unstayed masts

[matelot]

I often sail with a friend who owns a boat with a carbon fibre re-inforced unstayed mast. This has been severely tested strength wise (too embarrassing to relate /forums/images/graemlins/blush.gif ) and not found wanting, so I often wonder why more boats dont have them. Its a truism to say that planes dont have stays to hold the wings on any longer but nevertheless its a valid point to make. The other argument I've heard is all about slack in the forestay etc, but if you can design round aeroplane wings bending under load I'm sure you can do so for boats as well.

So why no more unstayed masts?

 

[TigaWave]

It's all about reducing the cross section, unstayed masts have to be a lot thicker, which doesn't help the aerodynamics.

 

[saltwater_gypsy]

The engineering at both deck level and at the foot of the mast is much more elaborate and rugged (and expensive) than with a stayed mast

 

[snowleopard]

Not so, the forces on the hull with an unstayed mast are actually lower than for a conventionally stayed mast and cost less to construct.

There are two reasons why unstayed masts are uncommon -

Cost (hand-made carbon fibre construction costs 2-4 times as much as conventional extrusions)

Conservatism. I well remember a poster on here saying at the time of Team Philips' first structural failure "any fool can see that an unsupported structure like that isn't strong enough". I presume he only flew on biplanes.

True, unstayed masts are thicker but they have less windage than a conventional rig with all the stays.

They also stay up rather better than stayed masts. Look at any stayed rig and count the number of components that could fail and bring it all down, then think about fatigue, hidden corrosion, missing split pins etc.

 

[damo]

Many thousands of boats do have unstayed masts....junk rigs, cat boats etc

The design is maybe a couple of thousand years old, so you could say it is actually quite successful /forums/images/graemlins/smile.gif

 

[Lakesailor]

Not quite sure how you work out that the loads are not higher on the deck and keel.
My experiments with an unstayed mast in my trimaran proved to destruction that the keel step and the deck-level aperture are under terrific strain.
Only fine weather will provide the opportunity to see if my modifications to reinforce the keel step and manufacture a steel frame to bond the deck and hull stresses together will have been succesful.

 

[Billjratt]

Part of the unstayed survival plan is possibly a monohull's ability to "roll with the gusts".
A cat however, isn't so accomodating...

 

[flaming]

From a performance point of view it has been tried a few times. Basic issue is that a rig without a foresail is slower upwind, the slot effect means an increase in efficiency. And a foresail needs a forestay. So that's one stay. To be able to sail upwind the forestay needs tension, which either means that the mast has to resist massive lever arm forces, or support in the form of a backstay or swept back spreaders is required.

For the '99 admiral's cup the French brought a boat called krayze kyote (or similar) which had a rig with no side stays but a forestay and backstay. The fact that it hasn't been heard of since is a good indicator of the sucess of the idea.

Basic problem is that even unstayed carbon cannot get the stiffness required to shape sails. An unstayed rig will have great off the wind performance, but to windward the stayed rig will power away.

 

[grumpy_o_g]

If you're interested in seeing what happens to an unstayed carbon-fibre rig when it's tested to destruction have a quick look at this page , especially this video of it being tested to destruction.

Although a glider wing does have some flex designed into it to act as "suspension" it would be impossible to reduce it much without thickening the wing considerably. Even though it's narrow the wing also gains some rigidity from the skin - the wings are only a 2" or so thick though so a lot thinner than a carbon mast. They have to hold 9G and the glider in this case weighs 750KG and has a wingspan of 20 metres.

 

[saltwater_gypsy]

quote Snowleopard "Not so, the forces on the hull with an unstayed mast are actually lower than for a conventionally stayed mast and cost less to construct."

Check out a Freedom at the earliest opportunity. The deck reinforcement is massive and the heel of the mast is very strong to take high loadings from all directions. The forces on the hull may be less but th same forces are transferred elsewhere. Vertical loadings on the hull at shroud attachment points become horizontzl forces at deck level with an unstayed mast.

One of the undoubted merits of unstayed masts is the "Give" at the mashead in gusts.This applies as much to a Laser as to a Freedom.

 

[bbg]

Flaming got it right in terms of sail shape. You have much more control of sail shape with a stayed rig.

As for forces, Snowy may have been speaking of the hull. Obviously the forces have to be transmitted somewhere - and that is through the deck aperture and the mast step.

