F-Boat Hinges and Stainless Bolt Failure

 Edit 7-19-2024. Edit 7-21-2024: I originally posted the images just to share them with a metallurgist. They deserve some explanation.

The bolts are not from my boat. They are from an F-9A, a home-built version of the Corsair F-31. The hinge design is similar. I know of three F-9As that have suffered from this problem and no other F-boats. I don't know why this is the case, but I have educated suspicions.

When the bolts fail, even if over 50% of them are bad and the others compromised, the structure does not fail. It is very conservatively engineered. Although stress certainly plays a part, I suspect it is only the pre-load (installation torque) and not service loads. As long as the service load does not exceed the installation pre-tension, the bolt does not "feel" the change in load. The fitting experiences changes in stress, but the stress under the bolt head does not change. This is one of the primary reasons for pre-tension (in addition to preventing loosening, but they are related). As long as the pretension does not exceed the working load limit (WLL) there is no fatigue, not change in stretch, no movement, no failed seals, and no loosening.

These bolts are loaded in nearly pure shear, so the failure point, if related to sheer, should be at the flange/hull interface or even farther in, not at the head. The fore-aft loads from the amas are carried by diagonal cables. The view is from the aft hinge forward. The forward hinge is just visible in the distance.

So why are they failing at this location, and why only (I think) on the F-9A?

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 I asked the owner of one of the boats for more detail. This is what I go. He obviously put some thought into it.

All the ones (bolts, washers and nuts) I removed were 304 series (A2), it is isolated from the aluminium bracket with a glass reinforced acetal washer.  This sort of crevice corrosion is quite common in saline environments where a water film can get trapped and not flushed.  It is a significant area of interest in offshore oil and gas where the use of duplex steels (which have austenitic and ferritic phases) is common which are designed to be more resistant to this type of corrosion.  One of this issues is there is no great way to tell if a fastener has crevice corrosion.  PERN number, which is a ratio of  chromium, molybdenum, nickel, and nitrogen content is often used to compare the resistance of steel to this type of corrosion.  PERN is approx18 for A2 (304), 24 for A4 (316) and mid 30s for duplex.  I notice the farrier sailing manual suggests no maintenance is required except flushing the bolts with fresh water if feasible.

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[Corsair F-24 lower folding strut. A smaller boat, but a similar design.]

Sometimes the head of the bolts come off due to crevice corrosion under the heads. Only under the heads. The bolts are epoxied in, creating a good seal, but who knows.  Another possible difference with this F-9A vs. the production Corsairs is the used of glass-filled acetal washers in place of 304 stainless washers. Was the glass possibly abrasive? There should have been no movement. Some other chemistry, possibly related to why bolts more commonly break inside the laminate?

These boats, unlike the production version, used carbon in the beams. Some builders used it extensively. Carbon is quite noble and could be a factor.

This is more like what I would expect, but it is not what we see on F-boats.

This can also affect hinge pins (F-28R).

The F-28R and the F-9A contains some amount of carbon, but whether it was used in the affect area is not known (these boats are on their 2nd owners). Certainly not near the failed pin.

 

More F-9A parts

 

 

 

 

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I'm puzzled as to why these problems do not seem common in the Corsair production versions. I'm glad, mind you, just puzzled. As more information becomes available, I will revise this post.