain deck cleats if the ground holds well under the excessive strain.

I used drogues frequently to overcome the slot effect between trimaran hulls--and cats have the same problem. Drogues from the stern minimise the effects of sailing to and fro behind the anchor, or around the anchor on changes in wind or tide.
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Old 25-09-2019, 13:49   #15
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Re: Use of Sea Anchor

Quote:
Originally Posted by Mike Banks View Post
Sea anchors are too big for your purpose--what you need is an inexpensive drogue which will keep your vessel heading into the current, or if the wind is really it will lie at an angle to directly downwind but with less moving about.
Mike, I just misinterpreted the meaning of your first sentence. You have experience with this so my only point was that any drag device cannot orient the boat to the current, only to the wind direction.
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