Mounting gooseneck bracket to mast

[sanibel sailor]

Gooseneck broke. Had a new bracket fabricated. The side plates are 1/4” thick. The mast is 5/32” thick. I was initially planning to drill and tap 1/4-28 machine screws, but the welder suggested 1/4” SS rivets. He has a rivet installation tool that size. Looks like they have shear strength of 1700 lb and 2100 tensile. Should be quicker and easier with less chance of stripping a hole or breaking off a tap in one.

Advice?

[keelsidedown]

By no means a SME on this, but I'd ask myself some questions first: how wide are the plates? Does the plate have enough "grip" with a hole drilled dead center? Looks like the plates are "contoured". Was the aluminum ground or bent to achieve the contour? What is the thickness at the thinnest part if ground? If bent, what was the radius? How much sail area is going to be yanking on the boom attached to the attachment? How long is the boom?

Beyond that, make sure the "welder" is using hydraulic riveting equipment that's metered so you know you're getting consistent seating (sure, these are going to be pop rivets, but these are towards the top end limits for mechanical leverage compression). Find an inspection hole and use a bore scope to check before and after on each rivet. Don't use a single rivet between mast attachment points.

[sailormed]

Let your welder use his rivet machine. After that ask him to make 2 “C" shaped aluminium pieces that embrace the gooseneck from the front of the mast. One above, one below. Fix this to the mast with rivets as well. Drill and tap the C piece and the gooseneck. Bolt together and use Tefgel. This will surely hold. Maybe over engineered but you will never doubt the construction.

[sanibel sailor]

Side plates are 1/4" x 1-1/2", contoured in hydraulic press to fit curve of mast. Not quite perfect but very close. I had cut 1/4" off the bottom of the mast due to corrosion and used that as a pattern. Holes will be staggered about 3/8" in from edge, 7 on each side, two above/below the cross pieces and three in thee middle section. Holes all lined up can create a opportunity for a crack to "zipper". Mainsail 250 sq feet, boom 13 feet. Much more robust than stock piece which just slid in sail track. You can see where it tore out the edges of the track.

I have ordered rivets with proper grip length. I will be doing install myself with the rivet tool my welder has to lend which is a big two arm device, not the small gun type. It has tips for up to 3/8" rivets so should be sufficient. I don't have access to a gauged hydraulic rivet tool. Did not know such a thing existed. I may check with a rental place. I was planning on positioning it and holding it temporarily with Clecos in the middle, then fastening sequentially toward the ends.

[meirriba]

Is the new bracket made of aluminum or stainless steel?
I am thinking of electrolysis issues, e.g an alum. bracket and steel rivets (or bolts) or s.s. bracket pressed to alum. mast.
I think that in either case you need some protective measures to avoid future problems.

[roverhi]

If you decide to drill and tap, which I'd reccomend, use 1/4" x 20 fasteners not fine thread. Aluminum is soft metal and the fine threads don't have enough meat to hold well and prone to cross threading. Of course use TefGel, Lanocote, LocTite or ? on the fastener threads.

I'm not a fan of pop rivets.

[thunderhoof]

I'll stay out of the rivet /bolt debate but I would like to suggest that you taper and round off the top and bottom of the side pieces. I am fearful that they will snag the sail and bruise your hands while handling the sails with the square corners I see.

[sanibel sailor]

Originally Posted by thunderhoof

I'll stay out of the rivet /bolt debate but I would like to suggest that you taper and round off the top and bottom of the side pieces. I am fearful that they will snag the sail and bruise your hands while handling the sails with the square corners I see.

Good idea.

[kmacdonald]

Originally Posted by roverhi

If you decide to drill and tap, which I'd reccomend, use 1/4" x 20 fasteners not fine thread. Aluminum is soft metal and the fine threads don't have enough meat to hold well and prone to cross threading. Of course use TefGel, Lanocote, LocTite or ? on the fastener threads.

I'm not a fan of pop rivets.

I agree. For a 5/32" thick mast there will be three threads engaged and that is enough. It will be about 50% stronger than rivets and much more resistant to shock loads.

[Jim Cate]

If you are going the rivet route, thus eliminating the chance of removing the gooseneck fitting in the future, you might as well bond the plates to the mast with epoxy additionally. Should add significant strength, little cost and not difficult.

Jim

[sanibel sailor]

Originally Posted by kmacdonald

I agree. For a 5/32" thick mast there will be three threads engaged and that is enough. It will be about 50% stronger than rivets and much more resistant to shock loads.

Just curious where you get the strength figures from? I found a formula to calculate it, but it made my head hurt.

The fine vs coarse debate got me reading. A little knowledge is dangerous and if there's a skilled machinist out there, please speak up. Coarse threads better for threading into lower tensile strength material (e.g. Aluminum) to prevent stripping of the internal thread as well as if corrosion is expected. Fine threads are better when the length of engagement is short (or if threaded material is stronger than screw). Generally recommended length of engagement- how deep a hole- is 1x to 1.5x screw diameter to match screw strength to tapped thread strength. That would be either a #6 or #8 machine screw for my mast. Machinery's Handbook says 3 threads is not much of a grip, should be 5 at least, 7 better for aluminum. Having said that, most hardware on my mast is screwed rather than riveted.

[kmacdonald]

Three threads engaged is routinely considered adequate. More would be a little better but not by a lot. And yes, course threads for aluminum. Jims idea of using epoxy is also good but makes it a little more difficult to remove. (it would have to be heated) The majority of the force is shear so tensile strength (pull out) is not as important.

[Jim Cate]

Jims idea of using epoxy is also good but makes it a little more difficult to remove. (it would have to be heated)

I made the recommendation only if he was going to use rivets as he described. The convenience of easy removal is well shot if it is riveted, so might as well add the epoxy bond.

I'd be more likely to use screws (and in fact did so on our boat) because removal is likely to be desired eventually.

Jim

[sanibel sailor]

Thanks for all the advice. Decided to use rivets. Liked the epoxy idea conceptually but it is almost 90F by noon now and it would need to be thickened and I foresaw hardening epoxy with not enough fasteners in place with an enormous mess. I used 18 1/4” SS rivets, at about 2k pounds each, that is roughly twice my displacement.
Held in place with hose clamps then drilled and tapped 2 8-32 machine screws, mounted boom to check clearance and fit, then went to town with rivets. Used big rivet tool, that thing really gets some squeeze on it. I feel very confident in it.

[Mauna Kea]

I don't know if my previous post went through or not. I won't get into the debate of rivets or bolting, but looking at your photos it strikes me that you are not using the groove in the mast to take MOST of the load. All you need to do is to have a vertical 'talon' piece welded between and on the back of the upper and lower horizontal webs (the one with the holes). That talon piece would fit snug into the groove and greatly reduce the load on the fasteners. With this modification, the fasteners (rivets or bolts) will mostly work in traction rather than both traction and shear.