Rotating Free Standing Mast Design

I should also ask, is this the hydrostatic righting force? If so how do we estimate (and add) the force from that sudden deceleration and change in direction of rotation? or tell me why this is negligible.

 

We are refering to the transverse righting moment, the moment during roll. This is because the roll moment can actually be quite constant--you sail at an angle of heel, you can stay that way for hours, even in strong winds. Momentary heel of upwards to 45° and 60° during gusts--at max. righting moment--can be common, so such loads are very real.

You are right to say that the longitudinal righting moment during pitch can actually be much greater, but is it also much less common, even rare. Boats actually are pretty dynamically unstable when pitching, and oftentimes when descending down into a wave, will have a tendency to yaw into a roll before the load in the rig increases too greatly. This is more the norm, so the transverse roll moment prevails for design purposes.

I have been involved in situations where I've had to analyze the pitching moment on a rig to determine the possible causes of the rig's failure--the deceleration of the mast as the boat came to a stop in heavy waves could have caused a shroud to backstay to part company with the mast, and the mast going over the side. It does happen, but rarely. So, as with many things in design, we engineer to the most common everyday loads, and we do not try to design/engineer against all risks. Note that if you are sailing your boat and pounding through the waves and pitching agressively, you really shouldn't be doing that. Change course and ease up on the boat and the rig. I am sure you drive your car prudently--it's designed to be driven by responsible drivers, and it is not designed to be driven at 50 mph over curb stones.

I hope that helps.

Eric

 

That helps although i will have some clarifying questions after digesting it.

It is easy to say "don't do that" but when on the windward side of a land mass in a storm you have no choice but to beat up wind, you need to beat up wind. MOB or response to call for help. Swells not coming in the the same directions of waves and wind. That is the quickest way out of the storm. I am not saying this is comfortable or fun but it is often necessary for long periods of time unless you are coastal cruising and have harbors or shelter anchors to retreat to. This is rarely available in large regions. And even when they are available, entering them in a storm or at night is more dangerous then staying at sea and sometimes staying there means beating while you tack up wind to hold your position.

 

I will agree, I feel and see that all the time. So how do we predict at what force a boat will do that, then see if that value exceeds our safety factor?

 

You can't predict it--the loads are too uncertain, so we just don't go there. That may seem like a cop out, but if you cannot define the loads, and prove them in some way, what are you supposed to do? How fast does a boat decelerate when pitching down into the back of a wave? What kind of wave--short steep ones or long shallow ones? How fast is the wind blowing, what is the fetch, what angle is the boat making to the waves, how big is the boat, how heavy is the mast, is it a wide shallow boat or a deep narrow boat, how quickly will the boat yaw, is it under control of the helmsman who can make snap navigation decisions, or under the control of the autopilot which can't---the questions and variables are endless, and in most cases discrete loads cannot be defined.

In the end, we concentrate on the transverse righting moment because it is so easy to define and because heeling loads can be considered discrete and constant--we design the rig accordingly and assign a safety factor for the unknown situations. In general, over time, we have found that racing boats can get away with a safety factor between 1.0 and 2.0 in their rig designs, but cruising boats will get a safety factor between say 3.0 and 4.0 in their rig designs. Such boats survive over long periods of time, even as they venture into bad pitching situations. I am of the opinion that if you have to assign a safety factor over 5.0, then you probably don't know what you are doing and you shouild ask more questions or do more study in the design.

Eric

 

Maybe i am confused about materials.
If i am pounding a blunt nail into a block of wood or a beneteau (..ooops did i say that out loud?) I hit that nail with the hammer by bouncing the hammer off the head of the nail. Not pressing on the nail with my body weight. I am less worried about damage when using a static long duration force at a lower intensity, right. This is a terrible analogy i know.

Stayless masts are CF. So is there something about CF where longer durations of (in this case stress, strain and compression? maybe more) at a lower force is more of a problem than many abrupt forces for acute or momentary durations? What fatigues CF the most?

Does the fact that CF bends and is light aid in reducing the forces in this sudden change in pitch moment. Of course much of that bending has been lost over the years to ensure good sail trim, right?

I would argue numerous days of the constant force in a crossing. not hours.

