Snatch loads - Rule of thumb.

[Snowgoose-1]

I'm reviewing my life lines, tethers strong points etc. I can't find any useful guide for calculations.

I'm thinking just to double up , I.e. I m 14.5 stone fully loaded young, lithe, fit extremely strong 76 Yr old single hander.

So I will go for something like 30 stone minimum. Any thoughts ? Also thoughts on elastic tethers.
TIA

 

[Fr J Hackett]

Another thread with a title that belies the content

 

[B27]

Jackstay webbing is normally heavy 25mm stuff with a breaking load of 2 or 3 tonnes.
Because your tether might be pulling sideways on a fairly straight jackstay, the tension in the jackstay can be much more than your weight.
Then add a factor for shock loads.

 

[srm]

Your figures are probably OK for a single tether strong point, though shock load will depend on the acceleration of your body and how far you have traveled before the tether pulls tight. Keeping the tether as short as possible will reduce shock loads.
However loads at both ends of a jackstay could be significantly higher depending on the angle the jackstay makes at the tether point. I can not remember the calculation, hopefully someone else will, but seem to remember that an angle of 60 degrees gives end loads equal to the point load and 120 degrees plus generates a tension / end loads of four or five times the point load. The latter being the more likely figure for a jackstay laid along the side deck or coach roof.

Any mathematicians out there who can give the formulae? I used to know how to calculate it, but have been retired for 16 years and have not needed to think about such things for even longer.

 

[B27]

See also the thread on the PBO forum I started today:
Strength of fitting bolted to GRP

 

[bedouin]

I thought jack-stays, tethers etc all had to have a breaking strain of 2000 kg but I can't remember where I saw that

 

[Supertramp]

As with most boat gear that can be subject to snatch loads, I would fit the strongest I reasonably could. At least matching anything being replaced. You can calculate and experiment with shock loads (there was a thread recently about dyneema and knots) but really how do you predict the maximum in such out of control situations?

Any system will fail at its weakest link and perhaps you want a tether or jackstay to fail before you do. Off subject a bit but 'm reminded of old boats were the gear was heavy but ropes or sails usually failed before structural damage occurred. I understand that MOB gear is a little different.

Elastic tethers introduces a whole new field of possibilities. The recent MOB threads suggest short and not very elastic.

 

[rogerthebodger]

The force induced by snatch loads is all about the mass and the deceleration.

For the same mass the force depends on the deceleration the faster the deceleration the higher th snatch load fora set mass

The first question will be what is the level of deceleration second question is what is the mass you are stopping

 

[Supertramp]

Hence a bit of elasticity increases the deceleration time and reduces the instantaneous force needed over more milliseconds. Eg webbing or rope versus wire. But you still have to translate that into all the component parts of the system. B27 in post #3 2 or 3 tonnes is the right ball park. Not sure how an exact figure helps, other than making sure everything is specced above it. The recent MOB thread was interesting in considering tether length and where to anchor - less distance to fall means less acceleration/deceleration but greater inconvenience.

 

[rogerthebodger]

Hence a bit of elasticity increases the deceleration time and reduces the instantaneous force needed over more milliseconds. Eg webbing or rope versus wire. But you still have to translate that into all the component parts of the system. B27 in post #3 2 or 3 tonnes is the right ball park. Not sure how an exact figure helps, other than making sure everything is specced above it. The recent MOB thread was interesting in considering tether length and where to anchor - less distance to fall means less acceleration/deceleration but greater inconvenience.

Hence fall arrester design

 

[Sandy]

Also thoughts on elastic tethers.

I am required to wear one on one of the vessels I sail. The elastic keeps the tether out of the way when walking about the deck and does not interfere when needing the length when working on the bowsprit.

On my own boat I prefer non elasticated.

 

[ylop]

I thought jack-stays, tethers etc all had to have a breaking strain of 2000 kg but I can't remember where I saw that

There is an ISO standard for preventing MOB which requires jackstays to be able to take 2000kg (in line or upto 30deg deviation). Static tether points are upto 600kg depending on the design category of the boat.

 

[jwilson]

Seen 6mm eyebolts seriously distorted and almost pulled through deck by snatch loads from two simultaneous MOBs on S/S jackstays. It was a bit violent: boat ended up inverted. I would say 600kg spec a bit flimsy. Modern strap jackstays will probably cushion some of the snatch load.

 

[rogerthebodger]

Seen 6mm eyebolts seriously distorted and almost pulled through deck by snatch loads from two simultaneous MOBs on S/S jackstays. It was a bit violent: boat ended up inverted. I would say 600kg spec a bit flimsy. Modern strap jackstays will probably cushion some of the snatch load.

6mm eye bolt is far too small for any safety equipment hat could take snatch loads.

I have 12mm U bolts on my safety jack lines in a steel deck with 6 mm thick back plates under the steel dick

 

[zoidberg]

I thought jack-stays, tethers etc all had to have a breaking strain of 2000 kg but I can't remember where I saw that

OSR/RORC Special Regulations.

But.... but.... those are determined by a committee. Individually, all nice fellows - even the 'honourary fellows'. We all know about the horse designed by a committee, don't we?
Being a capable and self-reliant singlehander, perhaps read Brion Toss's e-book 'Falling' and do a self-briefing*.

*Works for me, and I'm a lithe, fit, extremely strong singlehander who's just a tad less young than the OP.

 

[ylop]

Seen 6mm eyebolts seriously distorted and almost pulled through deck by snatch loads from two simultaneous MOBs on S/S jackstays. It was a bit violent: boat ended up inverted. I would say 600kg spec a bit flimsy. Modern strap jackstays will probably cushion some of the snatch load.

600kg isn’t for a jackstay end point (that is 2000kg) it’s for a fixed eye, which as someone ^^^ explained gets a lower load than the jackstay anchors.

 

[B27]

Seen 6mm eyebolts seriously distorted and almost pulled through deck by snatch loads from two simultaneous MOBs on S/S jackstays. It was a bit violent: boat ended up inverted. I would say 600kg spec a bit flimsy. Modern strap jackstays will probably cushion some of the snatch load.

so, 6mm has been tested fairly firmly, and found just adequate, so 8 is plenty?

 

[zoidberg]

Why on earth would someone want to shave the specs on this to 'close to the bone'...? Why not 'As beefy as I can accommodate'...?

And.... 'modern strap jackstays'. What tested spec for these? How do you allow for inevitable UV degradation? How do you measure - or even guess - what that might be?
At what age do you get cold feet and replace them - a year, five years, six months?

 

[geem]

Why on earth would someone want to shave the specs on this to 'close to the bone'...? Why not 'As beefy as I can accommodate'...?

And.... 'modern strap jackstays'. What tested spec for these? How do you allow for inevitable UV degradation? How do you measure - or even guess - what that might be?
At what age do you get cold feet and replace them - a year, five years, six months?

Our jackstays get fitted when we need them. 12 years old but only fitted for overnight passages or when the weather is narly. They are the equivalent of a few months old in UV exposure. Heavy duty webbing with heavy duty shrink-wrap over the stitched end loops provided extra UV resistance.

 

[zoidberg]

You reckon.....

"There are old pilots, and there are bold pilots, but there are no......" ( complete at leisure )