How much mast can be unsupported by rigging?

[Quandary]

The issue is not the strength of the mast which looks adequate but the method used to fit the sleeve it fits in to the hull to avoid the risk of tearing the floor out, a lever this long will transmit big loads so the hull is likely to fail long before the mast breaks unless the support it is well braced or triangulated in some way. A couple of simple shrouds might be the safest and cheapest way to do this.

 

[Niffler]

The issue is not the strength of the mast which looks adequate but the method used to fit the sleeve it fits in to the hull to avoid the risk of tearing the floor out, a lever this long will transmit big loads so the hull is likely to fail long before the mast breaks unless the support it is well braced or triangulated in some way. A couple of simple shrouds might be the safest and cheapest way to do this.

Fascinating I never would have thought that there is a big sideways load at the foot of an unstayed mast.

 

[Owl]

Go on, I'll bite.
How do you work that one out?
Work out the compressive stress in the leeward side of the mast for a cantilever unstayed mast and then try again...

Depends on the boat and how you use it.

Most of the load in a stayed mast is the compressive load of the stays. For the boat, the mast is being pushed through the bottom and the chainplates are being pulled off the deck. Get rid of the stays and you lose most of the stress.

The mast needs to be strong enough to knock-down the boat, ideally a little bit stronger. This means unstayed masts are riskier on a cat and easy enough on a dinghy. In my case - a 'traditional' GRP clinker dinghy - I can't hike out so that limits the force applied. It is junk rig, so very fast and easy to reef - I can reef when I see the gusts coming in - which again limits the force. If the wind is too strong I don't go out, again limiting the force.

The mast I've got is a very old aluminium windsurfing mast. It is about 53mm diameter and about 1.4mm wall thickness. I actually figured it would buckle first time out but it was cheap, available and very light so worth trying. During tests I found that the main problem is flexibility when close hauled - the mast bends back rather than the sail coming in. However, it was plenty strong enough to cope with 20k gusts in a fair chop so overall I think it is ok.

All the figures for mast design are available in Practical Junk Rig - expensive but good value when you find out how much information is packed into it. There is also a spreadsheet available through the Junk Rig Association that shows what many of the members are using on their boats against size, weight, beam etc.

 

[Owl]

Thanks, junk was another option but making the sail myself would be more complicated. Does it make any difference to the integrity of the mast if it is in a collar at deck level or in a sleeve?

FWIW I figured that using two attachment points (partners and keel step) means the burried bit of the mast can bend as the mast moves, which limits the stress concentration at the partners. A sleeve would result in no flex below partners and a big stress concentration in the mast. This is just gut feel.

Junk rig was easy but time consuming to make. A small junk rig has almost as many parts as one for a 40' yacht so takes a similar amout of time to make. Designing all the details also took a lot of time. However it was worth the effort!

 

[Quandary]

Fascinating I never would have thought that there is a big sideways load at the foot of an unstayed mast.

Well if you only sail downwind there wont be, however if you want to go the other way all the loads you are converting to provide forward motion are from the side.

 

[DownWest]

FWIW I figured that using two attachment points (partners and keel step) means the burried bit of the mast can bend as the mast moves, which limits the stress concentration at the partners. A sleeve would result in no flex below partners and a big stress concentration in the mast. This is just gut feel.

Junk rig was easy but time consuming to make. A small junk rig has almost as many parts as one for a 40' yacht so takes a similar amout of time to make. Designing all the details also took a lot of time. However it was worth the effort!

Not just gut feel. A tube would concentrate the stress at the point were the mast exits the tube. A keel step and partners at deck level allow the mast to flex between the two. OK, it won't be much, but in theory the mast will flex a bit to windward between them, so the stress is a bit less at the point above the partners. Probably negledgable in your case.

Niffler. the load at the keel is about the same as the deck, just in the opposite direction.

 

[bignick]

Depends on the boat and how you use it.

Most of the load in a stayed mast is the compressive load of the stays. For the boat, the mast is being pushed through the bottom and the chainplates are being pulled off the deck. Get rid of the stays and you lose most of the stress.

