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 Post subject: Frame twisting
PostPosted: Thu Feb 12, 2009 5:46 pm 
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Site Rank - Deck Hand

Joined: Wed Oct 29, 2008 11:16 am
Posts: 10
Location: Jacksonville,fl
Ok so on my 17, i get a lot of frame twisting. Like, when a gust comes, the windward bow twists. i know on the 16's that when this happens, it means you have to tighted the frame and the tramp. Well, i've tighted my tramp, but how do you tighted the frame so that the hulls don't twist as much. or are the 17's just sloppy boats. I know that the standing rigging on them is fairly loose, is the frame the same way? Thanks for the help.


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PostPosted: Thu Feb 12, 2009 6:54 pm 
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Joined: Mon Jul 14, 2003 7:11 pm
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Location: Detroit, MI
The 17 is a sloppy boat, but there are some things you can do to stiffen it up.

Make sure that the "shoe" castings on the outboard ends of the crossbars are firmly seated to the hull. On older boats, glue them in. (You have to take the crossbars off the boat to get to the shoes.)

Tighten the crossbar bolts tight with a T handle allen wrench. Use anti-sieze compound on the bolts.

If you're really determined to remove as much slop as possible, you can "bed" the crossbars using thickened epoxy - but that's a really involved job that I would only do if I was racing the boat.


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PostPosted: Mon Feb 23, 2009 12:58 pm 
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Joined: Mon Sep 20, 2004 12:36 pm
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Location: Tri-Cities, WA
MBounds recomendations are right on track. Making sure the tramp is tight, though not as important as with the 14 & 16, will also add to the overall stiffness between the hulls. :wink:


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 Post subject: Re: Frame twisting
PostPosted: Thu May 14, 2009 6:56 pm 
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Joined: Sun Jan 20, 2008 2:13 pm
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>> The 17 is a sloppy boat, but there are some things you can do to stiffen it up.

I didn't realize how bad my H17 was until I compared it to a racer. The beam end castings were loose. It tightened up after I glued them but I then found it to be out of level, square and plumb. The fix was to bed the beams in thickened epoxy. I set it up level and square on the trailer, then I put thick crushable cardboard shims at the bolts so I could draw it to plumb by tightening the bolts just until it came to plumb. After the epoxy set I tightened the bolts properly. A bit of a hastle - but now the centerboards point straight down and it is way less sloppy.


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 Post subject: Re: Frame twisting
PostPosted: Sun Jun 07, 2009 11:43 am 
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Joined: Mon Jul 25, 2005 8:20 pm
Posts: 149
Location: Campbell, CA
Below is a link to a posting on this forum with cross sectional views to help you decide the best course of action.

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

The epoxy bedding is something I have not done. But in my view, it does not address axial hull twist. It would be very effective at maintaining squareness guarding against oscillations in the flat plane. But if the end castings are not rigidly secured to the hull, the hulls will twist regardless of how well the epoxy bedding was done. To see my point, mentally assemble an epoxy-bedded boat without end castings, then try flying a hull - disaster; both hulls fold under the trampoline. So a less extreme example, say the castings are installed but slightly loose able to move up and down by(.015"), in a year they will work loose to .050, two years .125, and five years .375. The bedding may slow this process, but does not prevent it.

I am assuming that if one is going to install bedding there is a loose casting problem. Before bedding, I prefer a wool and resin fix at the casting pocket. Or better yet, a 7/16 stainless bolt, as illustrated in the above post, which is strong and adjustable (just like the FX1 and H18)

Peace,

Dan Peake
2003 H17SE
2005 FX1


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 Post subject: Re: Frame twisting
PostPosted: Mon Jun 08, 2009 6:59 am 
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Joined: Mon May 09, 2005 10:25 am
Posts: 2619
Location: Jersey Shore
There's a couple "issues" with the 17 design that really limit how stiff the platform can be made.

The outboard end castings have to have enough clearance to allow the crossbar to slide over them. This clearance immedately introduces a level of slop into that connection regardless of how firmly the end castings are connected to the hull. (I had considered pop riveting the crossbar directly to the end casting, but never did.)

Second, the eliptical crossbar profile doesn't really lock into the round hull saddle (compared to like a H20 or tiger with their rectangular cross-section), so it wants to roll and twist in the saddle. This is compounded by that fact that there is only a single inboard bolt. This single connection point also allows the round crossbar to "roll" at that connection point pretty much regardless of how tight you get the bolt.

Third, the connections for the outboard casting to the hull are prone to loosening up over time (although this can usually be fixed).

Last, the thin elliptical crossbar profile is probably not the greatest shape for resisting torque, so the crossbar itself is prone to twisting.

