Mast bend in Masthead rig.

[feetup]

It seems obvious to me how you can obtain mast bend to flatten out the main on a fractional rig but what about a masthead rig? The only way I see that it can be done is to put some tension on the forward lower stays to prevent aft movement and force the mast to bend by cranking down on the backstay thus putting the mast into some serious compression, wherin it can only deform in the middle. With backswept spreaders there would be some forward force to get the center to deform forward as well.
I have spent much of my career trying to keep beams or members under compression from deforming and it is hard to wrap my head around this. It seems wrong on a few levels to me.
The loadings on the cabin top of a deck stepped mast forced into deforming this way must be enormous, coupled with the tension of the shouds they could equal the displacement of the boat!
It seems that the problem would be compounded on a boat without double lowers like the San Juan 23 and some others.
A baby stay would help everything out, but I have never sailed a boat with one , and it looks like a pain to tack a genoa with. I assume that you will need some form of beam or stay below decks to transfer the loading to the hull.

What am I missing here?

Feetup.

[bcooke]

The loadings on the cabin top of a deck stepped mast forced into deforming this way must be enormous,

Yep.

I am not one of the nerdy engineers around here but I think the greatly increased loads imposed by mast bending on a deck stepped mast and all the other problems it causes (as you stipulated) is exactly the reason why it is not really recomended and why serious race boats break all the time.

From what I can tell you are not really missing anything.

-Britton

[Figment]

With a deck-stepped masthead rig, I feel that mast bend is really a secondary product of backstay tension. Primarily it's about achieving headstay tension... minimizing headstay sag to maintain proper shape in the headsail.

Mainsail flattening or "leach dump" on a masthead rig is more about halyard/cunningham and outhaul tension.

[bcooke]

Boy, do I have a lot to learn...

[Robert The Gray]

I have spent much of my career trying to keep beams or members under compression from deforming and it is hard to wrap my head around this. It seems wrong on a few levels to me.
Feetup.

my $.02

The spar is not a building. In a building's structure the deformation of any one member must be minimized so as to maintain the integrity of the whole. When a structural beam is sized to small it is not just that it might catastrophically fail, but it's bending beyond a certain tolerance leads to cracked exteriors, jammed windows and doors, and a general sagging to the whole operation. A post in a building that deforms under normal live load requirements would be a problem indeed. The mast beam on a deck stepped boat is like that and should be designed to not bend.

I would think the aluminum spar is a bit different, both in use and material composition. I think the mast is more like a flag pole in that it can have some bending designed in to handle the loads. I think using the bend of the mast to influence mainsail trim is most likely limited to smaller boats (under 30') designed to be competetive in one design racing. I doubt many 35' ketches have bending spars.

My Santana 22 was mast head rigged with only one set of lowers, I put the mast bend in by tensioning the back stay, it was all I knew how to do. The most it would deflect was about 2"- 4". The mast was deck stepped but had a pivot/tabernacle at the base so the mast did not bear on a flat surface but on a pin. I would not try to bend a mast that was not designed to bend, I do not know if the San Juan 23 was. From what I have read even though the Triton is fractionaly rigged, the mast should not bend. It should be kept in column as much as possible using the aft lowers. A bending aluminum spar would have to be sized to bend under the right conditions, and have a pivoting bearing surface I think. Once the section size of the mast increases to a certain point I imagine bending becoming unworkable.
These are my suppositions. I am not an engineer.

r

[Hirilondë]

Only masts specifically designed to be bent should be. And most aren't. Even if your mast is supposed to the little you gain in sail shape and hence speed has to be weighed against how well your boat can handle the strain, and whether you can justify the damage it may cause for this little speed. My mast is an aluminum telephone pole, I adjust my back stay to keep my forestay taught but not like a banjo, and quit while I am ahead.

If you like bent masts buy a Narraganset Bay 'S' Boat :)

[FloatingMoneyPit]

Headstay tension---just how tight should it be? When I grab my furled-up sail and give it a shake, it wiggles a little more than my gut would like. When sailing upwind in a moderate breeze, there's a pretty good sag. Probably a foot or so, on a 33 ft luff. Nothing has changed structurally since last year (mast step, rigging, chocking, etc), so I'm not certain why this is happening. Cranking down on the headstay turnbuckle and backstay helps, but not enough. When I do the wiggle test on other boats in the marina, they seem much firmer.
I better stop typing before this sounds any more like a Viagra commercial.

[Tim]

There is no single answer that covers all boats, all rigs, or all situations. The specifics of the individual circumstances are important in order to determine what is OK, what is not, and what, if any, rig bending and under-sail adjusting one should be doing.

Most people set up the rigging way too tightly, particularly on older boats. Things just aren't supposed to be drum tight on most older designs. The rig doesn't need it, and the structures supporting the rig aren't designed to handle it. Stays should be tight, but not too tight.

Rigging on a cruising boat of the type we're generally discussing here should be set up tight enough to not seem sloppy at rest, and should then be adjusted only as necessary to keep the mast straight while under sail. This is what mast tuning is all about. A mast that is straight at the dock may form funny bends under sail on one or both tacks. Some adjustment is necessary to straighten the mast, but adjustment doesn't always mean extra tension. It just means "adjustment".

Beyond this, tight enough is tight enough. You don't need to be able to strum out a flamenco tune on your boat's stays. Leave the silly rig tension gauges on the store shelves where they belong. People who use these end up no better off than their neighbors, and also typically overstress and overtighten the rigging for zero gain. Besides, there's no such thing as a generic value that works for all boats, or even for sisterships from the same class.

