I made a structural model of it at work in beam modelling software. My worst case loadcase was 75kg lift, 7.5kg drag and a lift coefficient of 1.2 on 200mm of rudder 20kts (ie stupit steering).
As most people would expect the highest loaded tubes are the diagonal ones going from lower aft to top forward. On one side the diagonal tube go into compression, on the otherside it will go into tension. The steering load in this tube dominates all other loads so the tension and compression are almost equal at around 350kg. Most materials can handle this direct force, the tube size is governed by buckling limits.
I considered 4 different tube types, all standard sizes.
- In stainless steel I could use 19x0.9mm. (I was very surprised and disappointed 12.5x1.2 hit the buckling limit, stainless would have been so bling).
- In aluminium I got 16x1.6, I believe this is a smaller section than the bladerider (and cloned BR) gantry.
- 10x5 RHS carbon pultrusion
- 12IDx1.5 Carbon tube.
No aluminium welder could understand how simple the gantry was and they all quoted me around 20 hours. In anger I drew up the CNC parts below which made the whole gantry self aligning once the 6 tubes were cut to the right length. All that was required was to zap around the perimeter of each CNC part and the thing was assembled. Unfortunately the small, thin tube was still hard to weld, especially to the 4mm CNCed plate, so it still took around 8 hours, 2 pies and a bottle of john walker.
The results are good tho, seams sturdy enough. While I am told all gantries are destined to die, I can take immediate pleasure in having one of the lightest going around.
Sail 13 was a another friday, escaping from work. left work late and missed the wind. Sail 14 was quite a bit more interesting. It was blowing 10kts and I haven't had the boat flying in such light weather. I was surprised how hard i needed to work in such little breeze. I think I was undersheeting and not hiking enough on previous sails. This sail I was making a deliberate effort to pull the main on to 100% and hike out as far as possible. It payed off and the boat regularly flew - well until my weak abs gave in. 14kts boat speed in 10kts breeze = w00tage.
What about the rig tuning session?
ReplyDeletealways thought aluminum could outperform carbon in this application. What about titanium though?
ReplyDeleteI started commmenting, but it is turning into a post in itself.
ReplyDeleteComparison of readilly available stainless, alloy, titanium and carbon tubes.
I don't see aluminium outperforming carbon ever. Stiffer, stronger, lighter? Cost and ease of fabrication is the only benefit i see in alloy.
Titanium is heavier than carbon, and very very expensive so i don't think it will be an option.
But we will see in the next post.
One of the functions of the gantry is to possible the variation of the bias of the angle of attack of the rear foil, isn't it?
ReplyDeleteAnd why the gantry has to be so long (over 500mm - seen in your SLINO plans)?
G'day Jurgen,
ReplyDeleteOnly just saw your comment. I'll direct markla to your post.