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PostPosted: Mon Nov 14, 2011 11:15 am 
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I have read just about every posting on these forums that touches on the topic of the cross bars and the various issues that people have had with the plates at the outboard end of the cross bars. I have a few questions that I couldn't find answers to, or just wanted more info.
I recently purchased a used Hobie 17 and noticed that the fiberglass bump at the end of each bar was cracked. 1 had a small crack running top to bottom, 1 was repaired but the repair was cracked in the same manner, and the third was repaired but not well, no crack. The seller told me these bumps are just for looks and not structural. Well when I took the hulls apart for winter storage (I am in VT) I discovered that these bumps are not entirely for looks and hold the end plate the the cross bars fit into. 2 of these plates came off with the bar, the other 2 were very loose. In all cases it is the screws that are loose, the aluminum bars that insert into the hull below the bump are still snug.
I have used a Dremmel tool to open the bumps and have discovered what I think is the cause of the cracks. Water sits inside the hollow that the bars rest in and over time that water has seeped into the screw holes and into the bump. The bump is filled with open celled foam and it absorbed the water then froze and cracked the glass.
My plan is to remove all the foam from each bump and fill in with solid glass. My question is should I use west system or or regular polyester? I like the various products from west but if the boat was made of polyester that might bond better, or so I have been told. I will then re-seat the plates with 5200 and drill new screw holes. I am not going to mess with the pin holes as I do not want to create an alignment issue.
I am setting this boat up for long distance adventure racing so the weight/class legal issues are not important.

Thanks.


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PostPosted: Tue Nov 15, 2011 1:10 pm 
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Here's a couple links on the difference

http://www.redrockstore.com/resin.htm

http://www.boatus.com/boattech/TipResins.htm

There a some purists but from what I have read I think that Epoxy would be the way to go. I have not done any repairs with it yet but will be soon.


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PostPosted: Tue Nov 15, 2011 3:33 pm 
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Quarath wrote:
There a some purists but from what I have read I think that Epoxy would be the way to go.


Choosing to use polyester resin rather than epoxy resin has nothing to do with being a "purist." It has to do with using the more appropriate material for the job. Some applications favor polyester, some favor epoxy - it's that simple.

sm


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PostPosted: Tue Nov 15, 2011 7:06 pm 
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srm, so what is your preference? I have used 2 bits to get out the stuff inside. I used #446 sanding band to open up a hole about 1/2 to 2/3 the size of the surface area of the bump and then I am using #114 to get the foam and filler out of the cavity.
I have used West system in the past when I was younger with varying results. I have found it sometimes turns a yellowish color and was told that it was due to what it was reacting with. I have done as carful a job as I can to get out the foam and filler but there is still a little bit of filler in the seams and the corners. My concern is that the west may react with this and not be as solid or well bonded as I want. Of course whatever I do will be better than what was there, but I still want it to be strong. I would just use the polyester without hesitation but I have 2 concerns about it. First, being in VT I can easily get the stuff at the hardware store, CarGroom, I think it is? I have used it on other boat repairs and it is good but are all poly products the same? This repair is more critical than others I have done recently so I am concerned about quality. Second, what I like about West are the various density fillers and other products they sell, while with poly I don't think there is as much out there for fillers, etc. Also poly doesn't blush so it is easier to deal with for a finished surface.
My assumption is that the boat is poly based so I should go that route but I am looking at all options.
Inspection ports are on the way and I will install them before going on to filing. I want to se where the pins go under the bumps. Anyone know what they are bedded in? Solid glass? Wood and glass? Foam? (hope not).
Also I sanded down the v shaped gash in the bow. It looks like below the dack the hull is a sandwich of glass/foam/glass. But lower down, near 1/2 way it looks like the foam is narrowed and then gone, just glass. This true? Or is it just the previous repair that I took out and damage to the area I am seeing.
If it is easy to post pics here I will.


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PostPosted: Wed Nov 16, 2011 6:17 am 
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A couple things...

First off, in this day and age, you can get anything you would ever possibly need for this repair off the internet and delivered to your doorstep in a matter of days. I wouldn't use sub-standard hardware store materials just because they are readily available locally.

