The split backstay is to alleviate a conflict with the mizzen mast which is no longer there. If it were a double backstay that went all the way to the truck (masthead) then you would get some lateral support but not much since the lateral angle would be pretty small compared to shrouds.
From the line drawings (CAPE NORTH 43 sailboat specifications and details on sailboatdata.com
) it appears this is a double spreader rig with the forestay attaching at the upper spreaders so the mast is adequately supported laterally.
When the boat pounds into waves the inertia effects of the mast, and stays and the sails
attached to them cause the mast to pump fore and aft. These inertia loads are in addition to the normal loads from the tension in the stays and from the wind
pressure on the sails.
At the masthead the forward pull of the headstay, sails and pumping loads is counteracted by the backstay. At the lower spreaders there are for and aft lower shrouds that resist the mast inertia loads, plus whatever sail and stay loads are transmitted down the mast from above.
At the upper spreaders the line drawing shows a fixed shroud
that is out of the transverse plane of the mast, that does not run over the tip of the lower spreader, and that anchors at the same location as the aft lower. This type of shroud
is not common, but it is not unheard of either. I do not recall
if there is a specific name for this shroud. I assume there is also a normal intermediate shroud in the lateral plane of mast though I can't see this on the line drawing. The out of plane shroud will carry some fore and aft load but not much since the angle with the mast is pretty small, 2-5deg.
Since the line drawing does not show a running backstay and normally it would be shown I assume the mast was designed to be strong enough without one. Given that there are double lowers, the mast is probably pretty beefy and there are the out of plane shrouds carrying some of the forestay loads, I could see it working fine without running backs, especially in light and moderate weather
. I personally would be more comfortable with running backs for heavy weather.
From the photo
in the initial post I only saw one set of spreaders. It was not apparent if that was due to the perspective of the photo, or if there is only one set. If there is only one set of spreaders, then all bets are off.
Ted Brewer, the boat designer
, is still alive as of several months ago though he has retired from most sailing activities due to health
issues, his or spouse, I don't know. His website is still up though it has been 16mo since it was updated. I would try to contact him and confirm that the boat was designed for use of the staysail without running backs. The following is contact info from his site.
Ted Brewer Yacht Design
604-796-3732 Fax 604-796-3738