When does depth affect ocean swells?

[rossad]

Be weary of the bigger depth changes on your charts 10 or 200 miles from the coast in any kind of blow. Continental shelf is where not to be in a severe gale sometimes 200 NM off land.... thats where the waves start to break heavy.

[zeehag]

Originally Posted by mrohr

It ain't necessarily so. Inshore waters indeed will have reflected waves resulting in confused sea conditions close in ,if the existing seas are breaking against a steep-to headland,but waves breaking on a sandy beach, or any gradually shelving bottom,mangroves,etc. do not reflect back out into the existing waves to cause "chop".In these cases the wave energy is dissipated on the beach.
Of course there are plenty of other reason to avoid shallow waters if possible.

east and west coasts are very very different in geology. west coast has back chop for almost 60 mi out. is more pronounced closer to beaches and cliffs.
100 ft in pacific coast can be too close to sail comfortably here.

[Jim Cate]

Originally Posted by newt

Jim,
A couple of things make this different. I have sailed a few oceans, and this one has a large robust fetch across from Japan and The Gulf of Alaska. That makes the typical waves here over my head in the cockpit. That combined with a very irregular underwater shelf, and Massive rivers flowing out of the continent (and out behind Vancouver Island) and the frequent gales- none of which I noticed while in Queensland. Yeah it really is like sailing on another planet.
The shallow water further mixes things up, and you get what they call "the washing machine". I must say I thought I was pretty good coming up here from the Gulf stream and the Atlantic. I have taken a little humble pie....

G'Day Newt,

Points all noted! And yes, the Queensland coast is pretty well protected and generally free of severe weather, barring the occasional cyclone. But the NSW coast has no protective reef and has long period swells from Southern Ocean storms, much as the PNW has from Arctic regions. The East Australian current flows southward against the general direction of the bigger swells, and the Continental shelf varies widely in distance offshore. I had the feeling that this combination wasn't all that different from the conditions in the PNW, but I'm surely not an expert!! I suspect that some differences in the details of the ocean floor may cause the apparently more severe sea states in your area (when away from the influences of strong outflows like the Columbia or the J de F straits).

I haven't had the pleasure (?) of sailing the PNW waters, but I have done a half dozen trips up and down between SF and SD, and a few down the Baja coast. I have not found there to be unusual sea states attributable to reflected wave action. Plenty of 6-8 foot wind waves on top of whatever ground swell existed, not particularly nice when northbound. I've done it harbour hopping up the coast, and I once did it in one long tack out and then back in. The latter trip was a bitch despite being well offshore... can't recommend that route!

Anyhow, I've enjoyed this discussion and thank Gordy for the posted description of wave/bottom interaction. Newt, I hope that your next attempt at the delivery goes well and brings the pleasure of a job well done.

Cheers,

Jim

[jackdale]

Just a quick comment.

In my trips around Vancouver Island, I find under "normal" circumstances things start to settle down around the 100 meters contour (approx. 300 feet).

BTW 6 feet = 1 fathom.

[Andrew Troup]

Good thread, interesting question and thought-provoking, thoughful answers.

I think there are a couple of further points worth floating here.

Firstly there's the important question of the direction(s) of energy propagation. This is not easy to determine without access to external information sources, because most of the energy transported by a complex seastate is contained in the longest wavelength ground swells arriving from far away, which are very difficult to detect and differentiate from what is built over them.

Unless they're abnormally high, (generally they're long and low) both their wavelength and their direction are effectively camouflaged by shorter wavelength swell trains from different directions. These in turn are overpainted with local wave trains of much shorter wavelength and with much steeper flanks, and whose direction reflects the local wind angle.

If the direction of the first-mentioned long-period swell trains is alongshore rather than onshore (as often happens on the east coast of NSW), the shallows will not be a bad place to be, in fact arguably they'll be protected to some extent from the energy of these trains, which cannot travel far in shallow water. It makes sense that it would be a habitual practice in such regions to sail with 'one foot on the beach'.

