Fixing leaky mast

Thread starter #1
Hello fellow Sunfishers.

With the rudder all done, I decided to tackle some other SF issues, and first on the list is the leaky mast.
This is the second time I have tackled this job. The first time was only three or four years ago, and although I thought I was careful taking things apart and then putting them back together with Sikaflex as a sealant, the mast now leaks at both ends. It does not leak a lot, but enough to bug me into fixing it.
Thread starter #2
So the first thing to do was to remove the end-cap at the base of the mast, which was fairly simple. I just knocked the pins into the cap using a nail set (picture #1) and then carefully used a knife to break the seal between the cap and mast (picture #2). For some reason I did not take a picture of the cap almost off, but working the knife around you can pry the cap up enough to pull it off with your hands. One thing to be careful of is to make sure the pins are in far enough that they are no longer engaged with the mast.



Active Member
Try using 3M 4200 next time. Both are polyurethane sealants but Sikaflex is
listed for concrete use. The 3M product lists 300 lbs. per square inch
adhesion so it should keep the caps from shifting and breaking the seal.
Thread starter #4
The next step was to take the masthead cap off. If you have a look at picture (3, 3A), you can see the wear from the halyard. I put this cap on when I first got the boat, and I have been surprised at the wear rate. I have since bough a smoother line for the halyard.

I used a 1/8" drill bit to drill the 1/8" aluminum rivets out. The first rivet gave me no trouble (pictures 4-6), and you can see the rivet happily traveling up the drill bit in picture 6. The other side however (pictures 7-9) gave me more trouble. For whatever reason the drill decided to drill off center which you can see in picture 9. I did not want to end up with an oblong hole in the mast, so I punched the inside of the rivet with the nail set (picture 10) and then carefully drilled the rivet head to as close to the mast as I could (picture 11). I then pried off the rivet head (picture 12) with a knife, and punched out the remaining piece of the rivet with the nail set (pictures 13-14) leaving an only slightly irregular hole. I then removed the cap using the knife to break the seal and then carefully pry it up far enough that I could twist it off with my hand leaving me with a cap with a lot of residual Sikaflex.


Thread starter #5
Try using 3M 4200 next time. Both are polyurethane sealants but Sikaflex is
listed for concrete use. The 3M product lists 300 lbs. per square inch
adhesion so it should keep the caps from shifting and breaking the seal.
Hi Webfoot1,

That's interesting. Reading from the product data sheet (Sikaflex 291) under product benefits I get:

Use above and below water line
- Resists salt water
- Fast strength build-up
- Excellent adhesion to gelcoat,
fiberglass, metal and wood

- May be squeezed or brushed into
- Stable
- Paintable
- Versatile packaging
- Excellent bond
- Fast tack-free time

nothing about concrete.


Active Member
Opps, I was looking at the self-leveling Sikaflex. Anyway, the caps should
have stayed sealed. Looks to me like the tight fit of the cap squeezed/scraped
the sealant away. I'm sure you'll get lots of recommendations for sealing the
mast with everything from expanding foam to inserting toy rubber balls. I've
always used 3M 5200 but then again maybe I don't sail as much as other people.


Well-Known Member
Now having five masts, I'm not sure how many are "sealed". :confused: Night-time condensation seems to find its way into just about everything anyway. :rolleyes:

Did Sunfish manufacturers' "seal" Sunfish masts? :(
Hi, I have successfully sealed one up and it has worked well for a few years. I took off the caps on both ends. I then took racquetballs, and using a liberal amount of silicone, glued one into each end of the mast, then put the caps back on. No water after several years of use. The reason I put the ball in at the top as well as the bottom is that the water cannot get in if the air cannot get out, so I tried to make it as water and airtight as possible.

I want to do this on my other mast too, but it is newer and has one of the internal reinforcing sleeves in it, and the raquetball won't fit in the base. I could probably find another ball to work, but haven't found one yet.



