I have now collated all the feedback and passed it on to Martin Thomas who is editing the new edition of "Heavy Weather Sailing".
It was great to hear first hand accounts from Randall Reeves, Jeanne Socrates, Tony Gooch, Tim Good and Susanne Huber-Curphey among many others. The new edition of HWS will appear in due course but should anyone be considering venturing into the more extreme conditions and want to know more about the JSD I have attached a copy of my summary below.
I would also like to hear of some of the more technical aspects from anyone that has used the JSD .
Boat length and weight
number and type of cones
approximate distance from the stern to the first cones
Tail weight used

+x

 

Matt.North - 16 Apr 2023

Hi - does anyone have any experience using heavy duty U bolts with large backing plates on the transom, rather than attaching to chain plates bolted onto the side?  It seems to me the strength would be better as the bolts are in line with the force rather than at 90 degrees?  
Thanks

Hi Matt,
I hope some engineering types weigh in.
My take is that most transoms and their scantlings were not designed for the massive forces generated by the JSD. Most boats I know have gone to a design along the side of a boat near the transom much like chainplates for the standing rigging. Even there, good engineering and evaluation of the distribution of forces should be appraised.
Note, when determining the holding power of a chainplate style installation, the holding power of the chainplate is generated primarily by the friction between the plate and the surface it is attached to and then distributed to structural components nearby: which is why the torque on the mounting bolts is crucial. The shear strength of the bolts holding the chainplate is far less important and should not come into play if the friction generated is adequate.
Contrast this with the point loading that even a fairly large backing plate would induce on a transom: which is not designed for that sort of load. In addition all forces will be borne by the two nut/threads of the “U” bolt (or the 4 nut/threads if 2 “U” bolts). I am sure that this can be looked up somewhere and would guess it does not come close the forces Jordan estimated (and that users have experienced) that the JSD generates.
My seat-of-the-pants thoughts, Dick Stevenson, s/v Alchemy