vertical down forces on stepped mast Please HELP

Discussion in 'Sailboats' started by zambant, Oct 25, 2008.

  1. zambant
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    zambant New Member

    Wow...fantastic site....99% of the stuff goes straight over my head...

    I have a 33 year old Jaguar 22 sailing boat which has been "butchered" by the previous owner.

    Can anyone give me a rough ball park figure to help me repair it.

    To all intents and purposes its the same as a Catalina 22 the specs for that can be found here

    http://www.catalinayachts.com/yachts.cfm?act=model&id=15&link=spec

    What i need to know is the APROXIMATE / BESS GUESS as to the down force that is likely to be found at the mast step....

    I need this to repair the bottom of the kingpost.

    You see I have spoken to catalina and they say " It will probably only be a few hundred pounds" through to a couple of mast manufacturers who say " over a tonne" to "no idea"

    Can anyone help...please :)

    Many thanks

    John
     
  2. daiquiri
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    daiquiri Engineering and Design

    It is funny that you don't thrust guys which have constructed your boat. :)

    Now, looking at the boat's specifications I see that shrouds are all 1/8" diameter. The nominal breaking load of a standard 1/8" stainless steel wire is around 7100 N (appx. 1600 lbs). The mast stays are typically pre-tensioned to about 15-20% of their breaking load. So in this case (and considering the higher pretension value - 20%) you'll have 4 shrouds pulling the mast downwards with appx. 4x1400 N (4x320 lbs).
    Since shrouds are inclined to the vertical (say, 20° mean angle of the 4 shrouds?), the net vertical component of this load will be about 5300 N (appx. 0.54 tonnes or 1200 lbs). This is the load acting at the mast step.
    Considering that you have to multiply this value by a safety factor (and I would give it at least a value of 2.0-2.5), it justifies what those mast specialist guys have told you.

    P.S.:
    Larsson & Eliasson use a mast step load value of approx. 85% of boat's displacement in their book "Principles of yacht design", which fits the numbers above with a safety factor of 2.0 .
     
  3. zambant
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    zambant New Member

    Thanks for you help

    Thanks for your help.

    The problem for me is the manufacturer no longer exists and the lower part of the kingpost was removed along with any evidence of what existed.


    I need to make up a support and decide where and how to fix it to the innerside of the hull....of course the loadiing is critical to deciding the size of the post and also decing how to spread the load into the hull.

    Once again thanks

    John
     
  4. Joakim
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    Joakim Senior Member

    You should add to those forestay, backstay, all halyards and other control lines attached to the mast (depending on how they are guided). I think these could easily double the load. Pretension can also easily be 25% or even more.

    Joakim
     
  5. daiquiri
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    daiquiri Engineering and Design

    Halyards are attached to the mast and transmit their loads to the mast. Therefore they do not add to the load on the mast step.

    Shrouds are subject to cyclic loads and have to be dimensioned so that their nominal load doesn't reach the fatigue limit of stainless steel. The fatigue limit is typicaly 35-45% of the maximum tensile strength (assume the mean value, 40%). Therefore, the maximum design tension load of the shrouds shouldn't exceed about 40% of their maximum tensile strength.
    The shrouds are pretensioned to no more than 1/2 of their maximum design load, because that assures the winward stay to be fully loaded when sails are under maximum pressure while the leeward shroud is at ideal zero tension.
    1/2 of 40% gives 20% of the maximum tensile strength, and that is the pre-tension value that shouldn't be exceeded if you want the shroud to work under correct design conditions.

    25% or more, as you say, is wrong because it doesn't take into consideration the material fatigue properties. You can pretension them to 100% if you want, but it will be all to your risk.
     
  6. Joakim
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    Joakim Senior Member

    25% is in many trim guides and I know some people use a bit more. Wise or not that could happen and the mast step should be strong enough for that.

    Joakim
     
  7. Guest625101138

    Guest625101138 Previous Member

    This is not quite correct. Fatigue life is related to stress RANGE not the absolute stress. Infinite cycle life is normally possible if the stress range is less than 40% of yield. That could be 0 to 40% or 50 to 90% of yield for any tension member. A good example of this are the studs in an engine cylinder head. Typically have significant preload relative to yield.

    So taking the size of the shroud and assuming only 40% of yield is not necessarily a good approach. The shrouds could be preloaded to 50% of yield and still happily achieve infinite life. They would have much greater preload than necessary providing they are correctly sized.

    The real limit on the mast down force should be set by the yacht's righting moment irrespective of the size of the shrouds and stays. The shroud and stay tension can be set when tight reaching into a strong wind with full sail. The lee side is adjusted till no longer loose with very slight preload and the mast straight in the longitudinal vertical plane. Obviously needs to be done on either tack and need to end up with the mast perpendicular to the deck when not sailing.

