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Old 12-01-2009, 21:28   #1
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Question falling off 30ft waves

hi
i was reading a discussion on another thread about the strength of a deck salon.One member questioned their strength if you happened to fall off a 30ft wave and land on your side.

my questions are thus-
how many of you have sailed in waves this big?
how frequently would you encounter waves like this?
what is meant by falling off a wave?how would this happen

martin.
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Old 12-01-2009, 22:37   #2
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Martin if you study this overly dramatic simulator video it will teach you that what is more important than wave height is actually “wave period.”

In tank testing any vessel performs worst for any given wave height when the wave period is around 0.9 to 1.2 x the actual ship length. In testing this is where we measure maximum G-Forces and stresses for the ship and personnel

The combined wave and swell height of 30 ft (vertical distance from trough to crest) is not that unusual in an extended storm when you throw in other influences like current or cross swells. It is how you take them that counts



One other note…. When it is stormy it is important to watch the bottom contours when you can get these kinds of sets 100nm out there in calm weather

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Old 12-01-2009, 22:49   #3
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I understand that this is not always a result of a tall wave, but an extra deep trough forming under the boat in "confused seas". The boat becomes airborne and slams down hard. Boats and gear can be damaged.

Slowing the boat down or bearing off can solve pounding, but falling off a wave due to an extra deep trough is something unavoidable. The water just disappears from under the boat.
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Old 12-01-2009, 23:11   #4
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if you were faced with waves similar to the ones in the simulator i'm assuming you would steer into them differently? he seemed to be trying to get maximum effect from the simulator. would this be correct?

so falling off a wave would be when the crest of a wave travels under the hull faster than gravity can bring the boat back down?in which case momentum would carry you deep in to the water ready for the next wave to swamp over you?
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Old 12-01-2009, 23:27   #5
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BassAck….You are exactly right!

Where people get into trouble in rough conditions is when they set up the ship’s speed and course to average sea conditions and then an extraordinary wave comes along and spoils their day.

Amateur mistakes by someone who was thinking more about varnish and schedules than worst case scenarios that could injure crew…..




Martin…that’s right, when their speed and wave shape conspires to make them airborne
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Old 12-01-2009, 23:43   #6
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Quote:
Originally Posted by BassAckwards View Post
Slowing the boat down or bearing off can solve pounding
so lets say your heading north and the waves are coming directly at you then turning N.E or N.W would help?- i just want to make sure my understanding of bearing off is correct.
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Old 13-01-2009, 01:19   #7
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Quote:
Originally Posted by martinworswick View Post
so lets say your heading north and the waves are coming directly at you then turning N.E or N.W would help?- i just want to make sure my understanding of bearing off is correct.
Yes. How much to bear off depends on wind and sea conditions, speed, and the boat. Basically you "hunt" until you find the most comfortable approach.

Quote:
Originally Posted by Pelagic View Post
Martin if you study this overly dramatic simulator video it will teach you that what is more important than wave height is actually “wave period.”

In tank testing any vessel performs worst for any given wave height when the wave period is around 0.9 to 1.2 x the actual ship length. In testing this is where we measure maximum G-Forces and stresses for the ship and personnel

The combined wave and swell height of 30 ft (vertical distance from trough to crest) is not that unusual in an extended storm when you throw in other influences like current or cross swells. It is how you take them that counts
Study Pelagic's explanation. Youre asking the right questions.
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Old 13-01-2009, 01:27   #8
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thanks for the replies.
they've made the subject a lot clearer,
cheers
martin
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Old 13-01-2009, 01:30   #9
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Rough Water Seamanship

Read as much as you can because everyone has different opinions.



Rough Water Seamanship Part I: Boat Handling
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Old 13-01-2009, 04:12   #10
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We got into some pretty big waves/swells, during typhoon Tip near the Philippines many years ago.
They were huge...but not really breaking as I remember.

We would be in the bottom of the trough surrounded by mountains of water that were certainly going to absolutely crush us.
Then a few second later we'd be right on top and could see forever.

When we'd "fall off" is was generally very fast with an abrupt deceleration at the bottom....not really a slamming or smashing, just a really fast deceleration.

We had a severely compromised rig and the forward engine mounts had broken so we were just bobbing around taking the waves on the beam.

When we would come down I think we'd get up to about 70 or 80deg heal...at first we were concerned about the cabin windows taking the brunt… but just before we'd get to the bottom she'd recover a bit and I cant remember ever actually falling on them.

This was all over 30 years ago and I'm sure I've forgotten more that I remember, but I do remember these where absolutely huge way, way over 30 ft and fairly steep...but not breaking.
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Old 13-01-2009, 06:36   #11
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Quote:
Originally Posted by James S View Post
We got into some pretty big waves/swells, during typhoon Tip near the Philippines many years ago.
They were huge...but not really breaking as I remember.

We would be in the bottom of the trough surrounded by mountains of water that were certainly going to absolutely crush us.
Then a few second later we'd be right on top and could see forever.

When we'd "fall off" is was generally very fast with an abrupt deceleration at the bottom....not really a slamming or smashing, just a really fast deceleration.

