sprit pole?

Discussion in 'Boat Design' started by wayne nicol, Aug 28, 2013.

  1. wayne nicol
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    wayne nicol Senior Member

    would anybody be able to lead me in a direction, with regards to an idea on size and/or rigidity required for a sprit pole approx 20' for an approx 200 sq ft sprit sail.
    am looking at a carbon pole for this.
    so that it is still vaguely manageable for a person.
    any sources would be great!
    thanks
    wayne
     
  2. michael pierzga
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    michael pierzga Senior Member

    Soller composites sell carbon unidirectional and biax sleeves. You might have a look if you decide to build your own.

    Good tube building tutorial video on the site and they may be a good contact for pre manufactured poles.

    http://sollercomposites.com/composites/carbon fiber uni-fabric.html#UNISleeve

    As far as the engineering scantlings.. I dont know.

    A scetch of how you will control, mount this pole would be good.
     
  3. wayne nicol
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    wayne nicol Senior Member

    thanks Michael
    thanks for the link- that just might be the way that I will go, might need to make it with a taper.
    as for the rigging, still in process.
    any ideas would be welcome

    was thinking of a shackle type arrangement at the peak, and it would be attached during the hoisting process, also so that I don't lose the peak during the early reefing stages,-pre scandalizing.
    might need to use a line controlled snap, as on a whisker pole, so that it can be scandalized in a hurry.
     
  4. Petros
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    Petros Senior Member

    carbon can tolerate quite large deflections, maximum strength would be an important factor to prevent pole failure.

    There is a trade off with material properties (both strength and elastic moduls or flexiblities), and diameter of the pole, and skin thickness. Without more specific details of what you have in mind in terms of size you can not really tell you the details of how to configure the pole.

    the maximum bending moment for such a configuration would be about 6000 in-lbs, at the attach point (includes a 1.5 safety factor), this would be in about a 20 kt wind, which is higher than most would leave their sail up.

    such a pole would need Section Modulus of 35. A larger dia pole would need less carbon thickness than a smaller dia pole, and it would be more light if you use a lightweight core material. A larger dia pole would also be more stiff. with too thin a skin however, it will suffer damage more easily during handling and transport.
     
  5. wayne nicol
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    wayne nicol Senior Member

    thanks, insightfull!!!.
    ok heres the plan.
    26 ' boat, currently a sloop rig, going to change the main to a sprit rig.
    need to shorten the mast, from 30' down to 20 ', which gives me about 18' from the goose neck up.

    have opted for a sprit for a number of reasons, higher performance than the other traditional rigs, less weight aloft, simpler rigging, anly downside is the reefing, but should be able to get in two small reef points, then it would be down to scandalizing.
    currently main is 170 and jib is 130 ft sq, so if the mod designers feel that the current rigging will be sufficient, then the main might land up around 200 ft sq when done, and jib will be a little less than 130.

    so the issue is the sprit pole-
    will probably have to build my own, am currently entertaining building the pole with a swoop in it , on the outboard side, and a short square section at the foot.
    the square section so that the pole can be secured against the mast, to ensure that the pole does not rotate, and the swoop is for when sailing on the "bad tack" the pole now on the back of the sail will not split the sail in half.
    kinda like one half of the wishbone boom/pole setup.
    just with current sketches, the sprit looks like it is going to need to be about 20 feet.

    going to make a mock up poly tarp sail, and check the exact length of pole needed, etc.and see how it all fits together.

    gotta talk to a designer more!!
     
  6. troy2000
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    troy2000 Senior Member

    I'm not a designer. But are you going to get enough improvement on the 'bad' tack to be worth the trouble of getting an asymmetrical sprit to stand proud of the sail? As a general rule, simpler is better....
     
  7. michael pierzga
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    michael pierzga Senior Member

    Something strange......a 20 foot pole on a 26 foot boat with a 20 foot mast ? What is this pole for ?
     
  8. wayne nicol
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    wayne nicol Senior Member

    you are correct troy, just an idea i was thinking of, to counter a common issue raised with sprits, however i have also read many reports that do not pick up any noticeable difference with either tack. just thought that if i was going to be building a sprit pole anyway from glass/carbon, it might be a consideration.,
    i do agree with you whole heartedly- simpler is better, ironically thats one of the reasons for settling on the sprit- now im just complicating it again!!:confused:

    but i am certinally no designer either, i have an idea, can do some very basic math and scale sketches, then i turn it over to the smart guys!!!

