Rigs and Rigging Weight vs Drag?

Discussion in 'Sailboats' started by RHough, Nov 28, 2006.


To increase performance of a rig, is it more important to:

Poll closed Feb 26, 2007.
  1. Reduce the Drag of the rig.

    5 vote(s)
  2. Reduce the Weight Aloft.

    1 vote(s)
  3. It Depends. (Classic NA answer) :)

    7 vote(s)
  4. Don't know

    3 vote(s)
  5. Who Cares?

    1 vote(s)
Multiple votes are allowed.
  1. yipster
    Joined: Oct 2002
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    yipster designer

    long as there are more than unstayed rotating free masts
    rigging, weight and drag would also play on this creasing mast
    just an example but on a farrier with apropriate rigging, why not
    looked for folding, hinging, elbow, telecopic or what do you have masts
    and like to hear some opinions on shown or other alternative systems

    oops, do realise opening a can of worms, start a new thread?
  2. grob
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    grob www.windknife.com

  3. yipster
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    yipster designer



    interesting site with history and concepts i didnt see yet, thanks
  4. rayk
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    rayk Senior Member

    An easier explanation for your customers may be...
    • Extra weight is only x kilograms more than your original standing rigging.
      This extra weight may be above the original CG, but the new combined CG will only be a matter of centimeters above the existing one.
    • As the wind increases drag in the rig will heel the boat, and slow it down.
      As wind speed increases, drag increases exponentially.
      Drag is a force of x kilograms pushing on the CE of the rigging.
      As wind speed doubles, the drag quadruples.

    Hope this helps:)
  5. RHough
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    RHough Retro Dude

    Your points are well taken as they relate to static stability.

    Obviously, reducing the weight of the standing rigging by 75% will have a small effect on boats where the weight of the rig is a small part of the total weight. On a 50 foot, 12,000 pound boat with a carbon mast, the weight of rod is a more significant part of the total.

    The Kevlar standing rigging was about twice the diameter of the rod it replaced. The new PBO rigging is only about 30% larger than rod for the same stretch per unit load.

    Where weight aloft comes into play is it's role in dynamic stability. The trade off must be in evaluating the effect of pitch and roll stability on boat speed compared to the drag penalty. AC boats use PBO as do more and more high performance racer/cruisers. I can quantify the drag, but I don't know how to estimate the effect of changing the pitch and roll period on boat performance.

    I want to think that the designers that specify PBO rigging have done their homework and have come to the conclusion that the weight savings gain more than the extra drag loses. I'd like to learn how to estimate the effect on the dynamics.
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    FAST FRED Senior Member

    "The argument is that it reduces weight aloft. What then is the trade-off? Can reduced weight aloft more than compensate for the increased parasitic drag of doubling the rigging diameter?"

    Parasitic drag doesn't increase.

    The problem is that small diameter wires all have about the SAME drag.

    Every single wire , halyard or lifeline "looks" about 1 inch in diameter to the breeze, in terms of induced drag.

    The higher drag is because the energy required to bend the wind around the wires and return is higher in small sizes.

    Thats why wires are gone from biplanes , but thick struts remain.

  7. rayk
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    rayk Senior Member

    Maybe the best way to start is a cross section model. (Pitch can be figured out using the same process in section.)

    Heel can be used to demonstrate the forces of gravity and bouyancy and the moment of increasing drag/windspeed in the rig.

    Trade offs in CG/CB and rigCE/transverseCF can be compared.

    :!: A model is simple to understand, but the values of a 'typical' racer/cruiser and the wide range of conditions it sails in will undoubtably create a negative result.

    Considering weight
    • The weight savings aloft will be marginal in relation to the displacement to start with.
    • CG will be a few cm lower. (3cm lower at 30 degrees heel is 1.5cm more righting arm.)
    • Righting arm has 'lost the weight savings' from CG and CB
    • Hull will ride millimeters higher. Heeled, the CB will be fractionally further from CG, increasing the righting arm.
    Now drag
    • Unlike AC racing the cruiser/racer is out and about in more variable windspeeds.
    • If diameter of rigging has been increased 30%, your drag is greater.
    • Drag is a force that acts on the CE of the rigging to heel the yacht about its transverse centre of flotation(in our tranverse model).
    • When drag increases a moment occurs.Nm Kgmf ftlb take your pick
    • When wind speed doubles, drag quadrules. This golden rule is vital.
      • if drag is 20lb@10knots and 80lb@20knots
        then increasing drag (not diameter) 30% is
      • 26lb@10knots and 104lb@20knots
        I made up the numbers to give an example, but whatever force you calculate is acting on a CE between the deck and masthead somewhere.
    • The drag on the rigging is in the direction of the apparent wind. Over the beam it will induce the maximum heeling moment. From forward of the beam it begins robbing speed as heel decreases.