    Hence the mast down force should be a function of the spread or spacing of the shrouds and the maximum righting moment plus the down components of the forestay and backstay tension. Weight of mast sails and tension in any halyards and sheets with deck mounted blocks having vertical force component could also come into consideration.

    The importance of stress range really becomes a significant distinction with a compression member. In this case the range could be from -20 to +20% of yield to give total RANGE of 40%. If you work from -40 to + 40 the component will fail. It would be twice the fatigue limit range and last about 1/4 million cycles. I know this from personal experience and other hard won knowledge.

    Rick W.
     
  8. daiquiri
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    daiquiri Engineering and Design

    Rick, maybe I didn't really understand what are you trying to demonstrate, but the above statement is not correct.
    The absolute stress is important when you talk about the pretension because it introduces a mean stress parameter. A negative mean stress will enhance the material fatigue behaviour, but a positive mean stress (pretension, which is the case here) is known to drastically reduce the fatigue limit of a structure, everything else being equal. It's effect can be estimated with experimental diagrams (Smith or Goodman Diagram - attached here).
    A pretension of 20% means that you have introduced a mean stress in the shroud, which will reduce it's fatigue limit by a certain amount, compared to a completely reversed cyclic load (zero mean stress). Typicaly, this reduction for steels will be around 20% - as you can see in the diagram attached.
    If you pretension the shroud to 50%, like in your example, the endurance limit drops to some 50-60% of the zero-mean-stress value. An infinite cycle life is still possible, but only if the shroud diameter is adequately increased.

    So, in my opinion, it is always better not to pretension the shrouds above the minimum necessary. If the rig manual tells you that the trim loads required are above the limit of 20% of shroud's max. tension (ok, give it even 25% - but no more), than you would do better to increase the diameter of the shrouds than to sacrifice their endurance. That's my engineering philosophy.

    What you wrote about the correct trimming procedure for the stays is prefectly correct and valid, but if the resulting loads end up to be out of these limits, an increase in shroud diameter is adviceable.

    And please note one more (and important) thing. I'm talking about pleasure boats, not about racers. Racing boats are designed for performance and minimum weight, the consequent loads are higher and life cycle requirements are much more laxed respect to that of vessels intended for an easy and safe cruising with your family.
     

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  9. Guest625101138

    Guest625101138 Previous Member

    I checked stainless steel and the straight plot is a good fit. For more ductile materials endurance limit is less sensitive to the average tensile stress. Cyclic stress range is the important consideration and has to be considered even in compression. I now have better appreciation of the significance of mean tensile stress for fatigue life particularly with stainless steel.

    Back on the topic of mast loads I am making the point that using the size of the rigging to determine mast loads is an indirect method that is making a lot of assumptions about the rigging design and correct adjustment. The righting moment of the boat limits the loads and the standing rigging should be set to suit the maximum righting moment. Anything more is too much and anything less will have lee side shrouds flapping in the breeze. If the standing rigging happens to be way over designed then it does not result in mast compression load being any more than needed.

    Rick W.
     
  10. daiquiri
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    daiquiri Engineering and Design

    Consider it my give back for your great and very instructing lessons on propellers. ;)

    I agree that many assumptions are hidden behind this kind of reverse-engineering, but the fact is that Zambant has a boat hull, he knows what rigging will be set up on that hull, but he doesn't have a kingpost for his mast. So, a quick preliminary estimate is needed for a load he can expect at the mast step. My calcs gives him a numerical value he needs as a starting point for the construction. A value which, incidentaly, coincides well with guidelines given by an important yach-design reference book, cited in my first post. Of course, anyone else's contribution with an argumented numerical value is certainly welcome.

    Now, about the lee side shrouds flapping - I've talked with some boatowners (because I needed that info about a year ago) which have experimented a little bit to find the optimum tension, and they told me that a pretension corresponding to some 15% have resulted in a non-flapping leeway stays. They have lately adopted a higher pretensions anyways (around 20-25%), but the value as low as 15% was, according to them, enough to make the lee stays sufficiently firm.
     
  11. philSweet
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    philSweet Senior Member

    Lots of neat rig tuning info, but is this how you design a kingpost? I am a total amatuer who managed to built a couple of boats before I had ever heard the word "scantling". They tend to look somewhat overbuilt. The good news is I can still look at them since they have survived twenty-plus years of indignities and near disasters.

