Best way to fasten something to a Sailboat Mast

[Bulawayo]

Similar information as I received from Spar Craft.

Originally Posted by Capt_Flrp

When I stumbled onto this forum, I was thrilled with the amount of knowledge shared and the lack of opinions stated as facts argued ad nausea. I do have to pipe up on this particular thread however. I would first agree with all preferring the rivet solution vs. threads of any sort. I do, however, have to point out the error in the fine/course thread argument going on. Assuming sufficient length of engagement, proper tap drill size, lubrication on taps, etc. etc., the fine thread is stronger than the course thread for the following reasons:
1) In tension (pulling against the threads) the stress area of the fine thread is larger, and in shear (pulling the fastened object along the adjacent surface) the minor diameter is also larger.
2) Fine threads have less tendency to loosen since the side of the thread incline is smaller and hence so is the off torque.
Other advantages include finer adjustment when needed (irrelevant in this ap), more easily tapped in hard materials and thin walls, and lesser torque required for equivalent bolt preloads.
On the other hand:
1) Fine threads are more susceptible to galling
2) Generally require longer thread engagements
3) Are more susceptible to damage and fouling
4) Metric versions are generally more difficult to obtain.
On the neg

[olepedersen]

Surprised no one mentioned adhesives,they are used in both aircraft and automobile construction.just sayinghttp://www.cruisersforum.com/images/smilies/whistling.gif

[olepedersen]

TOO much glue on the smileys eh

[four winds]

Agree with Capt. Flrp concerning the greater strength of fine thread bolts compared to course thread of the same. They are ubiquitous in high load machinery applications.

However, I just inspected the gooseneck on my boat. It is secured with eight 1/4 or 9/32 inch course thread bolts. I removed one bolt and had to turn it back in after coming out less than one turn to clear the corrosion. Then it came out easily. I think they are course because of issues listed in the "on the other hand" part of Capt. Flrp's post.

Also noted that the top of the bracket is very slightly pulled away from the mast. Not much, but enough to notice. Probably would be smart to carefully remove it and clean up, coat, and reinstall the bracket. I suspect the threads in the mast have some distortion from time in service under load.

Personally, I would have no worries using high strengths rivets here either. For 1/4 inch they have a sheer load of around 700 foot pounds and there are eight of them. Granted, stainless bolts have higher load values. But given the corrosion present and suspected thread damage on mine I think rivets would perform well in this application. Maybe better in the long run given the lack of attention my gooseneck has endured.

My opinion only of course.

[Reefmagnet]

Originally Posted by Capt_Flrp

When I stumbled onto this forum, I was thrilled with the amount of knowledge shared and the lack of opinions stated as facts argued ad nausea. I do have to pipe up on this particular thread however. I would first agree with all preferring the rivet solution vs. threads of any sort. I do, however, have to point out the error in the fine/course thread argument going on. Assuming sufficient length of engagement, proper tap drill size, lubrication on taps, etc. etc., the fine thread is stronger than the course thread for the following reasons:
1) In tension (pulling against the threads) the stress area of the fine thread is larger, and in shear (pulling the fastened object along the adjacent surface) the minor diameter is also larger.
2) Fine threads have less tendency to loosen since the side of the thread incline is smaller and hence so is the off torque.
Other advantages include finer adjustment when needed (irrelevant in this ap), more easily tapped in hard materials and thin walls, and lesser torque required for equivalent bolt preloads.
On the other hand:
1) Fine threads are more susceptible to galling
2) Generally require longer thread engagements
3) Are more susceptible to damage and fouling
4) Metric versions are generally more difficult to obtain.
On the neg

Your #1 in neg/cons is why it's best to avoid fine threads in aluminium and austenitic stainless steels. Fine threads can be stripped with less effort because of the additional mechanical advantage they have over coarse when tightened. I mentioned previously that it is easier to cross thread a fine thread if not careful. When screwing into a thin, curved surface like a mast or boom, even more so.

Sent from my SGP521 using Cruisers Sailing Forum mobile app

[roverhi]

Fine threads don't work well in soft aluminum because the thread ramps are so thin they are especially susceptible to corrosion and abuse. Couple that with cross threading issues and it makes fine threads a big NO NO in aluminum. Look closely at the illustrations of thread that Sip Tie posted and it's very evident why fine threads are not a good idea.

The spar makers I'm familiar with use Monel or SS rivets if it's a piece that they really really want to keep attached to the mast. Soft aluminum rivets are virtually worthless except to attach non structural items like conduit. Stuff you really want to stay attached, use Monel or SS.

Like drilled, coarse thread tapped machine screws to attach things to the mast. Unfortunately it takes a bit of time to drill and tap for the fasteners, something that's anathema to production work. It's the additional labor costs of drilling and tapping that's the reason the the spar makers go with rivets where they think they can get away with it. Doesn't take a lot of skill to drill and tap that a good center punch can't provide.

