DRAGONFLY Trimarans User Forum

Check your bolts that connect the beam hinges to the beams and the hull. I had one snap for no apparent reason on my 920. These are the bolts with the shallow domed head. It looked fine from the outside, but inside the beam socket on the main hull I found one of the bolts had no nut on it any more. The bolt had sheared off flush. Looking at the broken end it looks like it has suffered stress crack corrosion. I may replace all of them as they are only €2 each.

Just an update if anyone is reading. I have spoken to a specialist mechanical engineer in fatigue failure where I work. Her advice was to replace all the similar bolts immediately. The one failed bolt was a warning that needs to be heeded. The total cost is about €100 so not too expensive to avoid a potential catastrophic failure.

She also advised that my failure mode, right next to the nut, is typical of galling upon assembly starting the crack caused by being assembled without thread lock or grease.

I have just sent some photos to upload. Look at them. If you mean these screws and nuts, so it should not be a problem. I’d noticed that for some years ago on my 35’ and sent these pictures to Quorning. He said, that it is all right, and nuts are not necessary.
Please click on any of the photos to see them enlarged!

It appears the DF35 has different fixings/bolts.

On the DF920 they're indeed dome shaped



John Blaiklock tells me: "The A4 dome headed bolts that connect the beam hinges to the beams/hull are sold by www.seascrew.com. You need A4 or 316 and search for carriage bolt. These have the square section in the shaft of the bolt under the head."

Bo Wetzel wrote:It appears the DF35 has different fixings/bolts. On the DF920 they're indeed dome shaped

The hinge arrangements on the 35 and 920 are different. There is no equivalent to my broken bolt on the 35. My bolts are fixed vertically and the dome head is on the top (or underside) surface of the beam or beam socket on the main hull.

Whatever Quorning advise about the nut not being necessary, I don't want any more of these bolts breaking, so I am going to replace them all.