Unstayed Aluminum Mast

Hello, I would like to build a 20' Cat boat with a cat ketch rig. Each mast will be identical as each will carry identical sails with a sail area of 100 square feet, utilizing sprits, not booms. I will be using 24' aluminum tube with .125" thick sidewall. There will be about 20' of mast above deck. Cat boats are stiff and heavy for there given lengths. My question is will 4" outside diameter be strong enough or will I need to go to 6" outside diameter? Thanks for any input.

 

Thought about mandrel wound carbon fibre spars? Last I heard they were around $280-300 Australian dollars per unit, depends on how that compares to local aluminium prices for you, but at teh time that price was more than favourable when compared to aluminium extrusions here. Plus, you get a nicely tapered, very light spar into the bargain. The other beauty of teh carbon spars was you could have the wall thickness tailered to what you needed.
Of course, being carbon they have no tracks, but if you're going unstayed, perhaps a simple luff sock or hoops instead of boltropes would sort you out? If you went with carbon, you'd be able to handle them by yourself as well, making them very easy to remove completely if you needed that feature.

Regards, Andrew.

 

It's pretty easy to glue-on mast tracks to carbon masts. I've used PVC awning track and any number of adhesives that can stick to epoxy and PVC. There are products from Seka, Plexus and 3M that will do the job.

http://www.sailrite.com/Flex-A-Rail...e3eTaxmKbNaNe34Pa38Ta38Pa3j0?sc=2&category=51

http://news.nauticexpo.com/press/c-tech/sail-awning-track-21624-6858.html

If you snoop around on the net a bit, you can probably find it near you from an upholstery, or marine awning shop. You can get it in 8 foot lengths fairly easily and that length is UPS acceptable without extra charges.

 

Do you know what the righting moments of these boats are, roughly?

With that, you can estimate the maximum force that will be applied to the sails (I think the standard is to take the hull's righting moment at 30 degrees of heel as this "maximum"). Assume the force is applied at the center of effort of the sail plan, to calculate how many pounds/kilos of force are needed to produce that much heeling moment.

Now assume, this is pessimistic, that the total force applied to each sail is applied to the mast along the luff of the sail -- if you have 100 pounds of force on the sail, and the luff is 12 feet long, assume a horizontal loading on the mast of 100/12, or 8.3333 pounds per foot of mast.

Now it's just a cantilever beam loading problem, and you can look up the formula, um, online someplace to calculate how stiff your mast section needs to be.

Multiply by 3 for a factor of safety, shock loading or whatever, and you're done!

You need to know something about the righting moment of your hull, though.