    Basically this is what we are looking at,
    :) marginally reduce total displacement
    • 15000lb-100lb=14900lb
    • =.0067% reduction
    :( but marginally decrease bouyancy
    • weight is lost from CG
    • CG=CB
    :) microscopically increase righting arm
    • lower CG>righting arm increases
    • hull rides higher>CB moves out (mono)
    :( and lose speed/increase heeling moment by a factor
    • percentage increase of drag
    The question is what will make the boat faster, and quantifying the forces for a given boat is the only way to find out. To increase boat speed for a particular yacht in a variety of defined windspeeds, will give a windspeed limit, where drag has increased to cancel out the weight savings, and over come the improved stability.

    AC boats might not be allowed to shed weight in some instances but can still manage a net improvement in performance if the weight is moved lower.

    AC boats sail on a course that has undergone intense meteorological scrutiny.
    They only race within windspeeds that are not too light nor too heavy.

    As you said RHough, the scale of these boats magnifies small improvements. It is hard to replicate on a marina boat. Unless they can afford it ;)
    Last edited: Dec 21, 2006
  8. brian eiland
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    brian eiland Senior Member

    Haven't had time to read this thread yet, but a posting will get me on the list to receive new ones
  9. brian eiland
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    brian eiland Senior Member

    Wow, interesting to find this old subject thread that was headed into some very interesting subject matter. Too bad it ended abruptly. Thought I might restart it, and see what happened.

    I was interested in this subject as just recently the question of extra drag on my bare mast came up here:
    ...and I was looking for methods to reduce it.
  10. Paul Scott
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    Paul Scott Senior Member

    Tangential question: I'm getting the hull finished for a 14' by 36" canoe, and I'm concerned with self rescue. The hull part of the equation I'm pretty happy with, but pondering the length of the mast, which weighs about ~1 lb/foot. Unstayed- how much difference is there between 14', 15' or 16' above the WL? The thing's a telephone pole, but it's cf and I already own it....

    Going for ~55-60 sq ft, 2' hiking wings off of each side.

    Tried a lighter mast with a dipping lug, but damn, that thing was top heavy and almost impossible to right. Granted it was a 16' by 30" hull with a 20' mast and a a 5' by 1lb 3 oz cf top spar. But it did give me a fright for how hard it was to right from a mast in the water 90 degree knockdown.
  11. Paul Scott
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    Paul Scott Senior Member

    Here's the 16'er

  12. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I can see some practical advantages for smaller boats, small enough that you can raise and lower the rig by yourself while at sea. The synthetic rigging might be easier to handle and to maintain, maybe. This assumes it can be coiled to a smaller radius. This also assumes it can be spliced easier than wire or rod rigging can be swaged.

    Regarding performance. The increased parasitic drag has it's most undesirable effect upwind in medium and high winds when you sail with a flatter sail at smaller angles of attack. In medium winds the effect would be that your optimum would be slightly lower and slower. In high winds, particularly in chop or waves, the apparent wind angle is not so far forward as in medium winds, and so the ever increasing parasitic drag is becoming more and more of a heeling moment. The effect is that you can generate less thrust forward. If you are sailing a classic narrow keelboat that heels over alot, there might be a balancing out in medium air, but in ever increasing winds the ever increasing parasistic drag will eventually win out. The only other performance compensation is less pitching moment, but I really can't see that making up for less forward thrust. Downwind, of course, is a different story. For a race like the Transpac, good case for reducing weight aloft vs reducing drag. For cruising boats offshore, I think the performance differences might be small enough that the maintenance and reliability issues become more important.

