Wave Height . . . When Would You Turn Around ?

[Hud3]

Hurricane Dennis (1999) would probably fit Pelagic's senario. If I recall correctly, it stalled and then slowly reversed course twice off Cape Hatteras, hammering a section of the East Coast for about a week. I lived in Salisbury, MD at the time and recall that Ocean City experienced sustained, very strong on-shore winds for at least five days.

[Bash]

Originally Posted by Hud3

Hurricane Dennis (1999) would probably fit Pelagic's senario. If I recall correctly, it stalled and then slowly reversed course twice off Cape Hatteras, hammering a section of the East Coast for about a week. I lived in Salisbury, MD at the time and recall that Ocean City experienced sustained, very strong on-shore winds for at least five days.

yikes. okay

[dpons]

Originally Posted by Pelagic

To clarify dpons

When I have been involved as owner’s representative in tank testing large yacht models “Critical” wave periods (or frequency) have been found to be in the .98 to 1.3 range of ship or boat lengths at any given speed.

So, if I understand properly, are you saying that a wavelengths of about 98 to 130% of the boat length are most unconfortable as boucing goes?

[Pelagic]

Originally Posted by dpons

So, if I understand properly, are you saying that a wavelengths of about 98 to 130% of the boat length are most unconfortable as boucing goes?

Yes that is correct although the critical range is more between 80% to 130% based on hull design issues

This tank test video although not demonstrating critical wave periods for pitching shows how changing frequency and height can have dramatic effects in rolling.

http://www.youtube.com/watch?v=ewqaRMGv2mE

Same applies with pitching and my own tank test videos show same wave height but changed frequency to the critical range, where the model’s bow gets buried

[atoll]

hurricane dennis,surely if you are sailing on a 15 day passage,during hurricane season,with good access to weather you would have re routed long before the low actually became a problem?

[Pelagic]

Originally Posted by Hud3

Hurricane Dennis (1999) would probably fit Pelagic's senario. If I recall correctly, it stalled and then slowly reversed course twice off Cape Hatteras, hammering a section of the East Coast for about a week. I lived in Salisbury, MD at the time and recall that Ocean City experienced sustained, very strong on-shore winds for at least five days.

Thanks Hud, but that doesn’t really fit my example as I am sure all of us would have avoided a ‘named” storm and stayed well clear.

What brought this "Fringe decision" question to mind was recently helping a friend with weather info on his passage from Langkawi Malaysia to Cochin India. Normally about an 8-9 day passage for his boat

A day in, I noticed on the 7 day forecast an unorganized Low well ahead and NW of his track towards Sri Lanka and alerted him that it was generating 2m seas and 20knot winds according to the forecast. I was hoping it would pass ahead of him but advised he not be in a hurry

Another yacht already well into the area updated me as they were actually in that Low getting 3m seas and lots of +30knot squalls for the past 3 miserable days.


It was very slow moving and much larger than the forecast. They had been doing weather tacks to try and get out of it.

So I recommended to my friend to dramatically slow down and we would watch the movement.

It changed from a SW drift to a more S drift and after a day my friend and his wife started feeling the uncomfortable effects for 24hrs with weather projections of 6 more days of it in their path and quite nasty seas near his approaches to Sri Lanka.

My friend made the wise decision to turn around and do a 3-4 day time killing loop by backtracking and getting into comfortable waters.

The sad part of this story was a 3rd yacht left Langkawi 2 days before them but kept going into the worst of it. Something happened and they lost their boat but luckily were successfully rescued.

It made me think that maybe this could be a good discussion point about when is a good time to turn around.

The issue and discussion I think is:


Not what the boat and crew Can take,

but a recreational cruiser’s idea of

what the boat and crew Should Take?

[MoonlightShadow]

Good thread Pelagic. Really making me think. I don't think there can be a definitive answer as it all depends on one's mood and everything as much as what one thinks their crew/boat can take. My rule of thumb, would be to chicken out if the wave height was more than half the boats w/l length, but when you add a 1.5 period/moment thats throws my thinking way back to an earlier turnaround or smaller wave.

As a "cruise for plesasure" kinda guy now, I ticked the 2m box. I would go for the turn around perhaps earlier than most, because I am not in a hurry (eg: restricted by holidays from work for example), and am a bit "risk averse" and don't particularly want the challenge anymore. Comfort is good. My acrylic wrap-around high-vis pilot house negates against taking chances, which also suits my general persona today.

Different boat types, traditional long keel, flat bottom daggerboard, and different crews: young and fit, or old and infirm, different AVS, and all sorts of factors would bring different decisions. I would accept the moment and roll more on a boat that I had just re-rigged, than on one where I hadn't changed the stainless in a while. There are so many factors.

1.5 period and big waves isnt that uncommon in inshore parts here sometimes, where the estuary narrows, when the wind is easterly it brings them in and the bottom contour steepens them up with a little help from a wind against tide situation. We'd bash through them in a Pilot boat without much thought, as they are momentary and not continuous. However, I can't remember ever experiencing quite that relationship out at sea. Probably because I had bottled-out early.

From looking at your video, the waves do not appear steep sided. It would appear that the parametric roll is induced directly by the combination of moment and height (which is interesting and educational) as theoretically one could make a graph for every boat showing what it would be? In the same manner as AVS? Can one presume that the Metacentre and CG have pronounced affects on how this would differ from one boat to the next?

Not sure if I am making sense here, but do we ever measure the seas accurately anyway? It's usually gut feel based on experience. We form "an impression" of the waves. Or is that just me? I must admit I am not experienced in Ocean crossings, I have never done one. But in the North Sea here, most of our decisions, while based upon our techincal knowledge (or, as in my case: complete lack of it), are based upon the traditional seat of the pants instinctive feel, gained from all our receptors rather than mathematical formulae.

Sorry, I am rambling here, more thinking out loud (or on the keyboard).

[Vega1860]

Originally Posted by Pelagic

I think it would be interesting to Poll and discuss when a sailor half way thru making a +15day Ocean passage would decide that beating to weather is no longer worth it?

We never plan an up-wind trip but sometimes the wind changes. When we left the Strait of Juan De Fuca in September we ran into a southerly gale that blew us north a hundred miles before we could get under way again.

VIDEO