Recently did a complete rebuild of the 43 years old mast on my boat. The coarse threaded SS fasteners that were undoubtedly originally installed without benefit of Never Seize, LanoCote, or TefGel mostly came out with a little heat and a small impact driver. The threads in the aluminum were/are still good despite all those years in a marine environment. The few fasteners that I couldn't get out were ones that passed through thick aluminum like a cleat body or winch pad. The non threaded part of the fasteners corrosion welded themselves into the thick bodies of the cleat or pad.

NEVER EVER USE SELF TAPPING SCREWS IN ALUMINUM. Those are the only threaded fasteners that have pulled out or elongated holes in masts that I've worked on.

[MarkSF]

I've been using McMaster Carr high strength domed rivets in 18-8 stainless.

McMaster-Carr

The "domed for wide thickness range" type has the advantage that the mandrel snaps off flush, so they are sealed (mostly).

1/4" ones are rated for 2250 lbs each, tensile.

I have used them for a rigid vang, which has seen a lot of stressful use, and a couple of other mast fittings. All I can say is, so far so good.

I installed them with Lanocote to, hopefully, avoid corrosion.

The rigger did remark on what a pain it was to drill out a couple that were in the way, last re-rig.

[estarzinger]

Fine vs course: "fine threads are stronger when the female thread is strong relative to the male thread, and coarse threads are stronger when the female thread is weak relative to the male thread". see . . . http://www2.mae.ufl.edu/designlab/La...-Fasteners.pdf

Increasing fastener diameter adds more strength than fussing about threads.

Heli-coils are useful if you will need to remove stainless fasteners from aluminum (and almost everything has to be removed at some point later). They also increase the thread diameter in the aluminum, so increase strength while maintaining primary fastener diameter.

You can actually weld to aluminum masts. One major european mast maker (Rondel) does most of their attachment points that way. Yea, it will effect the heat treatment of the mast wall, but if you design the weldment properly you can easily compensate for that. It is quite elegant when done properly.

Adhesives should work well, and Hall spars has build a number of masts (carbon and aluminum) with glued on mainsail tracks . . . but they do not You can screw into carbon, but dont if you dont know what you are doing. You usually should reinforce the local area, and isolate the fastener from the carbon.have a great reliability record, because of unanticipated peeling and shock loads.

[Cheechako]

Originally Posted by Terra Nova

Plenty of misinformation here.

Use coarse threads. Much stronger than fine threads or aluminum pop rivets.

Yep common knowledge. Depth of thread/bearing area is more on coarse threads, also, fine threads often gum up in threading in aluminum and are not clean threads.

[four winds]

Certainly I'm close to overstating my welcome in this thread. But just a couple more thoughts before I excuse myself.

If one has worked with high strength structural aluminum then weak or soft is not a word one would use. It's use in auto construction dates back decades. In frame and suspension members once exclusively steel.

Once I thought I could straighten a bumper reinforcement with a block of wood and a hammer. It was barely bent. Wore myself out with a full size sledgehammer with zero effect on the aluminum. Ended up strapping it to the frame machine and used 60k pounds of hydraulic power to finally fix it.

The advantage of stainless strength is not realized when mated to a thin aluminum spar's resistance to sheer in the small area of the threads. As evidenced by my gooseneck being slightly pulled away at the top. Assuming I'm correct in thinking the threads are partially sheered.

Certainly a mast isn't typical aluminum either. But it is intended to flex. Structural aluminum is very rigid, and behaves more like hardened spring steel than most realize.

The disadvantage of stainless being a dissimilar metal can be eliminated using sealed structural aluminum rivets. Once set the mandrel stub is contained so no hole. The sheer plane runs the length of the set rivet and is more than equal to the same in the spar.

Monel would be my second choice. Bolts of any type a distant third.

Of course I realize many, including some pro riggers, would disagree with me. I'm OK with that.

Interesting discussion.

[Terra Nova]

Originally Posted by four winds

...If one has worked with high strength structural aluminum then weak or soft is not a word one would use...

You would be better off sticking to something you know about.

[StuM]

Originally Posted by Sdwcheney

I have a Nonsuch 30 with a free-standing mast. You do not drill into a Nonsuch mast. The preferred method is to use stainless steel bands with rubber or plastic under the band. This is the strongest way to attach to any mast.

How do you hoist your main with bands around the mast?

[Catalpatiger]

I personally don't think tapping threads into a mast is not strong enough. As a rule of thumb threads tapped into alloy the wall thickness should be 1 and a half to 2 times the thickness of the diameter of the fastener. And steel is 1 thickness.

[four winds]

I think that is exactly the issue, Cat. The wall thickness, except on very large masts, is much less than one diameter of the fastener. Though I did forget to measure mine when I had the gooseneck bolt out yesterday.