On the other side of the Tasman, there is a long fetch for the prevailing westerlies of the roaring forties. The entrained energy is directed onshore onto the west coast of the South Island of NZ, where there is no enthusiasm whatsoever for travelling close inshore.

There may be even more energy arriving from distant storms in the Southern Ocean, travelling alongshore (ie from the SW), but the energy from the west /norwest is sufficient, and from far enough away, that it is often problematic well offshore.

The second question: at what depth? is complicated by factors others have mentioned, particularly the rate of shallowing, and the presence of ocean and tidal currents which can cause upwellings which have major effects on seastate, often eclipsing any rules to do with local depth.

But any rule is also rendered difficult to apply by the difficulty mentioned above: discerning the period (or wavelength) of the longest, lowest component swell, which may contain the bulk of the energy.
Because it's THIS period which has to be plugged into the formula (assuming a gradual change in depth)

I think the difficulty of doing this might help explain why, among practical experienced sailors, the theory has become discredited, because the period they've inferred has not been nearly long enough to match the observed effects on the wave energy.

Add to this the fact, as has been observed by previous posters, that the rule relates to the depth when waves start routinely breaking forwards (rather than spilling backwards) without any other provocation.

This is is an order of magnitude more consequential than the changes which interest us as sailors - most of us have never seen a large wave breaking forwards offshore, and would not wish to (except on youTube, if we're given to masochism).

[senormechanico]

On our recent delivery from Washington State to Newport Beach California, we found a BIG difference in ride comfort when we crossed 4-500 feet depth (3 times).

[SV Liberty]

I avoid banks while sailing offshore, and I define a bank as anything that juts up much, much higher than the surrounding depths. The hourglass shoals off the east coast of the Dominican Republic are several hundred feet deep, but because the water that pours off them heads into the Puerto Rico trench at 20k+ feet deep it reportedly can cause rough seas (when we crossed to PR from the DR a few months ago, the weather was calm so I crossed the hourglass shoals, but I did it at the narrowest part to be "safer", and had no problems).

While motorsailing between Samana Cay and Mayaguana Island in the Bahamas, in 4k to 5k feet deep water, we blew an oil line exactly as we were closest to East Plana Cay. Sailing as close to the wind as we can in our Morgan OI 416 (which is not that close), I figured that we could clear East Plana Cay with a safe (but nail-biting) margin. We crept closer and closer to the reefs, eventually getting into about 195 feet depth, and as the bottom came up the waves became markedly steeper and rougher.

On a related note, however, a few days ago we were sailing from Guadeloupe to Dominica, and we were going to come close to a less than 200 ft deep bank - but I was on the leeward side of it, and I wondered if the waves crossing the bank, and then sliding into deeper water, would make them diminish (even if we crossed from deeper water onto and over the edge of the bank).

Anyway, no rule of thumb, except to avoid shallow banks at sea. Except, of course, when the kids have lines in the water, and want to catch the fish that reportedly congregate around the same banks that cause the waves that dad doesn't like. But since we never fish in less than 200' (the no barracuda rule), I suppose we stay deep enough anyways.

[Jbaffoh]

Originally Posted by Hud3

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.

That doesn't take into account the effects of current and tide. I've been in the Gulf of Alaska on 50 fathom fishing banks, where the effect of water flowing up from the depths and over the plateau would stack the waves much higher and shorten their interval. There, I'm talking about 50-foot waves in 300 feet of water. By running over the edge into 3000 feet of water, the waves settled down to 35 feet with a much rounder face and longer interval.

[newt]

All this is starting to make sense. I found a big difference between 3-400 foot depth and 5-600 feet on our last passage. Searoom is a plus too. The Washington coast is a huge leeward shore with swells coming from Russia and Japan. Kinda like the N. Zealand situation. Big long powerful swells with all the local rivers tides and wind thrown on the top.
Yeah thats it....

[zeehag]

newt--is why i like sailing 100 miles offshore--is way mo bedda--smoother rollers and less shore caused chop...better fishing as well.....