Ryan wrote:
Hi everyone
I've been trying to figure out my best option for a ~11m LAP mast for my offshore intended Alberg 30. I've been looking at aluminum flagpoles, lamp posts, straight tubes for a hybrid mast, as well as looking at hollow wooden masts. It seems like the options available in off the shelf aluminum tubes don't give a great fit. The best flagpole I've found is 0.25" thick and 8" at the base 6063 T6 (details attached). This seems like it may be on the weak side for the offshore safety margins. There is a slightly thicker 5/16 walled pole available but it is in two sections (not sure if this is an issue). I've found 24' 6061 T6 8x0.25" straight tubes for a hybrid mast but it seems like it would ultimately yield similar windage to a continuously tapered 10" hollow spruce mast.
Looking at various formulas for hollow wooden masts, it seems like I'm looking at a base diameter of 9-10" in spruce. Playing with the formulas it seems like doug fir's extra strength would let me go down to 8.5".
Performance and safety wise how important is windage here? Is a difference of say an inch in diameter at partner significant?
Weight wise all the options under consideration seem vaguely comparable coming in around 250 lbs. My understanding is that in this range the negative effects on stability are negligible and the extra weight may in fact be a boon to motion at sea?
Any thoughts are very much appreciated
Ryan
I am no engineer, Ryan, but have reasonable empirical knowledge. I think the 1/4"(6mm) wall thickness by 8" (200mm) by 11m flagpole will be strong enough for an offshore rig. I used the same section on
Arion and sailed the boat hard for at least 10,000 miles at sea without problem. Because
Arion's rig and steel hull were so strong, and the boat revelled in downwind sailing, I drove extremely hard when off the wind, often keeping up with substantially bigger
boats. I drove my first sail to destruction in four years but never had a structural failure in the rig.
If I was going to build an equivalent timber mast, I would not have a section of less than 10 inches or 250mm. Arion's sistership, Minke, has a mast built to that specification, and has cruised many miles offshore, currently hauled out in Fiji. Both boats set 36sq m of sail and displace 5 tonnes.