What is wrong with a Junk rig

Discussion in 'Sailboats' started by Mikthestik, May 14, 2016.

  1. brian eiland
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    brian eiland Senior Member

  2. sharpii2
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    sharpii2 Senior Member

    I did mention that in my post. If I simply stopped there, the eight Boomlets would indeed weigh four times as much as the Boom they replaced. But this seems to be the part you don't get. Even if this were the case, the 1.20 lb Boom would be replaced with 4.8 lbs of Boomlets. So, subtracting the 1.2 lb Boom(which has been replaced) from the 4.8 lbs of Boomlets leads to a net 3.6 lbs of added weight. So now this new rig weighs 22.8 lbs instead of the original 19.2 lbs.

    But I didn't stop there. Instead of planing one dimention down to 0.5 inch, I planed both dimensions down to 0.595 inches. Doing this gave me the same moment of inertia, but with a smaller sectional area, meaning a lighter Boomlet. So instead of weighing o.6 lbs each, they ended up weighing only 0.425 lbs each. 8 x 0.425 lbs = 3.4 lbs total. 3.4 lbs - 1.2 lbs = 2.2lbs. 19.2 lbs + 2.2 lbs = 21.4 lbs. And this is before I shortened the mast.


    If we have 8 boomlets at 1/2 the thickness then in total we have used four times the amount of material. Worse much of that weight is high up the rig, subtracting from RM, and adding substantial loads to the mast. The extra compression loads from them need to be taken into account, requiring a heavier mast section to be use further reducing RM, and the mast weight itself adds to compression loads.[/QUOTE]

    The additional load on the mast is significant but not all that substantial. Merely shortening the mast may more than make up for that.

    The added weight aloft is certainly a real issue, which shouldn't be taken too lightly. Pun. But, as I have said in many posts, good cruising boats are considerably different than good racing ones. Weigh aloft robs performance long before it steals safety. It also adds rolling inertia, which lengthens the roll period, but also makes for a far more gentle roll.

    Racing sailors are like athletes. Some actually are athletes. Discomfort, for the sake of potentially race winning performance, is a sacrifice they must learn to live with. Not at all true with cruising sailors. In fact, a more comfortable boat often ends up being a safer one, as the crew ends up less exhausted. I've even heard of slower boats reaching their destination sooner than faster ones, due to this factor.
     
  3. Skyak
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    Skyak Senior Member

    The reasons your exercise does not conform to my 'proof'
    -assuming solid constant section mast -just wrong, tapered at least, prefer hollow. The lug mast will have to be thicker to support the higher compression of the lug with the sail downwind of the mast.
    -assuming a classic chinese junk with a yard equal to your lug (well then yes it will be hard to make up the weight elsewhere) -I specified an elliptical planform that can use thin battens of declining length all the way up. The reason they can be so thin is that they are supported at the mast and the sheetlet. By comparison the lug yard must be thick because it is a beam in bending from the hoist to the leach which takes a huge force to hold flat.
    -the last point is I specified an equal safety factor to size components vs your guestimate. If you put the stress of an elliptical load distribution (proper trim) at design load of about 1psf you find that the lug you specified deflects too much to hold shape.

    I will do a sketch if I get the chance.
     
  4. sharpii2
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    sharpii2 Senior Member

    This has been an interesting discussion.
     

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  5. Skyak
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    Skyak Senior Member

    For the record, I never said 'Chinese' junk. I don't have the time or inclination to argue history. Of course you must have the opportunity to modify and optimise the lug. My motivation is to teach and learn not 'win'. The interesting point will be -does your modified lug have to become the modified junk? or take most if not all it's attributes? If you think about some dramatic picture of a lug sailboat in a blow -is the boom bending like crazy? Is the yard bent and straining despite the sail being obviously over twisted? You can 'see' the flaws just thinking about it.

    I can't say I thought of the elliptical junk before I saw Petro do it on a boat he built in two days.

    I am not sure what to make of your sketch. Sort of a balanced leg-o-mutin? Or an aero rig with an undermain? Or a poor mans windsurf rig. It gets me thinking about wanting to do a sail rig that can fold quickly to just a stick.
     
  6. sharpii2
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    sharpii2 Senior Member

    You could consider a Boomless Bermuda, flat cut. It would handily wrap around the mast, and could even be reefed this way. Probably the best fold up sail would be the sprit sail, as you get a lot more sail per given length of mast. Very popular for 19th century work boats for this reason.
     

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  7. Andrewski
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    Andrewski New Member

    Who knows the ratio formula for size of junk sail to hull. If i have a 48’ steel roberts, with a bilge keel, what sail power will i need?
     
  8. sharpii2
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    sharpii2 Senior Member

    Hi, Andrewski. Welcome to the forum.

    A simple if not over simplified answer to your question is the formula:

    ((vol.) ^O. 667)*12 to 18

    "vol." is the total weight of the boat divided by the weight of a cubic unit of sea water.

    Since you described the boat in feet, you will use cubic feet. Sea water weighs 64 lbs per cubic foot. So you devide the weight of your boat in pounds by 64 to get the vol.

    You then raise this number to the O. 667 power. After this, you multiply it by 12 to 18.

    This will give you an approximation. The actual number will be influenced greatly by the hull type, the rig choice, and the number of and skill of the crew.

    Wide hulls with hard bilges, for example, can carry more sail than narrow deep ones with soft bilges, if they have the same amount of ballast.

    Tall rigs are more efficient. Shorter ones are safer in knock-down situations, such as sudden shifting gusts.

    But if you stay within the 12 to 18 range, you probably won't be far from the truth.

    More sail can be easily handled on a rig with three or more sails than one with just one or two, with the same level of sail handling equipment, such as winches an roller-furling gear.
     
    Annode and bajansailor like this.

  9. Howlandwoodworks
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    Howlandwoodworks John Howland

    I sailed a 18' Prindle for years. It had an fractional sloop rigging w/ a full battened sail, one could adjust the tension on each batten giving the sail the desired shape. Tacking in a lightest of breazze I could give the boom a quick tug to move the foil to the windward side, it would make a kind of low tone snap as all the battens moved to the opposite side.
    A junk rigging could have the same unfortunate turn of events as a gaff rig and end up with the top and bottom of the sail in opposite sides of the mast in heavy unstable air.
     
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