Andrew, forgive me if I missed other posts, but what about deciding where the mast will end up on your boat and the structure to support it in the cabin? Have you decided on a sail plan? These may alter your needs as far as a mast goes. A Junk sail with a yard that extends beyond the mast tip may allow you to get away with a shorter mast. Differing sail plans may also introduce different loads on a mast, one sail plan may require a stronger mast than another. The materials you use to build a sail will also be important, will you be making the yard out of wood or carbon fiber?

As others have said, AI can be devilishly deceptive in some situations and this is one of them. The amount of data AI has to draw from in regards to Junk sails and the unstayed masts that employ them is comparatively non existent.

I would follow Arnie's guidelines and focus more attention on how the mast will be stepped in the boat. Again, forgive me if this subject has been covered, but perhaps AI could help with designing an interesting support structure for the mast in the interior of your particular boat.

Just a reminder, there are other AI models in existence, and these days they also have different "intelligence" settings for every model. Some models are labeled basically dumb and intended to be used for simple tasks so less power is required.

Good luck!

-Colin Clayton

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..."