I've had good results using epoxy from MAS. I've also had good results using polyester resins from uscomposites.com and fiberglasssupply.com. Both of these suppliers offer a full range of reinforcements, fillers, gelcoats, etc.

Next, in your first post, you say you're going to fill the humps solid with "glass." Epoxy and polyester resins are not "glass." They are resin systems. Glass (actually fiberglass) is a woven material which is made using fiberous strands of glass. So really, you're planning to fill the hump with resin, not glass.

Based on your description of the damage, the first thing I would do would be to fix the crack on the outside of the hump. It should be repaired by taper sanding/grinding out the cracked gelcoat and fiberglass and laminating in new fiberglass cloth using polyester resin. You pretty much can't go wrong if you laminate using the same resin system that the boat is made from (polyester) and using polyester resin will allow you to easily gelcoat the repair.

To fill the hump, I would probably use epoxy resin since you're not going to be able to add any glass reinforcement, also because gelcoating isn't required. Epoxy resin is stronger than polyester, so it's better suited for un-reinforced bonding and filling. I would not be concerned with compatibility issues with epoxy, it should work fine. I would fill the bump with epoxy and then insert the aluminum end casting while the epoxy is curing so that the two aluminum prongs are molded into the hump. Coat the prongs first with wax or release agent so the end casting isn't permenantly bonded to the hull.

These hulls are a combination of fiberglass/foam sandwich and solid fiberglass. It depends on the location. The bottoms and the high-stress locations are solid fiberglass. The broad sections of the sides and deck are sandwich.

sm


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PostPosted: Wed Nov 16, 2011 7:41 am 
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srm's advise is wise regarding resins, fillers, and glass reinforcements.

But as an engineer (naval architect, to be precise) - I can't help thinking that the cracking problem could actually be made worse by filling the humps with a hard block of resin.

Gel coat cracks when it loses its flexibility (age related) and the underlying laminate flexes. Flexing is a material's way of spreading out the load - if you concentrate the load in a small area (a hard spot), then the material deformation is increased.

By putting a very inflexible section (resin filled block) next to a more flexible section (deck), you create a hard spot at the junction - propagating more cracks.

If you're troubled by the cracks, then I think the solution is 1) do as srm says re: the exterior repair and 2) lay a couple of layers of fiberglass matting on the inside of the hump, overlaying the deck a couple of inches. Use epoxy for this job, make sure the surface is roughed up and don't worry about any filler left from your cleanout job.


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PostPosted: Wed Nov 16, 2011 9:02 pm 
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Srm, Quarath,
Thanks for the very useful replies.
I am aware that I can get resin/glass over the net, just wondered if the hardware store stuff was the same, or crap. By the comments I am going with crap. Thanks for the links. Also, what I did was go into the bump from the outside, where the cracks were. However I opened the hole up to 2/3 the surface area of the bump already. And I have hollowed out the bump as well.
My original idea was to fill the bumps completely with a resin/glass mixture. Knowing I would not be able to lay glass properly in such a small space I was planning on giving it a best effort with chopped up glass and resin. However this did not seem like a good tactic after checking out a few sites. So I was thinking of using west system. The comment on load dispersal is a good one and an angle I had not thought of before. I could indeed create a hard spot that would create additional havoc under load.
BTW the pins do not enter the bumps but go under them, just below deck level, from what I can tell at this point. The bumps have a "floor" at about deck level.
My main concern is to fabricate a strong repair that has necessary material for the screws to hold in, while minimizing the infiltration of H2O.
What about filling the bumps with a low density filler from west and the a layer of fibers and west over the top. Them inserting the screws wetted out with west and using 5200 to secure the plate. The 5200 would not be bonding to the west but to the gel coat from the factory.


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PostPosted: Wed Nov 16, 2011 9:05 pm 
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oh, as an aside. The foam inside the bumps was still wet when I opened them up. There is nowhere for the water to go. Also the foam was black and rotten smelling around the screws and it appeared there was a second skin inside the bump, almost like the filler was already glassed when put into the mold.


