Do you wear a life jacket? - Page 19 - SailNet Community
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post #181 of 215 Old 01-01-2014
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Re: Do you wear a life jacket?

Do we suppose that the force of the wave snapped the tether? That would mean the force of the water on his body was 2000-4000 pounds, equivalent to strapping a 150 hp engine to his butt and blasting him through the water at ridiculous speed. Improbable. The force on his body was probably more in line with a serious wave break, perhaps 500-1000 pounds.

Do we suppose that the impact of his body against the anchor point, without any stretch in the tether, contributed a major part of the force? More probable. Flying across the deck at 8-10 knots could account for all the force.

Sail Delmarva: Dynamic Tethers

A combination? Certainly. However, it seems quite probably that tethers with a little give in them could prevent this sort of breakage. Making them even stronger probably won't help; if the forces on the body are more than 5000 pounds the impact would be fatal.
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post #182 of 215 Old 01-02-2014
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Re: Do you wear a life jacket?

Quote:
Originally Posted by pdqaltair View Post
Do we suppose that the force of the wave snapped the tether? That would mean the force of the water on his body was 2000-4000 pounds, equivalent to strapping a 150 hp engine to his butt and blasting him through the water at ridiculous speed. Improbable. The force on his body was probably more in line with a serious wave break, perhaps 500-1000 pounds.
...
where do you get these figures from?
a cubic meter of water weighs already 1 metric ton, if it comes rushing at you at a certain speed, the force on your body should be higher than the mere ~250-500 kg you mentioned... even if not all of the force is transferred onto your body since water is a fluid and "sloshes" around you...
tethers usually have a breaking load of 20000 N which equals to roughly ~2000 kg/m2...
the carabiners must have according to EU regulations at least 22000 N of breaking strength if they are meant to be used for alpine climbing and there are no others on the market... if they do not comply with this regulation, they have to be marked with "not for climbing" - you won't take one of those to tether you to a boat with...

but you are right - if you get thrown around, the initial impact force on the tethers could easily be well above those breaking loads and a certain stretch in them would make them a lot safer...
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post #183 of 215 Old 01-02-2014
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Re: Do you wear a life jacket?

Quote:
Originally Posted by pdqaltair View Post
Do we suppose that the force of the wave snapped the tether? That would mean the force of the water on his body was 2000-4000 pounds, equivalent to strapping a 150 hp engine to his butt and blasting him through the water at ridiculous speed. Improbable. The force on his body was probably more in line with a serious wave break, perhaps 500-1000 pounds.

Do we suppose that the impact of his body against the anchor point, without any stretch in the tether, contributed a major part of the force? More probable. Flying across the deck at 8-10 knots could account for all the force.

Sail Delmarva: Dynamic Tethers

A combination? Certainly. However, it seems quite probably that tethers with a little give in them could prevent this sort of breakage. Making them even stronger probably won't help; if the forces on the body are more than 5000 pounds the impact would be fatal.
Vimes is right. I don't know much about the subject but the forces that I have heard about that can be generated on a harness line are on the order of some few thousands of kgs. I believe also that you are right and at some point those forces are so big that can also have a deadly impact on a body.

I noticed a change here in what regards the lenght of the lines here that from around 1.5m passed to just a bit over 1 m with two tethers. I believe it has to do with that and the charges generated: they would be much smaller on a shorter line than on a bigger one.

Those short lines are more uncomfortable and impractical to use specially at the wheel or at the cockpit where its very short length makes you have to clip and unclip at all the time. I have the two types on my boat and i only use the short ones to go forward in very bad weather (2 times till know ).

Nice study:

http://sail-delmarva.blogspot.pt/201...c-tethers.html

A good new year to you,

Best regards

Paulo


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Last edited by PCP; 01-02-2014 at 07:26 AM.
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post #184 of 215 Old 01-02-2014
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Re: Do you wear a life jacket?

The Admiral and I wear self inflatables every time we go out, they are compact and generally unnoticeable while under sail, because as Bill Cosby said "never challenge worse"
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post #185 of 215 Old 01-02-2014
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Re: Do you wear a life jacket?

this is my lifestyle, not my avocation. i have yet to fall into water...and i have been sailing almost 60 years.
no, i do not wear a life jacket on board.
iff it gets stinky on me out in the ocean i have the option of so doing. but normally i donot wear one.

oh yeah..i am essentially on board 24/7/365. my boat is my HOME. been living on board since 1990.
do you wear life jackets when you use the bathtub in your homes..........


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post #186 of 215 Old 01-02-2014
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Re: Do you wear a life jacket?

I am with Zeehag on this, long term liveaboard, never wear a lifejacket and only rarely feel the need to use a harness.

However I might wear one of these horse collar inflatable with a PLB as if I go over when I am singlehanded there is a chance of rescue if I can activate the PLB and stay afloat. Mind you the rescue services where I cruise often do not have money to fuel their boats..
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post #187 of 215 Old 01-02-2014
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Re: Do you wear a life jacket?

