Sleeving the Mast

A small, shallow crease was somehow formed on the front of my mast just below the partners. This crease was present when we bought Maybe. Perhaps the mast was dropped or the base slipped when stepping or unstepping it? After closer inspection, small hairline cracks were radiating from the crease in an outward direction. It was easy to see that the mast could fail if left alone.

The big question I had was how should this should be repaired. The initial thought I had was to have the crack TIG welded. Researching the web resulted in several so-called "experts" advising against welding saying that it would "remove the temper." Short of having an annealing oven, it sounded like a T4 temper was the best result I could expect from localized tempering. Short of the original T6 temper didn't sound too exciting to me. Plus the weld process could be too easily screwed-up if the wrong rod was used. I wanted to see if there was a "better way" available.

Consulting with Charleston Spar (Sparcraft), they recommended sleeving the mast from the inside using the very same J/24 (DM-375) mast section. Sleeving a mast is a common method for joining spars over 50'. Spar length is usually limited by the length of the temper ovens. With this lead, I again searched the net finding surprising little on the topic. One site that was most helpful is Brion Toss' SparTalk.

I assembled a short list of sleeve assembly specs as follows:

1. Splice plate(s) must fit absolutely tight against the repaired spar (cut the spar in half if needed)
   
2. The splice plate should be at least 3x the chord length
   
3. Use stainless steel machine screws (flat head undercut) of sufficient length to clear the innermost section
   
4. Drill, countersink, and tap the holes approximately 4x the diameter from the edge, hole, etc. and then 8x the diameter of the machine screw thereafter
   
5. Stagger the screws in the adjacent rows

I cut a small section off the sleeve piece and then cut the back channel off for a test piece. Sure enough, the fit is perfect if the front most portion is secured first.

After confirming the snug fit, I then cut a 4x chord length (~20"), exceeding the 3x recommendation. Going beyond this length could affect tuning.

Since the sleeve was still a single piece, it had to be compressed and pounded into the mast. WD-40 was used to prevent galling and help the mast slide easier.

Next, I proceeded to mark the frontmost portion of the mast and center punched each hole to keep the drill bit from walking.

Based on the thickness of the spar and mast, I chose to use a #8-32 UNC thread SS machine screw. I first drilled 3/32" pilot holes, followed by an undersized 1/8" hole, counterbore, and a good quality tap and some thread cutting oil. Using a slightly undersize 1/8" bit rather than a #29 bit for a #8-32 tap is adviseable with soft materials such as aluminum.

Run the counterbore slowly to prevent sharp edges forming on the counterbored holes. Also use a stop collar on the counterbore to control the depth. The finished product results in a near flush mounted screw.

Practice drilling, counterboring, and tapping of a leftover piece of spar to get the process down before attempting it for keeps. I found it nearly impossible to strip the Phillips head screws. There is incredible clamping power in those little countersunk screws!

Remember to start at the frontmost part of the mast to pull the sleeve tight against the mast and work your way outward from there one row at a time. It's also adviseable to drill a small hole at the extreme ends of a fracture to prevent a crack from propagating.

Working out from there was tedious. Mr. Miller soon helped out and made the job easier.

I used 1" spacing between the screws alternating the location.

After installing all the screws, I used some brake cleaner to remove the thread cutting oil from the suface. Then removeable thread lock was used to help seal the small space between the aluminum threads and the stainless steel screw.

The result was a professional looking repair that should adequately hold until I can replace the mast down the road. I've not seen any degradation in the repair after several races this year. If something does go wrong, I'll be sure to post back to this site.