sailing rig theory mind teasers

[markxengineerin]

First scenario, emergency response plan for various rig failures:

In the event of a shroud failure, we all know to tack. That leeward shrouds can be slack or close to slack in normal conditions is proof that they aren't doing a whole lot to keep the mast up, so it makes sense.
Forestay failure- aim dead downwind, this one is also intuitive because most of the aero force on the boat is in the same direction as the wind- unloading the forestay. But the tricky one is a backstay failure- is there a point of sail on most boats when the backstay can be "removed" with only minor consequences? I think the options are to point into the wind, point into the wind + drop sails, or just head upwind. Which of these, or something else, is best while temporary repairs are made?

Second scenario, just thinking about ideal rig tension.

In my mind, a mast being held up by pre-tensioned wires is similar to a bicycle rim, strung to the hub by pre-tensioned spokes. In wheels, the tension of the spokes has no impact on the stiffness of the wheel- the tension only needs to be enough to prevent "slack", which in the case of the wheel, can try to put spokes in compression instead of tension, obviously bad for a number of reasons.

I believe the same principles apply to our sailboat rigs. If true, it means:
1) there is no purpose for pre-tension other than to prevent slack from accumulating and allowing inertial/impact loads to build up.
2) The mast and sails are going to behave the same, neglecting second order effects, for any pre-tension level between "very loose, slack barely removed while sailing" and "very tight, almost breaking something"
3) The windward shrouds do not gain any tension as the boat heels over- the force balance is actually accomplished by a decrease in leeward shroud tension
4) The ideal pre-tension, for maximizing rig life, is a tradeoff between avoiding infrequent impact loads (too loose) vs. higher average stress almost always (when too tight)
5) There is plenty of wiggle room (awful pun) in this scenario, which might explain why riggers all do it a bit differently. I have seen numerous "rigging how-to guides", but none of them explained it like this, from a theory perspective.
What do you think?

[Lee Jerry]

You don't distinguish between racing and cruising (even though this is Cruisers Forum). In theory, the effects are the same/similar, but cruising masts are usually much heavier and stiffer so they will be reduced. However, looking at race boats can still be instructive.

Scenario 1 - get the main sheet on hard as quickly as possible. The next step might depend on the weather. If light or moderate, you could probably turn into the wind and deal with getting a spare halyard secured and then sails down without much concern. If heavy weather and going downwind, though, it may not be possible to turn across the wind to get upwind and you'd have to get the brace on and sails down staying downwind. What you do (in terms of getting sails down) kind of depends on the circumstances - day sailing a mile from shore will be different than cruising off soundings; and which direction is "home."

Scenario 2:
1. The purpose of pre-tension is to control the shape of the mast and get the proper shape in the sails, particularly the main, in response to the loads generated in the wind conditions.
2. No, but I think the easiest way to demonstrate this is to point out that that is why race boats tune the rig to the expected weather.
3. I think the slack in the leeward shrouds comes from the mast falling to leeward, with increased tension in the windward shrouds and reduced tension in the leeward shrouds.
4. The ideal tension is to get the proper shape in the sails, not extend the life of the rigging. See #1.
5. Lots of wiggle room for cruising boats, less for race boats.

[markxengineerin]

Scenario 1: So the main sheet acts to de-tension the backstay? I came to my cruising boat from sailing Lasers, and with that memory, this makes some sense. Laser main sheet and vang bar-tight = lots of mast bend aft, which would de-tension a backstay, if a Laser had one. But I think that only works going upwind? Is it better to drop sails or beat upwind with vang and main tight? I guess any headsails be immediately let fly/dropped/furled/dropped?

The purpose of pre-tension is to control the shape of the mast

Ah, that makes some sense, from a racing/speed perspective.

That said, I think you could end up with the same mast shape at a variety of different standing rig tensions - and if the initial shape is the same, I believe the response to any load will also be the exact same. If I'm wrong, an example might help explain. Would a racing "tune" involve a measurement of things like mast rake, bend, bend location? Is this done under sail or at the dock?

#3 Yep I agree with you there, don't know what I was thinking, confused myself!

[MicHughV]

This is am interesting topic for me. A structural marine engineer by profession, I have had access to some pretty advanced computer modeling software. These are not simple solutions to address nor remedy, with the ultimate goal of being to keep the mast upright and in compression at all times without bending.

Folks that design the the round the world racers and other go- fast boats, etc, also use computer modeling to determine what to do when something goes wrong, where and when and apply the results to the final design.

There many, many variables, far too many to address here. I look forward to reading responses.

