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Old 03-09-2015, 01:35   #1
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Wavesize vs danger on a sailboat

Hi,

I was recently out in some light-heavy weather where we encountered a couple of larger than usual (short) waves. Nothing special, perhaps around 3-5 feet which is more than what we're usually sailing in.

We were broad-reaching and we got a pretty good roll from it. (Small sailboat 27 foot). Not that it felt unsafe, but I could imagine that I wouldn't have been too comfortable if wave-size would increase much further.

Is there a formular to calculate when it's better to go stern-to-waves instead of broad reaching, or will a sailboat just power through most of it?

How big do waves need to get before its considered unsafe?
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Old 03-09-2015, 02:20   #2
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Re: Wavesize vs danger on a sailboat

It is really a combination of the height, but more critically the wave period between breaking crests.

When the period is around the same as your boat length for your given heading and speed, that is the critical and most damaging situation for that height
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Old 03-09-2015, 03:12   #3
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Re: Wavesize vs danger on a sailboat

I've heard figures ranging from a breaking wave with a height equal to your beam to a breaking wave of a height of 0.6 x Loa. If you are beam on being capable of capsizing a yacht. Mind you that's a proper breaker, not just a foamy top, and assumes you are lying beam on which would be a bit stupid.

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Old 03-09-2015, 05:19   #4
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Re: Wavesize vs danger on a sailboat

As Pelagic and Snow have suggested, it is more complex than size alone. Capsize is far from the only risk of big waves. Getting slammed by a short steep 3 foot wave could cause you to fall down your companionway and bump your head, while a big round greeny of 15 feet might not do much of anything. There's risk of seasickness, things can break due to pounding.

If you're primarily concerned with a wave overwhelming a boat and capsizing it, I would think a boat like an Albin Vega could handle quite a bit more than 5'. There are so many other factors to consider though, including your comfort level and experience.

Maybe a good judge of wave height would be if you ask yourself "should I be out in this?", it might be a good time to consider finding a place to hide.

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Old 03-09-2015, 05:34   #5
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Re: Wavesize vs danger on a sailboat

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Originally Posted by Snowpetrel View Post
I've heard figures ranging from a breaking wave with a height equal to your beam to a breaking wave of a height of 0.6 x Loa. If you are beam on being capable of capsizing a yacht. Mind you that's a proper breaker, not just a foamy top, and assumes you are lying beam on which would be a bit stupid.

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I read a test that was conducted not too long ago, that came to the same conclusion, that an average monohull of modern design could be rolled by a breaking wave of roughly the same height as the beam of the boat, now where I can see it happening to someone that isn't stupid is a disabled boat in a storm, loss of a rudder maybe.
But the take away is that a really huge wave of the right slope and period is harmless, but a relatively small breaking wave can do you in.
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Old 03-09-2015, 05:50   #6
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Re: Wavesize vs danger on a sailboat

Another interesting study recently found that the area under the righting moment curve from 90 deg to the AVS correlated quite well with complete inversion risk.

But it's interesting just how small a wave can be be to capsize you.

But it's more complex than just beam or length related to wave size.
And real breaking seas are rare. They are not the run of the mill whitecap. They are more like a wave breaking on a bar. Or a surf beach.

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Old 03-09-2015, 05:53   #7
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Re: Wavesize vs danger on a sailboat

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I've heard figures ranging from a breaking wave with a height equal to your beam to a breaking wave of a height of 0.6 x Loa.
I saw a rather extensive video about this and the "breaking wave equal to your beam" is what the latest research suggests. That is for causing a roll however. Less than that or even non breaking could still throw you around plenty. Being thrown around is pretty dangerous.
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Old 03-09-2015, 05:54   #8
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Re: Wavesize vs danger on a sailboat

Yep I've heard wave height 1/3rd the length of the vessel - which basically corresponds to the beam for most monos - is the number. I've been laid flat by 8 footers standing square and rolling in on the beam in a 25 foot boat so I reckon it's spot on. The good news is that they didn't in my case come close to capsizing the boat, just laid it flat for a few moments. I'd think you'd have to have a wave big enough to slide down or otherwise be severely broached to capsize, especially in a vega which has the rep for seaworthiness.

