Power-to-weight ratio vs rig efficiency

Discussion in 'Boat Design' started by modflod, Jul 25, 2014.

  1. modflod
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    modflod Junior Member

    Is there a power-to-weigth ratio that would include the rig efficiency into the calculation?
    Perhaps through AR of the main?
    My understanding is that neither the Bruce, RPI, Stability number or factor include this.
    For example, a multi with a very efficient rig, e.g. high AR, rotating wing mast, etc. could use a bit less sail area. This might not lower the CE.
    The less aggressive ratios would not represent the real potential of the boat.
     
  2. Jammer Six

    Jammer Six Previous Member

    I've never understood the blind quest for efficiency.

    A spider's web is extremely efficient, but wouldn't last more than an instant as a dock line. And you'd never get it tied.
     
  3. modflod
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    modflod Junior Member

    It has to to with available power. The amount of wind available at any given time is finite; The quest is how to use it the best.
    If you could use spider web material scaled up to the size of your dock lines, you could probably drag the marina out to sea with a tugboat before that line would part; therefore to tie only your boat, you'd need much smaller lines. Smaller lines would mean less room onboard, less weight, smaller diameter windlass and cleats(less weight, less room), which in turn would mean a lighter boat, which would reduce the scantlings (thinner hull skin, less reinforcements), which would lighten the boat some more. Of course I'm exaggerating here but in effect, efficiency will make your $ go farther or faster for any given wind.
    JF
     
  4. modflod
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    modflod Junior Member

    Or to make it even more real-world: With a more efficient rig, you could have the same hull, going the same boat speed but have a smaller rig: Smaller sails, easier to handle by yourself, shorter mast to go under bridges, less $ in purchase and maintenance for sail fabric, mast section and rigging. The list goes on...
     
  5. Jammer Six

    Jammer Six Previous Member

    My point exactly.

    You don't have, cannot get and will never have a dock line made out of spider web. In the same way, you cannot get more wind.

    With finite wind, a larger, less efficient rig that is more powerful is more powerful.

    I would choose the more powerful over the more efficient every time. They are not mutually exclusive, but the power is the point to me, not the efficiency.
     
  6. modflod
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    modflod Junior Member

    Power is a function of efficiency.
    What if you have the choice between a small powerful rig and a bigger, not-more-powerful-than-the-other rig (because it's less efficient)? Would you still pick the bigger one?
     
  7. Stumble
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    Stumble Senior Member

    It really all comes down to hp/$ or hp/lbs. more efficency makes the ratios better. And since the cost of developing a small efficency gain can be spread out over the entire industry the cost per boat is beneficial.
     
  8. Jammer Six

    Jammer Six Previous Member

    I've never been able to get another company to pay for my efficiency research...

    Since everything on a boat is a tradeoff, what are you willing to trade for more efficiency?
     
  9. Stumble
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    Stumble Senior Member

    Mast thickness, weight, and stiffness.
     
  10. messabout
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    messabout Senior Member

    I do not think that you should assume that a more efficient rig is more efficient. Depends a lot on the what, when, and how.

    What: what sea state are you operating in?
    When; during a force 5 or in a light air?
    How: are you going to windward or on a dead run?

    A high efficiency rig of X square feet will probably go well to weather. A lug, gaff, or sprit rig, of substantially more area, will almost surely smoke the high zoot rig when on that run.

    If that is not enough then consider that fancy (spell expensive and trim sensitive) rigs operate most efficiently within a particular wind velocity envelope.

    I am with you on the power to weight ratio concept, but there are too many variables to make it possible to cheat real world conditions with mere money.
     
