how about an engineering forum?

Discussion in 'All Things Boats & Boating' started by dionysis, Jun 25, 2003.

  1. SailDesign
    Joined: Jan 2003
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    Location: Jamestown, RI, USA

    SailDesign Old Phart! Stay upwind..

    Dionysis, assume the sail is rectangular, with an area equal to the real area, and the same luff length. This becomes a uniformly distributed load on the mast using whatever wind pressure will take the boat to max RM. Then apply a safety factor to the moments obtained and design to that moment curve.
    Steve
     
  2. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

    Saildesign, bending moments not in the free version NPS program
    but thanks for the great database, here a sample from it but what relation is there between diameter and wall thickness in bending strenght? and
    is the sample not a bit overdone for a 300 kg load on 2 pipes 4 meter long and supported on both ends? its not so easy, the shop guy started looking like he was getting a toothache when i asked :D

    NPS v2.2
    ========
    Material - Aluminum Pipe

    Outside diameter - 8.89000 cm
    Wall thickness - 0.7619999 cm
    Pipe length - 1.00000 m
    Material density - 2.71277 g/cm^3
    Flow area - 0.0042614 m^2
    ID surface area - 0.2314097 m^2
    OD surface area - 0.2792875 m^2
    Linear ID volume - 0.0042622 m^3
    Linear weight - 5.28313 kg

    Fluid velocity - 1.00000 m/s
    Fluid density - 1.0000000 g/cm^3
    Volume flow - 0.0042622 m^3/s
    Mass flow - 15354.86 kg/hr
    Static weight -
    fluid only - 4.26524 fluid+pipe - 9.54837 kg
     
  3. dionysis
    Joined: Jan 2003
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    Location: Tasmania, Australia

    dionysis Senior Member

    seems a good idea

    Steve.

    A safety factor of 2-3 say. for an ocean going vessel? and then adjust the section inertia up the mast so it stays within allowable stress and deflection. Is that the idea?

    thanks, dionysis:)
     
  4. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

    found a fairly good materials math site (gives f.e. bending loads on pipes i think) http://www.matweb.com that needs -free- registering for -not very active- forum. my respects for the pro NA / YD only rises, still wishing -easy- things were a bit simpler
    :rolleyes:
     
  5. MDV
    Joined: Feb 2002
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    Location: Singapore

    MDV Junior Member

    A good web site that I use often is www.efunda.com. It requires membership, but you can try it out for free for a certain amount of time.

    Michael
     
  6. Guest

    Guest Guest

    Eigenvalues vs pitch

    Note that a free standing mast will have first mode eigenvalues (natural frequencies of bending) at a much lower frequency than a stayed mast. If the eigenvalues match the pitch / slam natural frequency, there could be severe amplification of stress, and potentially fatigue failure.

    There is an article on use of FEA for vibration problems in the December 2002 issue of Desktop Engineering, www.deskeng.com.
     
  7. dionysis
    Joined: Jan 2003
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    Location: Tasmania, Australia

    dionysis Senior Member

    thanks for the info

    You have made a good point.

    I would have thought though, that if the mast is stiff enough the first mode frequency would be attenuated to a marked degree.

    Together with this, slamming frequencies will not be constant, so any resonance would be naturaly damped.

    Nevertheless, I take your point, and will look further into this aspect of mast design.

    Thanks for the pointer.

    Cheers, dionysis.
     
  8. Guest

    Guest Guest

    Pitch RAO

    You are correct that the boat will see a wide range of frequencies due to pitch, but it will have a significant peak in its pitch RAO. If this is in a frequency range close to those of typical wave spectra, and both are close to mast eigen value, you could have an interesting situation.

    You can do probability based rainfall fatigue analysis for something like a mast pretty easily, because the frequency-stress RAo is pretty simple, and then see if the probability of survival for the lifecycle is acceptable. It's kind of an interestingf analysis too.
     
  9. dionysis
    Joined: Jan 2003
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    Location: Tasmania, Australia

    dionysis Senior Member

    interesting

    We are talking about amplification of stress due to pitch resonance here. Not the so called normal inertias experienced by any rig.

    Would you agree that the range of wave spectra capable of driving the first mode of a mast, depend on the mast's length, and this in turn on the, (all things being equal) length of the boat.

    So if the boat is large, say over 60 ft, and not necessarily that heavy, the pitch frequency will be quite low. Higher frequencies would be damped, together with the added damping due to hull form.

    Add to this the damping effect of stressed sails, and the angles of attck etc.

    Again, this is not to say that this aspect of mast design should not be considered. Far from it, the more I think about it the more important it seems to be. So thanks for that.

    Another aspect is that, if the mast is tapered as it would be, it would introduce higher natural bending frequencies. This would also have an attenuating effect.

    I must say I will need to study some eigenvalue/frequency mathematics to give some numbers to this argument.

    Yes, it is an interesting kind of analysis.:cool:

    cheers, dionysis.
     
  10. dionysis
    Joined: Jan 2003
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    Location: Tasmania, Australia

    dionysis Senior Member

    by the way

    Do you know of any readily available, specific and mathematical treatments on this subject, either on the net or in a library say. This kind of fatigue analysis has to apply to hull strength design in general as well.

    Fatigue is one aspect of design that up untill now I have neglected.

    thanks, dionysis.
     
  11. Guest

    Guest Guest


  12. dionysis
    Joined: Jan 2003
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    Location: Tasmania, Australia

    dionysis Senior Member

    thanks guest, and cheers again. dionysis
     
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