Best fasteners in a mast -- Titanium?

[sailingharry]

So yesterday, I broke off my gooseneck. No drama, didn't even know it until the main came down. The rigid vang mostly supported the boom.


It seems of 12 5/16 screws holding it in place, about 7 broke off and the remainder pulled out. The broken ones won't move with a punch, and drilling them out will be challenging with limited chance of success. The pulled out ones have suspect threads. My intent is to raise it 3/4"and redrill.


I have an opportunity to consider attachment solutions. The original stainless lasted 26 years and at least 25,000 nm, so it's a good first solution. A friend suggests structural aluminum rivets, but I'm not convinced. And then I thought of titanium screws, available for about $4 each (see https://maps.app.goo.gl/G6ztYV4iWJsqmDoM7 ) -- not all that expensive if I get an upgrade for my money.



Any thoughts about titanium in this application? Any thoughts about any other alternative?

[hlev00]

My us spar cast aluminum goose neck is riveted with either Stainless or Monel rivets to the aluminum mast. Use tef-gel. Don’t use aluminum rivets.

[boatpoker]

Originally Posted by hlev00

My us spar cast aluminum goose neck is riveted with either Stainless or Monel rivets to the aluminum mast. Use tef-gel. Don’t use aluminum rivets.

+1
All metals of differing voltage potential on a boat should be treated with TefGel.

[SailingHarmonie]

The failure was not due to broken screws. That was a result, not a cause. Unless there is a basic design flaw, 12 5/16" fasteners are plenty strong enough.

The failure was due to the pulling out of the screws, which increased shear loading on the remaining fasteners, and that caused them to break.

Unless there were serious mechanical problems with the original installation, the reason they pulled out was that the threads in the aluminum corroded away because the stainless was not properly isolated from the aluminum when installed. With seriously corroded thread, all it takes is one bad accidental gybe to start the failure process--which might take some time to complete.

Properly installed and isolated with Tef-gel or something similar, stainless screws in aluminum should last far more than 25 years.

[SailingHarmonie]

Although I do not have the data in front of me, I would be a case of beer that any kind of blind installing aluminum rivet is WAY weaker than a properly installed stainless steel screw of the correct size.

They are widely used in installation of hardware on production lines because they are MUCH faster to install.

Be careful about installing screw that are larger than original. You want to be sure that you have enough threads in the thickness of the mast to reach full strength. Larger fasteners with coarser thread pitch can actually end up pulling out easier if you only have 1 or two threads in the mast wall.

[MicHughV]

I can only echo the above. Use fine threaded screws. Forget about rivets. Now and forever.
A few points to ponder....the wall thickness of you mast is likely not very thick, so when you drill and tap for a new screw there is going to be very little meat left in the wall of the mast to properly hold the screw.
Fine thread screws are available, but you'll have to go online to find these, as most hardware stores don't carry them. The same hold true for the tap required to cut the thread. An alternate is to consider a smaller size screw.
Next, slather the screw in 5200 prior to installing. Make sure enuff squeezes out to form a barrier between screw head and mast, for that matter the whole gooseneck needs to be slathered in 5200, as you do not want water wicking behind the gooseneck for any reason.

Were it me, I'd tape off the area of the gooseneck, so after the 5200 has hardened, removing excess 5200 is made simpler. You want a water tight seal around the entire perimeter of the gooseneck.

[Bowdrie]

Originally Posted by MicHughV

for that matter the whole gooseneck needs to be slathered in 5200, as you do not want water wicking behind the gooseneck for any reason.
You want a water tight seal around the entire perimeter of the gooseneck.

True enough.
On another tack, before rigid vangs most gooseneck failures were caused by inadvertent jibes, now things have changed.
Lighter, (thinner,) mast sections that hold fewer threads have not helped the retention of any hardware either.

[sailingharry]

Harmonie,


Some good points. And a bit to add.


My screws are 5/16 NC, spaced on 1.75" centers. I'm planning on splitting the difference (raising it about 7/8) and replacing in kind, so I'll be drilling into essentially virgin metal (I'd like to have more, but I think the resulting spacing of 9/16 to the old holes should be enough to impart full strength).


The old ones were NC, and seemed to have adequate thread engagement. I've always thought the finer thread on NF would be subject to strength loss in this corrosive situation. Is NF preferable? I think (but am not sure) that in automotive work studs are NC into aluminum but NF on the nut end.


I think I may have a bit of contributing behavior. I don't use a topping lift -- too many years of racing, I guess. My boom sits on the rigid vang. Sheeting it down tight with the sail down to stop wagging of the boom does impart fairy significant tension on the gooseneck (vs under-sail compression). If this is likely to be the cause, I'll need to ponder alternatives.


My rig "should" be well built -- it was built by Offshore Spars and 26 years later they are still in business (I've ordered stuff from them). I say "should" because Alan Poole (Saga president) had a bad reputation of driving his suppliers to cheapen products. Among other things, the masts were not primed and most have been stripped/repainted. Hmm...including mine... that means the goosneck was removed/reinstalled....wonder what impact that might have had?

[wholybee]

You could also use helical thread inserts. Usually used for repair, but can also be used to improve strength in softer material. The coil would be SS, so you would also avoid the future difficulty of removing a SS screw in corroded aluminum.

I have used them to repair threads in my boom where the hole/screw corroded and ruined the threads. Just drill to the correct larger size, insert the coil, and use the original size screw.

