Charles
The Hess designed Nor'Sea 27 is going to be a conundrum vis a vis a traveller due to the outboard pintle hung rudder and aft boom sheeting. Theres not much room on the cabin top for effective mid boom sheeting, your boat has end boom sheeting thus the 'rope bridle' (no traveller) arrangement to allow room for the large tiller to pass through/between !!!!
I think, there are only three possibilities to prevent your boom from lifting during a blow, etc. 1. Vang sheeting 2. 'stropping' the boom to the rail with a 4:1 tackle when the boom is 'outboard'. 3 'double duty' from a boom brake
The NorSea is a fair bluewater design, meaning that its scantlings (inbuilt build strength) are at least 3:1 or greater, so any modifications you make will have to be commensurate with those scantlings ... or youre bound to 'break something'.
The end boom sheeting that you currently have, plus your narrow stern, plus that tiller dominating the stern ... will simply not allow any functional traveller at the stern. The problem will be that such a traveller, always in the way of the tiller, will be next to useless when the boom is well outboard such as when deep reaching or running because of the 'pinched stern'.
Possibilities
Vang sheeting would a somewhat poor alternative to a mid-boom traveller due to the possible humongous forces involved .... from the 'leverage' of that long boom. If you do experiment with vang sheeting youre probably going to need at least an 8:1 (or better) vang system - quite easy to do by adding 'a doubler' to a 4:1 vang; plus, you'd need to run the tail of the vang back to the cockpit for convenient control, instead of constantly running to the mast base to adjust the vang. A doubler line is simply an extra strong line (or cable) added to a vang ........ run the second line 'from' the base of the current vang to a VERY stout block 'strapped' to the boom (near the present boom vang connection) and terminating in an eye (splice) several inches below the block that is 'strapped' to the boom ... the top of the 4:1 vang system connects to the 'strapped' block, the bottom of the 4:1 connects to the mast base. At 8:1 you should have sufficient power to adjust the boom when under full load. The downside is that the compression forces acting along the axis of the boom will be doubled .... may require a 'doubly strong' gooseneck connection (?).
But with such a long boom, dont expect vang sheeting to do all the work needed to prevent the boom from lifting .... that 'help' would probably come from a 'preventer' tacked from the cap rail (if strong enough to withstand the 'upward forces').
Preventer system - For when the boom is outboard. A 4:1 (or more) part tackle that is used from 'somewhere' near mid-boom to the cap rail, the 'attitude' of the preventer would be chiefly 'vertical' (from the cap rail to the boom) when the boom is 'outboard'. Got a cap rail track that can handle such loads ???? The preventer (used only 'vertical') could be run back to the cockpit for easy control to give sufficient 'downforce' to the boom. Downside is that for every tack or gybe, you have to release / disconnect the preventer from the caprail and reset to the opposite side caprail (or install two preventers ... 'rope salad' on the side deck to trip over).
A combo of both ... and you probably already have all the equipment!
Your mainsheet rope 'bridle' system should be sufficient for providing boom end downforce when then the boom is inboard of the stern. The only downside of a mainsheet bridle is that if you pull too much tension in the mainsheet, the boom aft end can also be pulled too far down .... thus the tendency to 'hook up the mainsail leech to windward' which causes SLOW beating and aggressive heeling .... (most folks over-tension jibsheets and mainsheets, and then wonder why the boat is 'cranky' !!!!).
Possible Rx:
A. Redesign your present vang system - the new vang would have a bodacious snap shackle on the end that connects to the mast base.
When the boom goes well outboard, disconnect the vang from the mast base and relocate the attachment to the cap rail (super strong cap rail track?) ... the 'attitude' of the relocated vang should be mostly 'straight vertical' from the cap to the boom ... for the 'least' amount of induced forces along the axis of the boom, connection to the boom can be heavy tubular webbing (need another bodacious snap shackle). Downside is you will have to 'play' the vang during gybing maneuvers; and, if you heel way over with a prevented boom and the boom/sail goes underwater you ARE going to break your boom! Obviously and for safety, the vang control line has to be lead back to the cockpit -- a good application would be a 'trigger' cam cleat (Harken) for instant release when needed.
B. My preference ---- Use a boom brake but have the boom brake connection to the boom closer to the mast than normal, the connection of the brake to the boom should be nearly directly over the cap rail when the boom is fully outboard. When needing more downforce on the boom, simply apply more force into the boom brake. Use a cabin top winch to apply sufficient force to the boom brake to apply the needed 'downforce' to the boom. I use a Wichard Gyb'Easy boom brake (just a fancy mountain climbing friction carabiner) and do use it to 'assist' the (mid-boom) traveller in F8+ wind conditions on my Tayana boat. If the boom is well outboard and does go underwater when mightily heeled over, the brake will 'slip' and the boom wont break (I hope). Problem with a boom brake is more 'crap' to possibly trip over and get tangled in when going forward when its dark and stormy; and, you may not have sufficient tension in the boom-brake friction control line to prevent or slow down an accidental gybe.
C. combo of A + B .... and/or Rigid boom vang !!!!
At least here's something else to think about ... as you really dont need the sudden 'power-ups' that can occur on a relatively tender boat that has an overly twisted sail. Who needs a 'surprise spreader / mast in the water' or a 'surprise goose-wing gybe'? ;-)