As far as forces on the hull go, I recall a feature in one of the magazines about the forces on the hull of an America's Cup boat. The boat had to be designed to take enormous loads from the rig, and therefore had to be extremely strong and rigid.

IIRC, the analogy they used was this: you could lift the boat up by the forestay pin and the two runner pins. Then, concentrate the weight of a fully loaded 747 on the mast step. The hull wouldn't flex more than 2 cm.

Taking all those rigging forces out of the hull means it can be lighter, but of course the loads go elsewhere.

 

[peterb]

Wings

It's amazing just how much some wings can flex under load. Nowadays wings are designed to flex in a controlled way, but back in the days of the Lancaster they were expected to be stiff. Never the less, when RAE tested the Avro Lincoln's wing to destruction, the wingtip flexed by more than 11 ft before it broke, and that on a wing only 60 ft long.

 

[snowleopard]

OK, some simple calculations.

Let's take a boat of 3 metres beam, 2 metres depth of hull, where the centre of effort of the rig is 5 metres above the deck. The leverage (couple in physics terms) of the wind acting on the mast is side thrust, say 1 tonne multiplied by distance from CE to CLR, in this case around 7 tonne-metres.

In a stayed rig this is counteracted by the downforce of the mast on the keel acting against the tension of the shrouds. If our staying base is 2 metres, i.e. 1 metre either side of the mast, we will have a force of 7 tonnes down on the keel and 7 tonnes upward on the chainplates.

In an unstayed rig there will be a force sideways on deck and keel but as these are 2m apart the magnitude of the force is only 3.5 tonnes.

The structures have to be different because of the different directions of the forces but the overall strength does not need to be as great.

 

[Lakesailor]

That's not simple enough for me. It won't fit on the back of an envelope. /forums/images/graemlins/wink.gif

 

[Lakesailor]

I accounted for that by building a mast with a sailboard mast top section. It bends away and spills the wind.

see the avatar. Or a bigger version here

 

[saltwater_gypsy]

Its actually a couple of 3.5 ton.metres or whatever. A sideways force of 3.5 tons at deck level and an equal and opposite force at the mast base. The structure to deal with these forces is not a trivial piece of engineering.
Its the Marmite thing again. If you want an unstayed mast, there are plenty of reasons to justify it. Just check out the Vendee Globe casuality list!!!!!!!

 

[bilbobaggins]

[ QUOTE ]
That's not simple enough for me. It won't fit on the back of an envelope.

[/ QUOTE ]

SnowPussie's expertise is in catamarans. I can lend you a couple of envelopes. BTW, you are allowed to use both back and front. No-one will mind.

As the odd designer will tell you, if you cross his path with Heineken, it's not at all difficult to build a ring-beam into a hull to carry and disperse those forces. And there are other clever but expensive solutions. If you want to race with a hope of success, you need very clever and very expensive answers. If you'd rather just cruise fairly swiftly, and without a lot of paraphernalia, then unstayed can quite readily do it.

As has been demonstrated.....



Clickit.....

 

[nickrj]

I know this thread is a little old, however I was searching on google for unstayed masts, and came across this. I had some questions...

Are there any examples of cruising monohulls in the 50ft range which utilise multiple masts? From a completely non-engineering background, it would seem a ketch or yawl rig would spread the load better, and be more suited to unstayed rigs. Is that a reasonable assumption?

What materials could be used other than carbon? Why couldn't the design characteristics of an airplane wing be used as a mast?

Obviously there is a desirable simplicity in unstayed masts, but then again there is now also a single point of failure... Is there any cost savings whatsoever, if you guestimate a 30 year lifespan for the boat, and that standing rigging costs are no longer a factor?

Who are the companies or designers that are experts in unstayed mast design?

Cheers, Nick

 

[Freespirit1980]

interesting to read the various comments about unstayed masts versus stayed masts, carbon fibre versus aluminium, single mast versus multi mast. I played a leading role with the Freedoms as the CEO of Fairways Marine that held the exclusive licence outside North America with Gary Hoyt.

Fairways built the Freedom 40/44, 33/35 and 28/30 that were also built by TPI in Rhode Island and the only 70. These were all cat ketches apart from the three masted 70 with wrap around sails and wishbone booms. Fairways instigated the Ron Holland designed Freedom 39 pilothouse schooner but never put into production. The schooner rig was the contribution of Rob James and John Oakley, both of whom sailed Freedoms extensively and were significant ambassadors for these boats.