[Rucksta]

Some things to consider

Monel rivets, sheer strength >4x stainless machine screws of same size
Close to passive galvanic potential in aluminium
Wall thickness of parent material not as critical as threaded fasteners

The original holes in the mast are probably munted or will be by the time you get the broken screws out.
Enlarging the hole may require a diameter greater than the gooseneck fitting can support - larger hole too close to the edge of the fitting.

Welding the munted holes shut and redrilling may create tempering issues in the mast

Inserts work great but are subject to minimum wall thickness to reach full strength

Applying an external strong back is an option if the gooseneck fitting still closely conforms to the new mast shape.

Drilling new holes spaced between the old holes raising or lowering the boom by a small amount may be a solution.

If the gooseneck fitting is drilled for countersunk fasteners a rivet may be compromised.

Your going to need a very powerful rivet gun to insert a 5/16 monel rivet


If you decide on titanium Tef Gel or similar is still indicated.

[stormalong]

Originally Posted by wholybee

You could also use helical thread inserts. Usually used for repair, but can also be used to improve strength in softer material. The coil would be SS, so you would also avoid the future difficulty of removing a SS screw in corroded aluminum.

I have used them to repair threads in my boom where the hole/screw corroded and ruined the threads. Just drill to the correct larger size, insert the coil, and use the original size screw.

I doubt that a Helicoil would have enough grip on a thin mast wall.

Two recommendations:

1) Stainless steel rivnuts. A quick google search found them up to 10mm. They are available in Imperial sizes too, I just didn't see them in my search.
With them you could use the existing holes. They require a special tool to install large sizes. The repair would be easy and last a very long time.

2) An even stronger repair would be a backing plate. This would require the mast be taken down and a custom backing plate bent. You could get the mast builder to bend you one.

When I rebuilt my mast about fifteen years ago I did option 2. To install I drilled & threaded a 1/4-20 hole in the backing plate and a corresponding countersunk hole in the middle of the bolt pattern for the gooseneck on the mast. With a strong but thin string I pulled the backing plate up to the matching center hole and screwed it in. Then I drilled and tapped through the mast for the gooseneck. That is about as bullet proof as you can make it. I used at least 1/4 inch aluminum for the backing plate.

And of course use Duralac or Tefgel. I prefer Duralac because it hardens a bit. Tefgel is a never hardening grease which can help screws back out on their own.

[GordMay]

For optimum thread engagement, in STEEL, a common rule of thumb is: length = 1.5 times the base diameter of the screw, or bolt.
In ALUMINUM, you want a length of between 2.0 to 2.5 x base diameter of the screw or bolt.
The idea is for the screw to break, instead of stripping out the threads, in the tapped hole.
So, for a 5/16-18 NC screw, in Aluminum, you need [2.5 x 0.3125] = 0.78125" length of engaged threads [thickness of mast].
Given 18 TPI [National Coarse], you need [length of thread x pitch = number of engaged threads]
[0.78125 x 18] = 14 engaged threads.

[Tedd]

Not a lot of love for rivets, here. That's understandable, since rivets are so often badly done. But it's important to understand that in a properly-designed joint the loads aren't actually carried by the fasteners, they're carried by the clamping force. In general, rivets give better clamping force than screws and have less risk of loosening over time. As a general rule, rivets are a stronger and more fatigue-resistant way to join metal parts together.

But that's a general truth. It's also true that designing a riveted joint is its own area of expertise. Simply replacing screws with rivets in an existing joint isn't likely to produce satisfactory results. You'd want to completely redesign the joint.

[thinwater]

Originally Posted by SailingHarmonie

Although I do not have the data in front of me, I would be a case of beer that any kind of blind installing aluminum rivet is WAY weaker than a properly installed stainless steel screw of the correct size.

They are widely used in installation of hardware on production lines because they are MUCH faster to install.

Be careful about installing screw that are larger than original. You want to be sure that you have enough threads in the thickness of the mast to reach full strength. Larger fasteners with coarser thread pitch can actually end up pulling out easier if you only have 1 or two threads in the mast wall.

a. I agree with your first post. Just reinstall it correctly with isolation.

b. I might end up winning that case of beer. In this case, bolts. You need a certain strength in a small space, and blind rivets of the correct size do not exist.

But if you were, for example, splicing a mast, blind rivets can be as strong or smaller than the 3/16-inch fasteners that would be specified to spread the load. Also, in the case of thinner spars (dinghies and beach cats) rivets are usually stronger than machine screws. I've done the testing and owned the boats.

Again, it ALWAYS comes down to proper installation.


(Monel rivets are not stronger than stainless. Specific rivet designs (structural vs. sealed, vs. cheap) can have many different features and strengths, but it's not the material. Just read the specs.)


---


In this case, just move the holes, thread carefully, use Tefgel, and torque properly.


Sailing Harry: NC is prefered for soft metals (aluminum). Google it. Stay with NC.



Another good way to prevent boom wagging is to use the topping lift, sheet, and a 3rd line to the side to create a triangle. In fact, I keep this 3rd line clipped up to the end of the boom. When the sail is down I bring it down to a stern cleat. I move the traveler to the other side (clears the walk-through) and tension the sheet. Takes seconds with a custom length pendant prepared. No motion, no stress on the gooseneck.

[stormalong]

Use RIVNUTs, not rivets. It is an ideal way to place a nut in a thin metal hole.

If you don't know what they are google it.