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Old 18-09-2015, 14:33   #76
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Re: Wavesize vs danger on a sailboat

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Originally Posted by chris95040 View Post
I notice my NWS marine forecasts don't tell you the wave period.

Is it possible to estimate this using wind direction, current direction, wind speed, and wave height?
See for example Fig 13.1 and 13.2 in http://www.metoffice.gov.uk/media/pd...and_Surges.pdf

Alain
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Old 18-09-2015, 14:51   #77
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Re: Wavesize vs danger on a sailboat

Trying to put a no. to it seems asinine. Depends on the boat and those handling her and conditions. I three foot swell may be terrifying to me and 20ft caps may not bother some one else.
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Old 19-09-2015, 01:11   #78
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Re: Wavesize vs danger on a sailboat

What about a catamaran? Where does the risk lie in the multihull arena?


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Old 29-09-2015, 21:29   #79
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Re: Wavesize vs danger on a sailboat

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Originally Posted by thomm225 View Post
The story on that video is that the crew is below decks working on a problem. (a massive leak I believe) So they may have been in a hurry and just did a quick set up with the jib and then got to working on the problem

The video was taken by the SAR Helo. The crew was able to repair the problem eventually and sailed the boat in without assistance. The Helo was unable to get in close enough to help due to the circumstances, and it wasn't to the point of abandoning ship
Thanks for giving the background on that. Where did you find it? Any word on what the name of the boat was? Where?
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Old 30-09-2015, 16:49   #80
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Re: Wavesize vs danger on a sailboat

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Thanks for giving the background on that. Where did you find it? Any word on what the name of the boat was? Where?
The information was below a video on youtube showing that boat.

I can't quite remember the name of the video, but for some reason, I can remember the lady that was singing the background music. (Hope Sandoval)

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Old 30-09-2015, 17:29   #81
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Re: Wavesize vs danger on a sailboat

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Originally Posted by thomm225 View Post
The information was below a video on youtube showing that boat.

I can't quite remember the name of the video, but for some reason, I can remember the lady that was singing the background music. (Hope Sandoval)

Must be the wrong link?
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Old 30-09-2015, 17:57   #82
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Re: Wavesize vs danger on a sailboat

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Must be the wrong link?
No, I just sorta forgot about waves and what not when I realized who was singing the background music...............

which was one of the great things about racing beach cats. We could usually get in early enough to enjoy the night life back in the day..............
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Old 30-09-2015, 18:00   #83
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Re: Wavesize vs danger on a sailboat

We are sailing a similar size boat and you truly do not want any breaking seas to come abeam.

In the best case scenario, you get shaken and scared, in the bad scenario you get wiped out and in the worst one you get completely rolled over. How far she goes will depend on how heavy your boat is, how much she grips water (long keel vs. racing keel but also the effect of the lee deck catching water, etc.), and how much she resists roll (rigging weight, etc.) to name a few factors only.

Waves offshore easily get 10ft and beyond and in a prolonged blow some of them will just break. This is not every day but otherwise pretty normal. Most of the time, the breaking part is not heavy enough to do serious damage but if there is more than one train or when there is some undercurrent, even relatively small waves can start to break in a very destructive manner.

So, in few words, just avoid breaking seas, especially abeam. Even a minor wipeout can lead to pretty much damage on the boat and frighten you and your crew.

Or else get yourself a bigger boat.

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Old 01-10-2015, 08:43   #84
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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.
..
That is very interesting, I mean that study about the relation between final stability and inversion risk. Can you point me to that study?

On my blog I have two lengthy posts about stability and I point the importance of that, that seems plain logical to me, but I would say that it is not only between 90 and the AVS but between about 70 and the AVS. Most of the boats that are capsized are not capsized by a single breaking wave but by a set of two that run close. The first one just puts the boat on its size or near and the second capsizes the boat.

That is easy to understand: the first wave takes almost all the stability of the boat that stays with a heeling of 70 to 90, the second one only has to overcome the stability between that point and the AVS (that is measure by the area under that point and the AVS point on a RM curve) and that explains the importance of that parameter.

