Boom/gooseneck repair

Moderator: GreenLake

Boom/gooseneck repair

Postby James4 » Fri Jun 02, 2023 7:50 pm

I recently purchased a '75 DS II. It apparently had no use for several years. On my first attempt to launch, screw heads popped off the chainplates and we limped back to the ramp. We're lucky the wind was very light and we'd only gone maybe 600 feet. We replaced the original #8 screws with #12/24's. On the second attempt to launch, months later, I pulled the boom out and the gooseneck fell off. I can see it had rivets that all failed. It also has lots of surface corrosion. Very happy this happened in the parking lot. I'm wondering if I should replace the rivets with aluminum or stainless. I'm also wondering about whether I can trust the attachments to the mast. They look fine, but I certainly don't want a failure there. 1) Would I be wise to replace all the rivets on the mast, and if so, aluminum or stainless? 2) Should stick with aluminum rivets for the mast/shroud attachment? That seems the most critical area. 3) A machinist neighbor says I should skip the rivets on the gooseneck, and instead drill and tap for screws. Thanks for any suggestions. Jim
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Re: Boom/gooseneck repair

Postby GreenLake » Fri Jun 02, 2023 10:12 pm

Aluminum rivets are preferable from the point of not introducing dissimilar metals at the mast/boom rivet connection. The fittings are SS, so you still need to use TefGel between them and the aluminum of rivet and mast. (sells in a really tiny container, but a little goes a long way, and it's indispensable).

My take on the gooseneck is that when the sail is rigged and sheeted home, the force on the boom will compress the gooseneck, more so, if you installed a vang. That means, the connectors are basically there for when you take the boom off the mast, and I believe you can get away with ordinary aluminum pop rivets. However, read back just a few posts here and you find a similar case discussed. In that instance, the owner decided to tap the gooseneck fittings and use machine screws. (With TefGel). So, you have options. The key is that the gooseneck fitting is a substantial piece of material, not as thin-walled as the mast or boom. That makes tapping and using machine screws a good option.

For the mast fittings, like shroud attachments, I would start with a close visual inspection. Do you see any loose rivet heads, any (powdery) evidence of corrosion? If not, you may not need to do anything. If you need to replace rivets, look up specialty fastener stores if you have them in your area or go online to try to find the most compatible style rivet.

I had both of the rivets fail on one of the spreader attachments, and failing to get matching rivets with a captive stem, I simply "spiked" the ordinary hollow core with a short length of aluminum rod. My thinking was that whatever extra strength a captive stem could provide, would not depend too much on anything other than filling the hollow space (and it also sealed the mast at that spot). I did that repair when I was a new owner and there hasn't been any other rivet failure (and the repair has held).
~ green ~ lake ~ ~
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Re: Boom/gooseneck repair

Postby James4 » Sat Jun 03, 2023 3:33 pm

Greenlake,
Thank you for your detailed reply. But I could not understand what you meant by spiking with aluminum rod. I'm pleased you haven't had any failures since the repair. And thanks also for the tip about Tefgel. I don't know what I'd do without this forum. Very helpful. Thanks, Jim
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Re: Boom/gooseneck repair

Postby GreenLake » Sat Jun 03, 2023 7:20 pm

Well, on a mast we would be talking about blind rivets, where a mandrel with a head is used to widen the part on the inside to fix the rivet in position and thus effect the connection between the two parts. The mandrel passes through the middle of the rivet and has a weak point at which it is intended to break off after the rivet has been pulled tight. For some rivets, this is close to the head, so only the head of the mandrel is retained. In others, the mandrel is either cut off or snaps off at the outer surface, so a piece of the mandrel is retained, filling the center of the rivet.

As a result, the rivet blocks water ingress when the mast is submerged, but it may also strengthen the rivet when loaded on shear. I couldn't get the right kind of rivet and simulated the effect by pounding in a short length of aluminum wire (or rod). Not sure it made a difference and at any rate, these were the rivets for the spreader attachment, which should not be as critical as those for the shrouds. However, curiously, it was those that had failed of all the rivets on the mast. Nevertheless, the repair has lasted too many seasons to count.
~ green ~ lake ~ ~
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