A quick rule of thumb that I have found works for me for aluminium tube masts is as follows.

Take a quarter of the circumference of the mast and multiply by the wall thickness of the tube, multiply by 0.8 of the diameter of the mast and then by 12000 pounds per square inch (approximately half of the ultimate strength of 6063 T6 aluminium alloy, this gives a two to one safety factor) This is a rough estimate of the working strength of the mast in foot pounds.

Multiply the sail area in square feet by the distance from the waterline to the center of effort of the sail, multiply by one (the pressure of a breeze of 20 miles per hour, about 16 knots, this being the usual point to start reefing sail). This gives the pressure on the mast from the sail plan.  The two should be approximately equal for design of a mast to suit a sail. For extra heavy boats or long distance cruising boats be a bit more conservative on the strength of the mast by reducing the strength factor to 8000 pounds per square inch. 

I hope this is of some help.

David

I would reckon the H28 probably displaces about 5 tons - a couple more than the Raven - and you are talking about putting quite a lot of sail area on it.  My feeling is that 152mm diameter is about right up to about 35 sq m.  Above that I'd go for a larger section and maybe with the extra displacement of the H28 it might be prudent to go a bit bigger.  I confess that I'm no expert on alloy and our mast database isn't as much use as it might be for sails of this sort of size.  However, I feel that I'm putting the absolute maximum on my section with a 35 sq m sail.  For peace of mind, it might be worth going up a size.  My section has a 5.2mm wall, from memory.

Your 250mm diameter is more the size you'd choose for a wooden mast.

As Graeme says, someone like David Tyler would have a better idea; however, as you are by no means the first person to broach this subject, a serious rummage through the fora might yield more information to help you in your decision making.

So the 200mm diameter alloy tube will be fine for the mast base? I read that 250 was necessary for a boat/sail the size of mine, but that could be for a wooden mast.

If that is the case I can start work on my mast in the next month or so!

Anonymous wrote:

How did you find the upwind performance on Fantail? I ask mainly because I live at Herald Island, which is even further up the Auckland harbour than most people have to travel. Especially with the prevailing easterlies it can be a menace even with a bermudan rig. 

I notice that SibLim has a more regular HM style sail. Was that a design consideration or personal preference?

I would say that Fantail wind to windward with her cambered junk rig as well as she did with her fully-battened mainsail and roller furling genoa.  And with a lot less grief.  With an ebb tide, it's fun sailing down from Herald Is as I know from first hand experience.  However, these long sails just to get out to sea do pall after a while - the locals here in Whangarei have to sail 13 miles to get past out of the harbour.

Actually, the sail I'm putting on SibLim is quite different from an HM sail, having much less of a triangle at the top.  I fancied trying a different type of sail and feel that the high-peaked yard on a cambered sail stresses the sail too much.  This is a personal opinion and is just as likely to be wrong as correct.  I wanted a more HAR than I'd had on Fantail, but without too tall a mast.  I also rather like the look of the flatter top.

How did you find the upwind performance on Fantail? I ask mainly because I live at Herald Island, which is even further up the Auckland harbour than most people have to travel. Especially with the prevailing easterlies it can be a menace even with a bermudan rig. 

I notice that SibLim has a more regular HM style sail. Was that a design consideration or personal preference?