Free-standing mast – Requirements

Following on from my previous post I thought I would describe the design process for the free-standing mast. Any design process needs requirements, so here goes:

  1. Strong enough to capsize the boat, with a safety factor. Normally expressed as a righting moment in Nm. For a yacht this will be approximately 1/4 of the beam (width) of the yacht multiplied by the displacement (weight). A dinghy is trickier since the crew move around to balance the boat – two crew hanging over the side can provide a large righting moment.
  2. As light as possible. A heavy mast will tend to capsize the boat. It also will be harder to raise and will make moving the boat on land harder. The weight of the bottom section doesn’t matter too much, but the top 1/2 or 1/3 should be as light as possible.
  3. As thin as possible. The mast will catch the wind with the sail up and down. When the sail is up a thin mast will improve performance. When the sail is down a thin mast will have less drag when rowing and manoevering the boat.
  4. I want to be able to sell the Wanderer eventually. There isn’t much of a market for junk rigged boats so the hull must be able to use the bermuda rig with as few changes as possible. This means that the mast must fit through the existing hole in the deck which imposes a maximum mast diameter of around 71mm. It must also use the existing mast foot.
  5. Cheap. This is a hobby.
  6. Easy to make, if necessary.

Strength

As described above the strength required is normally expressed in terms of the righting moment of the boat. Probably the best way to determine this would be put the hull in the water and measure it. In the absence of that we’ve got to do some guesswork.

I drew up the boat cross-section at 30º heel angle and made a guess at where the waterline and centre of effort would be. Based on that:

  • Crew weight is 150kg (1500N) at 1.07m from buoyancy: 1574Nm
  • Boat weight is 130kg (1300N) at 0.36m from buoyancy: 459Nm
  • Total righting moment: 2033Nm

At vertical:

  • Crew weight is 150kg (1500N) at .875m: 913Nm
  • Boat weight is 200kg (2000N) at 0m: 0Nm
  • Total righting moment: 913Nm

Clearly the 30º heel angle is the one to consider. To be rigorous we should consider other angles but this is good enough.

It would be good to use a safety factor of 2 or more on the righting moment but this could result in a large mast. If possible the mast should be stronger than this; however it is worth noting that a Leisure 17 has been converted to junk rig with a 3.25″ x 10swg mast.

A Leisure 17 weighs around 670kg with 250kg ballast. Sail area is a bit larger than the Wanderer. The yield moment of the 3.25″ x 10swg is 2626Nm – surprisingly low given that the Leisure 17 righting moment should be around 3800Nm. However the boat has been sailed for over 6 years without problems so the mast is correctly specified.

So overall we probably want a mast strength of at least 2000Nm. More would be good but less might be a bad idea.

Weight

I weighed the current mast and cables using my kitchen scales: 9.6kg. This is the target. Less would be great, a bit more is probably ok, a lot more would be a problem

Partners and Foot

The mast must fit into the same location as the existing mast.

The maximum width available at the partners (deck-level) is 71mm, unless the mast moves back behind the deck. However generally with a junk rig we want the mast forward of the bermuda mast so moving it back probably won’t be possible.

The mast foot fitting is a rack. This appears to be robust and it should be easy to fabricate something to fit.

Low cost

I’ve got kids and this is a hobby – there are lots of more important things to spend money on. The conversion needs to cost a few hundred quid, max.

Easy to make

If there is significant work making the mast then it needs to be done inside. I have a waterlogged shed that I can squeeze a 5.6m mast into. I have checked my sail plans and I’m confident that I can fit a reasonably sail plan onto a mast this length.

I don’t much like glass-fibre work so wouldn’t want to try carbon fibre. However, I’m happy with wood and metal work.

Summary

Strength: ~2000Nm

Weight: ~10kg

Length: 5.6m

Maximum section at partners (deck-level): 71mm

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