Edit. The waves weren't breaking. Breaking waves release a LOT more energy than the equivalent non-breaking wave!
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Old 03-09-2015, 08:19   #9
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Re: Wavesize vs danger on a sailboat

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Yep I've heard wave height 1/3rd the length of the vessel - which basically corresponds to the beam for most monos - is the number. I've been laid flat by 8 footers standing square and rolling in on the beam in a 25 foot boat so I reckon it's spot on. The good news is that they didn't in my case come close to capsizing the boat, just laid it flat for a few moments. I'd think you'd have to have a wave big enough to slide down or otherwise be severely broached to capsize, especially in a vega which has the rep for seaworthiness.

Edit. The waves weren't breaking. Breaking waves release a LOT more energy than the equivalent non-breaking wave!
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Old 03-09-2015, 08:36   #10
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Re: Wavesize vs danger on a sailboat

I think it was in Heavy Weather Sailing that I read this... a breaking wave on the beam is most dangerous (note : breaking) and the size threshold, beyond which the boat could capsize, was between 1/3 and 2/3 of LOA, depending on design.

Of course, significant damage to the boat and crew is possible at smaller breaking wave sizes.
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Old 03-09-2015, 08:37   #11
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Re: Wavesize vs danger on a sailboat

The OP is sailing in Denmark (ěresund?) so the wave period will be extremely short and the waves quite steep. 3-5 feet = 1-1 1/2 meters, not unusual, especially in Kattegat or around the south of Denmark.

As noted above - current findings are that a Breaking wave as high as the beam of your boat will capsize it if you are beam on.
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Old 03-09-2015, 08:37   #12
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Re: Wavesize vs danger on a sailboat

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Old 03-09-2015, 08:40   #13
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Re: Wavesize vs danger on a sailboat

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The OP is sailing in Denmark (ěresund?) so the wave period will be extremely short and the waves quite steep. 3-5 feet = 1-1 1/2 meters, not unusual, especially in Kattegat or around the south of Denmark.

As noted above - current findings are that a Breaking wave as high as the beam of your boat will capsize it if you are beam on.

Very similar to the seas one experiences on the Great Lakes. Short and steep. I've sailed in 10' seas with a period about the length of the 28' Pearson Triton I was sailing on. It was quite a ride.
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Old 03-09-2015, 08:46   #14
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Re: Wavesize vs danger on a sailboat

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Another interesting study recently found that the area under the righting moment curve from 90 deg to the AVS correlated quite well with complete inversion risk.

But it's interesting just how small a wave can be be to capsize you.

But it's more complex than just beam or length related to wave size.
And real breaking seas are rare. They are not the run of the mill whitecap. They are more like a wave breaking on a bar. Or a surf beach.

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I'm a little sceptical as that implies that the area under the curve from 0 to 90 degrees has no effect on capsize risk.

Here's how I see it : the breaking wave imparts a certain impulse to the boat. How far the boat heels in response is proportional to the area under the righting moment curve. If you run out of righting moment before the boat stops heeling, it capsizes.
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Old 03-09-2015, 08:50   #15
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Re: Wavesize vs danger on a sailboat

Breaking bs rolling. In a rolling wave, the boat just rides up and down like a cork. The actual water movement is up and down, not in the direction of the wave. In a breaking wave, the under portion moves up and down, the breaking portion moves laterally. The leading edge sets the boat to list heavily. When this amount of heal occurs due to wind, the boat (and sails if any up) spillway energy off. The boat may be pushed sideways as the keel loses bite, releasing even more energy. However, when the breaker (higher than beam) washes right over the boat, all freeboard takes the massive load of water in lateral direction. Where the wind pressure may be 1000 pounds, the water pressure may be 10000 pounds. What bite the keel still has, is trying to hold the boat on track, hence the much greater rolling moment due to a breaking wave than wind.
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