Legend or Jeanneau?

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Re: Spade rudder

Always seemed a bit of a dodgy argument to me. Given the top of a spade rudder is flush against the bottom of the hull, any bend in the stock will mean the top of the rudder pushes against the hull and jambs against it. The only situation when this wouldn't happen would be if the rudder was bent sideways only- unlikely given a yacht moves forwards.

Yes, a skeg might jamb the rudder, but isn't the point that it would take a larger knock to bend a skeg and rudder than it would to bend a spade rudder?

I think I must be missing something, as pleanty of designers seem to agree with Harle's reasoning, but what?

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Re: Spade rudder

Both a skeg and a spade rudder are both cantilevered - only one is a round shaft (normally, but sometimes not so away from the bearing surfaces) and the other a plated structure. I do not know any reason why the shaft of a spade cannot be designed to be strong enough just as a skeg can be.

John

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Re: Spade rudder

Perhaps he wasn't expecting the stock to be of rather badly constructed GRP. This argument goes back further than the 1979 Fastnet. If a spade rudder stock is made from beefy metal there is a good chance that it will survive impacts and other damage. If it's made of a non-metallic composite with unfavourable fracture characteristics then the chances are it won't.

I guess that when it comes down to it, a skeg could be knocked right off but the rudder would still function, after a fashion.

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Cantilevers

Perhaps it's a language thing. To me, cantilever = overhung. A spade rudder is overhung, i.e. it has a bearing at the tiller, one at the lowest point on the hull but nothing else. Lateral force applied to the rudder is resisted by the bearings that are probably closer together than half the total rudder + tiller length, so bearing stress is relatively high.

A skeg-hung rudder is not a cantilever, as it has a bearing at its lower end. The skeg can be made quite strong, providing considerable lateral and axial support for the rudder. A far stronger arrangement with lower bearing stress.

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Re: Spade rudder

If the rudder hits something, that object will be coming from more or less ahead, given a boat moves forward. A skeg is long in this direction, much longer than the diameter of the stock of a spade rudder. As strength goes up with the cube of the depth in the direction of bending, a typical skeg would be approximately 1000 times stronger than a rudder stock, if they were both made of the same material. Of course, a stocks typically steel, where a skeg is GRP, but you can still expect the strength of a skeg to be an order of magnitude or two stronger than a spade hung rudder.

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Re: For what

Word has it from people at the yard OPEL operate from - is that this rig is causing quite a lot of problems. - damage at deck level being mentioned. The compression forces must be huge!

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Re: Spade rudder

I guess downwind, with ocean swells is stressful for rudder stocks. In a perfect world one might have tried to blow downwind under twin jibs. That then leads to the issue of the rig - it just cannot be an optimum solution to have a rig without backstays, and simply rely on extremely aft swept spreaders - as others comment this at best puts huge compression strains on the rig, at worst renders it imperfectly supported.

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Re: For what

I'm sure the rig can work quite well. But if you were running in a gale and the boat ploughs into the back of the wave ahead, would you really prefer a boat witout a backstay over one with a backstay? I cant see how it is safer to engineer the backstay out. You have effectively reduced the angle of the stay supporting the masthead, and as such you have increased the load in stay, and by Newton increased the compression load in the mast. Most rigs built for strong winds do absolutely everything to reduce the compression load as it geometrically increseases the risk of sidewall buckling. You only have to see the efforts of the Volvo boats to use halyard locks to reduce compression despite the problems of engineering them. OTOH for coastal cruising maybe there is an advantage, in not having a backstay.

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Re: For what

The Volvo boats try to reduce compression loads because they use flexible masts inorder that they can alter their shape. Flexible masts can't carry comprssive forces (wich is why dingies with unstayed masts, such as finns cleat the halliards at the top of the mast).

However a cruising B&R rig is designed such that once the prebend is put into the mast, that shape is locked in, and you don't try to change the shape while sailing (Less efficient, but easier). Because you don't need a flexible mast, you can use a different mast section, optimised to carry compressive loads. The rigging will need to be a size larger, but so what. So yes, the loads in the rig will be greater, but its easy enough to engineer the rig to take them. And in exchange, you obtain a (potentially) much more efficient rig.

The advantage, from an engineering point of view, is that the loads on the hull can be much less, through the use of a space frame. The space frame will take all the rigging loads, eliminating the really large loads due to the rigging tension, and the only load transfered to the hull itself is the heeling force from the sails.



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Bergstrom

Jacket, I understand the logic behind the backstay-less rig, but having sailed with one on a Legend 35, I felt very frustrated not to be able to take out forestay sag upwind, and not to be able to run very square without risking mainsail damage.

i can't help asking why, if the rig is so good, no other production builders use it?

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Re: For what

Presumably the reduced stresses are why Opal are reported to be seeing compression problems in the deck structure.

I think we will have to agree to disagree - I like my backstay holding the rig up, I like to be able to flatten the main by bending the mast, and I loathe weight aloft, which is why the new boat is having plastic rigging!

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Re: Spade rudder

I assume from your occupation given in your bio that you know you are pulling my leg.

