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  #1  
Old 07-04-2007
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Wind vs Tide/Current

I've been sailing for a few years in the Pacific NorthWest, but until recently my moorage has had direct access to the ocean. What I mean by that is that once I rounded the breakwater, there was nothing between me and Hawai'i.

Several weekends back I bought a 24' boat for my fiancee which was located several kilometers east of the coast on the Indian Arm off Burrard Inlet. During the course of the brief sail from the boat's original mooring to her new home in my marina, I encountered something against which I assume I must have been sheltered previously by virtue of always having been on "open water." While clearing one of the narrower sections of the inlet, the water suddenly changed from what had been gentle 1-2 foot swells, to very short, almost jagged waves around 5-6 feet tall. There was also some really strange action on the tiller; not a consistent pull to one side, but current sort of throwing it back and forth.

It occurred to me - and I recall having read somewhere, I think - that there must be some interaction between the wind and tide that was causing this effect. The tide was flowing out (towards the West) and the predicted current at the spot in question was between 4 and 6 knots. The wind was blowing Eastward at about 15 knots. I assume because I was in the centre of a narrower section of inlet, the effect would have been more pronounced because, in part, of the Venturi effect.

So, after this long preamble, my questions are these:

1) Is there an effect when the wind is blowing against the tide? Does it have a name?

2) Is there a name for the particular type of waves I attempted to describe?
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Old 07-04-2007
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Hyperion,
You have encountered the situation that most commonly generates freak waves. Probably the deadliest version of which is found off the SE coast of Africa, where the Cape rollers meet the Sw'ly flow of the Agulhas current. Many a large ship, and I do mean large, has had her back broken by encountering a freak wave there. A freak wave is generally considered to be one that is impossible to generate by wind. In fact, their occurence was denied as impossible for many years. We now know differently.

One of the conditions needed for these to form is shallow water. "Shallow" in this case may mean anything under the 100 fathom curve. Shallow water waves, so familiar to the Great Lakes sailors, have a very short wavelength or period, and will build up very quickly to a surprisingly large height. The lack of depth of water, compared to open ocean, is what causes this phenomena. You, more commonly, observe it at the beach as the waves approach the shallows building in height as they do so.

Now, if you take a wind opposed current and add it to the mix, the effect is exacerbated. It makes for a most unpleasant ride, does it not? And, a big part of the problem, is that it is the type of condition that can blow up in very short order. Maybe twenty minutes of wind is sufficient to significantly change things.

The sensation you are probably feeling on your rudder is the result of the short, tall chop produced by these conditions. Rather than the boat riding up and down a swell the seas are passing quickly under the boat, and the rudder is being exposed as the trough passes astern. It has a sort of a weightlessness feeling to it does it not? That is because it is either out the water or the water is what might be described as thin or less dense. The rudder is less effective, must be held over for a bit longer until it's sporadic bite grabs water.

Once out in deeper water, the condition moderates. The danger, on a boat, is that conditions can be quite benign and you can go forward, only to be slapped right in the chest by a green sea six feet above the waterline. All while the boat is not necessarily pitching that much. Quite disconcerting, yes?

Bowditch has a chapter on ocean currents that is quite illustrative of their nature and interaction/forming related to wind. It is a good primer on the interaction of wind/water and moving masses of water. I am not aware, or do not recall, specific names for these conditions. A particularly spectacular, and dangerous, example of this occurs at the river Columbia bar. It is not for nothing that the USCG chooses to train their motor-lifeboat operations there. You've probably seen the videos of their forty footers rolling over 360 degrees. The videos were shot off Cape Disappointment.
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Old 07-04-2007
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Both the First and Second narrows into and out of Burrard Inlet run very hard indeed during spring tides. It is absolutely necessary to pay attention to the current tables, and to attempt to negotiate these narrows at maximum flood or ebb is a bit foolhardy even in the absence of any significant wind.

A strong contrary breeze can turn things into a total washing machine that is very dangerous for a small boat. That said, once becoming familiar with the area there are often back eddies or calmer spots at the edge of the passes and many do in fact pass through at off-slack times.

We must negotiate the First narrows each time we leave the marina on our 35 footer, and rarely attempt to do so if the narrows are flooding at more than 2 or 3 knots, and to "ride out" a strong ebb against a westerly is a interesting experience - do-able but with caution and careful conning.

The other factor in all of this is that these narrows are heavily used by commercial shipping, and the odds of meeting a large container ship, freighter, cruise ship or tug & tow are very high. There are traffic separation schemes that may well interfere with your plan to stay in "flat water".

The safe play is to try to transit at or near slack, as with any potentially dangerous narrows.
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Old 07-04-2007
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Sailaway, you've hit the nail on the head. Not pleasant at all, and this patch of waves seemed to come out of nowhere. Your suggestion vis-a-vis the rudder coming out - or at least part of the way out - of the water is consistent with the pitching we were experiencing in every plane. It was, as Faster writes, a "washing machine."

Faster - having spent absolutely no time sailing any further into the Inlet than the Lion's Gate Bridge, I didn't check the currents. Needless to say I will be checking in the future. The spot I was talking about in my post was nearly directly under the Second Narrows Bridge. It was not so bad once I cleared the bridge, but there were still a bunch of these "washing machine" type patches all the way to Mosquito Creek (where the boat is presently moored). I was amazed at how fast this was flowing.

Thanks, both of you, for your information.
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Old 07-04-2007
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Hyperion - which float are you on? We are at Mosq Crk also, B114. Mayhaps we'll run into each other one day.
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Old 07-04-2007
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Haha. Small world. I was looking at a Cal 25 on B dock about two months ago - it's probably still there by the looks of its condition.

My fiancee's boat is at D25, and my Catalina is at Burrard Civic (waiting for a spot to open at Mosquito Creek).
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Old 07-04-2007
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Freak standing waves, like the ones you encountered, are fairly common in any location where the current (tidal or river) is opposed by the wind. Similar things happen at the southwestern end of the Cape Cod Canal and at the Columbia River Bar. If you want to see truly amazing photos, check out the Columbia River Bar Pilots website.
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Pentland Firth wins my maelstrom of the month contest every month. At the north of Scotland, south of the Orkneys, it is where the Irish Sea and the North Sea attempt to resolve their differences. If memory serves, it's probably 15 miles across and the largest of ships take it off the "Iron Mike" and hand steer through there. I do not see how you could transit it in a north/south direction, in a small boat, and be able to have any prediction as to where you would fetch up. It is my opinion that the words, "roiled" and "maelstrom" were invented upon it's waters.
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