    Michael, i got that approx. measurement of the sprit pole from my initial sketches,based on the traditional sprit sail plan that i thought would work. i am absolutely sure that, that will all change somewhat, when the designers get hold of it, but on the slightly higher peaked traditional, sprit rigs, the sprit pole is quite an affair, one of the downsides of these simple yet very effective rigs.

    when i correlate the proportions to other smaller rigs of similar shape, with a similarly shaped peak, that measurement seems to be within the ball park, the pole can be a bit longer than the mast.

    currently the boat has a 30' mast, so i will be able to remove 12' off the mast, and possibly gain a few square feet in sail( although that is not the aim-( just the way the mast, and boom measurments work out!!)

    going to lose a bit to windward, but will gain in the reaches!
     
  9. messabout
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    messabout Senior Member

    While you are chewing on this deal, you can make some SWAG estimates of where the center of effort will land. Chances are that the CE will move aft if the mast is left in its present location.

    I have fiddled with sprit rigs, mostly sprit boom types. I like them for the sheer simplicity and ease of handling. I have also built some curvy single sided sprits for the reasons that you mention. If you go that route then the curvy sprit will need a lot more rigidity than one might contemplate. My several experiments revealed that the bent sprit would bend so much when cinched up, that the sail would not set well. This led to larger and larger sections of the pole untill it was so fat as to be clumsy. I went back to the moderately sized straight pole. Yes there is a difference from the good tack to the bad tack but not enought to put up with the outsized bent pole. Converting to a wishbone might have been a better option. But then it is a pain to loop it around the front of the mast.

    The bent single sider also needed a topping lift to keep it from drooping in the middle on account of the eccentricity of the poles weight distribution. I think that you could make a serviceable sprit from wood, maybe a birdsmouth type. I figure it would need to taper at both ends and be fairly fat in the middle. The birdsmouth build would surely cost less and work as well as a CF one. Not much heavier either. Working with most easily obtainable lumber at, 16 feet, you'd need to stagger scarf the strips. More's the fun.
     
  10. Milehog
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    Milehog Clever Quip

    I built a sprit rig for my sail boat but converted to a balanced lug after making a realistic assessment of the process of reefing in less than perfect conditions.
    The balanced lug is much more manageable.
     
  11. wayne nicol
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    wayne nicol Senior Member

    thanks messabout, i have done some initial "calculations" where the c of e will land, and it looks like it will be back about 8", from the original sail, but as soon as the rain clears up, i will get the original sail out on the lawn, and work from it, to date i have only been working with some drawings from a sales site, so not too accurate i guess.

    i could move the mast forward about 6 inches easily enough, but how would that effect the relationship with the shrouds etc, really dont want to start shifting those about!!

    good points on the curved sprit, so will toss that out, we have lots of high quality sitka spruce growing here, so that is an option, wont cost anything except time.
    i had thought about the cf pole being made with a double taper, just really not sure what kind of layup i would need to get the stiffness i would need, would probably use those bias weave carbon tubes/socks, that you pull over a mandrel and then wet out.

    if out of wood, and certainly birds mouth, any ideas on scantlings.

    what size was your boat milehog, how did you find the difference in the performance between the two rigs.

    from folks i have spoken to, the scandalizing is so quick, to drastically reduce sail area, but then you are left wrestling with the pole, but to take in a reef at the bottom of the sail will be more laborious, with moving the sprit first etc, then dropping the main halyard.
    but for me i feel it is do-able.- considering my gains with the shorter mast for my application.
    would love to hear more about your boat and ideas.
    thanks
     
  12. Milehog
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    Milehog Clever Quip

    Wayne, I didn't sail the boat with the sprit rig but after speaking with sailors having extensive experience on my sort of craft I made the change. The boat is a stitch and glue 4.6 meter (15') sail n' oar type.
    The balanced lug will pull the CE forward a bit and you can change the CE at will, depending on how it is rigged.
    Your talk of shrouds has me confused, forestay perhaps?
     
  13. wayne nicol
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    wayne nicol Senior Member

    nope, the side shrouds- 2 per side and spreaders.
    and a forestay( roller furler- but i think i will change it for a stay and hank on type sail.
    this boat is currently rigged with a marconi.
    and the mast is stepped on the coach roof.
    thanks
     
  14. wayne nicol
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    wayne nicol Senior Member

    thanks milehog,
    i hear a lot of concerns about the sprit, and they are all obviously valid, but i realise this is an exercise in compromises, so have set my mind at ease with its limitations, will just need to pay attention to those issues with a tad more diligence i guess!!

    the sprits do perform exceedingly well, and their simplicity is a big factor, and the lower heel angle is another.
    and i am a little biased- i do love the look of all those traditional sails, sprits, gaffs, lugs etc
     

  15. sprit
    Joined: Jul 2013
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    sprit Junior Member

    Look at the lightweight poles that pole jumpers use...
    Good luck.
     
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