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!

1 thought on “Free-standing mast – topmast stress concentration

  1. Pingback: Oops – I get things wrong | Martin's Blog

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