Just hold off from posting guys, whilst I fight with technology and transfer the remaining posts, then the floor is yours!

Well, dont think anyone can talk in absolutes, only relatives. Steel (Chrome Molybdenum alloy - much stronger than mild steel) bike frames would crack just as much as aluminium, mainly because the walls were so thin to make them light enough. So steel has to be designed around too. Heavy alu frames would last forever..

I happen to have a spreadsheet of tube strengths too, the 162dia/6mm wall tube is around 28% stronger than standard 150mm Dia/5mm tube. The most risk of course is the foremast, it gets thrown around the most in a pitching sea. Being shorter and carrying less sail helps.

Arne Kverneland wrote:
PPS: By inserting Paul's suggested dimensions, 200mm diameter and 6mm walls and still hoping for a Sigma of 240MPa, the breaking moment seems to end on 4212kpm. This will probably be higher than the righting moment of the boat, at least.

Just on a hunch I went and checked Gary's profile. I had assumed he was building a Badger but thats not the case. It's a stretched "Baby Badger". Gary did you just stretch her or did your add to the beam proportionately?

So the 200mm is rather too conservative. 180mm would probably be a better match however I'm just guessing as I do not have all the data. I do still think 162mm is cutting it rather fine however I seem to recall that Gary did not have much choice in diameter versus wall thickness.

What you do want to remember is that while righting moment is a good guide, the highest mast loadings are when running, not when beating as many assume. Hence the saying that the boat can handle more than the crew upwind but the crew more than the boat down wind. Then theres the stresses from knockdowns and (hopefully never happens) an inversion. The above being the case, I like to be very conservative in my calculations.

Gary's post is spot on regarding the problems with aluminium. As he points out, if you take care you can design around them but you do need to know that there are issues. You also need to know that all aluminium masts have a finite life span due to fatigue issues. An aluminium mast can only withstand a certain number of stress cycles (bending cycles) and then it work hardens to the point that it will crack and eventually fail. You can get around the issue by going over size as Graham has done but you then throw away some of the advantages of aluminium, namely weight reduction.

I suspect the Needlespar masts use some type high tensile aluminium that enables them to use the skinny sections that there masts so often have. However doing so must inevitably lead to a shorter life as the mast will be more highly stressed.

I used steel for La Chica's mast as it is a cost efficient (for me) and highly fatigue resistant material. The masts are hot dipped galvanized (that means galvanised inside and out) and epoxy coated so corrosion should not be a problem for a long time.

I used PJR's formula to design the masts. I first obtained the section sizes for a hollow wooden section and then calculated the steel equivalent. The masts came out lighter than solid wood but of cause heavier than any of the other popular materials. However not much heavier. The main mast dressed and ready for installation was just under 200kg and the foremast 97kg, these being the weights reported by the crane operator when the masts were stepped.

With regards to PJR's formula for mast diameters be aware that for medium to heavy boats it is pretty much correct but it does tend to give results on the heavy side for light boats. Arne has written quite a bit about this and if you go to his page, you'll find his writings on the subject (and much other useful information).


La Chica's Mast Specs

Main mast: Dia at partners: 235mm, Dia at masthead 100mm, LAP 11 500mm, bury 2 250mm, weight 200Kg, material is steel, wall thickness 3mm.

Foremast: Dia at partners: 195mm, Dia at masthead 80mm, LAP 8 700mm, bury 1 500mm, weight 97Kg, material is steel, wall thickness 3mm.

PS. Gary, your masts do sound light to me (I mean strength wise not weight) did you do the sums? I would have thought 200mm with a 6mm wall would be as light as one would want to go.

On 30/05/13 Graham Cox wrote:

Rather than go off topic on Lexia's OSTAR page, I thought I'd record a comment on a new thread. I I don't know anything about Needlespars masts, though I have noted that some alloy masts seem alarmingly slender. Arion's aluminium mast is 10.5m overall, with a dia at base of 200mm and is 110 at the truck. It is spun tapered with a wall thickness of 5mm from heel to truck, is not anodised and weights 200 kg. I don't know what grade alloy it is, another important consideration. It is not a marine section, being the bottom section of a very large flagpole. It was made in France. I did record the details of the manufacturer in an earlier post in 2011 but don't have the details available at the moment. Arion has not yet covered as many miles at sea as Lexia so we will see. My mast is similar to Tystie's in specs, which has stood up well. I think it might be a mistake though to think that aluminium masts can be lighter or much smaller in section that wooden ones. My mast is only 10% smaller in section than PJR's recommended solid timber mast.