Aluminium free-standing mast – some details

Now I’ve got two tubes that will fit together here are some thoughts about some of the mast details:

Overall design

Since I’ve got two tubes that fit together I’ll use them, rather than try to use wood.

Using the graphic method to find the lengths we get this:

Strength vs height for our aluminium mast

Since the yield moment of the two tubes combined is 2650Nm I’m using that as the target righting moment. This produces these tube lengths:

Main tube (2 3/4″ x 10swg): 174mm to 4062mm; length 3888mm.

Strengthener tube (2 1/2″ x 16swg): 0 to 2107mm; length 2107mm.

Topmast (2 1/2″ x 16swg): 3762mm to 5600mm; length 1838mm.

In practice I’ll probably extend the main tube down to the heel. This is a highly stressed part of the mast and a failure here would be a disaster.

Connecting the topmast

Since the tubes slide freely with one another the topmast can be removable. This will make transport of the mast much easier since everything will fit inside the hull.

The current design looks like this

Topmast joint

The topmast joint will need a taper between the two tube sizes so that all the batten parrells can slide easily over the joint. I can cut a section of the 2 3/4″ tube and machine it down in the lathe. I plan a taper of approx 1 in 20 which should be ok.

The taper sleeve needs to be attached to the topmast. However this is the most stressed part of the topmast so I really don’t want to drill any holes in it. First plan is to try epoxying the taper sleeve to the topmast. If that doesn’t work (aluminium can be hard to glue) I’ll have to try something else.

The topmast needs to have something to stop it rotating. I’ll put a tongue on the main tube and a recess in the taper sleeve.

The topmast will be supported vertically by an acetal bung in the main tube. This part of the tube is much stronger than it needs to be so drilling holes to locate the bung won’t be a problem

Masthead

The masthead needs to be light, easy to make and strong enough. On my other boat I just used lacing loops held on with machine screws and nyloc nuts and I’ve seen something similar to secure a topping lift on a Jaguar 21, so it should be plenty strong enough for the loads on a junk rig.

Since the top tube is 16swg it will need a bit of reinforcement. The easiest way is to use another section of the main tube and drill through both sections; something like this:

Mast head reinforcement

Not the greatest drawing but hopefully makes sense. The 84mm dimension is correct for a 10mm lacing hook, but I need to decide on the halyard block before I know which lacing hook I’ll need. The ends are tapered to make it look nicer and reduce windage slightly.

The mast will need bungs to keep water out in a capsize. I’ll get these on ebay, together with bungs for the yard and battens.

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