    Thought it might be worth looking at Reynolds numbers.
    For air, velocity 10 m/s and diameter 1 cm, Re = 7 x 10^3.
    That puts you pretty square in the middle of an inescapable Cd = 1.2 region for cylinders. The added roughness isn't going to hurt much, but it isn't going to help either. So if you double the diameter you are pretty much doubling the drag. Full stop.

    p.s. I have an Yngling, with a light fractional rig I can step by hand at the dock with the help of my daughter, but still too heavy and tall to raise and lower while at sea. If I were to convert the Yngling for offshore cruising with a Gunter rig of some sort, I think I would give serious consideration to a backstay that was made of some synthetic fibre rather than wire rope or rod rigging. As for the shrouds and forestays, I think they might best remain wire rope or rod rigging, because they would hold up better to wear, and they are short enough to still be manageable, and I wouldn't have to raise and lower the lower mast with such a rig. I would keep it up for a storm and sail under storm jib, etc. Interesting discussion.
  13. sharpii2
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    sharpii2 Senior Member

    First off, let's do some math.

    Assuming your cf mast is a non-tapered tube, its capsize moment works like this:

    For a 16 ft mast, it is 16 x 1 lb/ft x 8ft (to its VCG) = 128 ft/lbs.

    For a 15ft mast, it is 15 x 1 lb/ft x 7.5ft = 112 ft/lbs

    For a 14ft mast, it is 14 x 1 lb/ft x 7.0 ft = 98 ft/lbs

    The Yard, of course has to be added to this, so assuming it weighs 3 lbs and is positioned about a foot below top of the mast, its three likely moments are:

    15 ft x 3 lbs = 45 ft/lbs, for the 16 ft mast,
    14 ft x 3 lbs = 42 ft/lbs, for the 15 ft mast, and
    13 ft x 3 lbs = 39 ft/lbs for the 14 ft mast.

    Now we can add these two moments together to get total capsize moment to get:

    128 ft/lbs + 45 ft/lbs = 173 ft/lbs for the 16 ft mast,
    112 ft/lbs + 42 ft/lbs = 157 ft/lbs for the 15 ft mast, and
    98 ft/lbs + 39 ft/lbs = 127 ft/lbs for the 14 ft mast.

    As I hope you can see, you save only 6 ft/lbs by lowering the yard, from its 15 ft height to its 13 ft height on your shortest mast option, but 46 ft/lbs by shortening the mast.

    Now, if you shorten the mast, you are going to have to lengthen something, the Boom, the Yard, or both, to recover the sail area lost.

    Let's assume the Boom weighs 3 lbs also.

    We must lengthen it by 4 ft to make up for a 2 ft shorter mast. This way, we keep the Yard at is original length.

    If the yard was originally 3 lbs and about 6 ft long, it weighs about 0.5 lbs/ft.

    So, by adding 4 ft to its length, you are adding 2 lbs to the Boom.

    But the Boom has some height too. Let's say it is 3 ft above the mast step.

    If that's true, the capsize moment of the 16 ft rig would be (3 lbs x 3 ft) + 173 ft/lbs, or 182 ft/lbs.

    Using the same reasoning, the capsize moment of the shorter, 14 ft rig would be ((3 lbs + 2 lbs) x 3 ft) + 127 ft/lbs, or 142 ft/lbs.

    The taller 16 ft rig would be 28% harder (182/142 x 100) to self rescue than the 14 ft rig.
  14. CT 249
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    CT 249 Senior Member

    That seems to be an extraordinary heavy stick - what's it off? Even a Laser mast is something like .8lb per foot, a windsurfer carbon stick is about 0.25 lb per foot.

    An International Canoe isn't hard to right, even with an alloy stick with a carbon top section.

  15. Paul Scott
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    Paul Scott Senior Member

    Mr 2- thanks very much! Just what I needed- The only problem having a bookshelf full of boat and boat design books is you have to plow through them when you forget which book contains what....

    Chris- just a little proto I'm fooling around with. The mast was originally for another dipping lug boat, and for example, the goat island skiff crowd has found (with its lug rig) that they need a lot of sail tension to get better performance. Which means a lot of compression on a free standing rig. So Forte fabbed my 20' cf telephone pole, which I'm repurposing. I'll have to weigh it again, I've forgotten how much it weighs exactly. But I still have snowthrowing/shoveling stuff around it. And I need that stuff. It snowed yesterday.

    FWIW, the foto I posted above was of a quick and dirty IC ish proto that was designed and built to test the following:

    Whether a proa concept would have any advantages - like with not having to switch sides with the seat

    Whether a dipping lug was promising, especially as an air rudder, so I could get rid of the water rudder

    Whether surfboard ends/bows were worth pursuing

    Whether tying the seat to the mast would get rid of structure to deal with the monstrous twist a <legal> <normal> IC hull experiences between the mast and seat, so I could use lower tech construction and still hit the 50kg minimum.

    Different thread, methinks.
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