stayless wishbone rigs

jimi

jimi

Well-Known Member
Joined
19 Dec 2001
Messages
28,660
Location
St Neots
Visit site
How strong are they? My thougt is that stays spread the load and therefore the same amount of stress is taken over a larger area whereas with an unstayed mast presumably all the force is at a single point. Mind you it would mean not having to carry shroud cutters? Thoghts?
 
I know they are very strong, good etc etc. BUT! I'm sorry, they just dont look "right" to me. IMHO /forums/images/graemlins/grin.gif
 
Seems to have worked for the Chinese who are supposed to have explored widely in junks.

To me the least appealing feature is the stress-concentration on the mast at deck level.
 
[ QUOTE ]

To me the least appealing feature is the stress-concentration on the mast at deck level.

[/ QUOTE ]

That's exactly the point I'm making in the original post, both the stress on the mast and also the hull, presumably the hull is speecially strengthened?
 
They say the loads are less and I can understand why.

On a staylerss rig you have the weight of the rig pushing down and the sideways force of the wind. The former is supported by a strong mast step on the keel is latter by a much strengthened section of deck.

On a stayed rig you have both the above plus the down force component of the rigging loads which can be considerable.

Provided the mast itself is strong enough to take the stresses while supported only at the heel and partners then the rig should impose less strain on the hull and be safer. Because the whole rig tends to fall off to leeward in gusts, paricularly at the head, there is a certain amount of shock absorption as well. There are also fewer components to fail.
 
Think I understand that, however, as you say, both the mast and the hull must be stronger. I presume an ordinary boat can be modified or would it have to have particular charactristics?
 
I'm exploring what's known about this, and I'd be very interested in any pointers to real knowledge and experience - especially related to doing the calcs needed for the heel and mast-partner loads.

Fishing luggers went unstayed for a long time, and material science should be able to do a better job for us today.

I know the various 'Freedoms' used unstayed masts, both of carbon and Al alloy, so there's know-how out there.


/forums/images/graemlins/wink.gif
 
Think ringframe ; tying the step and deck area together and feathering the loads into the hull-bit like what shrouds do ,especially when through bolted to a bonded in main bulkhead.
 
Jimi
Try it with yours, all you have to do is screw down a bit of 4x2 where the mast goes through the top bit then cut all the wires.

I guess it's cause you're too tight to replace the rigging?

Ian
 
I'm not sure the hull needs to be stronger. The deck certainly does at the partners, but I don't see why the hull would need to be stronger.

On the contrary, the hull needs to be quite stiff on a stayed boat to prevent it bending from the stay loads, which can be considerable. I remember reading, many moons ago, an article in (I think) Yachting World about the latest generation of AC boats.

The article said that the compression loads at the mast step were enormous. How enormous? Imagine picking the boat up by the forestay and the runners. Imagine putting a fully-loaded Boeing 747 on the deck (actually concentrating its weight on the mast step. Imagine the boat only deflecting by a couple of cm (perhaps less?).

I might be a little off on the details, but that is a seriously strong hull - and it needs to be strong only to take the stay and compression loadings.
 
Spent quite some time sailing my mates Freedom 39. Very very easy to handle as tacking was a doddle. Crap on the wind because as soon as wind pressure built up the masts bent to leeward and spilt all the wind, never got more than 100 degrees between tacks at the best.

BUT the really difficult issue was tipified by an incident in St Malo when the crew of the outboard boat were creeeping back on board at 0300 rather the worse for wear and an enormous crash was heard on the foredeck. The next morning, I looked at our sheepish neighbours and casually asked who staggered on board and automatically reached for the mast stays - only to find we didn't have any!!

Oh and the other weird thing, SWMBO has sailed many miles with me and is rarely sick, but on this boat she was sick in a flat calm - it must have been some subtle change in motion caused by the masts being in unusual places.
 
As I sail under an unstayed rig I have looked into the stress issue fairly carefully.

A conventionally-stayed mast requires the opposing forces of thrust from the mast step upwards and from the chainplates downwards. An unstayed rig has a thrust horizontally to leeward at the step and an equal one to windward at deck level. The distance from centreline to chainplate is quite similar to the distance from step to deck so the forces on the hull will be fairly similar for the two types of rig. The difference is that the forces act in different places so the deck needs to be stronger in the unstayed rig but this is compensated for by not needing to be very strong below the step.

The forces are fairly small in terms of modern GRP construction. In my case the maximum forces come at the point where the windward hull starts to fly: in that circumstance the side force at the heel is 7 tonnes.

The much more serious issue is the bending strain on the mast at the exit from the deck. Mine is a carbon fibre tube around 1 ft diameter with a wall thickness of over 1". The mast weighs about the same as a conventional mast and stays put together. That's quite a lot considering it's mostly carbon.

The benefit is drastically improved windage and a much more efficient rig.

The question of the masthead sagging to windward is a design issue. A tapered cylindrical pole will sag to leeward if it is thin at the top but a wing section bends to windward which is weird the first time you see it. In either case it eases the heeling moment by opening the leech.

For those who are worried by the lack of stays, ask yourselves how many individual components there are in your rigging which could bring the mast down if they failed. Steel is subject to fatigue failure, Carbon isn't.
 
[ QUOTE ]
How strong are they? My thougt is that stays spread the load and therefore the same amount of stress is taken over a larger area whereas with an unstayed mast presumably all the force is at a single point. Mind you it would mean not having to carry shroud cutters? Thoghts?

[/ QUOTE ]

When did you last fly on a plane whose wings were supported on wires? Sure you have to design differently for an unstayed mast but there is no reason why it shouldnt be stronger.

With boats there is another issue of course - aerofoils that arent solid and cant contribute to the support like wings do. To make it more difficult, the common rig is bermudian, so there is the issue of forestay support and windward performance. So whilst I have no doubt about the strength and suitabliity of unstayed masts for long distance, performance isnt as good as a conventional rig.

A pal had an accident in a Freedom with unstayed carboin fibre masts. Did serious damage to booms, sails etc and would undoubtedly have ripped the mast out of a conventional boat like my own. No damage to the masts at all. To say I was impressed is an understatement.

None of these comments would apply to ally unstayed masts which I believe have been an issue on Freedoms.
 
You must log in or register to reply here.

Share this page

Top