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 Post subject: H17 Crossbar Cracks
PostPosted: Sun Feb 07, 2016 9:34 am 
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Site Rank - Old Salt

Joined: Mon May 09, 2005 10:25 am
Posts: 3533
Location: Jersey Shore
Last year when I pulled my (2007) boat apart, I discovered cracks in the ends of the crossbars. The cracks were present at all four corners (front & rear, port & starboard). The cracks were very similar to the pictures posted by fastcat, although not quite as bad yet.
Image

I was fortunate in that I had spare front & rear crossbars, so I could still get the boat on the water. I took the broken crossbars to a welder and had the cracks welded and ultimately ended up using my spare front crossbar and the welded rear crossbar (I wanted to use the rear crossbar if possible since it has the newer traveler car system). But when I took the boat apart this fall, the rear crossbar had cracked again at the welds.

Looking at the crossbar, I think there is a flaw because the design of the tramp track causes a stress concentration under torsional load which makes it very difficult for the crossbar to resist torsion without spreading open (the 17 crossbar profile is different than other Hobies and not strong enough to withstand the "hoop stress" generated by the hulls trying to twist relative to one another). This cyclical spreading of the crossbar ultimately causes the crossbar to crack right next to the tramp track. In fact, I mounted a crossbar end casting to a 2 foot length of 2x4, inserted it into the crossbar and twisted and the crossbar spread open pretty easily.

So, the solution I'm going to try will be to add a stainless steel band around the outer edge of each end of the crossbars. It is a 5/8" wide, 0.025" thick band clamp held in place with a 10-32 machine screw. The hope is that that this will will add enough strength to the end of the crossbar to keep it from spreading open and cracking (or continuing to crack). The only issue may be the fit into the hull which could be a little tight due to the extra material, but I did a trial fit with a small section of crossbar and was able to get it installed. I'll update on the results after I put the boat back together this spring.

Image

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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Sun Feb 07, 2016 12:29 pm 
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Joined: Sat May 02, 2009 5:22 am
Posts: 585
Location: Columbus, Indiana
I usually drill a small (1/8") hole at the end of the crack before I weld damaged aluminium. Weld towards this small hole and use a heat sink if possible. That should keep the crack from spreading as you weld. Sometimes, I will form a thicker piece of aluminium that wraps around the tubing to act as a heat sink to reduce the chance of deforming the tubing.

This extruded anodized aluminium has been stressed in the forming process and can be a real pain to get a good weld. Don't sand or grind down the repair as that will weaken the weld. Leave it proud. Good luck! :)

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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Thu Feb 11, 2016 12:20 am 
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Joined: Mon Sep 20, 2004 12:36 pm
Posts: 788
Location: Tri-Cities, WA
Your crack looks very familiar. I have cracked 4 front cross beams and one rear one on my 2000 Hobie 17 Sport since buying it in 2002. I replaced the beams with new each time, but that is a bit expensive (though one was under warranty so the replacement was free). I've had my three best front beams and the cracked rear beam welded then reinforce with marine epoxy. Hopefully this will get me through the next few seasons.


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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Thu Feb 11, 2016 3:48 am 
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Joined: Mon May 09, 2005 10:25 am
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Location: Jersey Shore
I thought about quite a few repair methods and I doubt welding or epoxy are viable long-term solutions. Even if this brings the crossbar back to original strength, we know the original design is only marginally strong enough to hold up and would likely crack again.

That's why I'm hoping my band clamp solution works. It is a stronger material (stainless steel) than the original (aluminum), and much better geometry for transferring the torsional "hoop stress" across the tramp track area of the beam. As long as I can get th crossbars fitted to the boat, I think this is going to be a solid, permenant, repair.

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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Fri Feb 12, 2016 7:03 pm 
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Joined: Mon Sep 20, 2004 12:36 pm
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Location: Tri-Cities, WA
Time will tell, I break new beams about every 1.5 years. The weld was to bring beam up to original strength, the marine epoxy is the added strength.
On your band fix did you have to move the end knuckle up to accommodate the bands extra width? and put an electrical barrier between the dissimilar metals to avoid galvanic corrosion? I have 2 welded backups incase my fix doesn't work out, then hopefully yours does.


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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Sun Feb 21, 2016 12:51 pm 
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Joined: Mon May 09, 2005 10:25 am
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Location: Jersey Shore
I considered epoxy, but I really don't think it would add any significant strength, especially without any reinforcement. It can also be quite a challenge to get a good bond between epoxy and aluminum.

