My little experience about mast bending on my KF 20+:

Gulf of Triest this Sommer,  18 Kn gusting 24, little steep waves 0.4 m. I started conservatively to avoid too much stress on her 40 years hold spar. With 4 panels up we were making poor progress upwind. I wondered if the boat was undercanvassed so I raised the 5th and the 6th panel. The boat heeled to 15 degrees, stiffened up and started to go upwind properly. The mast bended at any gust. My alloy mast is a folding one. The dimensions are 10 cm diameter at the base and 6 cm the top. The joint between the upper part and the lower one is covered by a sliding alloy tube. The lower part of the mast is quite stiff while the upper part bends. I have a second original mast in one piece which measures 10 cm at the base and 6 cm at the top. Hasler itself developed the rig of the KF 20+ and sailed his own KF 20+ "Pilmer" upwind in F6-7 "in steep breaking sea" (see article below) without breaking the rig. Surprisingly, according to PJR the mast for a Kingfisher 20+ with a 21 sqm sail should measure 20cm at the base!

I wonder how "alarmingly" is really a thin bending mast on a costal cruising boat...

A thin mast would mean less parasitic drag on the starboard tack and less sail distortion on the port tack (am I right?) and while bending it could absorb gusts smoothly.

Could be a thin mast made of carbon fibre a good solution for a costal cruising boat? 

Ciao

Mauro

P.S: I see now that in the picture below the mast of Pilmer seems to bend backwards :)

P.S 2:  sorry for parasitizing this tread again :)

Anonymous wrote:

For my boat, I have decided that because the stronger of the two masts I can realistically build is still only about 3% of my displacement anyway,

And Arne wrote: These days I am content that I can design an amply strong mast, which adds no more than 3% to the weight of the boat.

An interesting discussion! I designed and built my masts using PJR minus 10%. They're built in basically solid spruce (there's a 5cm square hole up the middle) and glassed with some fairly heavy biaxial cloth in epoxy. With fittings, they come out at almost exactly 3% of boat weight. My first impression is that they are very strong and I have seen no obvious flexing when sailing (to date).

Hmm, more to think about. Arne's formulae, that we are using, are already lighter than PJR. I would also point out that you are using two poles, so the masts might be 6% all together? On the other hand, no matter how many masts one uses, the righting arm remains the same, meaning the mast still has to be just as strong even with less sail area on the mast (my brain hurts). On the other hand, the Sunbird masts appear to be too thin/light. Not only do they bend "alarmingly" but it seems they do fail with use. I am not inclined to use wood as my wood butchery, while good enough for many things, is not something I would trust for a mast build. Aside from my own failings as a builder, making sure the wood has no flaws in it is another concern.

Len wrote: I don't expect we will cross the Pacific (even half way to Hawaii) or travel the coast to Mexico.

I did travel the coast from BC to Mexico, in a boat with a grown mast that was significantly lighter than PJR minus 10%. It was fine!

I have heard that the trip from Mexico back to BC is less fun. Out to Hawaii and then BC seems better though still rough, still the current is the right direction. But time is the biggest factor right now. We shall see what the future brings.

Arne wrote:

I reef her 35sqm sail at around 6 m/s, so the resulting bending moment on the mast came out at only 386kpm. This is less than 20% of the M_b of the mast I use. No wonder I cannot spot any bending of that mast...

Sorry my post is out of topic but I find the mast bending issue quite interesting.

In laser (https://www.youtube.com/watch?v=v9-ZjYkzSxc) and finn dinghies (https://www.youtube.com/watch?v=atL4v8bi-fs) bending the mast is essential to trim the sail flat when the wind picks up. Moreover when the mast bends, the pressure will be spilled from the upper part of the rig which is also an essential element to depower the sail. Their vangs are incredibly powerful and can bend mast and boom brutally  It is also so by cat catches like NIS boats (beautiful boats from Bruce Kirby https://www.youtube.com/watch?v=3plWuqzTaQw) and Freedom boats (https://www.you tube.com/watch?v=iFle9YZCzLw). Modernd alloy wings bend a lot too (https://www.youtube.com/shorts/u-xwbcebUNE )!

A Laser sailor with more than 40 years experience told me that lasers brake their masts mostly because of hitting the ground while turtling or because of corrosion between the steel attachment of the goose neck and the alloy of the mast. Roger Taylor did not report about loosing the rig an his coromandel Mingming. 

My armchair guess is that the problem with breaking an unstayed alloy mast on a yacht has not much to do with the wind strength but with being capsized by big broaching waves and hitting the water violently. Probably discussing startegies to avoid this could reduce the stress putted on the mast size...but this is just my armchair guess...

Good luck with your conversion Paul!

Ciao 

Mauro

 

Back in 1990, when I started playing with the JR, there were two schools of thought regarding mast scantlings.