As the rudder is at the back of the boat and also largely protected by the fin keel, I am not very conversant with damage to rudders from collisions with objects when the boat is moving forward. It may be that the event is not very frequent.

If what you say is true about the strengths of skegs then I have to ask why it is that the skeg is normally the weakest part of a steel yacht, should it be so fitted with one. Skegs are known to fail at the root or the hull plating surrounding that through fatigue from the cyclic forces experienced and this is a particular issue with steel yachts which are normally strong in other ways. Vessels have been lost because of that, so spade ruddered boats do not have a monopoly on failure at all in the rudder department.

I would think you would find that a typical steel skeg is nothing like 1,000 times as strong as a steel spade rudder stock would be as you claim for the case of their being of the same material, simply because of the root issues. In the end a skeg's integrity is dependant upon the supporting structure at the root end, not upon its depth along its length, and a spade stock is just the same so dependant. There is no reason why a cantilevered plated skeg has any particular monopoly on strength over any other structural section, including a spade's round stock - either can be designed to be sufficiently strong. Of course, if you wanted to make the skeg over sufficiently strong just to be stronger than a spade stock ever could be, then one is welcome to do so, however there would not seem to be much point to that and is bad design.

John


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Re: Bergstrom

The rig's good, its the hulls that Legend stick them on that are the problem.

A B&R rigneeds a substantial space frame to carry the rigging loads from the shrouds. Legend don't seem to be building stiff enough frames into their boats any more, and so it's not possible to get enough rigging tension to support the forestay.

The rig is also better suited to much lighter hulls, as it then becomes faster to gybe downwind in a series of broad reaches. It then never becomes necessary to sail square, appart from occasionally in confined waters.

The summer before last I was lucky enough to sail a 28ish foot day sailer/weekender fitted with a B&R rig. It was a one off that had been designed to suit the rig, and sailed wonderfully. The forestay stayed reasonably tight up wind, and could be tensioned further with the mainsheet, dinghy style. Offwind was great fun- sailing a series of reaches is much more comfortable than rolling downwind in steep seas, and the worry of accidentally gybing is removed.

So yes, I like the rig, but only on a hull thats designed to take it. And such a hull is expensive, which probably explains why other production builders don't use it.

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Re: Spade rudder

Nope, I'm serious.

Fatigue failure is down either to poor design or poor construction. Both types of rudder are suseptible (I must learn to spell), and there's not a lot you can do except stick to boats whose build quality you are confident about.

The other is hitting things. Yes it can happen- I've done it (and spent 5 days stuck on a small, uninhabited island in the back of beyond repairing it in the pouring rain). Also, the number of boats that get lobster pot markers wrapped around their rudders prove that objects can swirl around the keel and hit the rudder. And in thie case of collisions, I stick by my earlier argument.

As you say, the strength of the rudder is also dependent on the structure its built into, but provided that is sufficient, which it will be, provided the designers half way competent, then a skeg hung rudder is stronger. Though if yacht designers are anything like bridge designers, then half of them aren't competent and shouldn't even be designing rubber ducks.

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Re: For what

<Presumably the reduced stresses are why Opal are reported to be seeing compression problems in the deck structure.>

Thats because, if the one I looked at at the London boat shows anything to go by, they seem to be trying to save money by not building a proper space frame to take the rigging loads. Not the fault of the rig, just the hull its bolted to.

I take your point about the rig not being 100% efficient and tweakable. I guess it depends what you want. When I'm landyachting or dinghy sailing I tweak like mad. When I'm on a yacht, I tend not to tweak so much, and so like a rig thats 95% efficient all the time, rather than 100% efficient for the 10% of the time I'm playing with it, and 90% efficient for the rest of the time.

I should point out that all my comments are based on a single, lovely, B&R rigged boat that I sailed, which went like a dream. It may be that the boat was so well designed that it didn't matter that it had a rubbish rig.


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Re: For what

... and I think I'm talking more about the boat, and the version of the rig it carries. Perhaps the worry is that most are fitted with a furling main, and none actually have the full Bergstrom Tripod - so the engineering is compromised.

And I still dont like the safety aspect of a rig that doesnt have a backstay...

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Re: For what

Just stating what I read.
It sure seemed to satisfy the Americans.They just think someone is building poor copies of the US Hunter in a shed somewhere overseas.

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Re: Bergstrom

I'm not sure I like the idea of tensioning the forestay with the main sheet.

As the wind increases you need incresing forestay tension to take out the sag. Lots of tension. So, you harden up the mainsheet as advised. Unfortunately as the wind fresshens, what we really want is lots of forestay tension, but to start easing leech tension in order to allow some twist in the head of the main. So, now we ease the main tension to generate twist, but now we've induced forestay sag, the draft in the jib has moved forward, and we've also induced a fuller jib.

Strikes me that we have two mutually exclusive requirements at work here.

However, if we could induce headstay tension some other way than by tightening the mainsheet we might be on a winner...

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I didnt like what I saw of the Legend at the boat show, and the above comments reaffirm the impressions I got. I would in any case be wary of buying the marriage of an American attitude to cost and profit, and British attitude to workmanship and quality. Ford, for illustration.

The French boats I have sailed on flotilla had all withstood the abuse they got very well.

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