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Hi all,
The SS bimini frame on my sundowner has broken. It seems to have fatiqued (and fully snaped in two) through a rivet hole underneath a joint fitting. Would a method of repair to be welding the SS tube and then redrilling and fitting the rivet or is there a better method.

Thanks in advance Scott

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2005 Four Winns Sundowner (Sold)
350 MAG B1

Hi Scott,

I hope you're enjoying your Summer, the Weatherman is promising below 0 deg C, a bitterly cold wind from Siberia and snow here tomorrow!

If your frame has gone in one place, it may not be far behind in others. I don't know if the '05 bimini frame is the same design as on my '04 Sundowner, but on mine all the tube -fittings were loose (the tubes are visibly a smaller outside diameter than the coupling bores; the rivets were mostly tight in the coupling, loose in the tube, some were vice-versa, as if they'd been tightened up too quickly; probably by a pneumatic riveter). The whole frame used to rock and roll about, many of the holes in the tubes and some in the couplings became elongated after 2 seasons ( ca 85 engine hours), it would have not surprised me if the tubes were also getting fatigued in places due to the constant side to side motion.

Prior to the last season I drilled out all the rivets and drilled new holes in the tubes ( where coupling orientation allowed this); I then coated the coupling/ tubes with a 2-k toughened adhesive, before re-rivetting them. This has now eliminated most of the rocking, what's left is due to clearances in the ball joints. If you're taking the frame off, I'd recommend doing this improvement to all the joints. My only concern when doing this was that perhaps FW had relied on the sloppy coupling joints to allow the frame to fit the boat; thankfully it still fits with the more solid joints!

No reason why the stainless tube can't be welded. You may get some discolouration, a little blueing, due to the heat. That may well polish out. If you are redrilling/ re- rivetting yourself my advice is:
- Use a Cobalt steel drill bit ( you should only need the one, rather than several HSS ones!)
- Buy or borrow a "lazy tongue " rivetter, you'll end up with one forearm like Popeye's if you try to use a small hand rivetter on 4.8 mm stainless rivets.
- If you do use a pneumatic rivetter, use it on the lowest air pressure, so it sets the rivets slowly and evenly, without snatching.

I hope this helps.

Regards,

Graham

Graham,
Thanks for the info regarding the bimini, i will start to make some inquiries regarding getting it welded, my frame has noticable rock and roll as well but thought nothing could be done about it, my boat has 315 hours on it so it may be a little further down the track in terms of wear.
What is 2 K toughened addhesive? seems like a worthwhile improvement.
I drive with my bimini stored in the sock most of the time.

Regards Scott

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2005 Four Winns Sundowner (Sold)
350 MAG B1

Scott,

I wish our Summer weather had been good enough to put 315 hours on in 3 years!

When I wrote "2 K" I just meant a 2 component adhesive (abbreviation came from my work with German colleagues, K = Komponente). I used a toughened structural adhesive as I was given a few twin tubes by a customer who make vandal resistant seats for the Railway industry. They told me this material cured very quickly with no shrinkage, but that I'd never be able to get the joints apart again though. I didn't want to use something that would shrink or be brittle, in case it itself fatigued in time.

Something like "Araldite" epoxy adhesive, which is fairly thick when mixed, or even one of the higher modulus 1 component moisture cure PU sealants would probably also do the trick.(the PU would be tougher and it would have been what I would have used, if I didn't have the 2-K material, but again with the PU once cured you'd never easily get the joint apart again without using some pretty nasty solvents). You might want to use something like the "FC" (= "fast cure") version of 3M's sealants, to speed your process up. (I also heard that having an open bottle of ammonia near uncured PU sealant accelerates the cure rate, as it reacts with the isocyanate very quickly indeed, rather than waiting for water to do that! No guarantee though!)

Another couple of hints;

I stuck some narrow stainless steel spacers on the tubes prior to assembly, using a hot melt adhesive, on either side of the rivet hole to ensure the tube remained roughly in the centre of the coupling bore as the rivet was set. Otherwise, the rivet would have pulled the tube to one side of the coupling. These spacers don't protrude beyond the bore and so the adhesive forms a continuous bead all aroung the outside of the coupling bore. I could also have used stainless washers, bent to the right curvature, but I didn't think of it at the time! I figured it was better to set the rivet before the adhesive was cured, so as not to potentially introduce any stresses into the joint.

I ensured the rivet hole in the couplings had adhesive in it plus adhesive on the rivet, to ensure any ovality in the holes was filled.

Good luck.

Regards,

Graham

hey scott
still havent got to the trailer yet btw

i no longer drive with the bimini up, but leave it dropped down with somw padding to protect the seats. i had some loose rivets, but worse was the screws loosening where the foward bimini strut joins the hull ( ball joint mount). the puny self tappers kept loosening, i think from the wobbling that occured whilst driving with it up ( albiet in the sock, as u have it). I drilled out the holes, sealed them, and used stainless bolts with a plate and nyloc screws underneath.

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Hi all,

Had a bimini guy here today from the local marina says instead of welding he would put a ss sleave with a couple of extra rivets as if it was welded it may distort or the brace coupling may not fit properly over the weld.

Bimini guy is saying that there was a join in the tube from the factory but i still beleive that it was in one piece and fatiqued and broken
see second photo showing rivet hole location under location of corner brace conector

does anybody know the original status of this tube

Regards Scott

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2005 Four Winns Sundowner (Sold)
350 MAG B1

Scott,

Last Winter I took all the couplings off my bimini frame, including the ones in your photos (including the missing one from the failed area in photo 2). It looks very much the same design as the one on my 245. The tubes were all one piece, I am sure I would have noticed if two tubes were butt welded together or joined together a different way. Many of the holes drilled in the tubes were a not totally "clean". If your's were the same, there may well have been stress raisers somewhere around their radius that would allow fatigue fracture to initiate. I think the line on the tube above the break in pic 1 may well be where the coupling has been rocking against it.

If you are having the tube sleeved, might be worthwhile bonding that in (as well as using rivets)

Regards,

Graham

Scott,

Just a thought; it's probably worth your while to drill out the rivet on the coupling and inspect the tube on the opposite side of the frame, where it's the mirror image of where the failure occured. If the frame was rocking like mine did before modification, the rocking that caused the fatigue was probably side to side, so there may be a crack beginning there also.

Regards,

Graham