Active Member
I figure if I'm turtled or capsized for all that long a leaky mast is down the list of issues. I think it's pretty watertight though.
I figure if I'm turtled or capsized for all that long a leaky mast is down the list of issues. I think it's pretty watertight though.
You don't need to capsize for your mast to take on a lot of water. As water washes over the deck and its gets into the mast step, the pounding action of the hull in chop seems to drive water into the mast. At the worlds this year, most of the fleet had the dealer running the charters drill holes in the caps to get the water out. It was windy and wavy and those are the perfect conditions for getting a few pounds of water in your mast, which is weight you don't want in a place you don't want it.


Active Member
Years ago we had a thread for someone did the maths and calculated the mast can hold two milk jugs full of water. I got thread bombed with people telling me it did not matter because the weight of water in water is zero? They could not seem to understand you're replacing something 400 times lighter (air) with water. Anyway, there have been new sailors here who can't get their Sunfish righted because of excessive water in the mast probably joined to other factors. I'm thinking that any water in the mast above the waterline raises the Center of Gravity which ain't that cool in a weight shift boat. Water sloshing is sort of like having a movable jug of water sliding up and down the mast.


Well-Known Member
Never had one sealed. Have always drilled a hole in bottom mast cap and let it drain.
Some recurrent intrusion would take place as the mast step fills with "splash-water".

While that's not a whole lot of water, it will slosh to the top of the mast in a capsize—exactly where you don't want it. :confused:

Racquet balls are heavy (where you don't want weight), so why not seal one half at each end, and retain the hole in the bottom cap? :)
Thread starter #16
Hello fellow Sunfishers,

Thanks for all the comments.

Back to my project, looking into the mast now that the caps are gone is a mess, so the first thing to do is get rid of the old Sikaflex. I first used a razor blade to gently cut/scrape most of the sealant out/off. It still didn't look all that great, so I used 3M caulk remover that was left over from a a bathroom project. I wasn't so sure it would work, but why not try? It took two rounds, but the remover worked, and when I was done, the mast looked pretty good.



Well-Known Member
. A racquetball weighs 1.4 ounces so I am not concerned about the weight.
Now that I think of it, a damaged (by cutting) racquet ball would probably allow water to seep through it. :oops:
Interesting, that a racquet ball has also been used to stop a driveline clunk in GM driveshafts. GM Slip Yoke Repair - Racquetball Method - HOW TO / TUTORIAL

Sunfish fleet use might consider used (bulk) racquet balls (eBay).

With the help of WD-40, and a bridle made of tape, I pulled an even lighter—and free—swim noodle through a mast. Re-pinned the caps on, and forgot about it. :cool:

While it was the ultimate "anti-sink" solution, it may have been sold-off with my Porpoise II. :(

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Thread starter #18
Hello fellow Sunfishers,

My Sunfish mast originally had plugs at each end, so I decided to go that route again. The original plugs were cork, and I was tempted to order cork online, but in the end I decided to make mine out of white pine (~7/8") which I have plenty of.

The mast is 2 1/4", so to get the right diameter plug I used a 2 1/4" hole saw mounted in a drill press without the pilot drill. I figured the walls of the mast and the gauge of the hole saw were pretty close, and I would end up with a plug the right diameter. The first plug I cut was a bit small, as I did not clamp the board to the table and the hole saw did not cut cleanly. On the next attempt I clamped the pine board to the table and the resulting plug was nearly perfectl. I use a four in hand rasp to round over the edges so that the plugs would not splinter when being tapped into the mast.

I decided that the plugs would expand and contract less if coated with epoxy, so that's what I did. I put small screw eyes in the plugs and hung them up with pieces of coat hangar to cure after coating them with MAS epoxy.

Once cured, I tapped them into place with a dead-blow hammer using the original too small plug as a buffer. I set the plugs just a little past where the end cap ends. I used Sikaflex to seal the plugs in place.