Another important point related with the final stability has to do with the time that the boat takes from coming from a 70/90 heel (first wave) to an upright position. When the second wave hits, if the boat has already recovered, the stability will be huge...if it is still on its side the remaining stability will be small and then the boat can be capsized by a smaller wave.

This contradicts the importance that some see on a big moment of inertia related with capsize resistance: With a big moment of inertia, if the total RM is identical, the heavier boat (with a bigger inertia) and the lighter boat (with a bigger arm) will heel precisely to the same point, when the boats are hit by similar waves (same energy), the difference is that the heavier boat will take more time to reach that heel point and more time to recover to the upright position making it more vulnerable to a second wave.

I am working on a new post for my blog about dynamic stability (the two previous were about static stability) that will address this and more points.
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Old 01-10-2015, 08:48   #85
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Re: Wavesize vs danger on a sailboat

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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.
That is valid but I believe that in absolute therms that regards motorboats not sailing boats with a keel. In what sailingboats are concerned beam is also an important factor but not the only one. That is one of the reasons that most of today's sailboat designs are beamy: to increase stability

That movie you saw was about sailing boats or generic, regarding mostly boats without a ballasted keel?

Regarding the size of the waves I believe that they refer not to the the complete size of the wave but to the size of the breaking part. I say this because I had been hit several times on the size by breaking waves with about the size of my beam (3.90m) and even if part of the wave went over the boat I never felt the boat was near the safety limit or that justifies to pass from sailing on the course (with waves on the side) to defensive sailing, running or sailing slowly upwind.

Of course, as I have said in what regards sailing boats beam is not the only parameter (even if an important one) and regarding the others (even if the beam of the boat is the same) not all sailboats are equal.
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Old 01-10-2015, 09:11   #86
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Re: Wavesize vs danger on a sailboat

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Originally Posted by Polux View Post
That is valid but I believe that in absolute therms that regards motorboats not sailing boats with a keel. In what sailingboats are concerned beam is also an important factor but not the only one. That is one of the reasons that most of today's sailboat designs are beamy: to increase stability

That movie you saw was about sailing boats or generic, regarding mostly boats without a ballasted keel?

Regarding the size of the waves I believe that they refer not to the the complete size of the wave but to the size of the breaking part. I say this because I had been hit several times on the size by breaking waves with about the size of my beam (3.90m) and even if part of the wave went over the boat I never felt the boat was near the safety limit or that justifies to pass from sailing on the course (with waves on the side) to defensive sailing, running or sailing slowly upwind.

Of course, as I have said in what regards sailing boats beam is not the only parameter (even if an important one) and regarding the others (even if the beam of the boat is the same) not all sailboats are equal.
Really interesting stuff, I'd like to see the relationship of the weight and depth of the keel (righting arm or righting moment?) to the speed of recovery. It seems the speed of recovery has a lot to do with your 2 wave theory. Also is it not also the steepness of the wave AND the volume of the part breaking that contributes to the successful knockdown? I know in my own case in steep breaking waves it was the feeling of the boat falling down the wave (suddenly losing flotation on the leeward side) AND the volume of water breaking that hit that seemed to do the math.
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Old 01-10-2015, 10:20   #87
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Re: Wavesize vs danger on a sailboat

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I think a lot has to do with righting moment.

In a previous boat, a Contessa 32 which has an incredibly high righting moment, I was broad reaching at night trying to stay off Tuamotu archipelago which were (in pre sat-nav days) somewhere to leeward; when a breaking wave (that sounded like a train) picked her up and while hanging from a stanchion, I saw the tricolour at the top of the mast illuminating the water it was submerged in. It was only momentarily and she popped straight back up.

This article in Yachting Monthly Understand your boat and her statistics has some good points and the difference in vanishing angle of stability between a Contessa 32 and a Bavaria 32 is well illustrated in a diagram

A great boat will compensate for asinine seamanship.
No doubt a great boat for the size not only with a great AVS but most of all with great reserve stability and that gives it a great dynamic stability but all is relative, even if a Bavaria 32 is far away to be a match in total RM (resistance to capsizing) I believe that a Bavaria 36/38 would be a match for it.

