Arrow Flight problem

[Kenkel]

I am shooting 55 lbs at 27 inches from my Robertson Stykbow.  I have been getting my arrows to fly straight with no wobble.  I just bought a dozen PO cedar shafts and fletched them up.  Now, my old arrows are cut to 28 inches so I started out at 29 with one of my new ones.  They are spined 55-60 just like my older ones.  The problem that I am having is porpoising arrow flight.  The nock shoots up in the air and porpoises to the target.  Will an inch make that much difference?  I tied a nock on so I will have to cut it off if that is the problem.  Any and all help is appreciated.

[Don Stokes]

Kenkel, there are several possibilities. Yes, an inch can make a difference, but usually not a lot with field points. Your shafts might not be the same spine, regardless of what they were sold as. If your fletching is different in style or height, it could be a clearance problem. If your brace height has changed, that could do it.

I suspect that you're underspined.

[Kenkel]

Thanks Don!