tabernacle question

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  • 24 May 2019 13:23
    Reply # 7388052 on 3251698
    Anonymous member (Administrator)

    The good thing with tabernacles is that they are fairly easy to make stronger than the mast because of the simple fact that they must have bigger sections. Bending strength varies with the cube of the cross section.

    During the short period of time that I used hinged battens, I actually made the hinge parts so that they sat on the outside of the battens, for the very same reason. These never failed.

    Arne



    Last modified: 24 May 2019 13:27 | Anonymous member (Administrator)
  • 21 May 2019 22:37
    Reply # 7354768 on 3251698

    Hi Scott,

    I use a simple formula of cross sectional area times strength times lever arm to calculate a moment that the section is capable of resisting, this can apply to any section shape. The simple way to do it is to take the cross sectional area of the major element, the side or the back , of the section multiply by the strength and then multiply by the distance between the center of area of the two sides. For round pipe sections I use an approximation of a quarter of the cross sectional area of the pipe wall and 80% of the diameter as the distance between the two sides. This seems to work out reasonably closely to the much longer and complex formulas normally used. I try to keep it simple as you can get lost in the complex formulas and make major errors that way. It may not be as accurate but I have found that it serves well and is simple and quick to calculate.

    I hope that this is of some help.

    David.

    Last modified: 21 May 2019 22:45 | Anonymous member
  • 21 May 2019 18:56
    Reply # 7354204 on 7352916
    David wrote:

    Hi Scott,

    [...] A much better shape is a top hat section that has wings on the forward edges of the side members, of a combined cross sectional area equal to that of the back face. [...]

    Hi David,

    Thanks for the response and the suggestion to use the top hat section. If I decide to use an alloy mast I will most likely use a beam made in this way.

    I have not been able to find any information on how to calculate the Section Modulus for this type of section.

    I found the math to calculate the Section Modulus for a C section. My idea was to determine if C section is strong enough. This would ensure that the top hat section is also strong enough. If I was able to get the equations for a top hat section then I would use the result directly. Do you know how I can calculate this value?

  • 21 May 2019 04:44
    Reply # 7352916 on 3251698

    Hi Scott,

    there is a weakness with the profile you have shown for the tabernacle. When going downwind the section is basically one sided and will crease along the thin forward edges of the sides of the tabernacle. A much better shape is a top hat section that has wings on the forward edges of the side members, of a combined cross sectional area equal to that of the back face. This also allows for bolting on a retaining bar at the top and bottom when the mast is in the vertical position.

    David.

    Last modified: 21 May 2019 04:45 | Anonymous member
  • 20 May 2019 15:26
    Reply # 7351705 on 3251698
    Anonymous wrote:

    I'm planning a tabernacle for the Brenda B, my Com-pac 19. The sail is in two pieces, roped with webbing and waiting for some heavier material to finish the short batten pockets by the mast. The 4 ½ inch x .125 inch 6061 t6 aluminum mast tube is on order and the snow is melting off the boat cover. My recent sailing has been from a dock on a lake a several miles from home which means I can be sailing in minutes, and with even less hassle with the new junk sail. While this increases my sailing time and allows things like for spur of the moment supper and evening sails, I miss time sailing out among the islands of Penobscot Bay. The tabernacle will make getting to the coast for a cruise or a temporary mooring much simpler than a keel stepped mast.

    Pictures of Daze Z's Slacktide tabernacles and Pete Hill's tabernacle write up in JRA mag #61 have given me some new ideas. I'd been thinking that to hold a round aluminum mast in a square tabernacle I would need half round shapes at the tabernacle's top and base to hold and clamp the mast. Slack tide does well with a round solid wood mast clamped square in a square tabernacle. It seems that that this would would work with hollow aluminum. Is this sound thinking? After seeing how the two tabernacles pivot I decided to use a wide heavy duty s.s. strap clamp designed for pipes to clamp the mast to a rugged hinge with the other flap of the hinge lagged to the back of the tabernacle. The intended pipe size is 4 3/8 inches and the space could be shimmed tight with leather providing a little cushioning.