We had a severely compromised rig and the forward engine mounts had broken so we were just bobbing around taking the waves on the beam.

When we would come down I think we'd get up to about 70 or 80deg heal...at first we were concerned about the cabin windows taking the brunt… but just before we'd get to the bottom she'd recover a bit and I cant remember ever actually falling on them.

This was all over 30 years ago and I'm sure I've forgotten more that I remember, but I do remember these where absolutely huge way, way over 30 ft and fairly steep...but not breaking.
This sounds similar to an experience I had in a 28ft monohull in the South China Sea during Typhoon Kai Tak in 2005. Compromised rig, lying ahull. But the seas were breaking too.
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Old 13-01-2009, 06:54   #12
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Quote:
Originally Posted by martinworswick View Post
hi
i was reading a discussion on another thread about the strength of a deck salon.One member questioned their strength if you happened to fall off a 30ft wave and land on your side.

my questions are thus-
how many of you have sailed in waves this big?
how frequently would you encounter waves like this?
what is meant by falling off a wave?how would this happen

martin.
Hi Martin,
It is easy to think the worst happens. It rarely does if you are a sensible sailor.
Re your questions:
1. Not waves for 30 foot - but certainly swells. As in my experience these were always further apart than shown in that video, they created downhill slopes to surf off down, but not steep faces to fall off.
2. You don't need to ever face waves like this today unless you want to. Modern weather forecasting and routing resources make it possible to avoid the places and the weather that create such waves.
3. Think the others have clarified this for you. In my experience one can just as easily fall off a 5 foot wave face. Even that - if you go airborn - is enough of a worry on the rig and your teeth!
Enjoy
JOHN
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Old 13-01-2009, 07:13   #13
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Thoroughly recommend study of Adlard Coles "Heavy Weather Sailing"

This includes some good descriptions of the problems that can be experienced when you get things wrong.
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Old 13-01-2009, 07:25   #14
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The only

The only time I ever actually had a boat fall off a wave and hit really hard was during the early nineties Perfect Storm. The waves were probably not 30 feet but somewhere between 18-25. The wave heights would have been nothing too bad in a gradually building storm but in this storm the wave periods were shorter than I've ever seen or experienced since. It was literally; wave peak, one one thousand, two one thousand, three one.. peak!

We fell of three wave faces, really hard, during that storm in a very well build Shannon but let me tell you it felt like we hit a brick wall. Stuff broke! We were not even in the bad parts of that storm so I can't imagine the waves bigger than they were and any shorter wave periods. The waves were cresting and the faces nearly vertical so we went through them more than over them, or that's what it physically felt like, but every now and then the bow would get knocked off just before the crest and we'd fall into the trough and then get pummeled by the next wave. We were rolled to close to 90 two or three times.

Unfortunately for us we were beating off a lee shore so we had it a lot rougher than we otherwise would have had we been able to trail a warp and fall off the wind or heave to.

This however was and is not a typical storm so I would not worry to much about being caught in one especially with today's weather forecasting. The seas and waves built so fast I've never seen anything like it, and I've sailed in a hurricane and with higher wind speeds but never with waves that short & steep. I honestly did not think it was possible to go from 20-30 knots & 5-6 footers to 45-55 knots and waves over twenty in a very short time frame but it is..

This storm would have been fine in this vessel had we had the sea room but we did not as we were delivering this boat up the New England coast from RI to NH.

I can assure you that if you ever experience short steep seas like this you want a STRONG boat. None of mass production boats I've owned could have survived this without some serious structural damage. Hell I had stitches and broken bones myself. I actually had a solid brass cabinet latch snap in half. In fact I feel quite comfortable saying I may not be here today had it not been for the robust construction of the vessel we were on. One hard trough smash on a production boat maybe but two or more is doubtful.

We were young and dumb and this storm snuck up on everybody but it still could have been avoided.

The conditions are all about wave period! I've sailed in some very heavy winds, 50+, with no problems (relatively speaking of course) and then some in the 35-50 knot range with massively short steep seas.

I'll take forty foot seas even cresting swells with long wave periods over 18-20 foot seas with short wave periods any day of the week.. In my experience height is not the only determining factor it is the period between wave peaks..

Of course these days I chose NOT to put myself in those situations..
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Old 13-01-2009, 08:00   #15
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[quote=Maine Sail;243458]The only time I ever actually had a boat fall off a wave and hit really hard was during the early nineties Perfect Storm. The waves were probably not 30 feet but somewhere between 18-25. The wave heights would have been nothing too bad in a gradually building storm but in this storm the wave periods were shorter than I've ever seen or experienced since. It was literally; wave peak, one one thousand, two one thousand, three one.. peak!

We fell of three wave faces, really hard, during that storm in a very well build Shannon but let me tell you it felt like we hit a brick wall. Stuff broke!

Marine Sail- you have had a experience that I would like to benefit from. What did break on the Shannon, what model was it, and would you beef up any stress point to make it even better?
I am not saying the Shannon is not a great boat- it is. But as a potential buyer of a Shannon from those years I would like to know what to look out for in used boats.
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