 

I feel demastings happen too often relative to what we would want , one of the reasons I am postponing my build, so i can save up for free standing masts and engineer fees. I have been converted to the "less things to break" philosophy.
So when traditional stayed rigs are made are they designing it for greater forces. How does a stayless wing mast differ from a ingred cutter or something else heavy and ballsy? and then relative to a light displacement coastal cruiser.
This comparison/measurement is quite useful and might help increase acceptance of the stayless system which is not as popular as it should be.

 

ohhh man. you are too quick for me, i am still on post 1. sorry.

 

If one designs the boat with that static long duration moment from heeling at 60deg as 1.0. And the safety factor is 3.0. But a few hours of beating up wind produces 3.0. then our safety factor was really 1.0. This is obvious.
So i am just making sure my safety factor is truly 3. Not trying to be a jerk, i am very precautious about safety. Just trying to "ask more questions or do more study in the design" as you said.

Perhaps we need to take an empirical (statistical observations of reality) approach rather than a theoretical (engineering equations) approach to arriving at those values because that is a great deal of variables. Sadly, no one funds such experiments or record keeping.

 

Well, that's just it, beating up wind for a few hours does not eat into that safety factor. If you design for a safety factor of 3.0 on the maximum righting moment that occurs at 60°, and you are heeling at 20° for a few hours, you are at a fairly low load all that time. Sure, some heeling and rolling occurs, but the pulses of load into the mast laminate are not all that high, and still a long way from breaking the mast.

Oh, that we could have the funding and time to produce empirical data on longitudinal rig loads. Using just one boat in a few different sea conditions would not be enough. You would need at least a few different boats, a long elapsed time to cover a variety of wind and sea conditions, a large geographic area to encompass different kinds of waves, and crews and support staff and measuring equipment and technicians to go out in harms way to measure rig forces in extreme conditions. You are necessarily going to put the boats and crews at risk. Dream on! Funding may be available for flying test pilots for new aircraft, but you won't find it in yacht design.

As for your question regarding design between stayed rigs and free-standing rigs, both types are designed to the maximum righting moment of the boat. The wind conditions and points of sail and the sail area are all immaterial, really, because the boat responds by heeling, and heeling induces the loads on the mast. That is why all we need is the heeling moment of the boat, equal and opposite to the righting moment. In a stayed rig and as the boat heels, the heeling loads induce tension in the standing rigging, and the standing rigging induces pure compression loads in the mast. The compression in the mast is equal and opposite to the sum of all the tension loads in the rigging. We have to design against the compression stress as well as the column buckling that could happen.

In a free-standing rig, the mast experiences bending--a totally different load phenomena--there is tension on the windward side of the mast, compression on the leeward side, and shear in between. There is also deflection--the masthead bends off to leeward and a little aback by the tension in the sail fabric, and you want the mast to not bend too much or too little--it has to be a Goldilocks mast, it has to bend just right. The engineering of the mast laminate is totally different than for a stayed rig, and the laminate schedule reflects this. If you are designing a wingmast that rotates, section shape and load orientation all play a part, and that has to be taken into account during the mast design.

Eric

 

Ohh no, Eric. You are pseudoreplicating. That one boat needs to be several similar boats plus replicates of the other designs. That is how you engineers have such impossibly great R2 values: pseudoreplication.
Hahahahaha

I agree that is why i said no one would fund it:

But remember there is more to stats than interpreting or modeling experiments. There are many instances where we can not run experiments for logistical, monetary, and ethical reasons. We have ways of estimating parameters on observations from record keeping of failures, observations using temporary strain gauges, etc. But yes, these efforts are expensive and unfunded.
Sorry, I digressed from the real topic.

 

Cadmus,

I think you are looking for a level of certainty that is impossible to find in boat design, and many other things we accept everyday.

Companies such as Lloyds have been keeping records of details of lost ships going back many centuries, they have developed guild-lines or "rules of thumb" for what is generally reliable and safe for various sizes and types of vessels. Much of ship design uses these tables derived from a large statical data base. It works pretty well but it can not guarantee that your particular boat will not ever be overloaded and fail. Much of that also depends on your own skill and judgement when operating your yacht.

