DRAGONFLY Trimarans User Forum

We were doing some maintenance this winter and discovered that some of the coach bolts which secure the stainless steel hinge structure to the main hull HAD SHEARED OFF!
I have photos if you want details. Check your bolts and replace if you have any doubts.

Please post detailed pictures!!!

We can't see how to upload photos onto the forum (Forum Admin: You sent them to me [Bo] and I will add them as I have done now)
Please click on any of the photos to see them enlarged!



Look at the rusty stains.
The sheared bolt was closest to the hinge pin in the bottom row on the port side.
We aren't sure about where the other bolt was located as we have now changed 6 of the bolts and only discovered the head was coming off when we got home.
We do plan to change all of them, but note that they are not all the same length. The bolts near the hinge pin are M10 grade A4 40mm long with full thread, coach/carriage bolts.
I can't seem to make the photos rotate correctly, but I think it shows what I mean.

Very curious that there were two different failure modes: One where the bolt broke in the middle, and the other where the cap or head separated.

What type of maintenance were you doing when you discovered these failures? Did they break when you were re-fastening, or when removing?

What manufacture year is the boat they came from?

We have had some new hinge pins made as there was significant wear. We had a stainless guy reaming the holes to fit the new larger pins. He said he touched one of the nuts and it fell off!!!
We thought we'd replace as many as we could and it was at that point that we found the one with the head tearing off.
So far we have only investigated 8 and found 2 complete failures. But need to get some longer bolts first.
The boat is 2004.

Were they seized (stuck) or was it possible the threads were galled? If yes, perhaps the stainless guy "touched" them with too much force? If he was being a bit heavy-handed it might be defensive behavior for him to blame the materials. Or perhaps there really was a defective batch of hardware?

I'm not saying they didn't fail, but this seems highly unusual considering you have two failures in one boat and I've never heard of this problem before. I have owned three Dragonflies since 2001 and follow the discussion board.

Also, I haven't seen that style of acorn nut used on the beam hinge plate bolts. Does anyone else reading have them on your boat?

My bolts and nuts look identical to these. I just looked at my hinge. If the stainless guy had to ream the holes, the bolt nearest the hinge would have been in his way.

I think the type of lock-tite used on these has to be heated to unscrew...

Please note that the bolts were not in the way of the reaming and I believe our fabricator when he tells me he only touched it. With regards to the locktite, I have been able to undo all the nuts I have tried so far without needing to apply heat. The locktite appears to be working well as they are moderately difficult to start to undo but not so much as to risk damaging the bolt, nut or thread.
Note that the bolt with the head coming off appeared fine when I removed it. It was only when we examined it closely at home and noticed a line of rust around the junction between the square shoulder and the head that we tried bending it by hand (girl fingers) that it began to come apart as can be seen in the photograph.

That's a bit scary.

This has happened to me as well. The problem is caused by stress crack corrosion, something which stainless steel suffers from in salt water.

I suggest that everyone replaces all their beam bolts every 15 years. It's an easy job and the bolts are not expensive. Look on the spare parts page here to find a couple of suppliers and a picture of one of mine that broke.

The bolts should be tightened to the correct torque for their material and size using a torque wrench. This information can be found easily via google, for example...

https://www.fastenal.com/content/feds/p ... 0Steel.pdf