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post #187 of Old 01-02-2014
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Re: Do you wear a life jacket?

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.

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Last edited by pdqaltair; 01-02-2014 at 09:23 AM.
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