Rotating Free Standing Mast Design

Discussion in 'Sailboats' started by Chuck Losness, Nov 14, 2010.

  1. petereng
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    petereng Senior Member

    Sorry Cadmus but "eat it" dosn't translate down here very well. Its a term not in my Enjineering Diktionary or wiki it seems. You can have any SA or Rm or ratios you like just give your engineer an accurate Rm curve and a sketch of what you are trying to achieve... Cheers Peter S
     
  2. cadmus
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    cadmus da boom hit'um 1ce 2often

    Ha ha ha ha ha
    Eat it: they loose the argument. They have to take back anything bad they said. They have to eat it comes from "eat their words" or "take back what was said."
    (not a friendly term. use only with friends who know you are kidding or people you hate)

    OK, so do all you stayless experts agree that the strength and ability to take a beating of a Westsail32. an Ingred 38 or Bristol Channel cutter can be reproduced in a rotating wing mast? even thought those boats are heavy and robustly rigged relative to the yuppie plastic ones popular today. And that the CF rotating masts will not be too heavy (2 being less than 150% of the original rigging, or a mass/height combination that will not bother a medium-heavy displacement metal boat with deep & large keel).


    And Thanks to Eric Sponberg, Yves-Marie Tanton, Peter Schwarzel, Petros, skyac and others for all chiming in on this thread. much appreciated.
    many thanks, Pete Cadmus
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Cadmus, I pretty much agree with what Petereng and Tanton have said. In fact, Petereng's prices are close to my own, and his philosophy is pretty much the same. I design round masts when the customer wants to keep the costs really low (they are less expensive to build) but if they have the wherewithal to spend a little more on tooling, I like to go elliptically shaped. I have some examples of these on my website, such as Copernicus' rig, which by the way, is the only retrofit mast I have ever designed for an existing boat. In that case, the owner had a stayed sloop and the new free-standing mast went into the same deck hole, so hull modifications were kept to a minimum. The mast was elliptical in section, which in my opinion has a little less drag and is more forgiving aerodynamically than a round section. Copernicus' mast does not rotate, and it does have a wood framework on the inside to shape it and build it, so it was heavy. But the boat sails way better than it ever did with the old mast (points higher, sails faster), which weighed just 5 pounds more, plus it had all the rigging weight too. Other mast designs that I have done with elliptical sections are the Herreshoff cat ketches, Sparhawk cat ketches, an A.N. Wright-designed cat ketch in New Zealand, and a few others, both custom and production.

    I typically use a factor of safety of 3.0 in my mast designs. That means that the stress in the mast wall will reach only 1/3rd of its ultimate strength at the maximum righting moment of the boat. And the reason I do that is this: In tubular composite structures like masts and poles, fracturing in the laminate occurs in three stages when they are loaded. The first stage is minor cracking of the resin in the intersticial spaces between the fibers. this starts as soon as the mast carries any kind of load at all. The more load carried, the more cracking. This cracking is never restored--it is permanent but it does not affect the overall strength and stiffness of the mast. It happens at very low load levels.

    As the load increases, eventually you will reach a point where some of the resin will crack off from the surfaces of fibers. This, too, is permanent and cannot be recovered, and it happens at an order of magnitude or more (i.e. more than 10 times the level) of the first level of stress. However, this type of cracking also generally does not affect or decrease the overall strength of the mast.

    The third level of cracking is when the fibers themselves break. This happens at an order of magnitude over the second level, and generally occurs at about 50% of the breaking strength of the material. As with the other levels, this type of damage cannot be recovered, AND it DOES affect the ultimate strength of the mast. Once you reach 50% of the strength of the mast, you will likely have done permanent damage to it, and it's overall strength and stiffness will not be as designed.

    I never want my mast designs to reach that level. The maximum load that the mast will see will be when the maximum righting moment of boat is experienced, in normal sailing situations. It is a self-governing type of load. A boat is a free-floating body, so the vast majority of the time, the load in the mast cannot get any higher.

    In really extreme conditions, it's a different story. If the boat should get rolled over by a big wave, all bets are off. One cannot guarantee that any kind of mast will survive a roll-over. Even stayed rigs part company with their boats in wave capsizes. It is really difficult to design against all risks because to do so really adds to both the cost and weight of the mast and rig. And what do you design to? How powerful is the load when the mast hits the water, or the water hits the mast? How long is a piece of string? Pick a number. You can design to whatever factor of safety you want, but you may not be able to afford to build it, and the boat may be too unstable as a result.

