Folks: I need advice on the appropriate thickness for aluminimum tubing used for a mast on a gaff rigged Heron. It has been suggested that a 50 mm tubing diameter with a wall thickness of 2 mm would be light enough yet strong enough. My local metal supplier carries a tube that is 3 mm thick and it certainly felt andlooked like it would be far too thick to use as a mast. He also can order in a 50 mm diameter x 1.5 mm thick tube but I would be quite concerned that 1.5 mm would be too weak. While I could take a gamble on either thickness, if anyone has experience with which thickness is best, please chime in.

Chris,

Have you broken another mast? Or is this just a performance upgrade? If its a break then does that suggest you need a strong mast?

Finding anyone who describes the thickness of the alloy seems tricky. It seems to me that most people don't make cylindrical spars either so trying to infer the stiffness of the spar from alloy thickness may be wrong.

I did find Z Spars (who don't do heron spars) list dinghy masts with thicknesses from 1.6mm to 2.1mm. So 2mm is presumably right ball park. Bear in mind the load at the gaff jaws is quite high.

Do you know what the sectional weight is? A standard alloy dinghy spar seems to range from 0.8 to 1.2kg/m

On a back of a fag packet calculation I get the weight of a 58mm diameter YSP Wooden mast to be 1.5kg/m

If your 3mm aluminum weighed less than 1.5kg/m then I guess it would still be lighter than wood! Aluminium weighs 2.7g/cm3 - which on the back of another fag packet gave me 1.196kg/m. Would obviously be important to check that, and check that its pure Al not Alloy. (Alloy may be better - just the weight was done for Al)

I'd be tempted by the 3mm. Although actually I'm going to switch to bermuden...

Thanks for the comments. I currently have a 50mm mast diameter made from souther yellow pine which is stiffer and denser than the spruce or deal which many use. I did not get great grain alignment when I laminated two pieces which resulted eventually in bowing of the mast. This mast weighs in excess of 3.5 kgs which is far heavier than any wood or aluminium mast I have seen to date. The reason I am going for aluminium is reduced maintenance and stability (no bowing). The 3mm thick mast would weigh in at about the same as my current mast so going to a 1.5-2 mm thickness offers me a weight savings of 1-1.5 kgs. Yes it would reduce my centre of gravity a bit and improve bouyancy a bit when capsized. For those who know my boat which weighs in at about 10-30kgs more than most others, you know that weight has not been one of my priorities. My concern about the 1.5 mm thickness is exactly the load it would see at the jaws for the gaff. Last thing I want is for it to buckle under load. The fact that other spar suppliers use 1.5-2.1 mm thickness suggests that 3 mm is not necessary so I'll wait to see if anyone else provides advice and to see if my vendor comes up with a 2mm thick stock.

My worry about the 1.5mm would be less the overall bend and more the direct load at the gaff jaws... but I may be wrong...

The 1.6mm mast I found was a standard bermuden - not a gaff rigged affair...

If the gaff isn't 100% vertical it leans on the mast which must be quite a load when you consider the lever action it has...

Useful link:
http://groups.google.com.au/group/heron-sailors/browse_thread/thread/085e7b2bfa836b35?hl=en-GB#

From the Ausies... they are still agreeing their bermuden specs... but lots of references to aluminium thicknesses etc in this post...