A few thoughts:
- As mentioned, these should be through-bolted, whether with a bushing or not.
- Drilling out a larger hole than was previously there can weaken the boom but only under certain conditions. A short description of what makes a spar strong ....
- Spars (such as a boom) use monocoque or semi-monocoque construction to derive their strength. An aluminum can or an egg are great examples of monocoque construction - it derives its structure from the integrity of its thin skin. If you upset that integrity by putting a hole in it or bending it, the strength is greatly compromised. Semi-monocoque usually implies some additional structure that lends its strength to a monocoque skin. Most airplanes and modern cars are like this.
- An spar (such as are used on these boats) is a semi-monocoque structure in that it relies on the "shell" for most of its strength, but the rigidity of the material itself (as well as some internal non-skin structure) makes the integrity of the skin not so critical. This allows a spar to fail partially and not necessarily dramatically.
- Monocoque construction is pretty much all-or-nothing in terms of structure integrity. When it fails, it fails magnificently. Stand on an aluminum can, and it will hold you up. Pinch the sides slightly and it collapses entirely.
- When you drill a hole (any hole), the spar is already in partial failure. But ...
- When you put something else with the same compression strength (or greater) in that hole with no gaps, most (but not all) of the strength is restored. (There is still the issue of the affected section having decreased resistance to elongation.)
- Monocoque (and semi-monocoque) construction relies primarily on materials resistance to elongation and compression.
- So, drilling a larger hole is OK if you fill that hole with a tight-fitting bolt. However, the spar is only as strong as it was previously with regard to its resistance to bending in the direction of the side with the hole. Make sense?
- So, actually, the BEST solution is one that is rarely used, but one which I have seen used. And, it is brilliant. This is something that, in my opinion, should be just standard practice ....
I'll describe it for a boom bail for a boom vang (for example) but it applies to any spar attachment. It goes like this:
- Re-drill the existing hole slightly larger and file the edges smooth. (This actually improves the strength.)
- Go to onlinemetals.com and buy a flat bar of 6061 (or possibly 7075 if you want to be hard core - though it "might" be more susceptible to corrosion and need replacement at some point). (I'm not affiliated with onlinemetals.com, but used to moor just down the road from then and they are GREAT to work with and happily do small orders and custom cuts.) Get a strip that is as wide as will lay flat on the spar, and as long as you can tolerate, though about 18" is fine. Using a longer strip allows you to add other holes later without much drama. I like 1/4" flat bar. File the outward-facing edges and corners nice and rounded for safety.
- Affix the flat bar to the spar using ample machine screws (pay attention to corrosion properties), staggered along the bar. Staggering is important to maximize strength. Make sure you mark the center of your original hole so you know where to drill and so you don't put a machine screw in the wrong place. Machine screws just need to be long enough to penetrate the flat bar and the spar wall completely.
- Now, drill your new hole through the flat bar. (Presumably you are doing this on both sides of the spar, so be sure both holes line up obviously.
- Remove the bar and file all edges smooth. This is surprisingly very important for strength and longevity.
- Reinstall the bar.
- Install a slightly-larger bail (or whatever) since the area is now built up 1/2" thicker (if using 1/4" flat bar). Using graphite on the bolt will decrease any tendency for the bolt to wear the hole larger over time (though this won't be a problem if the bolt fits snugly).
Why do it this way? Well, this is really intended to beef up standing rigging to perform in an environment to which it is not designed. But, the reason I like it is because it GREATLY improves the strength of the spar at the point of strain. Plus, the bar is replaceable. So, if it starts to wear or you need to relocate a hole, this is easily accomplished. You can actually drill quite a few holes in the flat bar before weakening it to the point where it is not stronger than the spar to which it is affixed.
A friend actually did a clever thing, I thought. Keep in mind, I have NOT done the math on this and have NO idea of whether it is stronger, weaker, lighter, or heavier than the original boom. He affixed a single 2" wide strip of 0.1875" thick 6061 aluminum to each side of the boom. This adds a bunch of weight, so he actually drilled all the way through the bars and the boom every 6" (I think). He put a thin sheet of plastic between the bar and the boom so that any unused holes were not inviting homes for bugs and such.
Now, he can actually run a bolt through any of these holes to attach anything to his boom he wants at any time. At first, I thought this was excessive and unnecessary. (Well, I guess I still think so.) But, the reason he does this is because he uses his boat for racing, for casual cruising, and ocean-crossings. This system gives him great flexibility to change up his rigging according to conditions. (He also has two travelers - one over the companionway and one in the cockpit.)
Anyway, I digress.
The point is, the flat bar trick is a great way to take a worn spar and improve its strength past its original strength, all while affording a number of other advantages.