    So, over time I have found that my factor of safety of 3.0 has worked really well. As far as I know, all my masts are still standing--touch wood!--except for the ones that have hit bridges or been run over by a fork lift truck (you'd be surprised how ofthen that happens--sorry, can't design for that!). So the prudent thing to do is to just make sure you don't get into extreme conditions, or that you employ seamanlike tactics in cruising with your boat. Pay attention to the weather, and be prepared to handle the boat prudently and keep extreme loads to a minimum. And as I have discussed time and again, both here on and my website, the free-standing cat ketch rig just makes the boat a whole lot safer so that you don't even get into extreme conditions. Properly designed and prudently sailed, cat ketches just cannot get caught aback, cannot get caught in bad gybes, cannot get caught in potential broaching situations and so won't get rolled by waves like other stayed rig boats do--they don't get caught! they are just inherently safer all around.

    I hope that sheds some more light.

    Eric
     
  4. petereng
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    petereng Senior Member

    Cadmus - so "eat it" is an abreviation of "eat their own words" thank you. Cadmus the aircraft industry flicked wires and soft sails very early in their development. When sail was commercial any development that got a boat across the atlantic faster was implemented immediately if it was a reasonable cost. eg "America" had cotton sails etc etc. Then the steamer arrived and sail died just like valves did when I was a boy due to transisters. The sailing industry has been stuck with wires ever since. Its even entrenced in some racing rules. As an engineer wires don't make sense to me for the job. But bespoke yacht projects need to be approached with the right money and good planning or you will get stuck technically or financially. So get good people involved and figure out the costs early. One project I was involved in, a 20m free standing elliptical 450x300 mast for a 60ft cat the mast builder decided they did not want to do the mast join as designed (and agreed to at the time) and they used heaps more material and put the mast weight (and labour & more material cost) up. All designs and costs need to be locked in as best as possible before you start. Cheers Peter S
     
  5. petereng
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    petereng Senior Member

    Oh and by the way, the boats you describe are heavy displacment boats and will not go faster because you put on a rotating rig. You are up against their hull speed limit. Don't over capitalise or over technicalise (if thats a word) your boat. On fast multihulls or small planing skiffs we can keep developing the rig and go faster and faster until we hit stability and scariness limits. On a displacement hull such as a Westsail 32 you just stop at its hull speed. Cheers Peter S
     
  6. cadmus
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    cadmus da boom hit'um 1ce 2often

    I am not planning on building this myself, i already said i can not afford the equipment to do that, why is everyone returning to that. I will have it done professionally. I am not going to extrude aluminum from bauxite either. I already have a headache building a hull.
    I fly hang gliders. Hang gliding is very much the form of aviation closest to sailing, we like to think of it as 3d sailing. We have Carbon fiber ones. And we have aluminum ones. We have stayless hang gliders and stayed hang gliders. If a glider is going to be flow on the bunny hills by people learning to land or should i say CRASHING we give them aluminum stayed gliders. They take a beating, over and over and over. The boats i listed can take a beating over and over and over. Thus the question, could they be fitted with a stayless mast and still take the same beating. It is a good analogy.
    I realize i just gave you heavy displacement boats. I plan on building a heavy to moderate displacement boat, as i said earlier. I am not trying to prove William Froude wrong by changing my mast:D, i am not trying to squeak more speed out of a racing rule system, i am attracted to stayless masts because of other saftey and logistical reasons. NONE are for going fast. The freedoms yachts were not designed for racing, they are popular for other reasons.

    -less moving parts.
    -less things to inspect
    -less moving parts.
    -CF stayless masts provide floatation preventing a complete capsize.
    -reefing foresails is not fun. Cat schooner doesn't have foresails. (although it would be nice to have a drifter on a bow sprit for super light days).
    -One can tack without messing with foresails
    -one can tack with a hand on the tiller and the other hand grasping a cold drink. That was a funny joke, but seriously, If i become incapacitated that will be nice for my wife in a storm: one handed tack.
    -when jibing normally there is no risk of hitting and damaging your shrouds.
    -if i am incapacitated my wife could, in a big storm, jibe the boat by roatating the wings without preventers. solo.
    -The self strearing advantages of twin masts are nice, not always usefull but nice.
    -no spiniker sets. Spiniker sets get hard on big boats. Especialy at night. Especialy when alone. double wing masts will out perform a boat using drag without needing a spiniker and without using drag.
    -REEFING. I find reefing a tradiational boomed mainsail easy. So does my wife. (i do not want a wishbone) Having 2 tradiational boomed mainsails is comforting in a storm, and just in every day handling. (in a big storm i feel the front sail on a catschooner is going to be too forward of the CE of the keel. so I will need to macgyver something. but that is off topic)
    -owning a bunch of foresails is a pain and waste of space. I am already building my boat smaller than most people for other safety reasons. Reducing sails is helpfull.
    -owning 2 identical mains and having one identical spare is easy.
    -i sew sails, having one pattern on board is possible. having one pattern for every foresail on a cutter or sloop is not.
    -some stayless masts bend at the top dumping the sails when a big gust comes, not a replacement for reefing but nice when one is late to reef
    -my friends with freedoms say they heave-to nice.
    -my friends with freedoms say LOW MAINTENANCE
    -my friends with freedoms are crap at some aspects of traditional sail tuning... they could care less.

