Re: [Amel Yacht Owners] Re: Rigging tune on Maramu caused damage
Peter,toggle quoted messageShow quoted text
I appreciate your detailed post and also the additional photos showing the underside of the cockpit sole and upper compression post details. From your photos, it appears that the plate welded on top of the compression posts has been bent into a curve as shown by the straight edge you are holding up to it. Perhaps the mizzen mast compression loads were quite high at one time. Have you laid a straight edge across the front side of the mizzen box along the cabin sole by chance to see if there is a depression into the cabin sole under the area of the box?
I need to study this in more detail on my own boat but it appears that the mizzen mast compression loading is borne by the outside portion of the fiberglass box and as you point out the footprint of this load is outboard of the positions of the compression posts. This means that the cabin sole and the plate welded atop the compression posts are responsible for transferring the loads between the compression posts and the outboard edges of the mizzen box. Certainly the metal plate welded atop the compression posts is not thick enough to be carrying the loads by itself so I am wondering if there has been some deterioration of a core in the cockpit sole directly under the box? Have you used something to tap the fibreglass cockpit around the area of the box to see if any portion of it sounds hollow? I would be curious to know the construction of the cockpit sole directly under the mizzen mast box. Would it be solid fibreglass, does it have another plate embedded, or does it have a low density core?
Looking at the gap you are measuring showing the curve in the plate atop the compression posts, the movement looks about right to allow the mizzen mast box to drop and be damaged as it has so I think that you have found the failure area.
I will plan to remove some of the insulation in my engine room to reveal the top plate of my compression posts to see how they look. I can tell you that the fibreglass mizzen box on Sueno is showing no movement relative to the aft cockpit bulkhead so there cannot be any movement in the plate, but I will check. I should arrive on the 8th. The shrouds on the mizzen of my boat can be deflected inwards/outwards with moderate pressure (perhaps 15 lbs.) 2” to give you some idea of the rigging tension. My broker Michel told me that it is likely that the rigging (which is original) has never been retuned since Sueno was commissioned so if true she has never been left with tight rigging. I personally do not like the rigging on my boats to be very tight, especially on wooden or fibreglass boats. There is a big difference in the short term loading of a boat under sail and a tightly tensioned rig that retains that tension 24/7 out in the hot sun (which temporarily softens/weakens the resin allowing it to change shape) etc. When under sail, only the windward rigging sees increased tension while the lee side slackens. When you preload a rig by tightening all of the stays, every stay adds to the compression load and sailing loads are on top of that preload. Of course you don’t want loose rigging banging around on the lee side but I think that people often tune their boats more tightly than is needed.
I am thinking that what will need to be done on your boat is to come up with a good way to carry the support of the compression posts, outwards so that this support aligns with the edges of the fibreglass box. Perhaps a new plate could be added on top or even embedded into the cockpit sole. Let me know what you find out about the construction and condition of the fibreglass cockpit sole up and under the fibreglass box. During the interim I would try removing all of the rigging tension on the mizzen and take another measurement across the top plate welded to the compression posts to see how much rebound you have. There is a chance that the plate welded to the top of the compression posts is not bent as much as it appears and will spring back. If you need to be sailing the boat for a while, I would remove the rigging tension completely, lightly jack up the bottom of the cockpit under the area of the fibreglass box and install some temporary support as close to the edges of the fibreglass box as possible, and try to keep the rigging preload down.
I don’t think that there would have been much benefit to Amel making the mast box and the aft cockpit bulkhead one pc.glassing it together. The attachment to me looks like it is perfectly adequate for the intended loads.