Removing holes from aluminum spars

One vital consideration concerning aluminum spars is that they derive all their strength from skin stiffness.  Aluminum can and will bend; but as the molecules shift about it's important to not give them anywhere to go that they should not be.  This means all extra holes in an aluminum spar will weaken it.

My boat came with the then-common 1970s slab-reefing system consisting of a genoa track mounted on the boom, a cheek block mounted on a slide, and a Cunningham hook above the gooseneck.  The single-line reefing system as depicted in the Schaefer catalogue and elsewhere makes infinitely more sense, not only for sail trim but for simplicity and even safety.  Properly located, the parts of single-line reefing will contribute to a very efficient sail shape for the reefed sail and give one the ability to effect the reef from only one place-- in my boat's case, the safety of the bridge deck and main hatch.

To remove the parts of the old system required the filling of some two dozen holes on the boom and elsewhere.  Certainly one could weld these and have an excellent-- though expensive-- reconditioned boom.  Honestly I never thought of that; but I knew I'd have to fill them somehow.

Then Jeremiah, in the shop, suggested just using aluminum screws, threading them into the holes, and breaking them off to file them smooth.  The sheer simplicity of this idea just blew me away.  After all, so long as the holes are filled with something that can't shift-- thus prohibiting the molecules from moving too far-- it does not matter how you fill them.

Amazing Grace, the C48, came into the shop for a refit and I got the job of preparing the booms for refinishing.  As part of the job the aluminum screws were ordered; as soon as the necessary holes were filled I took off with the broken-off bolts and reused them all on my own boat. After all they only need to be long enough to thread into the wall thickness of the spar.


This first photo is not great, owing to glare; but the aluminum screws sticking out of the side of the boom (spar to the left) are visible. Most of these were for the reefing track.

The black tape marks where the sheetline bail will go.

In this photo you can see where a few of the screws have already been broken off.  I sawed most of the way through each one and then bent them with pliers to snap them off.  These screws are 1/4-20 thread.  They can break off when you're about 2/3 through them with the hacksaw.  The best idea is to saw them as close as possible to the surface of the spar-- but you'll have to choose one thread of the screw as the hacksaw blade will choose one for you otherwise.

The red stuff is Loctite 'permanent' (not my blood; that's elsewhere).  A caveat: the Loctite will work only if the hole is the right size for the screw and decently threaded.  Don't count on the Loctite to fill gaps (or even to stick if there are gaps).  If there is too much play, you can use epoxy (perhaps with a little filling compound) instead.

Here is the side of the boom after the screws were broken off and sanded over.  Can you see where they were? --for you certainly can't feel where they were with your hand.

Here is a close-up of the side of the boom, with a better view of the filled-in screw holes (silver circles).  I sanded these by hand-- 80-grit production paper takes the nubs down quickly and easily.  You will see a trace of greasy-looking aluminum residue following your sanding-- this tells you that you are actually sanding the end of the screw.  When you see the surface of the boom going silver you've come to the end (as you see how I hit it here).

I changed to 120 grit and then to 220 to polish these off.  As I will shortly be applying etcher, primer and paint I'll be able to further fair these places, should they need it, at any point during the future stages.




I filled the holes for the winch bases on the mast as well.  Since those holes were a bit too big to accommodate 10-24s any more, I drilled them all out to 1/4-20 (I should have used 12-24 but didn't think of it!).  In installing the winch bases on the mast again, to avoid running afoul of the old (now filled) holes, I can relocate them.  The open holes shown here are the new ones drilled to reuse the old mast bases in new locations.

(The pole in the back is the fence, not the spreaders!)

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