Rivets or Machine Screws When Splicing a Mast?

[Chotu]

Originally Posted by Dave_S

We've done those.... This is new

Ha ha ha.

I can’t wait to come up with the next one. I’m sure I will run into it soon enough.

[Chotu]

Originally Posted by stormalong

Wow, the thread that won't die! 135 posts so far. The truth is that both are used professionally and both will work. Since the splice is at eye level it is easy to monitor.

That’s actually a very, very good point. It will be in plain sight all the time. Easily inspected up close at eye level.

Anything loose can be tightened right up. And stripped. Ha ha ha

[RaymondR]

The problem with both riveted and screwed connections on aluminium is that aluminium is a relatively soft metal and if the item riveted or screwed is subjected to high axial loads the aluminium will deform.

It is also a fairly stretchy material and consequently those fastenings closest to the join may experience much higher shearing forces than those spaced further away. The consequences of these circumstances is that progressive failure may occur.

This is occurring in my boom which broke a couple of years ago. I sleeved it and used about 100 3/16 CS metal thread screws and although the metal is T6 it is only 2.5 mm thick and the boom is beginning to bend at the joint.

During my between cruises layover I will be removing dome of the screws and doing the drill, tap, threaded and peened, 3/8" aluminium rod thing to increase the projected areas being subjected to shearing forces.

[Dave_S]

Originally Posted by s/v Jedi

According to post 127 above, it was the material that failed, not the fixings.

It will depend on a thousand things. You want a mast to flex evenly along its full length. But now you want to put an ugly (mechanically speaking) join in it and attach the 2 parts with a 5' sleeve and fixings, the two methods are mutually exclusive so somehow you have to make it work. There is no good solution, at best you might hope for one that holds up, not so in this (post 127) case, but you can't pin point a reason from this other than it was a design or execution issue and we don't know the design.

Realistically you will need (not want) the section that is sleeved not to flex because it is attached with fixings. So the section with fixings is inflexible (not really true but for this discussion we can say it is much less flexible). So the section immediately beyond the sleeve will be a stress point, you will need to relieve that but the $1,000,000 question is how to do it without stressing the fixings or the holes. The partial answer is to have the material take up the blending of the stress over a greater area by having a termination of some kind on the sleeve so the stress point blends out to a stress area. How much area can you get ? I don't know.

The mast in Question (post 127) has 2 sleeves and has countersunk fixings... Not a good start, so I am cautious of the standard, then how long are the sleeves as a ratio to the mast length. How lightweight was the mast to start, i.e was the mast on a race boat at its limitations of elasticity, was the designer expecting a one piece mast but the guy only had short tanks so they chopped it up. Did the anodising change the elasticity of the mast, what did the end of the sleeves look like, was it adhered to the mast- not just fixed...... Clearly the job wasn't right because not only did one joint fail but both failed and not just at one end but top and bottom of both sleeves, if I understand correctly.

I suggested a termination of the sleeve of something between flat and a narrow taper - a shape that would hopefully help the blend the stress best over the restricted area.

As a side note a mast that flexes will deliver more power to the boat.

[Chotu]

Originally Posted by RaymondR

The problem with both riveted and screwed connections on aluminium is that aluminium is a relatively soft metal and if the item riveted or screwed is subjected to high axial loads the aluminium will deform.

It is also a fairly stretchy material and consequently those fastenings closest to the join may experience much higher shearing forces than those spaced further away. The consequences of these circumstances is that progressive failure may occur.

This is occurring in my boom which broke a couple of years ago. I sleeved it and used about 100 3/16 CS metal thread screws and although the metal is T6 it is only 2.5 mm thick and the boom is beginning to bend at the joint.

During my between cruises layover I will be removing dome of the screws and doing the drill, tap, threaded and peened, 3/8" aluminium rod thing to increase the projected areas being subjected to shearing forces.

Correct me if I’m wrong here, bht this doesn’t apply to my mast splice, does it?

I don’t really follow the forces on a boom, but they seem like they would be axial.

