Free-standing mast – topmast stress concentration

There is a stress concentration in the topmast tube where it leaves the main mast tube. This wouldn’t matter, except:

  1. The topmast is thin aluminium so isn’t particularly robust;
  2. There are scratches in the aluminium at the critical point from when I made the mast;
  3. The topmast is already a bit over stressed in this design so as to reduce the weight at the top of the mast.

To stop the topmast buckling I’m making a wooden plug for the critical section from my last piece of decent Douglas Fir. The plug is in two halves glued together which allows the middle to be hollowed out. To allow initial working I screwed the two halves together.

After considering my options I made this on my metalwork lathe. This made it easy to get the outside to the right size along the whole length. I used metal-cutting tools – not elegant but gets the top done.

Cutting the outside of the plug

Next stage was hollowing out the middle. For reasons of speed I used my milling machine.

Cutting out the centre with a milling machine

Then finished off with a powerfile / mini belt sander.

One sanded, one to go

The mini-belt-sander is a very useful tool – I wish I had bought one years ago. Mine is Silverline – one of the cheapest – as it was light and compact for getting into awkward corners inside the boat. It has been excellent – highly recommended.

I’ve glued the two halves together inside a spare tube so it all lines up. I used Tightbond (waterproof PVA-type glue) as this should be fine in the conditions, is nice to work with and won’t stick to the spare tube (I hope!).

Gluing inside a spare bit of topmast tube

Once the glue has dried I’ll cut the square end off, neaten up the insides as far as possible and then put some wood preserver on it. It can then be inserted into the topmast.

Hopefully Custard will then be good for high-speed full-sail surfing!

Making the junk sail – part 4 – finishing off

Getting the sail finished was a bit frustrating but I got it done. The main cause of frustration was that I needed to have the boat ready for a family holiday in August and there wouldn’t be any time to test it unless I got it all done last week. However, it all got done in the end — phew!

Templates

I created paper templates for the jiblets.

Jiblet template
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Jiblet design

No – not the guts of a chicken. The jiblets in a Split Junk Rig are the small jib sections in front of the mast. Since they are in clean air in front of the mast they should be more efficient than the main sections (mainlets?). hence they can provide a critical part of the drive of the sail. So their design is important.

I’ve been playing with some prototypes. The established thinking is that an angled shelf foot design is best, with the caveat that the actual sail shape will be different to the shape you think you’ll get. This tends to make me think ‘why?’ and try to understand how to get a predictable shape, when I should probably get on, make the thing and go sailing.

Ok – so first I tried a barrel cut jiblet. This was built into a panel with batten pockets and tapes to see how it all worked.

Barrel cut jiblet
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Split Junk Rig sail plan

With the mast nearing completion it is time to finalise the sail plan. I’ve been using a scaled-down copy of the Poppy plan for the preliminary design. I’ve now followed through to make sure everything works ok.

As recommended I built a string and stick model of the sail. This is the first attempt – a copy of the Poppy rig. With old garden bamboo it looks pleasingly authentic!

Stick-and-string model – first iteration
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