Sail Area / Power

Discussion in 'Boat Design' started by dustman, Sep 2, 2020.

  1. dustman
    Joined: Jun 2019
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    dustman Junior Member

    Earlier in my journey to learn about sailboats I asked the question: how much power can I expect out of a given sail area? I was met with countless snarky and mostly unhelpful responses. So for those of you who want a baseline, here you go! From The Nature of Boats by David Gerr:
    Gerr Sail Power.png
    And here is a chart I put together based on Gerr's chart(wind speed in mph on left, sail area on top and the result in watts. 750 watts=1hp. The figures for 5mph are questionable extrapolations.):
    Screen Shot 2020-09-01 at 8.46.53 PM.png
    All those snarky comments weren't without merit, however. There are a great many factors that go into how much power your sails will produce in various conditions, arrangements, trims, aspect ratios, what angle you are sailing relative to the true wind direction, etc, etc. And how fast your boat will go with a given amount of power has a number of variables as well. I'm still learning all this myself but wanted to put this out there for those who are floundering because of all the abstractions being thrown about. It is all understandable, but there are more intricacies than one would first expect. Happy fantasizing and researching!:)

    And for those of you who bash people's ideas... Be helpful, leave your ego and bias at the login, but by all means be realistic. And think about how Hereschoff, Wharram or the ancient polynesians would have responded to your inputs. They may have never built anything:((or maybe they would have ignored youo_O). And for those of you who are truly helpful, I couldn't have come this far without you, your investments in other people's dreams are appreciated!
     
  2. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    Written with true grace.

    Thank you.

    -Will (Dragonfly)
     
  3. messabout
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    messabout Senior Member

    An ancient bit of wisdom says: A little knowledge is dangerous. This is not intended to be "snarky".
     
  4. Will Gilmore
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    Will Gilmore Senior Member

    Perhaps, a little less dangerous than no knowledge? ;)

    -Will (Dragonfly)
     
  5. CT249
    Joined: May 2003
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    CT249 Senior Member

    So someone who slagged off just about all designers by claiming that if they bought a production boat they'd be buying a "poorly performing boat that requires constant maintenance and monetary input" is complaining about other people being rude?

    I just read through Dustman's first thread. He started throwing abuse when he was getting perfectly reasonable responses.
     
  6. dustman
    Joined: Jun 2019
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    dustman Junior Member

    If I was in the wrong somehow I apologize, but:

    I've learned a lot since then, but still don't like the nature of some of the responses. You have to put yourself in the shoes of someone who is brand new to the forum, and/or just doesn't know much about sailing or sailboats, and then go reread those responses. Many of them are demoralizing. I try to keep my cool now, because I understand why some of you respond to certain things the way you do. But maybe some of you could phrase it in a way that is more helpful and less demoralizing(and snarky)?

    I'm not sure what's rude about this: "poorly performing boat that requires constant maintenance and monetary input". That is objectively true of many boats as far as I can tell(in something close to a third of the sailing life videos I watch people are having problems with their boats). It is by no means an attack on anyone's vessel or a personal attack. When I build a boat and go cruising I don't want to deal with those problems, or die because something breaks. Not to mention I am poor and can't afford to blow money.

    If you notice in the post above I tried to be somewhat tactful and somewhat light-hearted while getting the point across as well as giving "newbies" encouragement and letting them know they don't have to be demoralized by the less than helpful responses they get. Anyway, I was hoping this post would encourage self reflection in some of you and give dreamers a little hope, and some things to think about and study. The question I would ask is: are you here to be helpful or here to be better than other people? And what does it mean to be helpful?

    And for the life of me I still don't know why people were so hesitant to give me the above figures.

    Anywho, I look forward to everything and everyone this forum has to offer. Peace, love and fair winds.
     
  7. CT249
    Joined: May 2003
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    CT249 Senior Member

    The problem is that the power from a given sail area varies enormously according to an enormous number of factors. These factors include mast height (taller masts can reach the faster winds up high); the amount of wind shear; the sail outline; the sail draft/depth; the leach tightness; the sheeting angle as measured against the boat; the sheeting angle as measured against the true wind; the boat's speed and its affect on apparent wind; etc etc etc. Even large swells affect the wind; they mess up the flow and can also carry a puff on their front faces.