The fact of the matter is almost everything you that affects your safety is done in exactly this manner. The food you eat, the car you drive, the house you live in, the medicine you take, all have safety regulations based on a history of what is generally safe and reliable. As an engineer I have design aircraft, buildings, cars, boats and other structures based on a standard for what is consider the current "standard of care", or state of the art if you will, where the design loads were derived based on the history of usage. When a building is designed for a certain roof snow load or earthquake acceleration, the size of the loads is based on the recent history of snow or earthquakes for that region (I say recent because in most of the USA, we sometimes only have 60 to 70 years worth of reliable data, and we have to assume what would be statically significant for say 100 year event, with incomplete data). So we do not know for sure how much snow the roof is expected to support over the next 50 years, but we set a standard which is a likely safe design load.

With ship design we fortunately have records that go back many more years than buildings or bridges, many centuries earlier in fact, thanks to companies like Lloyds. So there are many unpredictable things than can happen when out in deep water in a private yacht, but the "best information" we have says if you design to those standards, you will be safe.

Cantilevered masts are relatively new, so there is not a long history of with them, but a good engineer could back analyze a conventional rig from the design tables, and used loads similar for analyzing the mast and the deck strength. And you will end up with something that will be strong and safe for the expected usage. I would never recommend you intensionally "test" that strength, but that is the best we can do with the available data.

The size of the safety factor in all design work is based on the level of uncertainty and practical considerations like weight and cost. For example, you can not use safety factors of 2.0 or 3.0 on aircraft that is often used in ship design or buildings, they would be too heavy and never get off the ground. So they use safety factors of 1.1, up to 1.5 for really critical structure (wing body join, landing gear, engine mounts), and do a lot more detailed analysis, and use reliable materials that meet a high standard for strength and reliablity.

You can always over design the strength of anything you are building or having built for you, for the sake of your own piece of mind. But that will add a lot of cost and extra weight which will affect performance. I have back analyzed the German Lloyds load requirements for off shore cruisers and I am shocked and surprised at the strength they consider a reasonable safety factor, especially after coming out of doing aircraft design. the strength requirement is very large, far more than what I would consider a "reasonable" safety factor. And this is why so may deep water cursing boats are so heavy looking compared to coastal cruisers or racing yachts.

But it is not up to me to determine these safety factors, it is my job as an engineer to meet them so it can be certified to meet those specific class rules. All I can say is that they have the data base to decided what is a reasonable safety factor and what is not.

There are many ships and yachts lost at sea with no specific information on the cause of the loss will ever be known. They investigate all available records and see if there was any clue as to the reason of the loss, without a wreck to study, or eye witness/survivors, they often do not know. That is why these maritime inquests can take so long, and some of the more noteworthy and public losses, such as Titanic for example, can be second guessed for many decades. They do similar investigations for aircraft accidents, and be it human error, bad maintenance, or bad design, almost always there will be some regulation change proposed to prevent future accidents. That is why air travel and cruise ships are some of the safest ways to travel now.

So you will either have to just trust the rating rules as coming with very reliable built-in safety factor, or you make it stronger and pay the price in lost performance and cost.

You already trust industry standard when ever you drive your car, cross a bridge, take medicine, go up an elevator, eat food prepared by someone else, use power tools, and just about everything else you did not make, build or grow yourself.

Why the sudden paranoia on yacht design? No different than any other safety factor. Consider this, when you eat out your food safety is often dependent on a teenager working in the kitchen who is presumably been taught food safety handling rules by the local health department. At least when it comes to yacht design, you have experienced and trained professionals who must meet a much higher standard of safety.

You can never guarantee absolute safety, it would be impossible, so we accept a certain amount of risk with almost every activity we do, from sleeping in our house, to sailing or mountain climbing.

 

Sudden? ohh no. not sudden. This is what my family and friends have always talked about around the table. I do this with cars, rock climbing equipment and hang gliders also... but not as much as with boats.

