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Hobie 18 Rear Beam Corrosion
http://www.hobie.com/au/en/forums/viewtopic.php?f=13&t=53984		 Page 1 of 1
Author:  damon [ Sat Feb 21, 2015 1:28 pm ]
Post subject:  Hobie 18 Rear Beam Corrosion
The rear beam on my 1992 Hobie 18 has really bad corrosion at the inner beam casting position to the point that it is nearly divided in two on the starboard end.

After I noticed it I checked out a lot of Hobie 18's of much older vintage and didn't find any of them with serious corrosion issues, even some that were kept on salt water beaches year round for 30+ years.

My boat only sails about 7-8 months a year about 70% in fresh water and is kept on a trailer unless I'm at a regatta.

Anyone ever seen corrosion this bad? I'm assuming this is still under warranty right?

More pictures.
http://www.thebeachcats.com/pictures?g2_itemId=112954

Image

Image
Author:  SabresfortheCup [ Sat Feb 21, 2015 2:16 pm ]
Post subject:  Re: Hobie 18 Rear Beam Corrosion
Warranty :lol:

You have two things going on there. The first is some serious galvanic corrosion, the second is a stress/fatigue crack. My bet? You had something made of mild steel that repeatedly sat in that position and came in contact with the rear crossbar in the track. Maybe someone used mild steel bolts/cotter pins or zinc coated bolts in place of stainless on the main sheet traveller, or you had something else... Nut/bolt/washer stuck in the track for a while, and collected salt water. Because aluminum is more reactive than steel, the aluminum corrodes twice as fast and keeps the steel from rusting. Ships place chunks of aluminum on their hull to keep the steel hulls relatively rust free underwater for this exact reason.

Once the aluminum in the track became thin enough, the crossbar developed a small stress crack originating at the thin section. Once started, it continued to propagate around the crossbar. Unfortunately, there's really no saving that one. You might try to weld it, but being that it's heat treated aluminum it will be significantly weakened. But the rear crossbar has significantly less stress than the forward one, so maybe it can handle it? That's for someone else to say, I'm sure someone's tried it.
Author:  damon [ Sat Feb 21, 2015 2:50 pm ]
Post subject:  Re: Hobie 18 Rear Beam Corrosion
I am definitely replacing the beam, since it's a 92 and everything has been upgraded/replaced over the years I'd like to find a really nice condition late model black beam instead of the multitude of old ones out there. Think I have a line on one but the owner has been snowed in up north lately.

The only thing I can think of is the stainless (steel) bolt that goes into the aluminum wing mounting plate. That is a steel to aluminum connection for sure. All winged boats have that so maybe something for other H18's with wings to check.

But wait until I find my new beam before starting a run on replacement beams. :lol:
Author:  srm [ Sat Feb 21, 2015 6:17 pm ]
Post subject:  Re: Hobie 18 Rear Beam Corrosion
Actually, mild steel is less galvanically reactive with aluminum than stainless steel. Anyway, it definitely looks like the crack originates from the outboard rivet on your wing bracket. A little salt water sitting in a crevice for a couple months at a time is all it takes for corrosion to develop. I would say this beam is toast. When you install your replacement beam, be sure to coat all stainless steel brackets and fasteners in tefgel anti-corrosion paste. Rinse everything really well after sailing in salt water and it should give you many years of service.

sm
Author:  srm [ Sun Feb 22, 2015 8:03 am ]
Post subject:  Re: Hobie 18 Rear Beam Corrosion
Looking a little more closely, the crack starts at the lower outboard rivet for the wing bracket. The crack then looks like it runs along the edge of the crossbar bolt reinforcement bracket (inside the crossbar). So really what you have is a couple things going on here that helped contribute to the crack growth - galvanically dissimilar materials (the SS rivets and the aluminum crossbar), a stress riser (the edge of the crossbar reinforcement bracket), and loss of material (an extra hole in the crossbar for the wing bracket right in the vicinity of the edge of the crossbar bracket).

I think if I were you, I would consider relocating the lower outboard rivet on the wing support bracket. I would probably move it inboard (towards the boat centerline) about 1/2". This would help to move that rivet hole away from the edge of the crossbar bolt reinforcement bracket.

sm
Author:  damon [ Sun Feb 22, 2015 10:58 am ]
Post subject:  Re: Hobie 18 Rear Beam Corrosion
Well at least I can confirm that the Hobie 18 is hugely strong (or overbuilt) because the pictures of the damage were taken before the picture in my signature was taken. :o
Author:  OlderBowman [ Mon Feb 23, 2015 8:51 am ]
Post subject:  Re: Hobie 18 Rear Beam Corrosion
I'd agree that the crack happened first and corrosion came as a result.
The anodizing on the beam is only surface treatment. Once salt gets to untreated aluminum, corrosion starts.
Author:  moncasta [ Mon Feb 23, 2015 7:41 pm ]
Post subject:  Re: Hobie 18 Rear Beam Corrosion
I'm curious when you take it off, if the inner bracket has failed yet, the part that the stainless steel bolt goes into. It can crack and fail, but the boat still mostly holds together, but is much less rigid. That could lead to more movement inside the crossbar and rubbing in addition to the corrosion. In the couple of failures I've heard of there wasn't' any cracking in the crossbar before the broken bracket was discovered. It's an issue found in winged boats...

Tom
Author:  SabresfortheCup [ Fri Feb 27, 2015 1:19 pm ]
Post subject:  Re: Hobie 18 Rear Beam Corrosion
My mistake, I didn't realize the crack went all the way to the rivet hole. In retrospect, it's pretty well apparent. Oh well, you win some, you lose some!

srm, good point about the stainless. My point was that dissimilar metals cause problems, but I'm more used to dealing with ships (mild steel hull/structure, zinc anodes). There too, corrosion or damage is more often the cause of cracking, not the other way around! Either way, I agree that the crack originated at the rivet hole. I knew that damage to the anodization/coating could result in corrosion, I just hadn't expected it to be quite so drastic.

damon, that is impressive indeed if you were sailing the boat with that crack! Fortunately, the stresses in the rear crossbar are largely related to keeping the hulls parallel, the trampoline tight, and sheeting the main. The wings add quite a bit though, I bet a few good jerks while on the starboard wing would've finished the job!
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