Agreed, the load paths are completely different.
When a stayed mast is upright, no sails and no heeling moment, the shrouds are in tension and the mast in compression.

When the sails apply a heeling force, the moment on the rig is taken by the compressive force in the (whole cross section of the) mast, an increase in windward shroud tension and a reduction in leeward shroud tension. Eventually, as heeling moment increases, the leeward shrouds go slack.

Now think about it in terms of the second moment of area. The shrouds are a long way from the neutral axis, so they are very effective. They are also usually stainless....

In an unstayed I.e. Cantilever rig, the entire heeling moment of the rig is taken by the compressive force in the leeward side of the mast and the tensile force in the windward side of the mast. They are both close to the neutral axis so are not as efficient, thus localised stresses are very high.

The deck and hull are no longer in vertical tension/compression, but you have now got higher transverse forces instead, which transfer the heeling/righting moment between the rig and hull.

I wouldn't agree that you lose most of the stress, it just resolves itself in different parts of the rig and can be heavily concentrated in certain places. I'm not saying that unstayed masts are bad, they're not, but they pose some very different questions to a stayed rig.

 

[Lon nan Gruagach]

Agreed, the load paths are completely different.
When a stayed mast is upright, no sails and no heeling moment, the shrouds are in tension and the mast in compression.

When the sails apply a heeling force, the moment on the rig is taken by the compressive force in the (whole cross section of the) mast, an increase in windward shroud tension and a reduction in leeward shroud tension. Eventually, as heeling moment increases, the leeward shrouds go slack.

Now think about it in terms of the second moment of area. The shrouds are a long way from the neutral axis, so they are very effective. They are also usually stainless....

In an unstayed I.e. Cantilever rig, the entire heeling moment of the rig is taken by the compressive force in the leeward side of the mast and the tensile force in the windward side of the mast. They are both close to the neutral axis so are not as efficient, thus localised stresses are very high.

The deck and hull are no longer in vertical tension/compression, but you have now got higher transverse forces instead, which transfer the heeling/righting moment between the rig and hull.

I wouldn't agree that you lose most of the stress, it just resolves itself in different parts of the rig and can be heavily concentrated in certain places. I'm not saying that unstayed masts are bad, they're not, but they pose some very different questions to a stayed rig.

Only true if the leeward shroud goes slack, something it really mustnt do. There will always be a situation (gust, bouncing off a wave etc) that will exceed the force required to make it only just slack. The shock loading as tension is reapplied will damage parts of the boat. So rig tension is set so the leeward shroud never goes slack so there is always more stress on the hull from a stayed mast.

 

[lw395]

Only true if the leeward shroud goes slack, something it really mustnt do. There will always be a situation (gust, bouncing off a wave etc) that will exceed the force required to make it only just slack. The shock loading as tension is reapplied will damage parts of the boat. So rig tension is set so the leeward shroud never goes slack so there is always more stress on the hull from a stayed mast.

Stress is the wrong word really as it implies knowing what area of material is carrying the load.
On a yacht where the distance mast heel to partners is similar to the half-beam, the force at the partner would be similar to the shroud load, becasue the lever is similar. On a shallow dinghy, the force might be a lot more than the shroud load. You are dead right about pre-load, although some dinghies manage very well with slack shrouds.
But making the hull strong enough is not hard.
The problem is the mast must be fat, heavy or both on an unstayed rig. Obvously you can reduce this effect with a carbon mast.
Many fine boats have unstayed rigs, but in general a stayed rig gives less weight aloft and that is always good.

 

[bignick]

Many fine boats have unstayed rigs, but in general a stayed rig gives less weight aloft and that is always good.

.....

...because less material is required in the cross section of the rig....
... Which is because the compressive/tensile forces in the wall of the mast are lower.

Mast material properties (Yield or 0.2%proof stress) don't change because of the design configuration. Higher loads require more CSA for compressive/tensile forces, or increase in I to resist bending moments.