Just to give you an idea of the G-forces that can be involved, I did a bit of research for a post on another forum:
As for numbers... here
are some for you:
a (g) event
3.6 crowd jostle
4.1 slap on back
8.1 hop off step
10.1 plop down in chair
60 chest acceleration limit during car crash at 48 km/h with airbag
70 - 100 crash that killed Diana, Princess of Wales, 1997
150 - 200 head acceleration limit during bicycle crash with helmet
So, I do believe that falling across 8' of boat rotated through 70˚ of heel from port to starboard might well leave you with g-forces of well over 10 G's. In fact, you can probably break 20-30 G's in a spinnaker broach.
If plopping your butt down in a chair can generate 10 G's of force... getting thrown across a boat by a spinnaker broach or accidental gybe is probably at least 10 G's of force IMHO.
G-forces don't require great speed...since they don't measure speed but rates of acceleration and deceleration. For instance, dropping a computer 60 CM onto the floor can generate up to 500 G's of force... since the deceleration cause by the computer hitting the desk and stopping suddenly is very high.
Here's a explanation using a Steel ball
and actual numbers:
For simplicity, suppose that you drop a steel ball, which has a diameter of 10 cm and a mass of 1.0 kg, onto a thick steel plate from a height of 60 cm. The ball will, for all practical purposes, bounce elastically from the plate.
Suppose that during the collision process the steel ball compresses one millimeter. [While this figure is a bit large, it makes things simple to calculate]
The speed of the ball when it reaches the floor can be found using energy conservation: GPE=KE therefore m*g*h=1/2*m*v^2 Solve for v = sqrt(2*g*h)=-3.43 m/s.
The time for the ball to stop can be determined from D=Vave*t, therefore the time can be calculated by dividing the distance traveled during the collision by the average velocity during the collision. t=0.001m/3.43/2=0.00058sec.
Finally, the acceleration can be determined from Vf=a*t+Vo
where Vo=3.43m/s,Vf=0m/s and t=0.00058sec
Since the acceleration of gravity is 9.8m/s^2 this will give an acceleration in terms of the gravitational acceleration of 5914/9.8=603 g's!
If you figure that you can reach up to 30 G's in a broach, you might want to consider the breaking strength of the tether and jacklines.... If you weight 180 lbs.... and reach 30 G's... you're effectively loading the tether and jacklines with 5400 lbs. for a very short duration. If you've got jacklines with a breaking load of 4750 lbs... they might not hold. If you've got jacklines with a breaking load of 6000 lbs., they might not hold, but the chance that they do is far better, don't you think?