    All of those are not related to speed.
    There is more to stayless than speed. many of Eric and Tantons designs are not about speed.
    One of the reasons your general public thinks stayless masts are only for racers is because we only see them on light displacement day cruisers or racers. AND because most the discussion of these sails is about squeaking more power out of rating rules. Infact I have never seen any stayless design on a boat with the displacement to LWL ratio that i feel is sea worthy, so thus the Ingred 38 proposal. I know i am eating a lot of your time, I hope this gets some attention and increases your non-racer business.

    Most of these benefits are valued from the fact that 2 people sailing on a crossing is really one person sailing as the other is always sleeping. I intentionally am keeping my boat small because sailing single handed (with one sleeping) is not possible in a boat bigger than 30-35'. I know people will disagree. Those people are not my wife. Those people are likely talking about fair weather sailing not poop-in-your-pants storms.I can extend that to 35-40' if I break the sails up and add some features to make life easy. twin rotating stay less masts are easy.
     
  7. cadmus
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    cadmus da boom hit'um 1ce 2often

    ohh OK. I did throw you off by protesting a round shape. sorry. I am a scientist. Although my PhD is in toxicology and ecology I study aeronautics and fluid flow for other reasons (hydrology & sediment transport & river morphology) and for fun (sailing & hang gliding). It kills me to see a round mast, only because of illogical eccentricity. Sorry if that threw you off. You are right that i will not benefit from a wingmast with a heavy displacement boat.

    It also kills me to see triangle pointy tops, only because of illogical eccentricity. I hallucinate and see exaggerated induced drag spirals in the sky while looking up. I do secretly want no backstay so i can put a roached sail on just because it is the right thing to do, only because of illogical eccentricity.

    I agree. Round is the way to go. sorry.

    One can say foil shaped masts and heavily roached sails are useless for a cruisers. Like cruisers only sail down wind or something. Like we turn on the motor to go upwind or something. I hear it all the time. WTF? I will often reflect on a week, especially in the inland passages of Alaska, and think 'I spent the entire week sailing to weather.' SO foil shaped masts and heavily roached sails are advantageous for cruising. Many people say cat ketches have limited upwind performance UNLESS they benefit from the foil/ellipse mast and a roached sail, thus the desire for an elipse.

    But PLEASE ignore this fact and note that the list of safety features is the REAL reason a world traveler (not a racer, not a day sailor, not a coast cruiser) would want 2 stayless rotating wing masts
     
    Last edited: Apr 11, 2013
  8. petereng
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    petereng Senior Member

    Hi Cadmus - So how high is the mast? is it to be buried or on a post? What size do you think it will be and how much sail will be on it? Whats the Rm? We're past the discussion and onto the details please!! I'm happy to do a back of the envelope calc for you. This discussion has negative chance of improving my business, I've had similiar discussions over the last 20 years on the subject to no avail yet. But I'm still talking anyway. Peter
     
  9. cadmus
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    cadmus da boom hit'um 1ce 2often

    Well, i can't pass up free consulting service. Thanks. This is like a magic genie wish. I can not waste this, big decisions.

    My goal of comparing the rigs of an Ingred, a well documented boat is not possible as i have no righting data for her. bummer. I only have the rigging specs (pdf above). I can try to find them. If i wait too long the genie will go back into the lamp.

    My short list is 20 boats right now. No doubt Tanton has some <40'ers that i am eyeballing but i do not know the specs.

    Lets use a Dix 38 as that is only slightly lighter than Ingred and the righting motions are published online.
    Righting Moment @ 60 deg 7497 kgm (54226ftlb)
    Specs: http://www.dixdesign.com/dix38pil.htm

    Sail Area (Main + Fore-triangle) 67.0sq.m (720.9sq.ft)
    LOA - 11.70m (38' 5")
    LWL 10.25m (33' 7")
    Beam 3.80m (12' 6")
    Draft 1.5 & 1.75m (4' 11" & 5' 9")
    Displacement 10000kg (22040lb)
    Ballast 2800kg (6171lb)
    photo analysis shows mast 47-49.5' (14.3-15m) off the cabin top but 2 cat schooners can sometimes be a little shorter than one sloop mast, right?

