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Old 05-05-2012, 08:11   #1
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When does depth affect ocean swells?

Perhaps I could get opinions out there- when do you consider depth too shallow for going over (as in a bank or shallow sea) when it is getting hit by swells from the open ocean. I got pretty beat up with 6-7 foot swells on a 30 foot bank, and I am looking to make another passage keeping my offshore depth to at least 100 feet.
What is the rule of thumb you guys run by?
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Old 05-05-2012, 08:29   #2
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Re: When does depth affect ocean swells?

Unless you absolutely have to such as crossing a bar for a safe harbour - try and avoid shallow areas with swells / waves passing over them. You will always get a nasty and confused sea, often steep and violent too with the added risk of hitting the bottom when you are in the troughs. If your chart says anything about overfalls, or breaking seas avoid it if you possibly can. Once I did not and both the boat and I still have the scars to prove it!
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Old 05-05-2012, 08:29   #3
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Re: When does depth affect ocean swells?

I have no rule of thumb: every bar and shoal is unique, since there's often issues of tidal current to factor in, and a slight shift of wind or swell direction can make big differences. There is a bar in El Salvador with shallow depths and breakers straight across. It was squirrely, but we were expecting that. However, the big ship entrance to Corinto, Nicaragua felt exactly the same with a strong onshore wind and outgoing tide, even though there was gobs of depth. So while it's good to avoid any shallow water, local knowledge of inlets is more important--read up on them in cruising guides, and try to go with the tide when possible. I try to avoid any areas where I might bottom out in the trough of a swell. It's inevitable if you cruise that you'll wind up making scary entrances. I can only testify that the hundredth one isn't any less scary than the first.
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Old 05-05-2012, 12:04   #4
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Re: When does depth affect ocean swells?

Newt,

The textbook answer is that an open ocean wave will begin interacting with the bottom (which causes it to become taller and steeper) when the depth is 1/2 the wave length. Down here typical Tradewind swells with an 8 second period would travel at about 12 m/sec, so the wave length would be about 100 feet. A 50 foot depth would cause steepening. A 15 second swell would interact at 100 foot depths.

In the real world, I'd play it safer and stand off the 200 foot contour or deeper. Remember Slow Speed Chase and the Faralones.
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Old 05-05-2012, 12:31   #5
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Re: When does depth affect ocean swells?

Thanks Hud,
I got into real problems with a 10 second swell at about 30 feet!. I didn't realise that the whole area around Cape Flattery is very shallow, and the shelf extends 20 miles out into the Pacific. One of the very important lessons learned in the last adventure. One that I hope not to repeat, or if I do, know why the seas are getting so confused.
BTW- the guys around here talk about the 100 fathom rule (700 feet!)
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Old 05-05-2012, 12:34   #6
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Re: When does depth affect ocean swells?

Any time you have current that goes from really deep waters to less than say 100 ft deep you will get rough waves and some upwellings. Along the west coast of the US the coastal waters get rough about the change in ocean depth no matter how far out you are or how close the really deep water is to land, sence there is a 1.5 to 2 kt current heading south all the way to Cal. If you look at ocean charts you will find the roughest areas are where points of land get close to really deep water. At 12 miles off shore West of the Straight of Juan d Fuca your about at that area of the pacific ocean. I have been advised to stay in 6 fathums of water at about 2 miles off shore traveling north to avoid the south current and sometimes in an eddie to make better time and out at 3 miles off shore or further in the shipping lane going south to get the added speed of the current. At 6 fathums off shore you need to keep an eye out for crab pots always, there usually set in a straight line and avoid one and avoid all them.
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Old 05-05-2012, 12:39   #7
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Re: When does depth affect ocean swells?

This question was a puzzle in a recent edition of Yachting Monthly, the excellent UK sailing mag. The problem -- a real life one -- had someone sailing in South Brittany in brisk conditions - a F7, I think -- down a piece of water between some islands and the mainland, where the water gets narrower and much shallower -- I think it was just 4 or 5 meters. They guy was afraid to do it, and the puzzle was -- would you dare?

My answer was -- hell no! Wind gets funneled in spots like that, and I can just imagine what kind of sea state you might get when 3 or 4 knots of tidal current heaves up over that bank, with 30 knots of wind blowing.

But I was wrong -- the answer to the problem was -- ask local people -- that bit of water is navigable in those conditions. The guy in the puzzle got through it without any difficulty.
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Old 05-05-2012, 12:44   #8
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Re: When does depth affect ocean swells?

less than 100 ft has chop from rebound coastal waters---i try to sail out about 40- 60 miles off coast...i like 100 mi best, but isnt always practical.
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Old 05-05-2012, 12:48   #9
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Re: When does depth affect ocean swells?

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less than 100 ft has chop from rebound coastal waters---i try to sail out about 40- 60 miles off coast...i like 100 mi best, but isnt always practical.

He is only going to the Columbia river 150 or so miles south
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Old 05-05-2012, 12:50   #10
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Re: When does depth affect ocean swells?

drat--was hoping he is coming to mexico so we can party....
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Old 05-05-2012, 12:57   #11
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Re: When does depth affect ocean swells?

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drat--was hoping he is coming to mexico so we can party....
Zee if everyone went to Mex the cost of living for boaters would double
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Old 05-05-2012, 13:01   #12
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Re: When does depth affect ocean swells?

already has, but who is counting--i am just passing thru....
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Old 05-05-2012, 13:12   #13
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Re: When does depth affect ocean swells?

Rule of thumb = don't!

But obviously that not always possible, just that as everywhere different (and every place different according to conditions - tide / weather / current / atmospheric pressure - and not always obviously "bad". the more in the mix the more unpredictable things can be) useful for events / possibilities not to come as a surprise (no substitute for local knowledge, not the same as simply being a local! - a chart will give a heads up, but won't convey enough about the reality)....and having a plan B is nice (turning around and running away works well!).

Got to remember that when the ocean meets the land something has to give - nice if that does include "you".

Being on the edge of the continental shelf (whether 20 / 50 / 100 miles out) can be worse than being inside. or it can be worse. depends.
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Old 05-05-2012, 13:21   #14
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Re: When does depth affect ocean swells?

I knew someone years ago who had ben up and down a lot and swore the best trip down the coast was the ..... hmmm... I think 20 fathom line..... Does that make sense...? me..? I was out 50 miles, but he laughed and said that's why it was so rough... I dont have a chart in front of me so need to se how far out that fathom line is....
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Old 05-05-2012, 13:44   #15
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Re: When does depth affect ocean swells?

Ooops! - meant to say:-

Got to remember that when the ocean meets the land something has to give - nice if that does not include "you".

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