Jumper strut vs. running backstays
 

I guess the title says it. It came up in conversation the other day. Can a JS take the place of a running stay?...or vice versa? I'm curious to know the limitaions of either.

It depends
 

Hi,
Most jumpers are trying to do two jobs: keeping the mast from bending to leeward at the top; and opposing the forward pull of the forestay. Runners are only about the second of those jobs. With a sufficiently large angle, and sufficiently small forestays'l, jumpers can do both jobs without help from runners. On some boats, the jumpers are calibrated to work up to a certain load level, at which point the runners need to supplement their efforts. On still other boats, different standing rig components handle the lateral loads, and the runners (or, rarely, a single, forward pointing jumper) take care of the forward pull.
So, no simple answer here. Did you have a particular boat in mind?
Fair leads,
Brion Toss

Not really, but I am changing the mainmast on my ketch. For the most part the figuring is done, but the thought to add a triangular jumper did occur to me. The new mast is substantially smaller fore and aft. I have figured for running backstays already. I was wondering if it would be worth adding.

Variables
 

Hi again,
It is all about the moments of inertia of the new and old mast, the vessel righting moments, the rig configuration, and how you want to deal with the pull of the forestay, and the shape of the mast. You don't just add jumpers because you think it might be good to. Or rather you shouldn't; people do.
Fair leads,
Brion Toss

Good answer, pretty much what I had thought. Thanks