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Old 25-03-2020, 15:11   #1
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Mast Step Bolts sheared

Lagoon 450 (2013) Doing a regular rig check noticed the two forward bolts on the mast step looked loose and tried to tighten. When I went to tighten realised that these two bolt heads had sheared off and that the step plate has rotated about 5ml to port. Bolt studs are still in place when viewed from underneath. Must have missed this in my walk arounds and was surprisingly also missed by rigger in recent (very thorough) rig check - or damage may have just occurred - but have not been in any extreme sea/wind conditions or had any impact accidents. Sailed from Caribbean to NZ last season - so quite a few miles have been done. Now planning to remove mast and inspect/repair. Wondering if anyone has had similar problem?
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Old 25-03-2020, 16:25   #2
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Re: Mast Step Bolts sheared

Yowza! I am not familiar with the Lagoon mast step design. Is that plate the only thing that holds the mast from shifting sideways? or is there also an internal plug (I hope!)?

I'd need to see the broken ends of ends of the machine screws to be sure, but it looks like classic crevice corrosion of the stainless bolts. They likely sit in a tiny little puddle of oxygen poor salt water. The threads give a stress riser, and the cracks start. It doesn't help that these are under significant tension from the upward load of the running rigging.

It's not an instant thing, it happens over time, and is really hard to detect--until failure. In this case, it is worth changing those bolts on a regular basis.

You can see pictures and an analysis of a similar failure here: Stainless Crevice Corrosion
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Old 25-03-2020, 16:34   #3
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Re: Mast Step Bolts sheared

these are not structural looks like, too short. Just inspected mine and they are firm and no mast foot rotation can be seen.

My boat is closing on 30k nm, same 2013 year and L 400 has same setup.

Please update with pic how it looks when mast removed and what is going on under there .
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Old 25-03-2020, 17:24   #4
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Re: Mast Step Bolts sheared

Thanks all- does look like a case of crevice corrosion (thanks billknny for the great article - a reminder of why we have to accept a ten year replacement of standing rigging even if all seems well).
The heads of the bolts I posted are just what I removed from the deck. The 30mm or so of the shanks are still imbedded and viewable from underneath. Note evidence of water ingress and corrosion is evident on all bolts. I also attach a pic of the ends of the machine bolt heads I have removed which seems to suggest a process of slow corrosion rather than sudden failure. I will post an update when I am able to haul out.
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Old 25-03-2020, 17:40   #5
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Re: Mast Step Bolts sheared

thanks. Inspection now on my list of things to do.
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Old 25-03-2020, 18:05   #6
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Re: Mast Step Bolts sheared

Crevice corrosion and/or bimetallic reaction with surrounding aluminium. It is worth spraying all SS bolts that run through alloy with a dewatering fluid on a regular basis, every couple of months - takes about 10 minutes to do the whole rig at deck level, including forebeam, goose neck etc. Next time up in the Bosuns chair spray the rig connections to the mast. This keeps the bolts free and able to be undone without breaking the head off, or destroying threads. Obviously use Duralac or similar when replacing bolts.
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Old 25-03-2020, 18:53   #7
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Re: Mast Step Bolts sheared

Quote:
Originally Posted by Tin Tin View Post
Crevice corrosion and/or bimetallic reaction with surrounding aluminium.
While there is certainly corrosion that can occur when stainless is in contact with aluminum, it ALWAYS affects the aluminium and never the stainless. The electrochemistry involved is always the same.

What CAN happen is the corrosion residue from the aluminum can lock the stainless fastener in place so tightly that it breaks under the force used in an attempt to remove it, but that was not at all the case here, the bolts failed in the absence of applied torque.
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Old 25-03-2020, 19:00   #8
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Re: Mast Step Bolts sheared

Quote:
Originally Posted by gstent View Post
Thanks all- does look like a case of crevice corrosion (thanks billknny for the great article - a reminder of why we have to accept a ten year replacement of standing rigging even if all seems well).
The heads of the bolts I posted are just what I removed from the deck. The 30mm or so of the shanks are still imbedded and viewable from underneath. Note evidence of water ingress and corrosion is evident on all bolts. I also attach a pic of the ends of the machine bolt heads I have removed which seems to suggest a process of slow corrosion rather than sudden failure. I will post an update when I am able to haul out.
gstent,

Pretty classic crevice corrosion appearance on the broken bolt ends. Old, dull corroded metal across most of the failure surface, with a narrow rim of shiny surface where the ultimate failure ripped the metal apart.

When you reinstall these, it would be a good idea to apply your sealant of choice from the head of the bolt on down to limit water intrusion around the threads. Then when tightening, turn the nut on the inside while holding the bolt still to maintain the integrity of the seal.
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Old 25-03-2020, 20:01   #9
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Re: Mast Step Bolts sheared

Quote:
Originally Posted by gstent View Post
Thanks all- does look like a case of crevice corrosion (thanks billknny for the great article - a reminder of why we have to accept a ten year replacement of standing rigging even if all seems well).
The heads of the bolts I posted are just what I removed from the deck. The 30mm or so of the shanks are still imbedded and viewable from underneath. Note evidence of water ingress and corrosion is evident on all bolts. I also attach a pic of the ends of the machine bolt heads I have removed which seems to suggest a process of slow corrosion rather than sudden failure. I will post an update when I am able to haul out.

What is the plating on the mast support inside the boat? The corrosion ring is interesting.
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Old 25-03-2020, 23:15   #10
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Re: Mast Step Bolts sheared

To Gstent, OP, have you owned this boat from brand new, if so is this the first time these bolts have been tightened/loosened/inspected?
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Old 26-03-2020, 09:51   #11
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Re: Mast Step Bolts sheared

Quote:
Originally Posted by billknny View Post
Yowza! I am not familiar with the Lagoon mast step design. Is that plate the only thing that holds the mast from shifting sideways? or is there also an internal plug (I hope!)?

I'd need to see the broken ends of ends of the machine screws to be sure, but it looks like classic crevice corrosion of the stainless bolts. They likely sit in a tiny little puddle of oxygen poor salt water. The threads give a stress riser, and the cracks start. It doesn't help that these are under significant tension from the upward load of the running rigging.

It's not an instant thing, it happens over time, and is really hard to detect--until failure. In this case, it is worth changing those bolts on a regular basis.

You can see pictures and an analysis of a similar failure here: Stainless Crevice Corrosion
It doesn't help that these are under significant tension from the upward load of the running rigging

I don't think the bolts in question which bolt the mast step to the cabin house top experience any tension at all. The bolts anchoring the turning blocks will experience upward tension.
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