    Personally, with only .125 wall thickness, I would want to put an insert inside the lower end of the mast tube to prevent flattening from pressure.   6061 T6 is superb material, probably the best aluminum for both strength and resistance to cracking.    I would avoid drilling it if possible, and if I did drill, I would take great care, using a drill smaller than the final size and a fixed size chucking reamer, and paying great attention to the corners of the holes so they are extremely smooth.... Even the smallest irregularity here can be the point where a crack can start. (this based on aviation experience) Perhaps building the bottom of the mast up with carbon tow and embedding a bushing in it similar what Pete did on Oryx.... see JRA#61.   Also there is a Utube slide show on the KD 860 that has a couple clips of someone building a chainplate this way:   https://www.youtube.com/watch?time_continue=1&v=Bv-ath_xmjY


                                                               Howard

    Last modified: 20 May 2019 15:27 | Anonymous member
  • 16 May 2019 16:12
    Reply # 7345045 on 3251698

    I attempted to calculate a meaningful number for the strength of a tabernacle.

    I hope this topic is a good place to ask for feedback. If anyone can check my math, in the excel sheet, I would really appreciate it. I taught myself what I could about calculating the section modulus and bending moment and how to use material strengths. This is very different from anything I studied in school. I have no practical experience with structures like this.

    From Arne's TCPJR book I determined that it would be reasonable for me to have a mast that supports a bending moment somewhere between 925 kpm and 1826 kpm. I intend to aim for the higher value.

    In the attached spreadsheet I did my best to work out this value for a 'C channel' cross section. One tab is for a Douglas Fir tabernacle. The other tab is for a 6061-T6 tabernacle. Both are 'three sided box' sections.

    I triple checked my formulas. The results indicate a structure several times stronger than what I expected from the materials. Did I make a mistake?

    For a 1.5" (38.1mm) thick Douglas Fir tabernacle surrounding a 7" (177.8 mm) mast I get a maximum bending moment of 7025 kpm

    For a 1/8" (3.175 mm) thick 6061-T6 tabernacle surrounding a 7" (177.8 mm) mast I get a maximum bending moment of 2175 kpm

    Thank you for taking the time to read my post.

    Scott.

    Last modified: 16 May 2019 19:43 | Anonymous member
  • 29 Jun 2018 14:46
    Reply # 6349946 on 3251698

    I've had to cancel my offshore sailing plans again, I relaunched about a month ago and everything looked fine, but on my way to Nova Scotia stress cracks have appeared in the mainmast tabernacle and are spreading rapidly. The cause seems to be a tortional stress which has been a problem all along. The trouble is that the lateral stresses are not being taken up evenly fore and aft and are causing a twisting moment which I hadn't expected. I'll be hauling out again for repairs and hopefully get things right this time, I was planning to write an article about these tabernacles thinking they were a success, now I'm glad I didn't. When I do get things right I'll share what I've learned but more work and more testing are needed


  • 10 Apr 2015 05:20
    Reply # 3290900 on 3251698

    Or steel that could be left ashore or epoxy coated and stowed as ballast. New plan, I'll be at making the lower 1 /10th plus of the aluminum mast square by snugly encasing and epoxying it it in a fiberglass covered plywood box, ½ inch marine grade, and then using Pete Hill's proven design. The inside corners of the box can be filled, at least top and bottom, to provide an even bearing surface all the way around the mast. Got the plywood yesterday and am making progress elsewhere. Today I fit and attached battens to the sail and was relieved to find that they all fit in the pockets despite a couple dry spots that had me thinking I might have to pull some stitches. I'll get there yet.
  • 09 Apr 2015 15:25
    Reply # 3288989 on 3251698

    I have sailed a pointy sailed boat once with a wooden mast counter balanced with a length of concrete bolted to it , it came down and up no trouble , probably acted as ballast as well   .     alloy tubes don,t do  well in  compression perhaps a hard wooden or steel former of a fair length in side might do . 

  • 25 Mar 2015 13:42
    Reply # 3265947 on 3251698

    I think that's a good idea, it would help solve the problem I had with my foremast tabernacle, which is a lack of torsional stiffness. Not a problem when the boat is in calm water or on the hard, but could be if you wanted to raise/lower the mast in less than ideal conditions. The lower the axis, the stiffer it would be. If I had thought of that before, I might have done things differently. 

    I think I have a solution to the twisting problem, will post pictures when it's done.

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