Do you wear a life jacket? - Page 20 - SailNet Community
Old 01-02-2014
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Re: Do you wear a life jacket?

Quote:
Originally Posted by zeehag View Post
do you wear life jackets when you use the bathtub in your homes..........
Nope, I don't. Then again, I'm reasonably sure that my bathroom at home has little chance of sinking in the ocean while I'm in the shower.

When on board and out of the slip, we wear pfds -- inflatable for my wife and me, vests for everyone else. (inflatables can be expensive!)

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Old 01-02-2014
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Re: Do you wear a life jacket?

Quote:
Originally Posted by capt vimes View Post
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...
I'm with you on the first part. Any mistakes were in the guessing about the scenario, which is all we can do. Since the impact part has been studied to death by the UIAA (climbing gear regulatory authority), I think it is simpler to detour to fall test data.

2000 jules is about equivalent to a UIAA test fall on a standard length tether (1.8 * 2 M * 50 kg * 9.8 = 1764 Jules. It is well known experimentally that 1" webbing cannot withstand that (only about 1100 joules for 2 meters of webbing), that 8 mm climbing rope can withstand that once, and that 10 mm climbing rope can withstand that ~ 10 falls, including a moderately sharp edge.

The impact force (experimental determined) if using a rigid object will exceed 4000 pounds with webbing and will be 1200-1600 pounds with climbing ropes. A harness and human body (which will deform a few inches) will reduce the impact about 100-500 pounds, depending on the example.

Since sometimes the harness fails, this further supports that the forces are very high. The military has found injuries become common at about 10 Gs in a full body harness; sailors have much less harness and more force.

-----------

Previously I had suggested that a Screamer (Yates) would be a good candidate for force reduction, but I withdraw that idea. The problem is that a Screamer is single-use and that lost MOBs were often during rollovers with repeated high impacts. An elastic tether would be more rugged in actual use. Screamers are fine for industry where only a single fall is contemplated; after that the guy kisses the ground, goes home and kisses the wife, and takes the rest of the week off.

(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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Last edited by pdqaltair; 01-02-2014 at 01:47 PM.
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Old 01-02-2014
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Re: Do you wear a life jacket?

pdqaltair - i made some mistakes...
J is not only kg.m²/sec² but also N.m and if we calculate now the force of a "fall" with 2100 J which is stopped within 10 cm you get 21000 N of impact force and that is not a small number...
and it is quite in accordance with this graph were the force is shown in relation to a "sturzfaktor" which is basically the height of the fall divided by the length of a rope - in our case this factor would be around 1 but the speed would be less than that of a free fall with 9.81 m/sec² acceleration:
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Old 01-02-2014
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Re: Do you wear a life jacket?

If there were only such a thing as dynamic webbing. I looked briefly at some products sold as recovery straps and such, but they didn't have the stretch characteristics of dynamic rope. Rope is problematic because if you step on it it can roll. I'm happy enough using 8mm rope for my tethers, since it is too small to create much of a roll hazard and I'm not sailing the southern ocean. It is still tougher than webbing (can absorb ~ 50% more energy) and much easier on the ribs.

Perhaps dynamic rope for the helm work station only would make sense. It always seems to be the helmsman that breaks the tether (guys on the jackline have the jackline for energy absorption--they only need to worry about getting stuffed under the bow wave!).

(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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Old 01-03-2014
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Re: Do you wear a life jacket?

Yes, I wear an inflatable life jacket and tether when I sail alone, and, with crew, when conditions are anything less than calm. I know, if I go overboard, getting back on the boat will be a challenge and hypothermia could set in before I do, but it would give me a chance at survival, whereas watching the boat sail away will not.
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Old 01-07-2014
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Re: Do you wear a life jacket?

I just found these pictures from my neighbor's driveway after the last hurricane.

I wonder how much force was exerted on that piece of rock and concrete.
Wonder how much a tether would have helped to keep it in place.

The lesson from the Icarus story is not about human failing.
It is a lesson about the limitations of wax as an adhesive.
If you have an engineering problem solve it.
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Old 01-07-2014
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Re: Do you wear a life jacket?

I believe we are all in agreement. I single hand 90% of the time in the Chesapeake bay. I wear it 90% of the time. I also trail a 100ft polypropylene line behind boat w/ a bowline tied off at end. Just in case I do fall off I can grab hold of line and hopefully drag myself back to boat. I have not tried that yet. You also must make sure you can climb back on boat if you do fall off. I DO NOT have a swim platform. I have a stowable ladder tied off a stantion that I can release and climb back on the boat. I also have a portable VHF radio tucked into my PFD on challenging days. . I do not have roller furling and when I go up front to pull down jib I feel safer w/ vest. Safe sailing!!
billdix is offline
Old 01-08-2014
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Re: Do you wear a life jacket?

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Originally Posted by billdix View Post
I believe we are all in agreement. I single hand 90% of the time in the Chesapeake bay. I wear it 90% of the time. I also trail a 100ft polypropylene line behind boat w/ a bowline tied off at end. Just in case I do fall off I can grab hold of line and hopefully drag myself back to boat. I have not tried that yet. You also must make sure you can climb back on boat if you do fall off. I DO NOT have a swim platform. I have a stowable ladder tied off a stantion that I can release and climb back on the boat. I also have a portable VHF radio tucked into my PFD on challenging days. . I do not have roller furling and when I go up front to pull down jib I feel safer w/ vest. Safe sailing!!
i went overboard once... the autopilot decided to tack, when i was still lying in the main on a close haul having a nice snooze there...

nevertheless - we also had some sort of line towing just in case, and i can tell from my own experience that you just cannot imagine how bloody fast this line comes rushing past you...
even at only 5 kts, it is frightening when you up to your nose in water.
you must be really very, very desperate to try and grab this line especially if you have no gloves on.
even if you manage to get hold of your bowline by slinging an arm through, the chance of a dislocated shoulder or even worse an injury is very high!

and then comes the really tricky bit:
just imagine - your probably injured and at 5 kts, the speed will drag you under water and i doubt very much, that you would be able to drag yourself back to the boat. the resistance the water gives you is something you have to figure yourself once...

btw:
when i eventually was back on board, we immediately undid that line.
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Old 01-08-2014
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Re: Do you wear a life jacket?

Quote:
Originally Posted by billdix View Post
I also trail a 100ft polypropylene line behind boat w/ a bowline tied off at end. Just in case I do fall off I can grab hold of line and hopefully drag myself back to boat.
Dream on... :-)

As Capt vimes attests, unless you go over when hove-to, or fore-reaching at under 2 knots, hauling yourself back aboard while under sail or power isn't gonna happen...

It's just SO much simpler to focus on not falling off the boat to begin with... :-)
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Old 01-08-2014
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Re: Do you wear a life jacket?

I understand. Yet I have read dragging a line is a good idea. Maybe my dragging behind boat will force a change in direction causing the boat to go into irons? then drag myself back? Never been overboard but thinking ahead
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