The forces on my mast are almost strictly compression. With a little shaking involved.

So those fasteners are just holding the splice piece inside the mast, keeping the butt join in alignment. Which I’ve said earlier in the thread. The forces are in the butt joint.

There might also be another reason for yours failing.

2.5 mm is 1/10 of an inch. Not even.

The number of threads per inch on a coarse thread is 8. The number per inch on a fine thread is 12.

The number of threads you have going through your 2.5 mm material then, is less than one if you use coarse thread. And is 1.2 threads if you used fine thread. Not good. That’s probably why it’s failing.

My material is 6.3mm or 2.5 times thicker. So I’ll have 2 full threads going through it if course or 3 threads going through it if fine thread.

[Dave_S]

Originally Posted by Chotu

Ha ha ha.

I can’t wait to come up with the next one. I’m sure I will run into it soon enough.

Hope so, this is fun 🙂

[Patrigo]

I have not read all of the above, so I do not know if anybody has suggested Rivnuts??
Since I first heard of them a few years ago, I have used them almost exclusively. Check them out if you are not familiar with them. Easy to use and easy to check or replace.

[MWGDVC]

Chotu,
I know this is a bit late, as you have handed this task over to your fabricator, but just some points to think about…
I have built a few things, some of which are in orbit right now.


I have come to ask myself, how would I solve this problem if it were my boat?
I would make a very close fitting inner sleeve with a longitudinal split to allow it to expand in diameter to match the inner diameter of my mast extrusion.
Next I would assemble the pieces and undersize match drill just the row of fasteners 180 opposite the split of the inner sleeve. Use a “V” block with the appropriate size hole as a drill guide to assure the fasteners are installed straight.
Remove the sleeve, drill, deburr, and tap the holes in the sleeve to accept locking helical inserts using the appropriate STI tap per the insert’s specifications. Wet install the inserts using zinc chromate or 23377 primer to obviate corrosion between the stainless insert and the aluminum mast sleeve.
Drill mast holes to close fit clearance,(see machinery’s handbook for guidelines), deburr, reassemble pieces, and install hex head bolts with washers underneath to hold the sleeve in place for subsequent match drilling. Tighten snugly, repeat match drilling the next row port and starboard of the initial row.
Repeat the disassembly/ reassembly process until all holes have been drilled, deburred, tapped, and inserts installed.
Countersink close fit holes in mast using a microstop tool and the correct degree (matching my chosen fastener’s countersink degrees) countersink bit with the correct size pilot.
Sneak up on the correct countersink diameter with the microstop. Each click allows the tool to go .001” deeper, but the CS diameter change is dependent upon the CS degree angle.
As my mast’s cross section is an ellipse, the microstop will most likely need to be adjusted for each position around the mast. Reduce the depth and sneak up on the final CS diameter each time.
The round cross section of the mast means the countersinks will be oval, rather than round as they would be if on a flat surface.
My preference is a margin (the ring visible around the fastener’s head) of .002”-.005” around the fastener head. Since this application is an oval CS, this would be greater in one axis, but I wouldn’t want any portion of the fastener head above flush.
Deburr mast sections, clean thoroughly, and assemble mast, torquing fasteners to 80% of final torque.
Once completely assembled, remove fasteners one at a time, wet install fastener with threadlocker, and torque to final torque.
I know this is a time consuming, fiddly process, but I would want to do it right the first time, especially as the parts involved are expensive, and critical.
I would choose as shallow a countersink fastener as I could obtain, as less mast material is removed in the countersink. Draw an isosceles triangle at 82 degrees, and another at 100 degrees, with identical base dimensions, and you will see what I mean.
I am in no way offering advise, I am merely outlining the process I would follow if I were to attempt to splice a mast.

[RaymondR]

Originally Posted by Chotu

Correct me if I’m wrong here, bht this doesn’t apply to my mast splice, does it?

I don’t really follow the forces on a boom, but they seem like they would be axial.

The forces on my mast are almost strictly compression. With a little shaking involved.

