Surviving A Lee Shore

Hi Justin,

I generally try to avoid rivnuts if I can but acknowledge that they are sometimes the best solution. The beauty of them is that they provide threads in a surface that is too thin to otherwise thread. Of course, it is still effectively a blind rivet so if a similarly sized rivet would pull out of the material, this will pull out also. The real annoyance to me with these is that if the bolt ever corrodes/galls, then you can easily spin the rivnut in the hole and it is a real bear to drill one out, much worse than a rivet usually. Loctite is in order here for a few reasons but if you pick something too strong, you will find that you have to drill everything out just like rivets but that it will be even harder.

If you decide to look at drilling and tapping or also looking at the bolted part of the rivnut installation, there are 4 failure modes that you need to consider:
– The first one to check is always bolt tensile strength. This is simply a matter of the shank strength of the bolt versus the load on it. Bolts in stable joints are typically torqued so that they are at ~75% of shank yield strength. To prevent fatigue, you then need to make sure your maximum load is below this point (tension from torque is a bit inexact so you should leave some room here). There are times where you torque to lower values but it is because of one of the other factors.
– Bolt thread strength. A good rule of thumb is that on a coarse bolt, 5 threads engaged is enough (3 is actually usually full strength) and on a fine one, 7 is enough. This is easy enough to calculate too based on tension, thread stress area, number of threads and material yield strength.
– Mating thread strength. This is usually a much bigger deal as the mating threads are typically made out of a weaker material. If the material is the same or stronger, then the same rules of thumb apply but if it is weaker, you need to calculate it. On a mast, this is going to be your limiting factor, the wall is just too thin and it is a weaker material so you will be limited on the torque and corresponding tension you can apply. In these situations, you ideally have a locally thicker area of material or thread into something behind the material such as a nut with a proper backing plate.
– Head strength which can mean a few things. For something like a socket head cap screw, this is not a worry in a properly designed joint but for something like a low head, button head, or flat head allen drive, this is a big deal. Sometimes the head is too weak for the tension applied and basically snaps off. More commonly, you are limited in how much torque you can apply before stripping the head out.
While the calculations are not hard, I generally recommend to people that they use a bolt torque calculator such as http://www.futek.com/boltcalc.aspx which takes into account the first 3 factors. Once you have done it a few times, you get a feel for it and don’t have to explicitly check each factor each time.

For your application, I would recommend starting by figuring out what the max tensile load on the fasteners is. You can then run the numbers for whether screws could work (I expect number of threads into the mast is going to be your limiting factor). Also, you can look at rivets where you need to look at tensile strength and also the strength of the mast underneath them. Finally, with rivnuts, you need to consider both the rivet and the bolt calculations as you have both things going on at once. This is the work that I would need to do to feel comfortable with the answer on my own boat. As a sanity check, you should also make sure that whatever you come up with is stronger than your mainsail track system.

And putting a trysail on a 26’er means you are braver than I am.

Eric