After breaking my 2016 Chinook boom 18 months ago see Boom Slayer post and then breaking the brand-new Chinook boom replacements twice in a row this year, I ended up with enough unbroken boom bits and parts that I could attempt to rebuild one. Follow along with this 5-post series.

STEP 1: PREPARING PARTS
First, I assessed which one of the broken booms could be more easily fixed. The one that broke in the middle of the main body looked like a good candidate for splicing.



I cut the broken ends with a hacksaw and removed about 3/4" total. I also removed 6" of EVA grip with a scraping painter's tool while making sure not to gauge into the carbon fiber. I then used Acetone to remove the contact cement residue.


I had a couple of broken tailpieces from the last 2 broken booms which I cut about a 9" section to use as a brace between the 2 broken sections of the boom. The tailpiece diameter is almost perfect to insert into the boom. It just needed a slight sanding on one side to help it go deep enough inside the slightly curved boom in that area. I wanted the brace to go deep inside both sides of the broken boom to ensure good support.


I dry-fit everything together with the intact tailpiece fitted and locked onto the boom. I then measured the boom on each side from boom head to boom collars to adjust symmetry and find what exact gap I needed to maintain between the 2 broken sections of the boom. This ensures that I would be able to close the boom collars over the corresponding holes on the tailpiece.


Something I forgot to take a photograph of is the piece of EPS foam I forced inside the clue end of the broken boom piece to prevent the resin from sliding down the inside part of the tube and permanently gluing the tailpiece inside the boom during the next step.


Tomorrow: GLUING THE BRACE

Great post, was actually wondering if someone was going to try fixing those! Good detail on the foam plug to keep the resin out, I for sure would have forgotten that and then quietly taken the post down...

So if you are gluing the brace with the tail piece on, is there a particular tail setting that you are using? It seems to me that at short settings, the boom arms are being squeezed inward, and at long settings, less so or even outward force. Would these forces affect the boom tube joint, or is it better not to have the tail piece off when gluing?

Thanks, guys!
Good question, David. To be frank I didn't really think about this. You are one level above me here! Your assumptions are making sense. Maybe setting the tailpiece at mid-point might be the answer... I set the tailpiece at the shortest setting when gluing the brace. If the boom would crack again, I don't think it would happen at the repair site but rather at the boom clip bend. My last boom cracked right there and maybe the most vulnerable part of the boom since it seems to be flexing the most when the boom arms are spread out.
Check Next Step in the next post.

STEP 2: GLUING THE BRACE
I prepared a small batch of thickened epoxy (with silica) to a peanut butter consistency. I wanted the resin to bridge the thin gap between the brace and the inside of the carbon tube for a sturdy joint. I glued the parts while the tailpiece of the boom was in place and locked inside the boom on both sides to maintain proper alignment between collar prongs and corresponding holes on the tailpiece. I glued each side separately and oriented the boom accordingly while curing because I wanted gravity to work in my favor in having the epoxy filling as much of the inner space in the joint as possible. I also created a tapped dam around the joint to make sure I could add enough epoxy while keeping everything somehow clean.


During the last side glue-up, I placed a couple of small wooden shims to help the boom maintain the proper spacing between the 2 broken parts.


Once the resin was cured, I removed the tape+shim and placed a small piece of EPS foam in one of the visible holes of the tailpiece section I used as a brace to prevent the resin to flow inside the tube during the next step with the Carbon fiber layup.


TOMORROW: GLASS THE GAP

When you glass over the joint are you going to taper out stagger the marinating? Not sure if it matters here but we usually taper splice pieces to avoid a stress riser.

How many of these booms do you have now?

Another good observation, KM. The following post will provide the answer.
I had 3 broken booms; 2 are unrepairable and one has just been repaired!

STEP 3: GLASS THE GAP
Prior to glassing a few layers of carbon fiber, I tapered the joint to a smooth slope to strengthen the repair by maximizing the carbon fiber overlap on the repair. To manage a consistent taper around the joint, I started sanding 8 facets and then knocked down the facets' edges all the way around until I obtained a smooth conical taper.



I wrapped a single continuous layer of carbon fiber that I had fully wetted with epoxy onto a flat surface prior. The wrapping is a bit messy to do and looks a bit uneven with loose strands on the surface but that didn't really matter as long as there was no air or gaps between each layer of carbon.

How much fabric did I use? I approximated the number of layers, the length, and the taper of the carbon fabric so it would fill the thickness difference between the brace and the outer surface of the boom tube plus a bit more. Knowing that I can cover 2.5mm thickness with 8 layers of that carbon fabric, I calculated that I needed about 12 wraps to fill the gap with the first layer at 71mm circumference and the last at 91mm (remember C=2πr ?). I divided the difference between the small and large tube circumferences by 12 and added all the resulting circumference values for each layer to obtain the total length of needed carbon fabric. I then cut the fabric with a taper on both long sides to accommodate for the taper of the area that needed to be filled (Starting with the smaller end first).


I kept everything nice and tight by wrapping electrical tape around the carbon layup when it was still wet, starting from the center and out on each side to help excess resin to ooze out by the edges of the repair.


TOMORROW: FAIRING

STEP 4: FAIRING
After 24h, I removed the tape and revealed a tight but slightly uneven surface (probably due to the uneven force I used while wrapping the tape). This wasn't a big deal since I just needed to sand down the excess to regain an even tube diameter matching the original carbon tube dimension.


I proceeded to even out the surfaces the same way as I created the joint taper earlier: Sanding facets and carefully knocking down their edges to a smooth tubular geometry. I wasn't worried about sanding down a couple of layers of carbon as I knew there were still plenty of layers of intact carbon fiber doing their job below.



Then, I prepared some thickened epoxy to fill small pinholes to fair the surface.


Most of this fill was removed during the final sanding to reveal a smooth and uniformed surface.


TOMORROW and LAST: INSTALLING GRIP

This is nice work. Looing forward to seeing what you break/fix next.

KM wrote:
Ha! Thanks. My plan for 2022 is to break nothing at all!

STEP 5: INSTALLING GRIP

Happy Monday! Final installment of the Boom Fix series.

I ordered EVA grip from Chinook. Unfortunately, the grip they sell as a replacement is a tad thinner than the original grip they use on their boom (Why?!...). I cut a length of grip about an inch longer than I needed. I tapped the original EVA grip to keep things clean and brushed on a thin layer of contact cement on both the repaired boom section and the grip material.


After 20 minutes or so, the glue was tacky to the touch and ready. I carefully aligned the grip to the existing seam and methodically pressed down all around the diameter of the boom while making sure I didn’t trap any air bubbles. I used a worn and slightly rounded flathead screwdriver to press the EVA material along all the seams for a tight glue line.


I trimmed the excess grip material on all seams but not too close so I wouldn't cut through too far down by mistake. I then removed the protective tape at each end of the repair.


Finally, I used a 120-sanding pad to remove the excess grip material at the seams and sanded a bit more of the original grip near the seams to make smooth to the touch the transition between the different thicknesses of EVA.


Except for a break in the EVA graphics, the boom almost feels like the original under the hands. I haven't yet sailed it but if it holds, this saves ~$900 worth of equipment. Time will tell!