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PostPosted: Thu Nov 17, 2011 5:38 am 
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The pics in this thread will probably be helpful to you...

http://www.hobiecat.com/forums/viewtopic.php?f=15&t=11944

sm


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PostPosted: Thu Nov 17, 2011 5:47 am 
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srm wrote:
The pics in this thread will probably be helpful to you...

http://www.hobiecat.com/forums/viewtopic.php?f=15&t=11944

sm
That's the thread I was thinking of. Thanks, srm.


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PostPosted: Thu Nov 17, 2011 7:38 am 
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excellent pictures, thanks. Not sure how I missed that as I have gone through the forums several times, but thanks again. Really interesting to see the whole picture, inside and out. I keep asking my self why the designers ever thought this was a good way to build this connection, seems like it is destined to fail at some point. Fortunately the pin holes in my boat are in pretty good shape, only 1 is elongated, the others are all round and fairly snug. Has anyone ever tried bolting the cross bars through the deck? Probably wouldn't work but it is a thought. Got my curved inspection ports yesterday, will cut the holes in a few days. Surprised at how little curve they have, hope they work out ok.


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PostPosted: Thu Nov 17, 2011 8:52 am 
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vtwave wrote:
I keep asking my self why the designers ever thought this was a good way to build this connection, seems like it is destined to fail at some point.


After studying the pictures more throughly, I'm somewhat inclined to agree with you. The hull construction at the crossbar endcaps is quite complex and yet the connection is really not that good. The fact that the prongs on the endcap largely bear against foam is disconcerting. Plus the fact that any water that leaks past those prongs will enter direcly into the foam core of the fiberglass sandwich is not good. Also, the screws for the end cap only grab onto maybe 1/8" thick fiberglass and again, you have a direct leak path for water into the hull structure.

One thought that comes to mind for repair would be to remove the foam from inside the hump and fill with epoxy as you're talking about doing. But also bond a threaded stud (or studs) into the endcap. Then drill an additional hole in the aluminum end cap and tighten the end cap against the hump with a nyloc nut and washer. This would hold the end cap against the hump better than the self tapping screws that are already there. It's very common for those screws to loosen, and when they do, the end cap wiggles and wears out the prong holes causing the boat to get sloppy.

sm


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PostPosted: Thu Nov 17, 2011 10:44 am 
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you know, come to think of it you could do several options like that. One option would be to back the inside of the bump with reinforcement and then using the two screw holes insert a bolt going from inside the bump through the plate and then just bolt the thing together. Then fill the bump with west. A great benefit of this would be that I could assemble the boat before putting in the west so I would be sure the alignment is correct. That would be a huge advantage. My biggest concern would be if the bolt head ever came loose inside the hump you would never be able to get the plates off as it would just spin with no way of stopping it. A solution to that problem might be to weld a piece a SS bar onto the bolt making a T shaped piece that would not be able to spin. or better yet weld it to both bolts forming a sort of U shaped piece that would not be able to move. I am liking this more and more. Can anyone think of a pitfall to this idea?


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PostPosted: Thu Nov 17, 2011 10:59 am 
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I think I would skip the backing plate as it is probably not necessary. You're going to fill the whole hump with epoxy anyway, so that's going to spread the load across the mounting surface. There really isn't THAT much load trying to pull the end cap away from the hull anyway (as evidenced by the fact that it's only held in place by two or three self tapping screws).

So what I would probably do would be to get two or three pieces of 1/4-20 stainless steel threaded rod, about 2" long. You can get this through mcmaster-carr. Drill out the existing screw holes so the rods will fit into the hull and end casting. Fill the hump with epoxy and then insert the threaded rods into the humps 1" to 1-1/2" deep (watch out for the epoxy overflowing). Once the epoxy cures, you'll have two solid threaded studs sticking out of the hull that you can then tighten nuts down on to pull the end casting in snug.

If you could get another stud mounted down by the prongs, that would be really good, but not sure if there's enough room to do that.

sm


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PostPosted: Sun Nov 20, 2011 7:31 pm 
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wasn't really thinking a plate per say but rather a few washers and possibly some West. The surfaces inside the bumps are uneven and not nearyt flat. If I could do better I would. Like I said I want confidence in this repair. I worry about rod backing out.


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