What has been lost in many of these posts is the critical difference between the wraparound sails with wishbone booms and the later fully battened mainsails on a track up the back of the masts that allowed Gary to fit his innovation of a self tacking foresail jib boom as first fitted on the Gary Mull designed Freedom 45 and 38. Why did Gary go away from the wrap around sail and wishbone boom? In my opinion, it was driven by the belief that the more conventional 45 and 38 were more acceptable to the American market place. The wrap around sail and wishbone boom with outhaul allowed the sail to be flattened or more shaped according to the point of sail and was much more efficient than a mainsail on a track up the mast. Think of the aerofoil shape of an aircraft wing to appreciate the wraparound sail. At Fairways we ran tests and found the wraparound sail to be over 90% efficient compared to a tracked mainsail at 60%.

The cat ketch rig with one mast right forward and an identical mast amidships gave a sail plan that had the slot effect. It is true that Freedoms going to windward were faster if not sailed too close to the wind. On a reach, the Freedoms were in their element and achieved speeds that far exceeded a Bermuda rigged boat. They had the added benefit of being self tacking and easy to sail shorthanded. All the sails were controlled from the cockpit including hoisting and lowering and reefing and outhaul to shape the sail. To reef, the halyard was lowered to a marker and the reefing line was pulled tight, a manoeuvre that took seconds only. These features made the Freedoms so easy to handle and improved safety. The Freedom 70 had three identical masts with all controls led to the cockpit.

The second generation Freedoms with their fully battened mainsails on tracks up the unstayed carbon fibre masts lost the off wind speed advantage of the wraparound sail but gained in ability to point higher. After I resigned from Fairways, I bought a Freedom 35 and later a second generation 38 to have a bigger boat. I was disappointed with the 38 much preferring the 35.

My strategy to sell somewhat strange looking Freedoms with wraparound sails, wishbone booms and an unstayed masts to a generally conservative sailing community was to persuade the most famous professional sailors of that time to race these cruising boats. And thus Rob and Naomi James for the ocean races and John Oakley for the round the buoys racing. John had been chosen to skipper Lionheart, the British America’s Cup challenger. He also headed the sailmaker Miller & Whitworth. Our primary target was to win the Whitbread which attracted the biggest global publicity. The Whitbread is mainly an off wind race, apart from the first leg to Capetown, at which the Freedom 70 excelled.

That the Freedom 70 never started the Whitbread was due to Fairways parent company, the Saudi Arabian Shobokshi Group, 4th largest in the KSA, not sending the committed capital. I resigned to try to force their hand. The other Whitbread competitors tried to have the Freedom 70 banned so Admiral Charles Williams and I played it up in the media to maximise publicity. We had agreed a compromise that the Freedom could not win the Whitbread prize but could take part and there would be a second prize that all participants could win. Flyer went on to win the Whitbread and had had a friendly race with the Freedom from Newport to Lands End beforehand. The Freedom 70 crossed in 13 days trouncing Flyer by four days. The winds were perfect for the Freedom 70 reaching the whole way.

In my opinion, the Freedom 70 would have beaten the other Whitbread participants and that success would have had a huge effect on the acceptance of the unstayed masts with wraparound sails and wishbone booms. We had arranged daily live TV broadcasts throughout the race.

Coming to the cost of building Freedoms with unstayed masts, actual boat costs were less than conventional boats because we saved on chain plates and all the rigging but against that the carbon fibre masts were more expensive. The strengthening of hulls and decks to hold the unstayed masts was simply done and inexpensive. The carbon fibre masts meant less weight aloft and a certain flex so a stiffer boat that would spill a sudden wind thrust.

I would choose the wraparound sail with wishbone boom and two masts every day over the unstayed single mast with fully battened main and the jib boom foresail. I wonder if there’s a visionary boat builder who would make what I would consider the perfect cruising sailing boat with that rig and sail plan. Perhaps a builder with one of Nigel Irens’s wave piercing efficient hulls. I arranged Nigel’s IT‘82 sponsorship for his trimaran that performed so well in that year’s Round Britain Race. Nigel later built his own unstayed sailing boats including Roxanne for his personal use. I own her smaller sister, the Romilly 22.