Hasler & McLeod’s Practical Junk Rig, PJR was published in 1988, and I used it a lot when designing my first sail in December 1989. Their scantling rules regarding wooden masts soon turned out to be too conservative and would have resulted in a too heavy mast for my little Malena, so I reduced the diameter of her mast as much as I dared.

The other school was that around yacht designer Alan Boswell and Sunbird Yachts. Their masts, made by Needlespar, were terribly thin in my view. As the photo below shows, the masts of the Sunbird boats bent a lot, even in a moderate breeze.

However, they still appeared to work  -  that is  -  for a decade or three. Then they could suddenly snap. In JRA Magazine 62 p.40, Jonathan Snodgrass tells how he lost the foremast on his Sunbird 32. If I remember correctly, that foremast was only 3 or 3.5”, and it was replaced with one at 5”.

My way of deciding on a mast diameter can be seen in Chapter 6 and 6.5 of TCPJR. There is no high science involved, but I just try to connect the mast’s strength with the size of the vessel.
These days I am content that I can design an amply strong mast, which adds no more than 3% to the weight of the boat.

Right now, just for fun, I tried to calculate the actual bending moment that the wind produces on my Ingeborg’s mast at maximum wind strength before reefing (close-hauled).
I reef her 35sqm sail at around 6 m/s, so the resulting bending moment on the mast came out at only 386kpm. This is less than 20% of the M_b of the mast I use. No wonder I cannot spot any bending of that mast...

Sooo... don’t spend too many calories fretting over this issue. Remember the warning sign that I have pinned on my wall, right beside me here:

Obsession with perfection
has always made
a fine anchor.

Cheers,
Arne

(..for details, look up my members album, photo section 8...)

Anonymous wrote:

Okay now that is a significant deviation for the same material - or at least material declaration! Didn't expect that. I took my values directly from Nedal, as both german subsuppliers kindly let me have the original technical drawings including material numbers from Nedal.

I think best to use the numbers from the place the alloy comes from. Both the alloy and the hardening process may vary from one place to the next. There may also be different engineering standard practices from country to country. The standard value for over building may be different.

However, I should not be surprised by the deviaton of material properties: in my job I designed several underwater acoustic domes from CRP. Due to own inhouse material tests, we found the suppliers material property values to be conservative by factor 3 (and it wasn't a cheap supplier)!

That at least, is nice to know. So the manufacture over builds, then we calculate an over build on top of that.... so what do we build for? The JR on a free standing mast is a wonderful (old) innovation. When seeking to promote the free standing mast, one does not want stories of failed masts.

It would be nice to have a data base of failed masts... I read of failed wooden masts, but all wood will eventually rot. I am guessing aluminum has a lifetime as well. 6061 T6 with a Tensile Yield Strength of 276MPa has a Fatigue Strength of 96.5 MPa (tested by bending to this pressure through 500 million cycles). I do not think they are saying 500 million is a limit so much as it is better than. Or, if we increase the pressure, it fails before 500 million cycles (they only want to spend so much time testing). Unlike steel which has a value below which fatigue is not a problem, aluminum collects fatigue at all pressure levels. It has been hard to find anything on aluminum lifetime with google. What physical attribute would one look for when inspecting their mast?  Everything I read about fatigue inspection suggests cracks.  So I guess a dye penetrant at some interval. Perhaps jacking the heel of the mast just a bit (keeping it still within the step) would help. Aside from cracks, corrosion pitting and the like are the obvious things to look for. I would like to hear from others what they have found at various ages of mast.

As far as I understand, 6063 is the most common (beside 6060, which is even worth regarding strength). I guess it is less an engineers reason rather than a matter of price.

Reading from your link above to Nedal, it would seem that 6063 is more workable or workable at a lower temperature. So taking tube and reducing it to tapered may be problematic with 6061 or 6082 (the blurb says they are equivalent in strength but then gives different numbers). They are already having to temper to get T6 hardening so I suspect cost of material has less to do with it as using less material would cost less as well.

...it becomes quite obvious that at least three values are needed to compare masts: yield strength, diameter and wall thickness. If only the diameter of a mast is given, the strength comparison to another mast can be easily off by 50% if not even more. In my eyes, such a blurry comparison would be close to useless...

In this case, I was assuming the highest yield strength I could find, which we are finding is probably over estimated because this is a tapered mast and so would have used a softer material, and worked backwards to wall thickness. I ended up with a wall thickness that didn't make sense. But, if the same manufacture builds in a safety factor of two or three, then it makes sense even if a weaker alloy is used. However, if I am building my own mast, I can't assume a built in safety factor and have to provide my own.

I am very grateful for Arne's calculation scheme, as it helps to give an close idea and kind of takes you by the hand for a good part of the path during the design process. But I am sure he

Yes very useful. And by using the same estimates across all boats it allows comparisons.