gantry material

this evening I saw the new harry potter movie for the second time in two days. prof. slughorn is a splitting image of a boat builder I have worked with...




karl mentioned on my last post that he thought aluminium would outperform carbon for gantries. I produced the above graph comparing the strength and cost of tubes of carbon, stainless, aluminium and titanium. The catch is only readily available sections were used. The metal tubes were sourced from http://www.onlinemetals.com and carbon came from http://www.carbonfibreexpress.com/.

Each type of tube has two columns. The first is the buckling strength of a length 550mm long (I mentioned in an earlier post that the minimum value for this parameter is 350kg). The second column is the cost of 2475mm of tubes (approx what you need for a gantry). The bottom axis is the weight of the 550mm long tube.

Filament wound carbon tube is the clear winner from the list. The tube pultrusion is next. I am sure I was told pultrusion has off axis material however I think for the marginal cost saving I would still go with the filament wound carbon - a little more robust.

First aluminium comes in at 50% more weight, but that only has a 0.9mm wall. You would probably only be able to glue such tubes together into sockets as the wall is so thin. However it is CHEAP!!! $25 for all the tubes you require. And just as strong as the carbon.

I always struggle to find an application for titanium (outside of high temperature/corrosive environments). This shows it has very similar strength and cost as the carbon, however its high density (4.51g/cc) makes it twice the weight of carbon, even with 50% less wall thickness. Stainless doesn't even get a look in before 3x weight. Rather cheap tho.

I am a little disappointed to see how far along the list my gantry is. But I am told bladerider uses 20x1.6 aluminium so I am slightly lighter than them. If anyone else wants to divulge what they use I will mark them on the graph above too.

3rd gantry / sails 13 and 14

i should have mentioned my new gantry earlier, it has been on the boat for around 5 sails now. Thanks to those of you who emailed suggestions.



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.
  

Interestingly the 10x5 RHS turned out to be the lightest option - lighter than tube. I think it would be easier to join too compared to tube. However I have access to some welders, and i don't like sticky so I went for teh aluminium.

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.

11 / wand / footy / jig / machines

A little bit of news:

Took boat out on Friday. 11th sail. It was blowing 22+ knots all day. I put my 7sq chicken rig up, and baring one large sustained gust at 21kts the wind remained at around 16kts for the rest of my sail. So I was underpowered most of the time. I recorded the boats top speed so far – 14.7kts over 2 sec just pipping N4rkla’s 14.666 from April. N4rkla took some photos and video, but nothing worth posting.

I have been testing wand response and there is currently a weight at the end of my wand. I figure the inertia might filter out some of the wave responses. During this sail the bungy came loose and the wand was bouncing through its whole range of motion for each cycle of the chop. The boat was bouncing about all over the place. This was hilarious to watch and a rather interesting ride. I was surprised how quick the height can change. Strangely nothing broke despite these uuuge accelerations and the foil stayed in the water. Pulling the bungy back on stabilized everything again. It will be interesting to see where this spring(shockcord)-mass(wand)-dampener(friction) study will lead.

I put the deck on my footy last night. Holes all over the place. I need to drown it in Tarzan’s grip to seal them all up. Might get it sailing this weekend. I will do a full post on the footies later this week.

I have sold the SLINO jig to a bloke in Melbourne. It will be departing shortly. It will be cool to see another one on the water. N4rkla are currently collating our design revisions and will post them shortly. Hopefully before the construction starts on the next one. I will be making the plans and Rhino model freely available on this site (and the world one too if there is additional interest).

I did a site visit of a marine machine shop earlier this week. They have such cool toys. CNC lathes, 6 axis mills, forging capacity. I need to hurry up and finish designing my new foils, but I got so much work on. But to give you an idea of how busy I am, I am writing this at 5:30am with 3 instances of Rhino open each doing a high res render for 3 different clients!!!!11!1!

Cool as ice...pfft

list of cool moth blogs that is much more accurate than any worlds form guide!

Thanks for the plug, but if you knew the real markla, cool would not be a word associated with him. His penchant for moths and heavy metal pale into insignificance compared to his uncool footy building skills and left handed-ness. Narkla on the other-hand is fuckin’ Fonzi cool! Just ask him…ayyyy!

It is true, after one too many days waiting for the wind to fill markl4 challenged me to a footy building duel, something to muck around with until it's worth heading out on the moth. Needless to say the building part is progressing slowly with the styrene sheet plate hulls finally watertight and keels being built now. One design swing rigs will be built next (with thanks to markl4's sorkplace for supplying mylar printing, little do they know), but we'll do these together because there is a knack to it (I'm a model yachty from long ago, including internet debates with DL about 12 years ago!!! maybe I shouldn't admit to that after being rated the coolest bloog...it's in the past now). Next hull build though, I'm not sure markla should heat cure the hull joins in the oven...styrene is a thermoplastic. Photo of the Fonz footy below, I can't find photos of your Elephant Man m4rkla.


On the IKEA moth front, we're in the process of updating the flat pack to incorporate lessons learnt and cool ideas, discussion to follow...

Disclaimer - N4rkla doesn't have nearly as cool (or thick) hair as The Fonz or womanise quite as much (if only I could)...

Before...
After...


Speaking of cool, how awesome is Alinghi 5! Can't wait to see video of it sailing. The yanks will have their work cutout to beat one of the fastest big multihull teams around. I "did" like the look of DoGzilla and thought it would be a contender, but with its latest mods and A5 now to compare against, it just doesn't do it for me anymore. I hope Larry doesn't take it back to the courtroom, settle it on the water.

business time

Boat’s odometer is up to 10. Got sick again on Friday afternoon, this time it was blowing 10-20 from due west. Westerlys here are typically patchy and gusty. As I was rigging a number of large bullets came thru being rather intimidating but the max wind speed moderated by the time I hit the water.

The wind was perpendicular to the channel I was sailing in so I got the longest runs to date – 630m – but not very fast. Typically the boat was sailing along level at a constant speed, but as I sailed into a gust the boat would accelerate, fly too high and crash. This is clear from the GPS track below.



I did have a good time and the long runs are very satisfying, but when I got off the water and saw the top speed it was a bit “is that it?!” (14.3kts). Reminded me a bit of this song…