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

    Eric, thanks for the info. I'm well aware of your excellent Design Ratios text; as a matter of fact, I re-read it yesterday before posting this thread.
    Problem with a VPP is that unless I don't understand them correctly (never used one), you need the hull to input it in. So, no use for comparing designs on the market.
    Do you mind expanding a bit on your statement that an efficient rig will not perform as well on a heavy boat?
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Sure, the speed of the wind coming into the rig is the apparent wind speed, and that is a function of boat speed. A heavier, slower boat is going to have a lower apparent wind speed than a lighter, faster one. Power generated by the sails is a result of apparent wind speed squared, so a lighter, faster boat is going to have more power per square foot of sail area (another measure of efficiency) than a heavier, slower boat. This, of course, happens with any boat, any rig. The real point I was trying to make is that you should assess a couple of different ratios at once, not just sail area/displacement ratio by itself, in order to get a better picture of performance.

    Yes, VPP results come from the designers, and not all designers even calculate them or offer them. You'll find a greater proportion of VPP results from the monohull racing fleets.

    Eric
     
  3. Steve W
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    Steve W Senior Member

    I disagree with the first paragraph, many times rig height is the only thing that keeps you moving in very light air, ie, a huge sail area on a gaff rig will get you nowhere when the wind is up off the water. Also in very light air, say, under 5k more sail area, particularly in flying sails but also in headsails becomes just too much fabric to keep aloft. My sons boat which I designed and built 27 years ago is brilliant in 5-10k but in under 5 will sometimes get passed by much slower boats when his big assym is hanging flaccid while bigger, slower boats with taller rigs sail on by.

    Steve.
     
  4. modflod
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    modflod Junior Member

    Eric,
    thanks for the clarification. One point I don't quite get is how the heavy/slower boat will be at disadvantage with a more efficient rig compare with the same boat with a less efficient one.
    Also, wouldnt assessing 2 ratios whose only difference in variable input is stability via CE and beam will at the end reflect those input (modified by the way the variable is treated inthe equation). For example, comparing SA/Displ with Bruce or RPI, in the end you're really comparing displacement, SA and initial stability. So might as well assess those 3 directly?
    Definitely questioning here, not arguing!
    Much thanks
    JF
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    JF--Certainly, for any given boat, a more efficient rig will render better performance for the boat than a less efficient rig. And yes, you can analyze as many ratios as you want to, to get a fuller picture of performance. Your original question asked if there was a single ratio that included rig efficiency as an indicator of performance, and the answer was no. You are correct that Bruce Number, RPI, Stability Index and Stability Factor don't include any element that correspond to rig efficiency. As stated before, to really assess rig efficiency, you have to have more knowledge about the boat than just the basic parameters.

    That is, let's say we have a single boat with a conventional rig of a certain sail area. It has a certain performance, and we measure that in the form of windward ability (speed and pointing angle) as well as reaching and downwind speed. Now we change that rig to a new wingmast rig with perhaps differently shaped main and jib, but of the same overall area. All the ratios are going to be the same, but we can surmise that the new rig will improve the boat's performance. We cannot judge that, however, merely by the ratios, rather we have more detailed knowledge of rig that tells us what we want to know about performance. We then measure that performance in order to see the differences and improvements--or degradations if there are any. I don't know of any ratios that would reflect the features of the second rig, other than perhaps, as I said, a sophisticated VPP program in which we can change the force factors as they relate to the rig design.

    Eric
     

  6. Skyak
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    Skyak Senior Member

    Rig efficiency? For starters the rig can be more efficient in many different ways depending on what limited resource you are dividing by. Wind speed? Righting moment? Weight? Rig forces? Money?

    To get to a useful power to weight ratio you need to get to thrust, the forward component. The rest of the sail forces are a hindrance that require heavy gear and ballast to counter. And of course all of these depend on apparent wind angle.

    So there is no power to weight ratio because "power" is a function of so many variables the ratio would be useless. Sail rig characteristics are hard to quantify, then you have to calculate the interaction with the rest of the boat to get the value.
     
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