Closing halyard exit slots in aluminum masts

Discussion in 'Boat Design' started by Manateeman, Nov 12, 2019.

  1. Manateeman
    Joined: Oct 2019
    Posts: 31
    Likes: 1, Points: 8
    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Hi. AdHoc. The mast has never been stepped in this boat. This will be the first mast. I’m concerned about the openings due to the number and location...loss of compression strength and the issue of the forces on the sidewalk of the spar because they are so close to the deck collar.
    I will post a drawing and moms tomorrow when hopefully I find out what alloy was extruded.
    As far as welding...well there are heavy mast steps welded on both sides of the mast and the top is tapered.
    I’ll see if I can find out more about how Pacific Spars welded it.
    I’ll take some photos.
    Thank you for the reply.
    Mark
     
  2. Manateeman
    Joined: Oct 2019
    Posts: 31
    Likes: 1, Points: 8
    Location: St. Mary’s Georgia

    Manateeman Junior Member

    8B36EFF1-3DFD-421E-A25C-708C5ADC238D.jpeg I made a drawing of the existing halyard slots. 7’ 9” is the distance from the heel to the top of the welded mast collar.
    The collar is about 4” high.
    As you can see, two of the exits are close to the deck collar.
     
  3. bajansailor
    Joined: Oct 2007
    Posts: 378
    Likes: 60, Points: 28, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Mark, I think that both Ad Hoc and I are thinking that you are worrying unnecessarily - the mast did not collapse in it's previous life with these existing slots / apertures in the wall, so why should it do so now in it's new life, assuming that nothing has changed with the mast?
    Especially as it will now be effectively about 7' shorter than before, hence less sail area, hence less loading compared to before (all else being equal).
    Re the deck collar, in a perfect world with the mast set up absolutely vertical with equal tensions in the stays on each side, there should (in theory) be no side force at all on the collar.
    It is not a perfect world though, and the rig will not be set up absolutely equally on each side, and wire can stretch a bit.
    But even so, the resultant side forces that will be experienced on the deck collar should be fairly small relatively (?).
    And there are no slots or apertures in way of the deck collar.
     
  4. Blueknarr
    Joined: Aug 2017
    Posts: 703
    Likes: 115, Points: 43
    Location: Colorado

    Blueknarr Senior Member

    Bajansailer is absolutely correct in stating that you are worrying mountains about mole hills.


    Bajansailer, there is quite a bit of lateral force applied to a mast while sailing. Dockside the static load from the stays should be purely compressive and columnar with no side loads. However. Once a sail is hoisted, the wind will load it horizontally. This lateral loading is the root of sailboat's heeling and leeway.
     
  5. bajansailor
    Joined: Oct 2007
    Posts: 378
    Likes: 60, Points: 28, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Oh I appreciate that these lateral forces can be huge - however they are countered by the tensions in the rigging wires.
    If they were not, the mast would fall overboard pretty quickly.
    I am sure that if the rig is set up correctly, the actual side force locally on the collar where the mast comes through the deck should be relatively small (especially so in comparison to the forces elsewhere in the rig).
    I guess that if one was applying simple beam theory, with an assumed uniformly distributed sideways load between the mast in way of the collar and the first set of spreaders, then there would be a deflection.
    And I suppose that a worser case scenario can be envisaged where the load is considered to be a point load at the centre of effort of the sail plan, rather than a uniformly distributed load?

    And the important thing here is that the mast survived happily in it's previous life, hence no reason why it shouldn't enjoy a second life now with Mark on his fine ketch.
     
    Last edited: Nov 14, 2019
  6. Barry
    Joined: Mar 2002
    Posts: 1,136
    Likes: 78, Points: 48, Legacy Rep: 158

    Barry Senior Member

    When the wind loads the sail the up wind stay will experience an increase in tension creating more compression in the mast
    The horizontal component of the stay tension will resist a portion of the lateral wind load at the top of the mast and the bottom of the mast will experience a significant portion as well.

    The heeling of a sailboat is a result of the both lateral forces of the mast but I expect the heeling moment resulting from the force at the top of the mast may be more than due to the longer moment arm

    Manateeman has mentioned failure due to compression
    Rigid materials rarely fail due to compression. You will rarely if ever find a failure limit that shows a value for yield for compression.
    I was going to say never but that does leave some room for exceptions
    Short wide members when under compression will fail when shear strength limits are reached. Long slender (unsupported along its length) members will fail due to buckling, a stability issue, which then fails in tension due to bending stresses
    So the company that developed this mast would more than likely designed the mast Area Moment of Inertia to resist buckling as this is how a long slender member will fail. Ie long before you have to worry about enough compressive forces reaching the ultimate shear stress limit (like a short wide member) the mast will have buckled
     
  7. Manateeman
    Joined: Oct 2019
    Posts: 31
    Likes: 1, Points: 8
    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Greetings all from a humbled boatbuilder. Please excuse me when I use the wrong term. Compression, for example.
    My go to text is Principles of Yacht Design by Larson, Eliasson and Orych. I like reading it because it reminds me how much work a naval architect does before they put pencil to paper. They have my utmost respect. Because I do not have an engineering degree, a lot of this book is way beyond my comprehension. For example, there is a section in the chapter Rig Construction titled Holes In The Mast. It’s clear to someone who understands all of the mast formulas and all the calculations in the chapter on Hull Construction. Most, and often, all of it, is beyond me, but clearly holes simply weaken the spar and further, there are areas of the mast where no holes are appropriate.
    I need to add more holes above the deck and I’m trying to follow the logic behind where the original holes were placed.
    In the chapter on Hull Construction, the section on Loaded Areas details how the deck must be strong enough to resist and distribute the transverse and longitudinal forces ...not just from the shrouds, but from the mast.
    I think it’s in a book by Brian Toss where he uses a piece of spaghetti to show two and three point fixity. I might be again using the wrong examples but if the deck needs to be ‘exceptionally stiff and resistant ‘ then isn’t the mast also stressed in this area of fixity ? I have no idea how to estimate the loading but the holes keep staring back at me.
    I spoke to one of the gentlemen who was with Pacific Spars. He will give me more information when I send him photos.
    He confirmed the extrusion is 6061-T6. The taper and all the mast steps were TIG welded. I’ve examined each carefully and cut into four welds while removing the lower steps. Finely done welds.
    Again, thanks to all who have taken the time and effort to reply. I truly enjoy this forum.
    A grateful manatee. Mark
     
