Saturday, October 16, 2010

Gantry thoughts

Following John's HHPDO report of his gantry failure (and the subsequent pix posted carnage album) and after reading Dave's thoughts on pre-designing a gantry failure point:

             "So how can I design a design failure location into my         gantry? Add a Shear pin to would release the bottom gudgeon at the pin? The strut and cassette assembly would fly up. 8^( Would this still break nearly everything? Add a SS pivot at the top? Release the bottom gudgeon at the stern? The whole gantry would fly up......."

...and re-reading Dan's report on two exploding gantries at the Gorge, I thought I'd chime in on my experience and observations.  When I built my first rudder system I was following Bill's lead with his added admonition to beef up the cassette since he had experienced the rudder ripping out the back wall of his cassette. My resulting system was successful in that in occurred no failures (even at the heavy air of the Weymouth Worlds.) The pictures below and to the right was taken in the spring of '08 during the second or third time out. Obviously the boat is flying fine, albeit a little low. This was with both the mainfoil and rudder foil as non-production elements, built by me with lots of assistance from Gui and Bill (using their molds and employing their expertise on laminate schedules, etc.)

Clicking on the image above will bring up the original image size and the gantry can be seen more clearly. This gantry and rudder have since retired to the spare parts bin.

During the winter of '08/'09 I re-designed the rudder system to lose some of the mass and play (no cassette, just a vertical with gudgeons), incorporate an improved worm gear, and to feature a forward cant angle similar to the M2's geometry. I built a new strut using John Z.'s skinny vertical section (the low drag winner in Bill's tank tests) and fitted a much smaller Beiker-designed/Bora-built horizontal (that I got just a few days before packing up for the '09 Worlds.) The horizontal and vertical sections seem to work well, and in fact the horizontal is near the size that's currently in vogue - the span is only 775mm, approaching that of the new M2 "small rudder." So with an improved adjustment system and a new, but not tested rudder and gantry, I went to the Worlds in Oregon. Unfortunately, the gantry decided to come apart not once, but twice. The culprit was the G10 plugs I inserted into the tube structure. Either due to poor surface preparation or insufficient contact area, or both, one of the inserts came out (probably the lower tension member) and caused a cascade of failures. Adam snapped a pic of me swimming back in and I commented about the fiasco in this post.

I did repair the gantry and sailed the rest of the season with the "agricultural" - thanks Bear- setup without incident. This spring, in preparation for the NAs I decided to replace the gantry with a third version, keeping the geometry but losing the massive turnbuckle, the fairing (which never seemed to work as well as I hoped), and incorporating a different pin adjustment system as suggested by Phil Stevenson in this post. The tiller and swivel system was completed and works way better than the old set-up, but I wasn't quite able to finish the gantry. Nevertheless, the rudder system was in the best shape since the boat was built. So, of course this season's failure wasn't the rudder but the mainfoil, occurring during the first race of the NA series (why didn't it happen during the almost four weeks of daily practice leading up to the trip to Harbor Springs?)

At any rate, the third gantry is just about complete. The geometry has a slightly wider top base, with the top attachments fitting around some beefed up G10 tangs instead of employing any inserts. The lower tension member has just a threaded barrel that accepts the male clevis for gross adjustment. The female barrel is embedded such that it can't come out. I've retained the triangulation joints and I believe that they are way beefier than required. The bushings for the 1/4" ss rod are made of 1.25" long delrin, turned so that they require a press fit into the vertical tube (and definitely not capable of vibrating out.)

So what do I think would happen if I ran aground at speed? My rudder vertical is about a long as the mainfoil strut so any forces should be taken by the mainfoil and centerboard well. If for some reason I hit a log that missed the mainfoil I would expect the boat to stop suddenly and damage would be taken by either the lower gudgeon on the rudder (it happened once at the Gorge on the 2nd to last day) or to the aluminum plate on the hull, perhaps ripping out through the transom (unlikely.)  The point of this whole post is that I think one should not design a failure point, but just build the whole kit and kaboddle sufficiently strong so it won't break. Wishful thinking, I know, but why anticipate failure when anticipating a perfect build is so much more rewarding?

Thursday, October 14, 2010

Winging it, part 2

Picture by Thierry Martinez
Interview at Sail Anarchy
And a comment from John Z: "I got to speak at length with Steve Clark after his talk at dinner (last weekend at the HHPDO).  The wing doesn’t look all that complicated; Steve was happy to discuss all of the details.  I don’t know that an average sailor could gain anything from sailing with a wing but is certainly is cool and I think about it all the time.  Transportation will be a bitch.  I’m a model airplane enthusiast … I want to build one; looks easier than building a sail.  Happy to share sketches with you once I make some up."  – John Z

My homebuilder itch needs scratching. Right now I need to sort the foil and continue to drool over the wing...

Sunday, October 10, 2010

Addiction Withdrawal

I'm just miserable reading the temporary results of the HHPDO being sailed in Rye.

According to the event scoresheet the winds yesterday for the four Saturday races were 7, 12, 15, and 17 knots. Compared to last year's 25 knots that is very pleasant sailing! The other surprise is John taking a 2-1 in his Hungry Beaver. There seems to have been some attrition yesterday, and today's results haven't been posted yet, but I'm hoping to hear some tales from John or Peter. Why, oh why, couldn't I be there???

My plan for the winter is to again sort out a competitive mainfoil for my boat. The plan is to buy and fit a M2 strut and foil (my well is a little further aft that the M2's well, but has plenty of room to fit a cassette to house the centerboard.)  Now, this isn't cheap to say the least (around $3K) but I'm convinced that will get be back sailing and out of the back of the fleet.  To support this habit I'm in the process of selling some stuff, including my 10 year old Miata.

First up, though, is the set of carbon shrouds and rear wing bar support by Southern Spars that I won last year at the Gorge. These have never been used or installed, only taken out of the bag to inspect them. Retail price is now AU$530 on the Sailing Bits website.   Current currency conversion say that's about US$520. Highest bid over $350 takes 'em.

Saturday, October 9, 2010

The Wing

I might as well chime in on my take about solid wings. I think they should be allowed to develop. My expectation is that they would be neither popular nor harmful to the growth of the class. Even if Bora or Adam would win the Belmont worlds using a wing in some races, my bet is that they would use a traditional soft sail and mast combo for some or most of their races. And I doubt that spending lots of money for a custom wing would earn anybody a trip to the podium. No, winning requires a more expensive commodity: time on the water. 

Wings would allow tinkerers to continue to experiment. Whether it's Karl's fully articulated mainfoil, Gui's dihedral rudder foils, or Adam's solid wing, diversity is good. One idea may sooner or later prove to be the next best thing for the class. But we'll never know unless we let the tinkerers play.

O yeah, and Bernoulli said nothing about compressible or non compressible fluids. Only that total energy is conserved and that an increase in kinetic energy must result in a decrease in potential energy. When applied to moths there is a corollary: an increase in moth related expenditures must result in a decrease in expenditures elsewhere (food, clothes, living...).