Keel stepped mast issue?

[GeoBay]

I have a 1983 Niagara 31 with a keel stepped mast, single spreader rig with navtec rod rigging and proprietary navtec chain plates. All rigging and chain plates soundly bedded. The boat is sitting is nicely on its cradle with mast off. When looking at the main bulkhead where the bottom meets the structural molded fiberglass pan the gap is tight and square. However when the boat is launched, mast stepped and fully rigged a small gap starts at the bottom about 1/8" then when I fine tune the rig after a few sail days the gap will increase to about 1/2". Everything is just fine the boat sails well no strange creaking or noises coming from the rig or around the bulkhead, even when sailing in 20n winds and rough waves. The Niagara's have 2 deck ties that connect the top of the bulkhead (inside the boat) to the cabin top through to the outside bolted securely. The bulkhead at the top where it meets the cabin top never moves or shows any kind of gap at all. At my marina there are 5 Niagara 31s and 3 of them show no signs of movement the 4th boat shows a small gap about 1/8"max and of course mine at 1/2"max. I have talked to other people about this and they say all boats flex. The mast step, mast stringer, and keel sump are solid. Noticed this about 4 years ago and assumed that the hull was sagging under the rig tension but now thinking perhaps the cabin top is rising?. I have read on other posts that some keel stepped mast boats have tie rods that go from the mast collar down to the mast step to counter rig loads however this boat wasn't designed that way, the Niagara 31 is a Frers design. When the boat is hauled there are no external signs of cracking or crazing anywhere near the front of the keel where the mast step is located. Any thoughts on how to remedy this?. Attached is a picture of the bottom of the bulkhead when fully rigged and a summer of sailing.

[TeddyDiver]

No picture but you right about the deck and bulkhead rising when the rig is tensioned. A tie rod is just to prevent that, or if the bulkhead is strong enough can be bonded to the bottom.

[pcmm]

Sounds to me like the stringers under your mast step are deteriorating. If the gap opens to 1/2" thats enough to indicate that there is some crushing of the structure underneath.

This is a Niagara 35 getting its mast step rebuilt:

https://www.youtube.com/watch?v=MY4Czpc8AXY

[psk125]

Something is obviously moving. If the bulkhead was going up, (to make the space at the bottom bigger) it would have to be lifting the cabin top and deck as much as a half inch. That would be quite noticeable on deck as the hull/deck joint would be opening for that to happen. So the bulkhead going up is not likely. That leaves the possibility of the step going down. Deterioration of the step is possible, but what is more likely is the "banana-ing" of the hull from the downward pressure of the mast as the shrouds and stays are tightened.
Reinforcing the longitudinal stringers to tie more of the hull together with the mast step over a longer span might help keep this from happening. It could be that the boat was built with such stringers but they have come untabbed or delaminated. It might be helpful to check this out, under the cabin sole. The video repair seems to show what looks like a relatively short section- perhaps a foot to 18" longitudinally. To really keep the hull from bending from the mast's downward pressure, however, several long stringers (4 to 5 feet?) that were all well bonded to existing structural members might be needed. You might want them to extend well forward and aft of the actual mast step. This might entail removing the cabin sole to get to something nice and solid to bond to. It would certainly be better to add the new stringers while the mast wasn't stepped (and the boat wasn't torqued). Re-tabbing the bottom of the bulkhead would probably be a good idea too. Tie-rods might also fix this gap problem, but you would need something really solid to attach the tie-rod to, and if the hull is flexing, attaching to it won't solve much.

[GordMay]

A deck tie down keeps the deck from being pulled up on keel stepped masts, where the mast does not sit on the deck. When the halyards are led aft via turning blocks attached to either the mast collar or the deck surrounding it, the deck will try and pull up as the line becomes loaded. The deck can also be pushed up by the compression of the hull, typically, caused by rigging tension.
Installing one or two deck tie rods (sometimes called a “panting stay”), between the deck collar and the lower part of the mast (or mast step) will compensate will compensate for these upward loads.




When the mast is keel-stepped, it is important to
secure the deck to the mast, especially when the
halyards are led via turning blocks attached to the
deck collar. Otherwise, the lifting force of the halyards
could lift the deck. Seldén traditional deck ring
incorporates a “tie-rod system” to eliminate this
lifting action.

the halyards are attached to the
collar on the deck, it is essential to
compensate for the load they exert
upwards. It is therefore necessary
to fit one or two deck tie rods
between the deck collar and
the stepped part of the mast.

From ➥ http://www.seldenmast.com/files/exce...08-E-57-59.pdf

[GeoBay]

Thank you all for your input to this issue. I am going to try and attach the image from my original post as I couldn'tget it to work the 1st time.
There is more about this subject that I didn't mention in the original post but I'm trying to find some pictures that were taken but having trouble finding where I saved them.