Tuesday, October 6, 2009

Gantry #1 RIP

I've got five days on the water now and had my first major breakage. While blasting downwind my gantry decided it wanted to turn left while I was turning right. What actually failed was one of the attachments to the hull. This was two weekends ago and I've spent the last week and a half designing and building a new one. When I built the first one there was almost no engineering involved, so I decided I'd put in a little more time in the design department. I definitely underestimated the side loads created when foiling at high speed and how easy it is to load up the gantry with less than smooth steering (which right now is pretty much the only kind of steering going on).

The question is: how much load is there really? If you assume a Cl and speed for the rudder, you get a load that if applied to you and the boat, would quickly flip the boat/vaporize the rudder/accelerate you sideways quickly enough to make you black out. This doesn't seem to be a realistic load case. I think a more reasonable load case is applying the full righting moment and resultant sideforce to the rudder with a safety factor. Any more than this in theory would capsize you.

A second thought: how many gantries actually fail in buckling versus a failure of the attachment points/joints? It seems like it's pretty easy to design a tube for good buckling strength, but designing an attachment or joint that can handle all the multitudinous load cases is the tough part. Given my gantry failure data set currently has one point in it, I don't have much to go on. Anyone have any thoughts?

Congrats to Karl on getting married! I hope this doesn't affect his mothing time too much.

I'll post some pics of the new gantry as soon as its on the boat.

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4 comments:

  1. Doug CulnaneOctober 7, 2009 at 2:06 AM

    Normally the gantry fails at the joints. So make the material on the joints about as thick as the tube walls. I used UD and lashed it round like a boy scout would lash two bits of wood together. You do not need lots of material to make bullit proof joints.

    The tubes are about the same as a tiller extension. Mine are much smaller but fatter walls...

    The above is not very scientific but the dynamic loads are hi and hard to calculate.

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  2. TeknologikaOctober 7, 2009 at 2:49 PM

    The failure points are typically the attachment points. I have seen 1/4" stainless bolts sheared, and 5mm pre-preg carbon plates snapped.

    Bruce

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  3. NatOctober 7, 2009 at 9:46 PM

    That makes me feel better. I was getting loads that were exceeding the shear strength of a 1/4" bolt and was starting to wonder if I was being over conservative. Most gantries probably have 1/4" bolts attaching them, but since this moth has relatively low freeboard, especially aft, there's not much of a footprint to attach the thing to, and the loads get multiplied pretty fast.

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  4. MARKLAOctober 8, 2009 at 8:01 PM

    All gantries are destined to die.

    My first two gantries have failed in buckling midspan (where you would expect buckling failure to occur)

    My study indicated the diagonal tubes are highest loaded carrying 350kg when foiling and steering stupitly at 20kts.

    See:

    http://perverted-moth.blogspot.com/2009/07/gantry-material.html

    http://perverted-moth.blogspot.com/2009/07/3rd-gantry-sails-13-and-14.html

    http://perverted-moth.blogspot.com/2009/03/launch.html

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