Quote:
Originally Posted by capt vimes View Post
where do you get these figures from?
a cubic meter of water weighs already 1 metric ton, if it comes rushing at you at a certain speed, the force on your body should be higher than the mere ~250-500 kg you mentioned... even if not all of the force is transferred onto your body since water is a fluid and "sloshes" around you...
tethers usually have a breaking load of 20000 N which equals to roughly ~2000 kg/m2...
the carabiners must have according to EU regulations at least 22000 N of breaking strength if they are meant to be used for alpine climbing and there are no others on the market... if they do not comply with this regulation, they have to be marked with "not for climbing" - you won't take one of those to tether you to a boat with...

but you are right - if you get thrown around, the initial impact force on the tethers could easily be well above those breaking loads and a certain stretch in them would make them a lot safer...
I won't pretend they were carefully calculated, in part because we have no idea what hit the poor guy.

First, I chose 2000-4000 pounds as a first pass water force number because I suppose the tether failed at 5000 pounds (should have been that strong) and I wanted to leave something for impact. I could have used a larger figure.

I picked 150 hp because it might take about that to generate 4000 pounds bolard pull at those speeds. Outboards of that size are propped for high speed. 20-25 pounds per hp is a common starting point.

I did a quick calculation of water drag (F=1/2pv^2C), used 3 meters/s, an area of .7 meters, a Cd of 0.7, and got 490 pounds. We should add for skin drag, but it will be a smaller number. Check my work--I'm sure someone can estimate a better figure. Water is forceful, but not tons on a human body. I believe 500-1000 pounds is generous. Thus, most of the force came from tether impact, which was avoidable.

It just seems to me that tether impact is the greater portion and the only way to get to bone crushing figures. Yes, shorter tethers really help.

(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")

"Well, I just climb up to them."

by Joe Brown, English rock climber




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post #188 of 215 Old 01-02-2014
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Re: Do you wear a life jacket?

Well... The impact energy calculates 1/2.m.v^2... (m is the mass, v is velocity)
That means that a cubic meter of water traveling at 2 m/sec (7.2 km/h) results roughly in 2000J or 2000 kg.m^2/sec^2 of kinetic energy which gets transferred fully onto a solid, immobile object...
A sailor standing on the foredeck is now not immobile, which results in the object of say 80 kg of mass getting accelerated to the speed of the wave because the mass of the man is rather small compared to the mass of the water...
That in result means he will be pushed into the tethers by 2100 J or 2100 kg.m^2/sec^2 plus additional pressure from the water still pushing the sailor...
If the tether now has no stretch, all of that energy is transferred via the harness to the sailor in an instant... If the man is stopped in a tenth of a second the force on the harness and therefore the sailor equals to something of 21 kg.m^2. Now divide this with the area of the harness in m^2 and you should get the actual pressure on your body from the harness in kg...

I hope i did not make any mistakes here...
I think you have been right, the forces are really not that high as i initially thought...

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Re: Do you wear a life jacket?

Why not use rock climbing ropes for tethers? They have give. Because they're not flat and you'll slip?

I hate when people give that crap how you have to wear one because it's irresponsible to you mom or wife or whatever. They are a good idea in a lot of situations for sure but it seems to me for those times you have to wear one, cold water being the number one thing I'm reading, you'd be better off wearing a wetsuit. It provides floatation and keeps you warm. I find it irresponsible that anyone that feels that have a significant chance of going overboard and the water is cold would not wear a wetsuit.
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post #190 of 215 Old 01-02-2014
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Re: Do you wear a life jacket?

Quote:
Originally Posted by pdqaltair View Post
Do we suppose that the force of the wave snapped the tether? That would mean the force of the water on his body was 2000-4000 pounds, equivalent to strapping a 150 hp engine to his butt and blasting him through the water at ridiculous speed. Improbable. The force on his body was probably more in line with a serious wave break, perhaps 500-1000 pounds.

Do we suppose that the impact of his body against the anchor point, without any stretch in the tether, contributed a major part of the force? More probable. Flying across the deck at 8-10 knots could account for all the force.

Sail Delmarva: Dynamic Tethers

A combination? Certainly. However, it seems quite probably that tethers with a little give in them could prevent this sort of breakage. Making them even stronger probably won't help; if the forces on the body are more than 5000 pounds the impact would be fatal.
It seems I did not entirely understood your post. I thought that you were talking about 1000 pounds as the forces that are taken by a tether. If I understand correctly what you mean is that is the force if the tether is completely stretched. If a sailor is projected by the wave on a lose tether then that force can be multiplied several times, therefore the importance of short tethers.

That's more like it?

What I know is that a tether should have a breaking load resistance of about or over 2000 kg

You probably know this old study that revealed some frightening conclusions regarding tethers failures under dynamic charges?

"We were somewhat surprised that there were so many tether failures. 47% of the tethers failed in
such a way as to endanger the wearer. Failures were both in the hardware, stitching, or sometimes both."


http://offshore.ussailing.org/Assets...ea+Studies.pdf

Even so on the conclusion they fail to recommend a minimum breaking load under static circumstances, one that could take into consideration dynamic ones.

Regards

Paulo


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