Gantry #2

So here's my second attempt at a gantry. Its got a different control mechanism too. I told myself when I launched the boat that I wouldn't change anything till I figured out how to sail it, but breaking my first gantry accelerated my development program a bit. The new one has held together through two days of sailing in 0-5 kts so still haven't been able to really put it to the test. The adjustment mechanism seems to work. The biggest problem is now that I can adjust rudder angle easily, it's easy to put it in the wrong place.

There's a million things I would do differently for the next one, and might get the chance to do soon, as Karl's new rudder is coming along and he's thinking about a new gantry to go with it.

It weighs about 725g with the adjustment mechanism which is about 200g heavier than the last one. I think I could get about 150g out of it easily for the next one. What's a good weight for a gantry?

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.