Fore and aft vibrations in the rig

[phorvati]

Hi I have a C&C38MkII with double spreader, single lower shrowd rig. All stays are Navtec Nitronic 50 rod. They look really good but I will know for sure after magnafluxing. I plan on going cruising and will do a rig assesment in the next few months. Navtec recalled the SS screws, as well as the aluminum tiebars used to tie the navtangs at the spreaders.
At this point I know i have no galling (turnbuckles screws are original SS). The few times I was out in gale conditions I felt that the rig vibrates too much, fore and aft only. I am no expert but with a smaller ericson I had before, (dual lower shrowds, and single spreaders) I never felt this kind of vibration. Would putting inner forestay at the 2nd spreader with runners help prevent vibration and make the rig more seaworthy? Or would solent stay be enough?
I saw a steup in Bill Siefert's book "Offshore Sailing" where he has a one piece running back stay, a low stretch high tensile rope leading through a block aft to a winch. Then a shock-cord running from the first spreader is led to a toerail block, just aft of the mast(about where the aft lower shrowd would be) and then to the runner via a snatch block. So this shock cord would take the function of an aft lower shrowd. Meanwhile the babystay which is part of the setup already would counter the shock cord. Any thoughts would be greatly appreciated.
Thanks,
Petar

Of course it is hard to tell what is going on without experienceing it, but from your description you may be experiencing "pumping" which would certianly be percieved as a heavy vibration. I would check to see that you have adequate backstay tension to keep your mast "sprung" and not moving fore and aft. Because you only have single (in-line) lowers, you need to have adequate tension on the rig fore and aft to keep it from moving. An inner forestay, coupled with good backstay tension, can negate the pumping, IF that is the problem. Be sure that your rig is tuned correctly as well. The basic idea is that you want the rig to be gently curved, the inner forestay pulling the center of the mast forward and the backstay and mast step holding the top & bottom aft. Your uppers & lowers hold it side to side. Backstay tension in this case plays the role of keeping the mast from moving, and also allows you to control the sail shape, to a small extent. As your wind picks up, so will the tendancy for the mast to pump. So you increase your backstay to both keep the rig adequately tensioned, and flatten your main to depower it... I'm reaching the limit of my ability to explain this, so someone else - jump in!