[wholybee]

Originally Posted by markxengineerin

First scenario, emergency response plan for various rig failures:

In the event of a shroud failure, we all know to tack. That leeward shrouds can be slack or close to slack in normal conditions is proof that they aren't doing a whole lot to keep the mast up, so it makes sense.
Forestay failure- aim dead downwind, this one is also intuitive because most of the aero force on the boat is in the same direction as the wind- unloading the forestay. But the tricky one is a backstay failure- is there a point of sail on most boats when the backstay can be "removed" with only minor consequences? I think the options are to point into the wind, point into the wind + drop sails, or just head upwind. Which of these, or something else, is best while temporary repairs are made?

Second scenario, just thinking about ideal rig tension.

In my mind, a mast being held up by pre-tensioned wires is similar to a bicycle rim, strung to the hub by pre-tensioned spokes. In wheels, the tension of the spokes has no impact on the stiffness of the wheel- the tension only needs to be enough to prevent "slack", which in the case of the wheel, can try to put spokes in compression instead of tension, obviously bad for a number of reasons.

I believe the same principles apply to our sailboat rigs. If true, it means:
1) there is no purpose for pre-tension other than to prevent slack from accumulating and allowing inertial/impact loads to build up.
2) The mast and sails are going to behave the same, neglecting second order effects, for any pre-tension level between "very loose, slack barely removed while sailing" and "very tight, almost breaking something"
3) The windward shrouds do not gain any tension as the boat heels over- the force balance is actually accomplished by a decrease in leeward shroud tension
4) The ideal pre-tension, for maximizing rig life, is a tradeoff between avoiding infrequent impact loads (too loose) vs. higher average stress almost always (when too tight)
5) There is plenty of wiggle room (awful pun) in this scenario, which might explain why riggers all do it a bit differently. I have seen numerous "rigging how-to guides", but none of them explained it like this, from a theory perspective.
What do you think?

I highly recommend the book "Sail and Rig Tuning" by Ivar Dedekam. Pretension is explained that the windward shroud will take the load of the leward shroud plus half the load of the windforce. The mast head will only move half the distance compared to having no pretension.

I think the general recommendation is that if a shroud breaks, you hold course, and ease sails. At least right as it happens before you evaluate the situation. If a windward shroud breaks, you will probably lose the mast before you notice the shourd broken, as the mast will come down in the same event as the shroud breaking. So if your mast is still up, it's best to just reduce the load rather than a sudden change in load direction. The mainsheet through the leech of the sail will somewhat hold the mast while sailing upwind if the backstay lets go.

[markxengineerin]

I highly recommend the book "Sail and Rig Tuning" by Ivar Dedekam. Pretension is explained that the windward shroud will take the load of the leward shroud plus half the load of the windforce. The mast head will only move half the distance compared to having no pretension.

Interesting, anyone care to explain? I ordered this book and (finally!) Peggy's boat odor bible, thanks for the tip, I am pretty weak in this area.

[wholybee]

I was going to post the page that explains it, but not sure it would be allowed as a copyrighted work.

Basically, as the windward shroud takes a load and the masthead moves to leeward, that unloads the leeward side. So the windward shroud is no longer taking the load from the leeward shroud. So that load can be subtracted from the load the wind is creating.

[wholybee]

Originally Posted by Lee Jerry

You don't distinguish between racing and cruising (even though this is Cruisers Forum). In theory, the effects are the same/similar, but cruising masts are usually much heavier and stiffer so they will be reduced. However, looking at race boats can still be instructive.

Scenario 1 - get the main sheet on hard as quickly as possible. The next step might depend on the weather. If light or moderate, you could probably turn into the wind and deal with getting a spare halyard secured and then sails down without much concern. If heavy weather and going downwind, though, it may not be possible to turn across the wind to get upwind and you'd have to get the brace on and sails down staying downwind. What you do (in terms of getting sails down) kind of depends on the circumstances - day sailing a mile from shore will be different than cruising off soundings; and which direction is "home."

Scenario 2:
1. The purpose of pre-tension is to control the shape of the mast and get the proper shape in the sails, particularly the main, in response to the loads generated in the wind conditions.
2. No, but I think the easiest way to demonstrate this is to point out that that is why race boats tune the rig to the expected weather.
3. I think the slack in the leeward shrouds comes from the mast falling to leeward, with increased tension in the windward shrouds and reduced tension in the leeward shrouds.
4. The ideal tension is to get the proper shape in the sails, not extend the life of the rigging. See #1.
5. Lots of wiggle room for cruising boats, less for race boats.