    bajansailor likes this.
  8. Manateeman
    Joined: Oct 2019
    Posts: 31
    Likes: 1, Points: 8
    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Hi. Barry. Thank you for responding. So, what do you feel I should do about these slots...???
    Regards, Mark
     
  9. Manateeman
    Joined: Oct 2019
    Posts: 31
    Likes: 1, Points: 8
    Location: St. Mary’s Georgia

    Manateeman Junior Member

    Hi ,toBarry. Thank you for the reply. Well I think we both have a good understanding of welding aluminum. I wish there were some tests on mast extrusions using MIG vs TIG as well as different machine settings, rods, methods , welder skill, etc. I think it’s fair to say, a lot depends on who sets up the weld, their experience, the conditions under which they are welding...you certainly understand the huge variables. Please, let’s take a look at what I have. It’s a used spar, welded only with TIG. The alloy is 6160-T6. Both sides of the spar have steps TIG welded as well as the taper. They staggered the halyard exit slots, but the mast steps certainly have created HAZ areas up the spar. Pacific Spars did not weld the spreader support area, nor the boom gooseneck.
    You are not the only person who cautions welding on aluminum spars, but I don’t think categorically, this is a bad idea.
    As a matter of fact, I think caution is the word of the day because of all the poor preparation I have seen in aluminum welding. I really enjoy Welding Tips and Tricks. Here is a guy who simply says : I don’t know it all. Another example.
    A gentleman who constructs tuna towers in Alabama was gracious enough to give me his “secret” settings for anodized aluminum tube. Welders are artisans and cooperation benefits everyone. I would never state there is a right nor wrong technique only that we all would benefit from a more scientific approach and more importantly, a commitment to sharing information within a forum based upon simple human kindness.
    Manatees are not speed bumps.
    Respectfully , as you do have very significant credentials.
    I remain graciously appreciative .
    Mark the Manatee
     
  10. Barry
    Joined: Mar 2002
    Posts: 1,136
    Likes: 78, Points: 48, Legacy Rep: 158

    Barry Senior Member

    We normally had 5-6 people welding anodized aluminum tubing. The category on the anodizing was R5 which is an almost mirror finish. In order to get a structural quality weld we actually ground the weld bead area with a die grinder to facilitate the weld.
    I would expect the mast would be under considerable bending stresses due to the loading profile and that the halyard slots would be mainly oriented along the neutral axis, that the slots will not be a significant or relevant parameter to the mast failing.
    I would leave it as is. The load and subsequent stress calculations would be complicated and all the cross section parameters of the mast would need to be known.

    Probably the most simple conclusion to make is (and someone else floated it) that the mast is proven and the holes were not a factor
    And additional slots will not significantly reduce the strength
     
  11. gonzo
    Joined: Aug 2002
    Posts: 13,609
    Likes: 382, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Barry: your statement about compression strength is incorrect. Most metals, including aluminum, have equal strength for compression and tension. do you mean brittle when you say rigid? Aluminum mast alloys would not be classified as brittle.
     
  12. Barry
    Joined: Mar 2002
    Posts: 1,136
    Likes: 78, Points: 48, Legacy Rep: 158

    Barry Senior Member

    Not so
    When you take say a round cylinder with a diameter of say 12 inches and a height of 12 inches and compress it until failure
    The cylinder will normally slip-shear I think at 45 degrees. Ie when the max shear strength is reached

    So when you look at max strengths for a homogeneous structure aluminum steal etc you will see max shear and max tensile stress only being listed
     
    Last edited: Nov 20, 2019
  13. gonzo
    Joined: Aug 2002
    Posts: 13,609
    Likes: 382, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Compression and shear are two different things. Compression and tension are forces in opposite directions. Shear is at 90 degrees to them. Compression and tension strength in most metals is the same, that is why the tables only show tensile. In the case of brittle materials is when both compression and tension are tabulated.
     
  14. gonzo
    Joined: Aug 2002
    Posts: 13,609
    Likes: 382, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Exactly, that is the reason I asked if you meant brittle when you wrote rigid.
     

  15. Barry
    Joined: Mar 2002
    Posts: 1,136
    Likes: 78, Points: 48, Legacy Rep: 158

    Barry Senior Member

    Again not so suggesting that Shear is at 90 degrees to compressive forces

    Back in the early 70’s we had to take some civil engineering courses with our mechanical program. Probably because they had the “baddest” high tonnage compression testing presses as our university did contract work for large projects. In our labs we
    brought to failure many different cross sections and while the members if they did not buckle, Euler, then they would shear down a
    relatively small window of specific angle. So the metal, concrete, etc members would display slip along an angle. That is what shear is.
    Not at 90 degrees. But a compressive force creating shear yield stresses.
    Solids as well as liquids are virtually incompressible so how can you determine compression failure. Certainly with extreme compression you may see some strain, which by definition, is a reduction of say length, but really is not failure until it has yielded

    A side note as well, back in the slide rule days, we used Mohrs circle to determine stresses in members that may have multiple forces
    applied, torsion, compression with bending and when failure occurred it would often be that the shear strength was attained.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.