Engineering Question and Substitution

Discussion in 'Boat Design' started by Chotu, Feb 19, 2022.

  1. Chotu
    Joined: Mar 2018
    Posts: 83
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    Location: Florida

    Chotu Junior Member

    Hi,

    Materials are very scarce or out of reach financially to complete my project.

    I need a little simple engineering help with a conversion. Would anyone be willing to lend their opinion or expertise?

    My plans for the catamaran forward bow beam call for a mast section of 9.5” x 5” with 5/32” wall thickness. 18.5ft long.

    This section does not exist used anywhere and is $13,000USD to buy new. Crazy times we live in. So I must find a substitution.

    This is a standard aluminum bow beam/tube on the front of a catamaran. It’s purpose is to transfer the forestay load to the bows of the catamaran hulls. It is kept from folding up by a “seagull striker” which is simply a spreader like a normal mast has to stiffen it.

    Question: what size round aluminum tube can I use to replace the 9.5” x 5” mast section with 5/32” wall thickness? Moments are 25 x 11.

    Also, can I use a thicker wall so I can use a smaller diameter tube? It will look better.

    thank you!

    9D580F31-04FD-4B3D-AE2A-CDF7F1D1FC43.jpeg
     
  2. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    Buy a used mast section. Florida is full of them at salvage yards. It's where I sourced the mast and boom for my build. 20% the cost of new extrusions and included shrouds, vang, etc. Do you have moment specifications or just dimensions?
     
  3. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member

    Moments are 25 x 11 for the required section.

    Yes, lots of masts. I went to Don’s Salvage. However, they are all too small. None had 9.5” x 5” dimensions.

    Sailorman has no mast sections. They are not nearly as easy to come by in the right size as they used to be. The world is different.
     
  4. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member

    Note: I’ve found a 7” round tube with 1/4” wall thickness has a moment of 30, so I could just use that since they are readily available.

    I’d definitely rather have the nice mast section however. But time is running out. I have 8 weeks to get this rig up.
     
  5. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    I would be curious about how you would alleviate and the requirement to alleviate rotational forces with the mounting of the round tube.

    Is the spec for sparcraft s622 and do you have to buy a full section?

    What other beams are in the system, etc.?

    The beam cannot necessarily stand on MI data alone and the mountings to deal with rotation are perhaps key.
     
  6. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

  7. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member


    Very reasonable. And I did not consider this. I am curious however, where the rotational force would be coming from. If I go directly through the center line of the tube with the forestay, there is just a bending force on the tube. That bending is counteracted by the sego striker, which could be angled backward a bit so that it is more directly in line with the force being applied to the beam. Wouldn’t there be no rotational force in this case? The seagull striker does not need to be on the top like it normally would be, because that is the weak side of the standard mast section it’s supporting. The 5” dimension. Since the beam would be round, it can be angled more appropriately for the forces.

    Setting it up that way, I don’t see where the rotational forces are coming from in the first place.
     
  8. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member


    Sure. A square beam could certainly work. No problem there. I’m not sure about the looks. What do you think?

    If I’m going square beam, I’m almost inclined to go for a pultruded fiberglass beam in that case.
     
  9. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    Since it's just 18' have you tried calling any of the spar manufacturers to see if they have a remnant that long? Rig Rite may have something but are hard to get in touch with.
     
  10. willy13
    Joined: Jan 2022
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    Location: Canandaigua NY

    willy13 Junior Member

    The only problem I see with using aluminum rectangular tubing is that it has a sharp edge. You can buy tubing with a radius edge, but doubt you'll find it in your size. Not sharp enough to cut, but sharp enough to annoy me when I climb on a boat lift I have, and certainly sharp enough to open a bottle of beer, .
     
  11. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Rotational forces across the vessel. Port stern up, star bow down creates rotations in cats.
     
  12. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    There are more pages here that discuss.

    @Ad Hoc explains a twisting ruler analogy well, but this thread is really long

    the main thing is to understand the rest
    Of the structure-

    if there are only two beams; a round one is not going to stop rotation well between the hulls; most of this is beyond my paygrade-I understand it well enough to express concern, but not solve

    another way to look at it is tie four pencils together with rubber bands and twist the hulls in opposite directions and you'll see that a round beam does not resist those forces much; so the beam change is greater than just MI

    also depends on use; for a bluewater vessel; much more important

    calculating torsional stiffness hull https://www.boatdesign.net/threads/calculating-torsional-stiffness-hull.60040/page-7
     
  13. Chotu
    Joined: Mar 2018
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    Chotu Junior Member

    Of course. I have been in touch with everyone. Spar manufacturers are trying to charge $13,000 for this. Laughable. They required a picture of the boat first to see how much money I had. I’m not even kidding you.
     
  14. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member

    I don’t think I have to worry about that at all. Why would I worry about that? You’re talking about torsion?

    That’s not what the beam does. All the beam does is transfer the forestay load into the bow of each hull. That’s it. It’s not a necessary piece of the whole structure itself. It’s something to transfer the forestay load.
     

  15. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member


    OK. Now I see. You are just thinking of a different kind of boat. This boat is already built. The main structural beams are in place. There is no purpose to this beam other than to bring the forestay load and any anchor retrieval load to the bows. That’s it. That’s all it does. Oh yeah. It holds the netting in place as well.

    The structural beams are located in other areas. They are made from Fiberglass. They are an integral part of the boat.

    As usual, when people are way apart in what they are talking about, they are usually picturing different boats.
     
    fallguy likes this.
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