    Many of those factors mentioned above are themselves combinations of many different factors. For example, a sail's draft will often change in the position of maximum camber and the amount of camber every time it hits a wave or gets hit by a puff, and those factors vary depending on how high up the rig you're measuring and how much you have adjusted the sail; sometimes you may want more twist for the same wind strength. In fact you'll often want more twist on one tack than the other, to allow for wave effects.

    One of the more critical aspects is that a lot of sail trimming gets down to trying to ensure that the sail is developing as much lift as possible, without stalling. Stall involves turbulence, and turbulence is chaotic and unsteady and incredibly hard to predict even with the best computers and tools. In some craft, there can be only an inch of mainsheet tension making the difference between a sail that has largely stalled out, and one that is delivering maximum power - and that in itself can vary second by second. Good sail trimmers are often pulling the sheet on and off as the boat goes over every single wave, while changing the boat's course to steer it over the waves. A top sailor can do this vastly more accurately and much faster than an average sailor, so the amount of power they extra will be different.

    We can't simply compare sails by area alone. In offshore yachts, the light-airs headsail, the medium-airs headsail and the larger strong-wind headsail may all be of the same size, but they will vary in depth and therefore profile - and on top of that, good sailors will adjust them to be deeper or flatter. Sails can vary like wings; a chicken's wings don't work like an albatross wing, which doesn't work like a Cessna's wing.

    So the point is that given all these complexities, no one can give you the simple answers you want. The issues are so complex that the idea of relying on earlier rigs is excellent advice.
     
  8. Will Gilmore
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    Will Gilmore Senior Member

    The sail and its trim aren't the only factors that determine the power you can expect from a given sail. The stiffness and balance of the boat will affect both how much energy the sail is able to capture and how much of that energy can be transferred to forward motion.

    -Will (Dragonfly)
     
    Ad Hoc and DCockey like this.
  9. DCockey
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    DCockey Senior Member

    It is unfortunate that Geer does not provide any references for the sources of his information. It is very difficult to determine what the formulas and claims in his book are based on, what the underlying assumptions are, and how widely accepted they are.

    This is a critical point. Increasing sail area will not increase available power unless the boat has sufficient stability. At 30 knot wind speed most sailboats are or should be reducing sail area (compared to light air configuration) and/or reefing.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed, noted in a simple summary here.
     
  11. dustman
    Joined: Jun 2019
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    dustman Junior Member

    Most of those are (here comes the wrath:eek:) separate issues from what the sail itself is capable of delivering in various wind conditions.

    I'm trusting that Gerr had enough information to make an informed analysis. After reading Nature of Boats and some of his other writing he sounds like he knows what he's talking about. I would imagine he is assuming an aerodynamically efficient shape, etc. I didn't take that chart at face value initially, but after comparing that to the performance and sail area of numerous boats, taking into consideration their hydrodynamic properties, it seems to be a reasonable approximation. And unless one is racing for high stakes what I/we need is a reasonable approximation to move forward.

    Unless somebody throws a flat, rigid board up for a sail the differences aren't going to be as radical as they are being made out to be. And I reject the hypothesis that you have to be a highly educated engineer to build a decently performing boat.

    That being said, you all are making great points, hence this in my post above: "All those snarky comments weren't without merit, however. There are a great many factors that go into how much power your sails will produce in various conditions, arrangements, trims, aspect ratios, what angle you are sailing relative to the true wind direction, etc, etc. And how fast your boat will go with a given amount of power has a number of variables as well."
     
  12. Rumars
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    Rumars Senior Member

    The cookbook approach works if you want to build a moderately good boat. The more experienced the cook, the better the resulting product.
    The problem arises when someone does not want to follow the cookbook at all. Like for example insisting on a flat sail, when said sail has three supported edges that scream for luff rounding.
     
  13. clmanges
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    clmanges Senior Member

    Indeed, 'decently performing boats' have been built by people we might deem primitive, but overwhelmingly, their lack of highly educated engineers had to be made up for by generations of trial and error, with attendant losses of life and property.
     
  14. dustman
    Joined: Jun 2019
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    dustman Junior Member

    I'm not sure if you are referring to my sketch, if so that is an extremely basic sketch that leaves out many details. I'm not sure what you mean by luff rounding, would you mind explaining it and tell me what you think would cause this in my design? And I'm not dead set on a flat cut sail, but if I can make it work it would certainly simplify some things, and reduce cost.
     

  15. dustman
    Joined: Jun 2019
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    dustman Junior Member

    Fortunately we don't live in that time anymore.
     
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