The mast you designed recently, wasn't that starting with 7" tube?

Yes, it's 7" x ⅜", so that I could 3D print the deck ring on my 250mm square bed of my printer, and so as to minimise the size of tube going through the forecabin, but 8" x ¼" would be a better choice without those constraints.

My gut feel is that the Sabre 27 intended to go offshore in gale force conditions could use a 8" x ¼" x 5metre tube as the bottom section, 

Anonymous wrote:

I completely share the sentiments you have expressed which absolutely accord with my own experiences of my dealings with my new found friend A.I. 

Well, now...

I've just skimmed through the entire document (but not crunched any of the numbers), and it's all making perfect sense, as a way of calculating the tube sizes of an unstayed mast - if the loads that the mast must withstand are known perfectly. And as Graeme says, they aren't. The tube sizes to cope with the righting moment can be calculated/estimated quite closely, but just as one example, the actual loads generated when you run downwind slap bang into a big wave that was caused by a wind from another direction cannot be even estimated, let alone calculated. So we start with some idea of sizing given by calculations, sure... but then we have to adjust the results, employing some empirical thoughts and experiences which are just as useful. To put it in a very simplistic way: if the mast doesn't fall down, you made it strong enough, but it might be uneconomically over-strong. If it falls down, it wasn't strong enough. How do you know which side of the line between the two you'll fall? Only by looking at past experience, and applying some gut feel, and as high a factor of safety as is thought appropriate. This will always be an inexact science.

My gut feel is that the Sabre 27 intended to go offshore in gale force conditions could use a 8" x ¼" x 5metre tube as the bottom section, as a practical stock size that can be bought at reasonable cost, and will not need all the work involved in adding an internal timber or aluminium doubler. But that's only a gut feel, and I'm not going to make any calculations to back it up. If there's a reason to prefer a smaller diameter, due to space or manufacturing constraints, then a 7" x ⅜" tube might be needed, but it will cost more. Andrew, you might ask your AI for an opinion on those.

In conclusion, I'll just quote two of Arne's empirical rules of thumb which I agree with, and which AI doesn't seem to be fully aware of:

"The wall thickness of the lower alloy mast section should be somewhere between 2.5% and 5% of the outer diameter. (I would anyway not make a mast with less than 2.5mm walls, to avoid denting and buckling.)

This rule seems to produce sensible mast scantlings with neither too thick (heavy), nor with too thin walls.

The top section’s bury into the lower section should be at least 4 (better 5) times the inner diameter of the lower section and at least 10% of the top section above the joint..."

There are two matters the I would like to comment on, not as an expert or even  an experienced person when it comes to designing masts. I have been more a believer in looking at what others have successfully done and reported, and use that as a basis rather than design from first principles. (With enormous gratitude to our clever and generous pioneers of the modern Western junk rig).

Consider these two comments merely the raising of questions, nothing more.

The first relates to the use of A.I. I have no background knowledge in this area, merely learning the hard way how incredibly useful it can be, and how pitifully deceptive it can be if you allow it to take control of the project. I have been in a dialogue with Chatgpt for many hours each day for about two or three months on end now, in trying to build a web-based system (a different sort of project, but same principles I think) which is fairly well outside my technical knowledge. Here is what I have learned:

A.I. is not particularly intelligent. It is trained, and in possession of vast amounts of data, but at times it can display appalling ignorance and stupidity.  At first, I thought it was brilliant and all-knowing. I would describe it now as an idiot savant. You can make good use of its incredible ability, but beware. It is sycophantic, verbose and plausible, each in the extreme, and trained to be that way. If it gets the slightest hint of what direction you want to go in, it will lead you in that direction with a careless optimism that is hard to resist. It will go down every useless rabbit hole until it gets a hint of what you want to be told. You can get it to do the calculations, or coding (in split seconds) and produce voluminous reports reflecting your own ideas. As a critic, it is poor; sycophantic agreement is more its style. It is a donkey in need of a bridle. You can get it to do the hard yards, but you have to be the engineer. It can be lazy and forgetful. It will never say “I don’t know” unless absolutely driven to it – otherwise, it is merely plausible, especially when outside of its knowledge (training) domain. (Ask it to draw a picture of a split junk rig!) You can get a lot from it, but must maintain a healthy scepticism and at all times beware: it is trained to tell you what it “thinks” you want to hear, in the most shamefully plausible fashion. (The disastrous A.I.-driven targeteering by the US Military forces, which resulted in the double-tap destruction of a school for little girls in Iran on 28 February 2026, is a glaring example of the dangers of relying, irresponsibly, on inadequately supervised use of A.I.).

My suggestion is that once you have done all the calculations, and designed your mast from first principles, be sure to look at and compare with what others have already done successfully, with masting a vessel of similar characteristics to your own.

The other point is a matter of curiosity. I have never understood why mast designers begin and end by consideration of righting moment. I would have thought it obvious that a mast must sustain the righting moment, but I would have thought also that this steady force is actually the least of the design considerations. A mast must withstand great dynamic forces, the shock of a sudden knockdown, for example, if the vessel does not yield quickly enough. Even more important, it must withstand the shock of coming to a near sudden stop when running down wind, full speed, under full sail. The forces involved in a pitchpole (or the onset of one) must surely be greatest that a mast must withstand. The forces on a mast in the fore and aft direction must, therefore, surely be much greater than the more gentle sideways force of being hove down by wind from the beam?  I suppose this is all taken into account by the use of hefty fudge factors, but it seems to me to be a very indirect approach, and not really all that much to the point. But I don’t claim to know much about designing from first principles.