Wave height

[rslotpole]

I’m sailing a 40 foot sailboat through waves of 6 feet every 9 seconds for 4 hours. How rough will the trip be? I’ve used 1 sec per foot of wave but not sure this will hold up in 6 feet

[Montanan]

The wave height of a surface wave is the difference between the elevations of a crest and a neighboring trough.

So six feet indicates three feet upward and then three feet downward.

Waves have lengths and also have velocity / periodicity, e.g., 9 seconds.

You could be riding a moderate elevator up to 1,600 times during your 4 hour trip if not moving through the water [not making way], that is to say, the waves traveling under you, e.g. moored. Depending on your direction of travel you will interact with the waves with different relative periodicity.

As to if that is perceived to be rough well that depends on your direction of travel relative to the waves [into, with, or crosswise] and how your vessel performs under such wave conditions.

[rslotpole]

Thanks. Waves are from 110 and we will travel north

[LakeSuperior]

If you are headed into 6 ft waves in Lake Superior you will be going zero in a 40 ft sailboat.

The 6 ft wave measurement in a weather forecast is the average of the highest 1/3 of the waves (measured from peak to trough.) This is the significant wave height. Consequently, there are waves higher than 6 feet in that sea state.

Most important is the periodicity of the wave train. If they are space far apart in distance (time) i.e. fully developed then 6 ft waves are not so bad. These are typically found on ocean size bodies of water.

On Lake Superior 6 ft wave are rarely fully developed and so are close together making the sides (profile) extremely steep. They have become known as "square waves" and they are not much fun to be out in.

[rslotpole]

We will be traveling from Boston to Manchester. Expect to motor sail

[barnakiel]

Depends on the boat. A long deep narrow will go fine if you ally enough force. But wet.


A light flat will be very wet, very slow and very noisy. No sleep.


b.

[dougweibel]

Wave period makes a huge difference. 6' at 6 seconds, 6' at 9 seconds and 6' at 12 seconds are completely different experiences. 6 at 6 will be quite a bit of motion and 6 at 12 will be pretty gentle.

Direction also makes a difference. If the waves are at 110 and you are truly traveling north then the waves will be behind the beam and not impede your speed, but Boston to Manchester is northeast so the waves will be ahead of the beam and you can expect them to slow you some.

The final factor is the hull shape. Barnakeil describes the difference well, though that description applies more the shorter the wave period.

I wouldn't hesitate to make a 4 hour sail in 6' at 9 seconds with a wave angle of 65 degrees off the bow. Depending on your hull I'd expect maybe a half knot penalty. If the period is shorter things will go worse. If it is 6' at 6 seconds it will be unpleasant and slow!

The final thing is if you have a single metric you may be misled. The waves will be composed of wind waves and swell, often with multiple components to the swell. If you are looking at 5' of swell with 2' of wind chop producing an average 6' sea that is a much nicer situation than a 2' swell with 4 1/2' of wind chop. Some forecasting sites will give you the height and period of the various components and this is very helpful. You can dig the components out of Windy.com but cannot look at them all at once.

[Thomas1985]

What boat? I can tell you on our Downeast 38 six four to feet by six seconds is rough, but fine.

We were just in 5-8 with 10 footers mixed in and it was white knuckle for the crew. Got a new high solid paint job on the hull from the wife

Earlier this year we got in solid 8-10 with 12 footers mixed in. However, period was longer. This was considerably more comfortable than the steeper 5-8 last weekend.

Great Lakes can be very choppy.

[Thomas1985]

I will also add...

when it's 6 feet by 6 seconds we don't pass many other boats. Many must feel that's pretty rough in their boats. Both small craft advisories we've sailed in on Lake Michigan we were mostly alone out there.

Last thing... remember it will be rough for hours or even days. Be prepared to deal with whatever weather for a long period. Its easy to watch a 7minute youtube video of 12 feet and think "I got this". What you don't see is the other 16 hours where it keeps doing it...

[Montanan]

The old saying "It is the journey not the destination that is important" holds true to navigation.

I often adjust my course to ease the motion of the boat to enhance the comfort and safety of everyone onboard. Let the sea state recommend the best pathway over the surface. Don't plan to follow a direct course towards your destination, often the course that may have a shorter time [and / or distance] is least pleasant. Be willing to bear off and find out if your vessel becomes more pleasant. The helmsperson should seek guidance from the perception of comfort and ease of the shipmates.

Sailboats not being able to sail [and motor sail] directly upwind of course bear off from wind driven waves. Whereas motored boats tend to have a propensity to bash their way against the waves

As pointed out previously, expect that there will be yet larger waves mixed in, which can become a bit like riding an express elevator up to the penthouse and then back to the basement.

Too me swells are like rollercoasters, waves are jolting.
Running with swells reminds me of catching waves when surfing during my college days, each swell is unique and some rides are better than others, the occasional induced gybe keeps things "lively".

Bon voyage.

[psk125]

People have mentioned the direction of the waves making a difference. The direction of the wind will too, along with how strong it is. Hull shape, wave heights, wave periods, sail area...Lots of variables.

[Johan Leopard51]

Offshore imho must look at SWELL and wave and wind and frequency.

I agree Montanan advice : unless it is a really short hop (5nm I’d just do direct and slow), comfort determines heading, not destination. We’re power cat but when on other sail yachts we often have more fun and comfort 40 degrees off destination if that gives better apparent wind and not bashing short chop/waves.

So example off Cape Town right now is 35 knot SE wind 3 meter waves (SE been blowing a few days), main swell 1.8meter SSW 10 second. I would not choose to be out in that but if I had to be, then I’d prob be heading NNE to have good sail, swell behind and tolerate the rocking of the larger waves that are at least then less frequent. Also helps that NNE heads you to shelter in this case

If destination is straight with/against swell AND wind, we never head straight, the cats are more comfortable at an angle (for me).