When it comes to shock loads I must admit to being a bit unsure of how to calculate the possible loadings as depending on where you are stood on the side decks when you fall would change the angle of pull on the attachment point and therefore the affect the loadings.
Rightly or wrongly I have my jackstays longer than most, my theory is to allow the crew member to freefall into the water with the jackstays only taking the load of pulling the person from a standing start to whatever speed the boat is travelling rather than taking the shock of the fall combined with the pulling force.