In fact it is the razzing by family and friends that inspired this recent post. I am trying to show them and myself that a freestanding wingmast is as safe as a trad rig. On a purchase that big wouldn't you? Sadly "safe" "seaworthy" and "cruising" mean different things to different people. These words even mean different things to different boat designers. So i guess i planned to:
a-identify the most 'scary forces' on a rig and/or the forces or motions that frequently cause failures.
b-look up the maximum 'scary force' a sample of boats can handle (or if i have to, figure out how to estimate this for a series of boats maybe using multiple regression on the specs I can find, maybe by inquiring about specifics of the recommended rigging. I should really crack open my rigging books before i blabber too much)
c-ordinate a bunch of boats by their ability to handle whatever forces i found important.
d-look at where in that ordinated list I see safe boats switching to unsafe boats.
e-Then see if a stayless mast can be made to the specs I call safe for the type of hull I am planning to build without too much mass or girth.
Obviously I would need to normalize to displacement or length or some other variables. Or pick a series of boats with similar specs.
shootin from the hip here. Not easy. a big time sink.

If you got a better way to do it i am all ears.

If there is a rating rule or standards system that is going to let me discern the difference between a safe and unsafe boat, that is a great way to do it. But that is rather subjective (my opinions might be different from the next guy). I have frequently been inspired to read up on ABS, Lloydes, DNV, Etc. My experience with standards is that you have to pay to see them and you only see the advice, not how they arrived at it. Coming from the sciences, where everything is peer reviewed, repeatable, methods/analysis are explained and everything is publicly available, I find these proprietary standards hard to trust and harder to pay for.
Please, if you can direct me to standards or classifications systems that are free I would love to read them.

Problem again is that my idea of safe is not the same as all the people making those scantlings and specs. It is WAY different than the people buying beneteaus (all of which must have passed some rating rule system). If you feel a classification society or standards system is good at discerning "south sea crossing worthy" from "coastal cruiser," and maybe a few categories in between, I would love your (and others) opinion on what standards or ratings to consider.

 

Hi Cadmus - I have designed and/or have built many free standing masts mainly for cruising catamarans (6m high to 45m high). Conventional rigs very rarely break the mast section. Usually a metal fitting fails then the mast falls over. By going free standing you remove the problem of all those connections and fatiguing of small parts. If this boat is to be a ocean going one then a round section is the best bet. A wingmast is designed to provide more power. Something that a cruiser can acheive by adding a bit of sail area vs making a more fragile mast. Plus when the time comes to build your mast the mould will cost the same as the mast and you will not build your wing mast due to its large total cost. When cruising you will not be interested in continually trimming your mast to get the most power. In fact if you don't trim it properly it will have more drag and poor sail shape. So for cruising you need to keep it simple. I've designed many wing masts for people then the project falls over when the mould costs come in. If you use a round section then there are many people that can wind or mould your mast as the moulds already exist. Plus if round it will be as strong fwd/aft as it is sideways. This means that the strength of the mast will be huge for its purpose even in the event of the fwd accelerations you seem to be worried about. By the time you make a free standing mast stiff enough to perform as a mast vs a fishing rod, it becomes 10x stronger than needed for even extreme sailing loads. By the way I'm a yacht structures engineer who has been designing masts for over 25 years. All of the scenarios you describe can be calculated for and understood and designed to if required, good naval architects and enginers can estimate all that you speak of. The righting moment others are talking about here is the standard method for determining the nominal moment applied to the mast. Say Rm30degs or Rm45degs... for a monohull of course!! Once this is determined a safety factor is applied on strength to define the design moment. For free standing masts these can be lower than conventional because conventional is limited by buckling but free standing is limited by deflection (& strength) so it performs well. I design the tip to deflect ~15% of the height for most designs. This is tuned to the boat sailing requirements and sail shape of course. But if we agree that 15% deflection is OK then we shall find that the strength of the mast is hugh compared to the Rm requirement. This also is based on a CF mast or hybrid CF glass mast as the strength of fibres are much bigger than say aluminium. eg a typical laminates that I build have UTS > 1000Mpa whereas 6061-T6 al has a YS=300Mpa. So we are 3x in front already and much lighter. If your SF is set at conventional levels say 3x then the mast will be too heavy, you need to build a big section to get a light mast that is stiff in bending & light. Hope this helps. Regards Peter S carbon-works.com.au

 

Cadmus, If you haven't seen it already, you can read my article on Free-standing masts on my website, "Some Thoughts on the State of the Art" where I make all the arguments for free-standing rigs:

http://www.sponbergyachtdesign.com/StateoftheArt.htm

Eric