Paraw conversion: How to control a flexible spar?

Discussion in 'Multihulls' started by pbmaise, Feb 20, 2015.

  1. pbmaise
    Joined: Jul 2010
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    Location: Cebu the Philippines

    pbmaise Senior Member

    For a quick primer on the paraw rig see:

    http://en.m.wikipedia.org/wiki/Paraw

    It has a crab claw behind the mast and jib in front.

    After conversion my 65ft trimaran will be largest in terms of overall dimensions and weight.

    The issue I wish to address is how to control a 70 ft long flexible spar.

    Most spars, including booms, are controlled using a single line and termination at or near the distal end.

    Ideally I want one single control line that branches out and then terminates at several points along the spar.

    The issue is the farther each line connects along the spar away from the focrum, the greater the amount of line that needs to play out/in when moving.

    Thoughts?
     

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  2. pbmaise
    Joined: Jul 2010
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    pbmaise Senior Member

    Here is a picture of Lipton's sailboat Shamrock.

    en.m.wikipedia.org/wiki/Shamrock_(yacht)

    Note the lower spar's length.

    I doubt one control line was used, and what is the best way to reach out past the stern?

    One thought I have is to have paired lines. So that the branch line going to the farthest distal point is also going to the point closest to the focrum (mast).

    However these would cross.

    Has the world seen a paired boom? Just a thought in progress but a paired boom with a lower boom below the main boom could terminate.

    Yes a really stiff 70 ft carbon fiber spar would be nice. Tell me if you know of one.
     

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  3. pbmaise
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    pbmaise Senior Member

    There does seem to be some work in the area of pairing a boom with a mechanism to shape the boom for optimum performance.

    My chief concern isn't shaping, rather it is the boom will shape too much underload.

    http://www.google.com/patents/US4261276
     
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Some boats use boomkins at the stern.
     
  5. pbmaise
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    pbmaise Senior Member

    Thank you Gonzo
    At first I thought you were trying to insult me. Boomkin aka bumkin are traditional sailing terms.

    The 10 foot tall radar tower at the back of my vessel is an issue. However, it is good to see partnered spar with one above the other below. When reinventing the wheel it is easier if you don't have to invent everything and see other ideas.

    This has given me the idea of using the stern mast step to mount a boomkin at about 45 degrees. The primary purpose is to help keep the distal point of the 70ft spar in line.

    These are ideas in progress.
     

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  6. cavalier mk2
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    cavalier mk2 Senior Member

    Take a look at Chinese junk rigs, what you are after isn't too different. Separate lines off the spar could control twist.
     
  7. pbmaise
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    pbmaise Senior Member

    Thank you for tip Cavalier

    I found on this page some Phil Bolger drawings.

    http://www.thecheappages.com/junk/tutorial.html

    Of particular interest was a detail on controlling the junk sail twist with multiple lines that interconnect on blocks.

    With the crab claw sail I will use there are no battens, rather the lower spar can be thought of like the trailing edge of the junk.

    Dang it! If see a drawing of Vallhala. I have been physically following that boat around the Pacific and now am slowly modifying my rig more and more to match.

    Philip
     
  8. pbmaise
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    pbmaise Senior Member

    Yesterday I brought down the 70 foot long piece of bamboo that will serve as the lower spar of the crab claw sail. It weighs a lot, but no where near what the original aluminum boom weighed.

    BTW This has given me a possible lead on the real reason for the unexplained first demasting of my rig. It occurred prior to my purchase. I destinctly recall now the huge boomvang I had inherited when purchasing the Boat. It was about 7 feet long and packed with heavy coil springs. When I took this assembly apart in Thailand, I discovered two of the springs had failed owing to corrosion.

    This means for the first 8 years the Boat sailed the Pacific, a strong boomvang and springs helped counteract the load from the weight of the boom and sail. Once the springs broke, the weight was transferred entirely to the topping lift and hence to the top of the mast.

    Looking at the local paraws I see all employ a boomcrutch or gallows to support their spars when not flying.

    http://forum.woodenboat.com/showthread.php?147393-boom-crutch-vs-topping-lift


    My thinking is moving towards a combination boomkin and boom crutch.

    Time for me to diagram.
     
  9. pbmaise
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    pbmaise Senior Member

    Here is an update photo of my new boom crutch.

    Thanks for the ideas.

    The kids are standing on the now treated bamboo. There was still a final coat of paint left, and ropes are not the final lines.

    The two big spars hold the sail. A strap sewn every 18" onto the sail spreads the sail load along the entire spar. Within the sail spars, I installed aluminum tubes filled with dried bamboo and microbead. Then more bamboo is wedged around the aluminum pipes. They still flex, however final running rigging lashed every 36" is not shown yet.

    All spars are epoxy sealed at the top and epoxy painted. I left a weep hole at the bottom and a boric acid greating for any visitors. I don't expect them to last forever. However, I can make another 50 sets before I even come close to the cost of carbon fiber.

    The boom crutch is vertical in the background and has a triangular spreader bar giving it added strength. This piece of bamboo was originally bent near the spreader bar. The high horizontal force of the spreader made the pole straight. Had I started with a straight section of bamboo, the spreader would have introduced a bend.

    Within the crutch there is another spar. This internal spar was made for me using fiberglass, epoxy, construction grade honeycomb, and Dyneema.

    End fittings are different for each spar. The largest spar is using a new Harken spinnacker pole attachment insert. It simple locks onto the pin for the original boom.

    The smaller pole incorporates a 3/4" stainless turnbuckle. The exposed eye bolt is free to turn, drain moisture, and locks onto a 1/2" U bolt through the larger spar.

    Now here is where it was difficult. To install the U bolt I had to drill two parallel holes through epoxy plugs. Further, the U bolt is normally in compression not tension. To prevent the U bolt from pushing out the bottom, I made an adapter sleeve from stainless heavy wall pipe to transfer compression load to the top of the lower spar. Load is first spread to a large washer, then to a block of bamboo epoxied onto the top of the lower spar. To spread the loads more, the lower two meters of the large spar contains a section of that construction grade spar in the boom crutch.

    Normally a boom crutch is fixed vertical. However, owing to the sizes, loads, and weights, designing the attachment of the boom crutch too some thought. Instead of permitting the boom to rise up and over the crutch, I decided to afix the two. This eliminates the need to untie and retie the boom. The rear radar tower and wind generator tower forced the boom cripple to be so tall, that it isn't practicle.

    Therefore, the boom crutch is designed to pivot and handle both compression loads and tension loads.

    The 3/4" boom crutch has a stamped graded shackle embedded with pin exposed. I used the other eye bolt from that turnbuckle and imbedded it into the mast step. The shackle pin then securely holds the crutch in place.

    The top of the boom crutch has a 3/4" stainless swivel that also handles both compression and tension loads.

    The kids in the photo are standing on the spar in the longterm storage position. The majority of the weight is still carried by the boom crutch using a block and tackle.

    The only part I needed from the hardware store was 8" of stainless pipe.

    Note: I still have on the stainless pipe clamps. These I tightened every day as the bamboo shrank on deck. I keep them in place for added strength.

    Philip

    Hot Buoys SV
    The world's largest single mast paraw rig
     

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  10. Alex.A
    Joined: Feb 2010
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    Location: South Africa

    Alex.A Senior Member

    Have you seen Hans Klaars ontong java or the Wharram catamaran wakataitea?
    Some info to be found on wakataitea on Wharram builders and friends.
     

  11. pbmaise
    Joined: Jul 2010
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    Location: Cebu the Philippines

    pbmaise Senior Member

    Attached Files:

    Last edited: May 7, 2015
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