So those fasteners are just holding the splice piece inside the mast, keeping the butt join in alignment. Which I’ve said earlier in the thread. The forces are in the butt joint.

There might also be another reason for yours failing.

2.5 mm is 1/10 of an inch. Not even.

The number of threads per inch on a coarse thread is 8. The number per inch on a fine thread is 12.

The number of threads you have going through your 2.5 mm material then, is less than one if you use coarse thread. And is 1.2 threads if you used fine thread. Not good. That’s probably why it’s failing.

My material is 6.3mm or 2.5 times thicker. So I’ll have 2 full threads going through it if course or 3 threads going through it if fine thread.

Hi Chotu,

Whilst the forces on the boom are primarily bending loads from the vang and the almost mid positioned main sheet these forces in a beam translate to tensile, in the lower flange, and compressive in the upper. Similarly the forces in a mast will include lateral forces, particularly from the lateral forces imposed by the main sail at the boom gooseneck.

My boom repair was done on the boat away from my home base and was the best I could do in the circumstances. Some parts I got right (Making the sleeve in three pieces and configuring the design so that the sleeve expanded inside the boom sections) The boom was OK at the finish of that cruise but has taken on the bend during this past winters cruise. I will be doing the drilling, tapping, threaded rod and peening exercise during this summers layover.

I used metric threaded screws which tend to lesser pitch and, after countersinking, just about all the thread is in the 3.0 mm sleeve. I would have preferred solid rivets however there is no practicable way to install them in a closed section where the inside is not accessible.

If joining a boom I would prefer to weld the section with a sleeve inside. If concerned about creating a circumferential weak spot I would castlellate the sections the joint and also slot weld into the sleeve. Deep castlelatting the ends of the sleeve would also reduce any stress riser tendencies at their ends.

My boom broke because there was corrosion caused by the SS attachment for the vang which is always a problem where the aluminium has SS fixings or fastenings. If I retain the boom I will be making a new fitting from aluminium T section.

In view of all this I guess my answer to your original question as to rivets or screws would be neither.

[Baby tug]

I think I’ve said this before if it were my mast i would file the ends , grind whatever an angle on the ends put them together to create a Valley, tig weld , They make all kinds of different filler rod , weld , grind , sand smooth , depending on what it all look like , if i was worried about stress cracking , would cut a piece of plate in a diamond shape and weld one on each side of the mast to give it the added strength it might need , just me �� ��

[neilpride]

This is absolutely marvelous, we use it even when new material comes already assembled,and we take it apart and put it back together with this product.

[Boatwright]

Research "structural blind rivets". These are not the regular pop-rivets one buys at the hardware store. The mandrel is retained in the core of the rivet which gives a much higher sheer strength

Monel is said to be less subject to electrolytic corrosion in contact with aluminum than SS.

I would also be in contact with a company that fabricates masts for pro recommendations.

[Chotu]

Originally Posted by Boatwright

Research "structural blind rivets". These are not the regular pop-rivets one buys at the hardware store. The mandrel is retained in the core of the rivet which gives a much higher sheer strength

Monel is said to be less subject to electrolytic corrosion in contact with aluminum than SS.

I would also be in contact with a company that fabricates masts for pro recommendations.

That sounds reasonable. But the rivets are not in any sheer condition at all. They will be in tension.

Would they still be better?

[Chotu]

Originally Posted by neilpride

This is absolutely marvelous, we use it even when new material comes already assembled,and we take it apart and put it back together with this product.

We will add it to the list of materials.

[Chotu]

Originally Posted by Baby tug

I think I’ve said this before if it were my mast i would file the ends , grind whatever an angle on the ends put them together to create a Valley, tig weld , They make all kinds of different filler rod , weld , grind , sand smooth , depending on what it all look like , if i was worried about stress cracking , would cut a piece of plate in a diamond shape and weld one on each side of the mast to give it the added strength it might need , just me �� ��

This is really interesting actually. Why don’t people do this? Why is it always rivets and screws?

Certainly my welder would be a lot more comfortable with this.