To answer your questions, no, I did not move the end casting at all. However, I did a test fit this weekend and there was no issue installing the crossbar into the hull/casting. There was enough clearance to slide it on even with the addition of the band. To prevent galvanic corrosion, I applied a film of Tefgel grease to the band before installing. I disassemble my boat every winter, so I should have a pretty good idea after one season whether or not galvanic corrosion is a problem.

Image

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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Sat Jun 11, 2016 10:55 pm 
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Joined: Mon Sep 20, 2004 12:36 pm
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Location: Tri-Cities, WA
Well the epoxy reinforced weld held for about 2.5 months (~200 sailing miles) before re-cracking last week. The bar re-cracked next to the weld and the epoxy failed at the epoxy/aluminum interface. So I changed out to a spare crossbeam and added an end bracket (knuckle) I fabricated to the top ends of the forward beam attaching each with 3 - 1/4 20 SS bolts. https://www.facebook.com/photo.php?fbid=258182977879106&set=a.145239742506764.1073741829.100010622173515&type=3


Last edited by fastcat on Fri Dec 23, 2016 6:46 pm, edited 1 time in total.

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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Sun Jun 12, 2016 6:06 am 
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Joined: Mon May 09, 2005 10:25 am
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Location: Jersey Shore
What's the thought with this bracket/bolting? Trying to get the end casting to better share the torsional load?

I believe the reason the crossbars crack is because the profile of the crossbar is not good at handling the torsional load as the crossbar tries to twist (when one bow tries to go up and the other goes down). The groove for the tramp track basically tries to spread open and the crossbar doesn't have enough material/a generous enough radius around the track section to allow the load to flow around this area. Hence we see crossbars cracking right along the edge of the tramp track.

The strap I added goes completely around the extrusion and stops the crossbar from spreading open. If you lock a piece of crossbar in a vice and then mount an end casting to a piece of 2x4, you can easily flex the crossbar open by hand. Adding the strap makes it considerably more rigid. Not sure your solution will achieve the same thing, but I guess time will tell.

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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Sun Jun 12, 2016 11:02 am 
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Joined: Mon Sep 20, 2004 12:36 pm
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Location: Tri-Cities, WA
Yes sharing the load is the intent to put both the bottom and top of the crossbar under load. The bolts are bolted into tapped holes in the top of the aluminum end casting. The bolts are in at angles so to decrease stress on the threads. I took the boat out yesterday in 15 mph winds and put her through 25 miles of wave bashing averaging 11.2 mph with lots of hull flying. So far so good. The slight creaking of the beam has gone and it felt real solid. If this solution doesn't hold up to the test of time, I can always add your fix beneath mine, though I'd make the strap a bit wider to accommodate the 3 1/4 inch holes.


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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Fri Dec 23, 2016 6:45 pm 
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Joined: Mon Sep 20, 2004 12:36 pm
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Location: Tri-Cities, WA
UPDATE: Yahoo, the fix worked. After the mid-season front crossbeam replacement and knuckle addition (see above), I sailed the boat about another 500 miles in 5 - 25 mph winds averaging over 8 mph with top speed of over 21 mph (i.e., I drove the boat hard). :D Took the boat apart for winter and the front crossbeam repair weld held. The beam looks great. I'm thinking of adding the 'nuckles' to the rear bars also. They don't get the stress the front one does, but why not tighten up the boat even more.


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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Tue Jan 30, 2018 1:56 am 
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Joined: Fri Jul 28, 2017 2:16 pm
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Another update: the front crossbeam survived another sailing season with no cracks. This crossbeam was a welded repair and with the "knuckles" has now survived two seasons now. Previous new and repaired beams lasted 1-2 seasons. So I'm quite pleased.


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 Post subject: Re: H17 Crossbar Cracks
PostPosted: Tue Jan 30, 2018 5:38 pm 
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Joined: Mon May 09, 2005 10:25 am
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Location: Jersey Shore
Mine’s been holding up as well.

Two similar methods for achieving the same thing. Yours couples the top of the crossbar directly to the end fitting so that any torsional load on the crossbar becomes a shear load at the screws you added rather than trying to rotate the crossbar against the end casting, spreading it open. Mine simply adds hoop strength to the end of the crossbar so the crossbar can’t spread open as it twists against the casting.

In the end, if it keeps the crossbar from cracking, it’s doing it’s job. I would bet there are a lot of cracked crossbars out there that just haven’t been seen because most people don’t disassemble their boats every year.

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