Forget what the racers are doing. Race boats are designed to accommodate mast bend, both in the design of the rigs and in the way the chainplates, mast support, and other structures are designed. Bendy masts are indeed an important part of sail trim in those instances. The sails are even designed with this in mind. Untapered telephone pole masts from the 60s are not designed for bending. Under the CCA racing rule, in fact, mast bend was prohibited entirely.

Most masts are not intended to be tweaked and bent as part of the routine sailing process. Trying to do so doesn't accomplish much, and will only result in overstressing portions of the rigging. People who race on boats without masts designed for bending and high tension usually like to pretend they're the same as the more modern boats, and therefore crank on parts of the rigging in a futile attempt to keep up. Don't. It doesn't accomplish what you want, and will, over time, cause more harm than good. Racing requires a racing boat, or at least an acceptance that you are racing on a cruising boat, with its inherent race limitations.

Headstay sag is inevitable, period. Even the racers with hydraulics and 40:1 tackles end up with some headstay sag--it's just not possible to prevent it entirely. For racers, it's about minimizing the sag, not eliminating it. But race masts and sails are designed with this in mind. It just doesn't apply generically. That said, excessive headstay sag shouldn't be part of any sailor's repertoire. But there will always be some.

Most headstays on cruising boats end up with significant and noticeable sag, but as long as the rigging is not loose and floppy at rest, there's nothing to worry about. Tensioning the backstay isn't going to help enough to remove the sag, and will only overstress the entire setup. Obviously there are limits to how much a headstay should sag. But I suspect that too many people worry unnecessarily about what is an entirely normal and acceptable level of sag. There is no hard and fast rule or figure for how much is OK and what might be too much, so it's not possible to say what is OK in any specific case without seeing it in person.

A boat like a San Juan 23 may have a rig designed to accommodate some bending, since it is a 70s IOR-inspired design. Boats with single lowers and swept-back spreaders often have a bit of prebend as part of normal setup, whether masthead or fractionally rigged. Tensioning the backstay on a boat with swept-back spreaders does produce mast bend in the center; this sort of mast bend, assuming it is a designed function of the rig, is used to flatten the main and move the draft forward. Fractional rigs are typically more bendy than masthead, thanks to the unsupported top section, but plenty of masthead rigs are designed to be bent. In both cases, headstay tension is part of the equation, but the ability to fine-tune the sail shape is also part of it.

Typically, truly bendy masts are found only on boats with keel-stepped masts, as the mast partners are an important feature in achieving the proper bend in the proper area, and also in terms of the stresses on the mast step. Overtensioning a deck-stepped mast usually leads to problems with deck compression, compression post or beam problems, and elongated chainplate holes. These problems are extremely common in older boats thanks to the generally paranoid need among boatowners to crank down every bit of rigging to its limits.

[Ric in Richmond]

Question on spreaders.

When you grab the shrouds and pull fore and aft should there be NO movement of the spreaders or some minor tolerable amount?

Mine wiggle a little bit, but it appears to be in the joint...not the wood to the metal, but the metal to metal joint.

Haven't jumped on board and wiggled anyone else's spreaders to check.

[catamount]

Back to the original question about mast bend in a masthead rig -- with a "noodley" keel-stepped mast this is done by setting the position of the mast step relative to the partners and the head and backstay lengths. Pulling back on the heel of a mast that is fixed at the partners and at the head (by the stays), will induce a forward bow (that's bow as in bow-and-arrow, not bow as in bow of the boat) to the mast. This is called "pre-bend." Further bending of the mast is accomplished by cranking on the backstay, which compresses the "bow" of the mast. A babystay may also be used to induce additional bowing. Checkstays and running backstays can be used to control the amount of mast bend, as well as for providing additional support and to limit mast pumping. Some sailors worry about "inverting" the mast, which means the mast is "bowed" backwards towards the stern, or has taken on a fore-and-aft "S" or "Z" shape. They might use chicken stays to counteract the force of the spinnaker pole pushing back on the mast, for example. Inversion could also be caused by over-zealous tensioning of the checkstays. But most of you probably don't have to worry about any of this.

Regards,

[Tim]

Question on spreaders.

When you grab the shrouds and pull fore and aft should there be NO movement of the spreaders or some minor tolerable amount?

Mine wiggle a little bit, but it appears to be in the joint...not the wood to the metal, but the metal to metal joint.

Haven't jumped on board and wiggled anyone else's spreaders to check.

Older spreader bases tend to have a bit of slop thanks to wear, whether they are metal sockets with metal spreaders, or metal brackets to which are bolted wooden spreaders. The amount of slop, if present, should be minimal or hardly any, but there is frequently this small amount. It all depends on the setup. Some kinds are naturally more sloppy than others, while with other types of configurations even a small amount of movement means that something has gone quite wrong.

Can you clarify your spreader configuration? You say you have wooden spreaders, but I'm not exactly sure what you mean by the "metal to metal joint", unless you mean where the spreader base bolts to the mast. If that's the case, there should be no slop there.

[Ric in Richmond]

Sorry to hijack the thread...

The spreaders have a metal piece attached to the heel of the wood spreader.
The metal piece then inserts into a piece that is attached to the mast. My perception is the slop is in the metal to metal piece.

[/img]

[Ceasar Choppy]

Does your spreader attachment look like this?



Note the box that the spreader sits in is actually metal-- it was just painted over.

[Tim]

From the sound of it, I don't think you should be seeing a lot of slop in that sort of arrangement. However, it could be as simple as some loose bolts that need tightening. I'd take a closer look, though, in any event.