    "is it to be buried or on a post?" Not sure what you are asking. The original or the CF one. I would want it extending through the cabin to the floor stepped on or near the keel and I assume one would have extra ring frames in the forward 2 meters of the hull to support the forward one, also burred through the cabin. That is strongest.
     
  10. petereng
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    petereng Senior Member

    Your description is of a "buried" mast. Some yachts have a post that sticks up from the deck and the mast slides over it. A post solves the problems of sealing the bearing at the deck to keep out the water and improves the internal space. A rotating mast inside the cabin usually is hidden by a "chimney" or cabinetry which takes up space. You have to be able to access foot bearing and deck bearing for service and adjustment. The bearings are usually not small things. So we have a Rm=7500kg.m and a mast height above deck of 15m and bury of? (deck to keel 2.0m?) How high is the boom from the cabin? Cheers Peter S
     
  11. cadmus
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    cadmus da boom hit'um 1ce 2often

    Eric's run a carbon fiber round mast from the floor of the cabin, through the deck and up about 20% of the mast height. The wing fits over it so the bearings are above deck. That is most optimal in my opinion. I thought that was what you were asking.

    If that is not an option, bearings on the floor of the cabin and bearings on the cabin roof with the entire mast being one piece is fine. It leaks water but is fine.

    I assume those 2 methods are the safest.

    A post sticking up out of the deck that is not stepped the keel would mean i would have to build a huge superstructure on the deck to handle all that leverage. I think it is safer to tie it into the nice ring frames through the hull. I vote against that option. I realize now that is possible, but not needed if I like having the mast below deck.

    Maybe some rich guy in a blue blazer sipping wine cares about that. I like seeing keel stepped masts below deck (when they don't leak water). I would be honored to have a beautiful cylinder of carbon fiber majesty in my saloon and my v birth.

    Give me a minute on this one.
     
  12. cadmus
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    cadmus da boom hit'um 1ce 2often

    photo analysis says 2.372ft (0.72m)"
     
  13. petereng
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    petereng Senior Member

    I suggest a post, ie the post is inside the mast as you and Eric have described. The Post goes from cabin roof to cabin sole or keel. This removes the problems of sealing the deck. It creates problems in terms of the mast bearings but these will be covered later. Too big to do on an envelope. The other discussion is... as its a schooner should one mast be able to support the entire Rm? Or do we download it as the entire Rm will not be seen by a single mast? As you are conservative then a single mast should be able to support the whole Rm. 15m is not a big mast so its not going to be really big. Cheers Peter S

    For those out there watching of technical interest: there are two ways to design a rig. 1) Using first principles and doing an aero-elastic analysis eg you get an aero person to analyse the sails and get the edge loads. Then you give these edge loads to a structures person (eg me) and they calculate the structures. This is how top level rigs are done. 2) Using Newtons approach - We know that the heeling moment (aero loads) equal the righting moment or hydrostatic character of the boat. So the design can be based on the Rm and not know anything about the rig as long as we understand that the Hm=Rm so we sidestep the aero analysis. This is how conventional rigs are designed. There has been discussion about the forestay load and this again is claculated by newtons method. We know the design sag of the sail from the sailmaker so we calculate the forestay load from this sag. No forestay in this case so no problem with longitudinal rig loads as there are none. For Cadmus I am going to say that the mast is a cantilever with a distributed load up the sail track. This load is a multiple of the Rm (safety factor) I shall check it against his sail area to see what the heeling moment is capable of to keep it real. Otherwise its a few lines of calcs to size the mast. The only thing I can't do on the said envelope is a local buckling analysis of which I need to model it and run it through my FEA system. Cheers Peter S
     
  14. petereng
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    petereng Senior Member

    OK so lets say the following: Rm=7500kgm, boom at 0.75m, mast height above deck 15m, SA=35m2? how about boom length? 3.5m boom with 1.25m square head gives 39m2 and how high is cabin top from waterline? I'll assume that the metacentre (axis at which boat rolls around) is near waterline. Peter S

    33m2 boom 3m square head 1m long
     

  15. cadmus
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    cadmus da boom hit'um 1ce 2often

    photo analysis says 1.6m water line to top of cabin (much less to the deck/toe rail, 1.15m. but that is not what you want. top of cabin is 1.6m)
    (Freeboard at the bow, where the front mast stands is 1.44m waterline to deck, but you are just doing one mast for obvious reasons the previous mast is close enough and could be used twice. so ignore this measurement)

    i can not guess.
     
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