Your best bet is just to make sure your end caps are as tight as can be and same with the crossbar bolts. Having a firm tramp can't hurt either. Otherwise, as others have said, it is what it is.


sm


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 Post subject: Re: Frame twisting
PostPosted: Mon Jun 08, 2009 9:06 am 
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Joined: Mon Jul 25, 2005 8:20 pm
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Location: Campbell, CA
I forgot something above. I said that the castings must be rigidly secured to the hull, which is only half of it. The castings must also be clamping the cross-bar to the hull, i.e. holding down the cross bar end. SRM is correct that there is significant slop between the casting and cross bar (roughly 3/32"). So as Matt Miller has pointed out in other threads the casting is there to hold the cross bar down against the hull. And, since the cross bar and casting are matching elliptical shapes where they meet at the bottom, tightness and downbearing greatly help to reduce the twist of the cross bar, and help to reduce horizontal oscillations. I fully agree with SRM that an improved inboard bolting solution is required; it would greatly reduce the horizontal oscillations when combined with the casting fix and reduce loads on the casting.

SRM's Idea of riveting to the casting would probably improve things if the casting was positioned correctly, and fully anchored to the hull with reinforcing glass/metal etc... But as I keep pointing out, the casting will come loose over time, as does the inboard bolt. There needs to be a way to periodically tighten the casting.

Matt Miller points out that the 17 does get very loose over time, but also notes that 17's are nice and rigid from the factory. If that is the case, then the message here is that there is a need to solidly anchor the casting to the hull in such a way as to effectively hold _down_ the cross bar. An improvement would be to provide a method to periodically tighten it.

Peace,

Dan Peake
2003 H17SE
2005 FX1


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 Post subject: Re: Frame twisting
PostPosted: Mon Jun 08, 2009 7:07 pm 
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Joined: Sun Jan 20, 2008 2:13 pm
Posts: 35
The castings come loose from the hull. You glue them back on and the chance they return to exactly the right place is slim. I made certain my beams would slide under the castings before the super epoxy set. I thought about drilling and taping the castings to bolt the beams to them from above, but I feared the point load of the bolts might tear up the thin ally beams. Because I got the castings slightly too high - there was slop at the beam ends that I could feel when I shook the boat and it still wiggled!. So I waxed everything to allow for future disassembly and bedded the beams in thickened epoxy. It was a good thing I waxed everything because I had to do it over again because the boat was way out of plumb.

Because I waxed everything - not just the beams - the bedding will probably all fall out if I take it apart and I will have to redo the mess. The third try did the job as the boat has reasonable geometry and is tight - probably the tightest it has ever been.

I like the idea of a bolt at the outboard end - but you will probably have to do the bedding also to really get it tight and right. The Hobie attachment method is elegant and pretty but as Seymour Cray (Cray supercomputers) would say "elegance is for tailors" .


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 Post subject: Re: Frame twisting
PostPosted: Sat Oct 03, 2009 12:27 pm 
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Joined: Thu Sep 17, 2009 12:35 pm
Posts: 316
Location: Lake Champlain, Vermont
Dan (or anyone), Why shouldn't I place a THIN layer of SOFT rubber material in between the crossbar and the hull to reduce the slippage and fill the gaps as some are trying to do with epoxy? I know it has to be thin. Silicone wouldn't be my choice but some sort of rubber sheet. I will then re-fill the holes for the castings and call it a day. Thanks.

_________________
H18, H17 & Various motor boats


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 Post subject: Re: Frame twisting
PostPosted: Sun Oct 04, 2009 7:55 pm 
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Joined: Mon Jul 25, 2005 8:20 pm
Posts: 149
Location: Campbell, CA
Interesting idea, but I do not think you will be satisfied with the results. The results, in my estimation would likely be temporary, and only half effective. My opinion is that you would redistribute forces previously loaded on various pressure points, and lessen the overall movement. But the problem remains the same, loose castings: the factory did not provide a way to tighten them so, no matter what you do to the bed; rubber, epoxy, etc. the castings, will get looser and looser over time.

As I said earlier: imagine your fix, done to perfection, and then remove the castings and fly a hull. Disaster (I think the rear cross bar would tear out immediately from both hulls)- To truly fix this problem you must address the forces that started tearing the castings pins through all of that glass in the first place - as that is the primary origin of the problem. If you fix the casting drift, THEN bed for added rigidity, it would be amazing, but "bed" with out fixing the casting drift, and your hulls will rotate (axially).

That said, axial hull twist, mainly at the front cross bar, can probably be reduced by tightening your inboard anchor, and then over tightening the rig (not all that good for down wind performance) In this case, a bedded solution might provide an apparent cure (you're using the shrouds to pull the hull bed up against the cross bar at the casting end)- BUT if you remove the rear castings from this theoretical boat, and fly a hull - disaster.

My castings were professionally tightened using milled blocks of G10 and then filled, captured and bonded with glass and resin. The result: The condition is probably approaching new condition; they are relatively tight (in the world of 17's) but since there is no way to periodically re-tighten them, I expect that the oscillations will eventually worsen and require further attention.

Peace,

Dan Peake
Campbell, CA
2005 FX1


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