Effects of swell on sailing

[pcatterall]

Our group are considering moving from the Valencia area to lanzerotte.
One of the expected differences will be the larger and more constant Atlantic swell. How different will we find sailing there!?

 

[Yngmar]

Bigger and better? When going from the Atlantic into the Med this year, we've noticed the drastic difference in swell in the other direction. The Atlantic swell was often 3-4m, but such long intervals (distance/time between wave crests) that it was a more or less gentle up and down ride and apart from the odd bit of rolling when it was on the beam, everything was fine. It did not slow us down significantly (40 foot monohull).

In the Med on the other hand, the higher salinity means that swell can pick up dramatically fast and with a very short interval, which we found much more uncomfortable. In terms of sailing comfort, I would say one meter of Med swell is much worse than 3-4m Atlantic swell, and the one time we sailed in about 3m waves on the Costa del Sol, it was really quite unpleasant, not to mention very wet, due to the close and very steep waves. The wind also tends to create more current, so going upwind and fighting short interval waves was slowing us down drastically (making just 3 knots over ground with the engine at full throttle at one point).

So don't let the Atlantic's bigger waves put you off, you'll probably find them quite gentle. Unless of course it's also blowing hard and they come from three different directions

 

[MonniotC]

Someone please correct me if I'm wrong, but I always understood that the wavelength was a function of depth rather than salinity. Shallower depth equals shorter wavelength, and vice versa.

 

[RupertW]

Depth up to a certain multiplication of wave height, yes but given the Med is often 1000m or more the differences are, I think, much more to do with fetch - even East to West there is only a few hundred miles fetch in the Med and often far less. In the Atlantic (we spent 10 years around the West Country and West Brittany plus Canaries and cross Atlantic) the seas are often far far larger than I've ever seen in the Med but a lot less steep (and salinity matters for that) and far easier to sail in.

 

[[70521]]

Someone please correct me if I'm wrong, but I always understood that the wavelength was a function of depth rather than salinity. Shallower depth equals shorter wavelength, and vice versa.

Google is your friend.

https://en.wikipedia.org/wiki/Wavelength

 

[MonniotC]

Google is your friend.

https://en.wikipedia.org/wiki/Wavelength

Not sure what your point is. That link tells us all sorts of physics stuff about light waves, waves in solids, diffraction and all sorts of other physics stuff. However there is one small diagram showing sea waves becoming shorter in wavelength, steeper, and higher, as they approach a shore with the depth decreasing. Just as I said.

 

[William_H]

One problem of sailing parallel to a swell is the heel that you can get when on the leeward side of a wave. This is because many modern boats rely on form stability more than keel mass (as in a pendulum) to keep them up right. So the angle beyond level of the water of the wave adds to the angle of heel in flat water which can then be a bit alarming. ol'will

 

[macd]

Someone please correct me if I'm wrong, but I always understood that the wavelength was a function of depth rather than salinity. Shallower depth equals shorter wavelength, and vice versa.

It is, but there are also other factors and swell (as distinct from wind waves) is a special case. A critical factor with swell in open water is the age of waves, which (as suggested by RupertW above) is a function of fetch. Equally, if your premise is that the Med is shallower than the Atlantic, this is misleading. Its mean and max depths may be less (although 1.5 and 5km is scarcely shallow), but the Med actually has proportionally less extensive shallows. The latter is also one of the factors which contributes to its historically poor fish stock.

 

[pcatterall]

Thanks guys, setting aside the theory as to why the swell may be different in the canaries are there any other contributions about the real effect on sailing. To date I have the impression that the Canaries swell is higher but longer and less steep than we find here so in balance possibly more benign?

 

[macd]

Thanks guys, setting aside the theory as to why the swell may be different in the canaries are there any other contributions about the real effect on sailing. To date I have the impression that the Canaries swell is higher but longer and less steep than we find here so in balance possibly more benign?

Correct. However, even swells which are gentle to sail on can become extremely dangerous on approach to shore, and particularly to harbours with shallow approaches or bars. The Portuguese west coast is particularly hazardous in this way, and small boats frequently founder as a result. Most Canaries harbours are orientated so as not to be susceptible to prevailing swells (and the coast is mostly steep-to), but this factor should always be considered when choosing a destination on swell-affected coasts.

 

[PuffTheMagicDragon]

Short and steep seas in the Med are found where a relatively deep (>1000 metres) changes to a relatively shallow zone (<50 metres). Such an area is between Malta and Sicily, especially when a NW or NE is blowing.

 

[macd]

Short and steep seas in the Med are found where a relatively deep (>1000 metres) changes to a relatively shallow zone (<50 metres). Such an area is between Malta and Sicily, especially when a NW or NE is blowing.

That would be expected to exacerbate the roughness, but it's certainly not the only factor in play. The steepest, nastiest seas I've encountered in the Med, NW of Sicily, were in depths of 1km or more.

 

[RupertW]

That would be expected to exacerbate the roughness, but it's certainly not the only factor in play. The steepest, nastiest seas I've encountered in the Med, NW of Sicily, were in depths of 1km or more.

The seas from Halkadiki (our base) down the Sporades was unpleasant every time even in total calm with steep confused waves for 60 miles, but probably the worst was Sardinia to Menorca where the steepness was like a Solent chop magnified horribly but without losing the steepness.

 

[macd]

The seas from Halkadiki (our base) down the Sporades was unpleasant every time even in total calm with steep confused waves for 60 miles

Funny stuff, water...and certainly far beyond simplistic explanation, however tempting that might be. The passage you describe I've done many times and only once found it unpleasantly lumpy. Equally, I don't doubt your experience in the slightest.

 

[RupertW]

Funny stuff, water...and certainly far beyond simplistic explanation, however tempting that might be. The passage you describe I've done many times and only once found it unpleasantly lumpy. Equally, I don't doubt your experience in the slightest.

Fair enough - it was 6 return trips but all in one year (2016) and I'm sure your experience was correct too. It was one of the reasons we moved base.

 

[PuffTheMagicDragon]

There are two things commonly said about sailing in the Med and I am merely repeating them without necessarily endorsing them, although there is some truth about them:
(1) In the Med you tend to motor, in between gales and
(2) If you can sail in the Med you can sail anywhere.

 

[dunedin]

Rather than the swell when sailing in deep water, it would be the scarcity of safe anchorages that might be a concern about being based in the Canaries?

 

[webcraft]

Rather than the swell when sailing in deep water, it would be the scarcity of safe anchorages that might be a concern about being based in the Canaries?

There are a lot of affordable marinas though. Passages between islands can be 60 - 90 nm, so you only need a 24 hour weather forecast for the longest passages.

It is the wind acceleration zones that tend to upset softy sailors most in the Canaries.

- W

 

[Amulet]

Someone please correct me if I'm wrong, but I always understood that the wavelength was a function of depth rather than salinity. Shallower depth equals shorter wavelength, and vice versa.

My reading is that the shallow water effect only is relevant in very shallow water - I think I have read that the bottom has little effect on water at a depth of more than half the wavelength.. My understanding is shallow, so I might be corrected.

 

[AntarcticPilot]

My reading is that the shallow water effect only is relevant in very shallow water - I think I have read that the bottom has little effect on water at a depth of more than half the wavelength.. My understanding is shallow, so I might be corrected.

But ocean swells have wavelengths of 100s of metres. It's the ocean swell going into shallow water that makes the Bay of Biscay so unpleasant. Continental shelves are (by definition) less than 500m deep , and it shallows from thousands of metres deep over a short distance.