  11. modflod
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    modflod Junior Member

    point taken,
    I've been studying catamaran designs lately and the most recent designs I could put my hands on have the mast way aft, with a high aspect ratio main and a big foretriangle, presumably (on my part) because that foresail is more efficient based on the fact that there's column in front of it to delaminate the airflow. The designer states that the smaller rig will not translate into a loss of performance because of the higher efficiency.
    Independently of the veracity of that particular claim, whether it's marketing smack or actual improvement in rig efficiency, it led me to think that in a generic case where it would indeed be true, the ratios above would penalize that design unjustly, hence a ratio that would include the efficiency of the rig.
    I agree that it would also have to include velocity range, and then it's a whole can of worms
     
  12. Stumble
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    Stumble Senior Member

    What you are describing is almost the exact opposite of the modern trend in multihulls. At least racing ones. Massive high aspect mains with small blade jib are what I am seeing everywhere. Combined with massive asymmetric spinnakers for off wind.
     
  13. modflod
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    modflod Junior Member

    OK, might have been generalizing a bit... Apologies.
    Still, look at this VPLP 2013 design for Lagoon Catamarans.
    Close to 45% of the SA in the foretriangle and maststep at a full 60% of the LWL.
    Now I'm not saying it's good or bad, or that I like it or not. It just spurred the above thoughts on ratios
     

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  14. FAST FRED
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    FAST FRED Senior Member

    ONLY sail area will help in light airs, just look at the sail areas of 100 years ago .

    Tiny sails are for the racers where the best mediocre boat that rates small can win on points.

    Its easy to shorten sail in an over canvassed boat , hard to add to a Tahiti Ketch that needs force 5 to answer the helm.

    The real question is mainsail reefing size , 300-400sq ft is fine for the single hander with slab or similar reefing.

    The rolling window shade masts are easier , but at great co$t of efficiency .
     

  15. Eric Sponberg
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    Eric Sponberg Senior Member

    Modflod, I think you have a proper understanding of the Sail Area/Displacement ratio and the advantages of higher rig efficiency and what that does for overall performance. In a word, no, there is no ratio that takes into account the Aspect Ratio of the rig or the fact that the rig may include a wingmast. And the reason for that is, how would you rate the rig for its AR or its wingmast (or any other) feature, and can you get the general population to agree on such ratings? Rating such features within a simple ratio is totally subjective (which therefore has little meaning), whereas the standard ratios themselves are pretty objective (although, admittedly, a little limiting). So to truly assess any given rig or boat design, beyond just the ratios, you have to have specific knowledge of the boat, and include that knowledge qualitatively in your assessment of its performance.

    The one tool that might do that for you is a VPP program, because you have to input the rig and hull geometry into the program to generate speed curves. It also has to be a type of VPP program that allows you to adjust the lift and drag coefficients so that you can see the benefit of the more efficient rig. That is, you would have to be able to program various lift/drag curves for the different sail settings and different points of sail in order for you to make a proper assessment of boat performance. Unfortunately, most VPP programs, I believe, do not let you change the lift and drag coefficients. Or if they do, they are fairly expensive programs. I will stand corrected if anyone chiming in here can point us to a suitable VPP program at a reasonable price that allows you to change the lift and drag coefficients.

    One also has to be careful in assessing rig features with the other features of the boat, and most specifically, displacement/length ratio, or Bruce Number in the case of multihulls. A highly efficient rig on a heavy boat is going to have poorer performance than one might expect. So you have to look at both ratios together. In fact, you can plot Sail Area/Displacement ratio against Displacement/Length ratio (or Bruce Number for multihulls) for a population of boats and see how the data spreads in a chart. That at least gives some context of what the potential performance might be overall. And the more knowledge that you have of specific boats in the population about those other performance features, the better your assessment can be. But it is just really difficult to see everything in simple ratios--you just can't include everything objectively.

    For more information on the ratios, for the benefit of readers, I cover these topics in The Design Ratios, written right here on this forum a few years ago. I show how to plot the ratios against each other. I upload it again here.

    Eric

    (PS--to Jeff the moderator: I know I have uploaded The Design Ratios a few times in the last few weeks, but I sense that the topics under discussion in each case justify it. I hope I am not taking up too much space in the server.)
     

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