Originally Posted by MrHeadsocket
For my unbraked trailer I just use a standard breakaway cable (designed for braked trailers/caravans) that is attached to the jockey wheel bracket so it won't come off (unless the bracket falls apart!) And so far has been useful to stop the trailer rolling down Littlehampton Harbour's slipway when I unhitched it!
OK let's assume the slip is 1:8 its at an angle of about 7degrees.
Let's assume trailer and boat combined has a mass of 1000kg and gravity exerts of force of 10N/kg perpendicular to the horizon.
I think that means the force on the secondary coupling is 10 x 1000 sin (7) which is about 6500N - so 65% of the boat+trailer weight.
Breaking strain of 3mm wire rope seems to be in this sort of region. So no major surprise it was fine.
But what happens if the pulling vehicle is travelling at 60mph (26m/s)... And the load is applied suddenly... Let's assume the trailer was doing 60mph and stopped in 1 second flat it decelerated at 26m/s2 if that's a 1000kg load that means a deceleration force of 26000m/s2 or 26000N. That sounds to like a wire with a breaking strain of 2.6T is needed...
Is 1second too short a time or too long? I don't know...