Not to be obtuse but thin walled metal structures are better served with sheet metal screws than machine screws. Clearly rivets and rivet nuts are a more elegant and possibly stronger solution but you can't beat locktight and sheet metal screws for ease of use while up the mast. The course and large depth of the thread (only one is engaged BTW) offer a pretty good purchase. As tattooed to my brain in collage a sheet metal screw will develop the shear strength of the base metal *pi*diameter of the hole* the thread pitch. If memory serves aluminum is something like 30Kpsi and if we consider a 1/4" hole that would be 2355 lb/screw to pull it out IF the thickness of the base metal is at least as thick as the thread pitch. I'd bet the mast is that thick. That is ultimate strength so take a 1/4 of that for a safety factor (588 lb/screw) and locktight to keep it from working out.
Note that that is the force to pull the screw out along the screw axis not to shear off the screw. That is just the shear strength of the screw material*(d/2)^2*pi*thread pitch. Assuming the same size aluminum screw, that would be 147 lb/screw.
I guess my observation is that given you have enough screws they are much easier to use, are reusable, don't cause the corrosion that SS, and are plenty strong for mast work. The ease of use being key when up the mast.
C'mon Bill not one mast manufacturer I know of recommends sheet metal screws nor professional rigger. Over the years I have spoken with US Spar, Isomat, Charleston and others.
This is what Brion Toss has to say on the subject just so we have a "professional" opinion.
"Sheet metal screws are for, well, sheet metal, and even thin-walled spreaders are a bit thicker than that. So even for extremely lightly-loaded items like flag halyards we never even think of using sheet metal screws; if the metal is too thin to take a screw happily and here the ideal is a wall thickness roughly the same as the screw diameter then we'll use stainless rivets or, preferably, Riv-nuts."
Brion Toss likes stainless rivets inserted with Tef-Gel, fine threaded machine screws, with the proper thickness, and Riv-nuts. Rivnuts work fine but I don't know any spar manufacturers, except for some carbon guys, that use them and we don't see mast toppling over on a regular basis.
That being said I have seen self tapping sheet metal screws torn right out of masts before. There was a guy over on Sailnet a few months ago who even posted some pics..
I just installed a Harken Halyard restrainer on a Cape Dory 36 two weeks ago. Harken recommended fine threaded 10-32 machine screws or SS rivets. Being that this was at the top of a spar with the furler foil in the way my rivet gun would not fit so machine screws it was and the mast section was as thick as the 10-32 machine screws used. You need to know what your doing with the tap but machine screws but they can work very, very well in a spar provided the thickness is there. My preference is SS rivets then fine threaded machine screws I have not found many situations where a Riv-Nut would have been required but they certainly exist...
I have seen masts put together with both SS rivets and fine threaded machine screws and both handle longevity just fine. Our 1979 Proctor spar was built using SS rivets. Not a single one is loose or failed.
I suspect Johns friend may have used aluminum rivets or simply installed them incorrectly. You need the right SS rivets, when are not cheap, and the right rivet gun.