In what regards modern boats to have a match in seaworthiness and stability for that size we would have to look among very sportive offshore racer-cruisers, like the Sunfast 3200 that has a smaller AVS (137) but a bigger max GZ bigger (0.81m) and a more rounded stability curve. Well probably the Contessa 32 has just a bit more stability due to the superior weight but would be far away in stability in what regards the Sunfast 3600 that has about the same weight, a bigger stability curve and is a much faster sailboat.

In the end you are right, the Contessa 32 has a great seaworthiness, one should know the stability characteristics of his own boat and as the comparison between the Contessa 32 and the Bavaria 32 stability curves show, not all the boats are the same even if old ones are not necessarily better than new ones. In what concerns GZ curves (weight for weight) newer ones are normally better.
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Old 01-10-2015, 10:40   #88
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Re: Wavesize vs danger on a sailboat

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Really interesting stuff, I'd like to see the relationship of the weight and depth of the keel (righting arm or righting moment?) to the speed of recovery. It seems the speed of recovery has a lot to do with your 2 wave theory. Also is it not also the steepness of the wave AND the volume of the part breaking that contributes to the successful knockdown? I know in my own case in steep breaking waves it was the feeling of the boat falling down the wave (suddenly losing flotation on the leeward side) AND the volume of water breaking that hit that seemed to do the math.
The speed of recovery has not to do with the RM of the boat but with the relation between the RM available and the weight of the boat (a heavier boat would need a lot more RM to return to the upright position than a lighter boat).

To have an idea of how fast the boat will re-right itself you should look to the GZ curve that is independent of weight (A RM curve is a GZ curve with all values multiplied by the weight of the boat) and to the values between 60, 90 and the AVS point.

I believe that are a lot of things that (in what regards to Dynamic stability) contribute to a a boat having a better resistance to capsizing and vice versa but it is too long and complicated to post here but you can be sure of one thing: all the boats are not the same.

Regarding my own, that is a sportive boat with a relatively high ballast ratio (for a bulbed keel with 2.3m draft), I know that in very steep waves hitting by the side, when they are steep enough the boat falls on the side of the wave to the bottom, but I would say that it is more gliding on the wall, with a relatively soft movement and the boat does not heel more than 50/60 (with 5/6m very steep waves). I believe that the way the boat glides laterally the wave has not only to do with a big GZ values at 50/60 (probably the max GZ is not far from 60) but also with the keel being narrow escaping that way the tripping effect that would have happen if the boat had a keel with a large underwater surface.
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Old 01-10-2015, 10:50   #89
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Re: Wavesize vs danger on a sailboat

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There is no simple formula for what you are asking. There are some rules of thumb that others have posted.

In engineering terms you are interested in the wave vector force component that can exceed your vessels righting moment.

Its relatively simple to calculate what that static force threshold is for different heights above your boats center of gravity / bouyancy.

What is not simple is using that information while sailing or calculating the dynamic values in real time.

...
It would be easy if all things were done in a static way but the the boats are in the water and react differently when they are hit by a breaking wave. It all comes down to what part of the wave energy is converted in a rolling moment and witch part is dissipated by other boat movements that not a rolling one, for example, gliding laterally. That is the hard part to calculate and the reason why some boats that have not great static stability curves can be very seaworthy (like the French voyage centerboarders).
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Old 01-10-2015, 13:01   #90
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Re: Wavesize vs danger on a sailboat

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Must be the wrong link?
This is where I first saw the Keel Boat video. Still haven't found the one with the info on the SAR Helo, but this one is decent.

The Red Icelandic Fishing Boat is depending on a single 900hp Yanmar I believe I read some place, and you gotta love the sailboat racers keeping the hammer down with as much sail up as possible for the conditions. Then there's the dude holding the boom near the mast on the sailboat breaking through the surf line

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