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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Making the junk sail – part 1

Finally making the sail. The end is in sight!

Fabric

The sail is made from 65gsm polyester ripstop. This is the same fabric as I used in my previous boat. The reasons for choosing it were:

  • It is cheap;
  • Since it is coated it is impermeable to air, plus it doesn’t fray when cut;
  • It doesn’t stretch (much) on the main thread axes – along the fabric and across the fabric;
  • It does stretch on the bias (diagonal). This helps take out creases when using barrel cut camber;
  • Polyester is UV resistent and doesn’t sag when wet;
  • It seems to work ok.

The main downside is that the coating will probably wear off in time which will allow air through.

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Jiblet design – update

I’ve been continuing my experiements on the jiblets.

Leading edge angle

The first issue was the approach angle of the leading edge. The symmetrical aerofoil has a leading edge angle of 90º. This means air will not flow round the leading edge of a sail with this shape.

The larger the angle the more forward thrust we’ll get from the sail but the lower the angle we can go into wind (larger tacking angle). Conversely a smaller leading edge angle will improve the angle we can go into wind but will reduce thrust from the sail. Ideally we’d optimise the angle for Velocity Made Good (VMG) given the performance and drag of the boat, but we just don’t have enough information so a reasonable guess will have to do.

The hard-sheeted jib angle on my hull (Wanderer dinghy) is approx 12º. To allow air to get past the mast, plus allow for the extra drag of a junk rig I’m assuming that the jiblet angle of incidence will be around 15º. This means that the maximum leading edge angle should be around 30º (45º – 15º = 30º).

Playing on airfoiltools.com I came up with this shape that seems to look about right – and something like a sail.

9% camber at 35% chord; 1% thick from airfoiltools.com
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