    Suppose you are draft limitted in a narrow channel approaching a bend and around the corner comes a big boat doing 30 knots. He rolls you 30 degrees with his bow wave then you roll back just in time for his dinghy davits to collect your upper shrouds. Part or all of your rig is going away with the yacht. Your deck, kingpost and hull should survive this. Try the following scantling rule. Figure the worst-case rig loading -In the case described, maybe 2.8 x breakingload of rigging wire. To this add the weight of a schoolbus the same length as your boat. This will keep your hull out of trouble with regards to rig incidents. I doubt a kingpost designed this way would be more than five pounds heavier than one designed mearly to withstand "normal" loads indefinately. It's important to design for the occasional abnormal load. Even something as mundane as getting the bow of your boat stuffed under a dock can put a sickening load on your rig.
     
  12. daiquiri
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    daiquiri Engineering and Design

    I could argue that boats are not designed to remain unscratched after impacts like this one, but I do respect your view. **** happens and we all know that.

    Now, if Zambant tried to create a mast step according to your design criteria, do you think the rest of the structure (the existing structure of a commercial fiberglass boat) would be capable of withstanding that kind of impact loads? The guys at Catalina have told him that he should expect "a few hundred pounds" of load at the kingpost. It tells you everything about their design criteria. It is a commercial fiberglass boat and was designed according to common scantling rules. It would then make little sense to make a kingpost capable of resisting an impact of the mast against a bridge when the rest of the structure would probably not have the same strength. The kingpost would survive for future generations of scuba divers but all the rest of the boat will probably not.
    It would probably make little sense to tell him to dimension everything for 3 or 4 tons of load either. It is not just the kingpost to support that load. It is the whole boat.
    Did you see the mast of that Catalina? There is a link to the boats spec's in the first post. Which part do you think will fail first in a situation like the one you have immagined? I bet on the mast, not on the kingpost. ;)

    Your reasoning makes sense when applied to a design of a one-off (or few-off) steel boat. You can in that case start with the idea of creating a very safe impact resisting structure. And you can harmonize the whole structure to that design criteria, eventualy making a boat that will survive a reasonably high-energy impact.
    BTW, if only my clients were like you. The first thing they usually ask me is "how much will it cost?" and "where can we cut these costs?". And then you know how it is going to end. Cut, cut, cut... :)

    But your reasoning is correct, from the safety point of view. An example of good old design school, diametraly opposite to the modern engineering approach based on failure analysis and functionaly programmed failures of the product. :)


    P.S.
    I'm the one who always says that boats should be designed having in mind tractors, not airplanes. ;)
     
  13. Paul B

    Paul B Previous Member


    Guessing the RM at 1 degree at 100, one 200 lb crew hiking out, and a CPW/2 of about 3 feet Skenes calculates it at:

    RM 30 degrees :

    (100 x 28.65) + (1 x 3 x 200) = 3465


    P = 3465 x 2.78 / 3 = 3210 lbs max

    The photo below of a Cat 22 shows the post sitting atop the forward end of the centerboard trunk. The post looks to be about 3" x 3", but turned down a lot to look fancy. I believe the ones Catalina used were teak.
     

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  14. marshmat
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    marshmat Senior Member

    Shouldn't there be a tension tie rod in that photo, connecting the mast step and partner? Or is it just hidden behind the curtains?

    I think we're at the point in this discussion where the original question- how much force is on the mast step- has been answered, a few different ways now. The answer, depending who you ask, seems to be on the order of about one tonne. 1.5 tonnes if, like Paul, you take a conservative approach.

    But PhilSweet raised an important point up there about overbuilding. In a rig, excess weight aloft is generally a Very Bad Thing. But you're talking about a mast step / kingpost, low in the hull. The boat can take a fair bit of extra weight in this location with virtually no impact on performance. And the mast step isn't exactly a heavy component to begin with.

    At the risk of having Daiquiri accuse me of "failure analysis".... my engineering profs taught me to always consider what will happen if, despite your best efforts, somebody does something stupid and breaks the thing. If something must fail, I would much rather the mast fail above deck, than the mast step give way.
     

  15. Paul B

    Paul B Previous Member


    Haw Haw.

    This is a Catalina. Don't look for some things you might expect.

    To be fair, the post acts in both compression and tension. It is through bolted (or screwed) to the external mast step and the interior pan.

    OK, what holds the interior pan in place? Don't ask me...


    I sailed these little boats (swing keel model) quite a bit as a teenager and can say they are not very good going to windward in over 20 knots and big lump. Your VMG in those conditions isn't much.


    My guesstimate of RM 1 = 100 might be too stiff. I know some tender Quarter Ton 25 footers in the 70s were down around 120, and I'm sure the Cat 22 is not as stiff as those boats.

    I calculated it two ways with my guesstimates (Skene's way and Tim Stearns method) and it was 3210 vs 3205, so close enough.
     
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