Pretension is required to prevent or reduce the mast from falling to leeward. Pretension and rig tuning is different from adjusting backstay tension during a race. The backstay adjusts sail shape. But correct rig tune keeps the mast from bending or falling to leeward. And a slack leeward shroud will cause shock loads that can damage the boat or cause a shroud failure.

[Lee Jerry]

Originally Posted by wholybee

Pretension is required to prevent or reduce the mast from falling to leeward. Pretension and rig tuning is different from adjusting backstay tension during a race. The backstay adjusts sail shape. But correct rig tune keeps the mast from bending or falling to leeward. And a slack leeward shroud will cause shock loads that can damage the boat or cause a shroud failure.

A large part of rig tune, like half, is to control pre-bend in the mast. (The other half is keeping the desired mast shape under sail*, as you said.) The purpose is to "match" the luff curve in the main sail as required for the conditions (fuller or flatter). There are several rig options to control pre-bend, which may include: upper shroud tension with swept-back spreaders; lower shrouds (fore, aft, both); mast base location / blocking at partners if keel stepped mast; baby stay (fixed). There are then options to change the mast bend under sail, which may include: backstay; running backstays; check stays; baby stay (adjustable); vang. IMHO, on a mast head boat, the primary purpose of the backstay is to change headstay tension (sag); on a fractional rig the primary purpose of the backstay is to add bend to the mast.


* Some boats want to allow the top of the mast to fall to leeward. Some boats want to allow the middle of the mast to fall to leeward.

[fxykty]

This all really basic and applies to any sailboat with shrouds. Racing boats will generally have known values that they want to repeat or improve, while cruising boats can settle for good enough.

Easiest way if not using a tension gage with know values is to go sailing with mainsail and jib in breeze that is at the point of putting in a mainsail reef (or second reef if enough wind for first reef). Tension the leeward cap shroud (the uppermost one) so that is is just firm. It can sway around slightly but should not be loose on its pins. Tack and do the same with the other cap shroud. Tack and check the first one you adjusted. As a double check, go below and try opening and closing any doors that are in athwartships bulkheads. If any are tight or don’t fit, then ease the same number of turns on each side until all the doors fit.

To double check mast centeredness, count the turns from fully lose to just tight - if both cap shrouds are the same length then both sides should have the same number of turns.

That puts your masthead (or hounds if fractionally rigged) in the middle of the boat and allows the mast to fall off slightly with load.

Next you tighten the intermediate shrouds, working from the top down if you have multiple spreaders. These are tensioned just enough to keep the mast in column - a straight line when looking up the mainsail track. Tack several times to tension and check both sides.

Any mast will stay standing up with no backstay if either the spreaders are swept aft or, for inline spreader masts, a topping lift and mainsheet are tensioned and/or a mainsail is hoisted and not more than 30* off the centreline with a tensioned mainsheet.

Losing an intermediate shroud or backstay is not necessarily fatal to an overbuilt cruising mast, but losing a cap shroud or forestay probably will drop the mast. A headsail and it’s halyard may support the mast but will likely stretch and allow the mast to over bend and break.

[neilpride]

As for losing the backstay, a strong dynema topping lift.
Nothing assures you that it will hold, but it is better than nothing.

Breakages occur mainly on racing boats, when large spinnakers are hoisted in strong breezes.

The percentage of broken masts due to backstay breakage is low, it runs continuous with no stress points, and the tension is not comparable to uppers or lowers.

As for proper tensioning, it varies from boat to boat, I've tensioned uppers on some boats in a scary way, but that's just the way it is., now for some time insurance companies ask riggers to use precision LCD tension gauge meter when tensioning and setting the rigging.

It was not mandatory, but now it is.

[markxengineerin]

Basically, as the windward shroud takes a load and the masthead moves to leeward, that unloads the leeward side. So the windward shroud is no longer taking the load from the leeward shroud. So that load can be subtracted from the load the wind is creating.

Got the book, diagram makes sense. I misinterpreted the reading about bike wheels- the situations are in fact the same- the stiffness of a wheel to a load is independent of the spoke pre-tension as long as no spokes go to zero tension.
The stiffness of a mast to wind load is independent of rig pre-tension as long as no shrouds go to zero tension
The windward shroud does reach higher peak tension for a given wind load, as pre-load goes up. So it is desirable from a rig life point of view to minimize pre-tension, but not go so low as to ever have total slack in leeward shrouds. Going to slack on leeward shroud would not increase windward shroud load (neglecting shock), but would matter a lot for displacement of the top of the mast.

Also clear from the diagrams is that loading on the windward shroud increases above its pretension the same amount as the leeward shroud decreases below its pre-load. I will test this out with a loos style gauge next time I have steady wind and calm seas, it would be nice to see the theory in action.