How Heavy? How Light?

Discussion in 'Boat Design' started by Southern Cross, Jun 13, 2013.

  1. Southern Cross
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    Southern Cross Senior Member

    Building materials have progressively become lighter and stronger.

    With the development of materials such as Graphene and Aerographite, whether there is a marine application or not, we seem to be approaching a threshold.

    Many a Cat sailor has seen there capsized boat get blown downwind in a strong breeze. I once saw a Hobie 16 with two crew picked up and slammed against a moored fishing boat by a sudden squall.

    Hypothetically, say you were assigned an ocean going racing Trimaran to design, the current French contenders for example, using a material with the properties of the above, how light could the boat be before it became too light and consequently unseaworthy?

    In other words, if you could rebuild Banque Populaire with a super light material, how heavy must it be to circumnavigate?
     

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  2. El_Guero

    El_Guero Previous Member

    Crew weight + 100 pounds / crew member ....

    :D
     
  3. Southern Cross
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    Southern Cross Senior Member

    Really? That's all? What about in a squall or a gale, Force 7+? I've seen smaller, fully crewed boats laid flat....
     
  4. El_Guero

    El_Guero Previous Member

    IF

    superlight

    and a French team is on it ....

    :D

    But, honestly? I don't have any idea how much to cut weight on a racer, and it be sea worthy ...
     
  5. Southern Cross
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    Southern Cross Senior Member

    Ah. Got it. Everyone is always so serious in this Forum. Went right over my head. Wasn't expecting some humor.
     
  6. El_Guero

    El_Guero Previous Member

    There aren't many on this forum who could adequately address cutting weight off of Banque Populaire .... That think almost flies as it is ....
     
  7. dskira

    dskira Previous Member

    We are a bunch of funny guys :D
     
  8. Petros
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    Petros Senior Member

    to go fast, the boat must be light. to be comfortable in heavy seas, it must be heavy. seldom can you compromise and still meet these two conflicting desires.

    If in a really light boat, you must head for shelter if the weather gets heavy. Think "seaworthy" is a relative term, many deep water racing sailboats are on the ragged edge of breaking up, you often do see many breaks ups of racing boats. this is true with most forms of mechanized racing, you either break and DNF, or win. If you are stronger and heavier than the other boats you will not win. If you could make materials even stronger and lighter, than they will make racing boats out of it.

    I would not really consider a racing sailboat "seaworthy" for pleasure cruising in the open ocean. Perhaps others do, but it just depends how much safety margin they are comfortable shaving away. So it depends on what do you mean by "seaworthy"?
     
  9. Southern Cross
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    Southern Cross Senior Member

    Thanks Petros.

    I guess my question is a little more simple than I made it sound.

    Say there was a way to replace current carbon building materials with a resin infused Aerogel, Aerogel being several times light than air, to achieve a hull that was significantly stronger and significantly lighter. Total displacement for the 70 ft tri less than 1000lbs.

    If such a material existed, how light or how heavy would the boat need to be to circumnavigate (this gives a wide variety of conditions).

    First off, if such a material existed, it would probably change the design process altogether. You might come up with something entirely different.

    Otherwise, I would think that some sort of ballast would need to be introduced, water, lead, or some other heavy metal just to keep it from blowing away in a breeze.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Ballast doesn't keep boats from getting blown away in a breeze, in fact quite the contrary. There's no easy way to answer your questions, mostly because they are to simplistic. How light, well damn, as light as you can engineer, given loads and a safety margin. On a racer, you can all but skip the second aspect (for the most part). There's no criteria for any weights that are too light or too heavy, assuming it floats and performs as expected. What you're asking in regard to Aerogel is precisely what has happen in recent decades, with carbon and other exotic fabrics. The result is very light and yes, ocean going craft. The ideal material would be 10 times stronger than steel and weigh as much as air, maybe even a little less. Yep, that's right a negative weight craft, that can scoot over a 100 MPH, with barely an appreciable wake.
     
  11. Southern Cross
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    Southern Cross Senior Member

    Par, I get that part. A knockdown for example. A displacement boat with a heavy keel will get knocked down and eventually pop back up. But in the same conditions I've seen catamarans blown sideways for a good distance. If these had been any lighter, as light as air, who knows how far down the bay they would have gone? Doesn't this also apply to a larger multi hull that is also super light? Yes, there are other ways improve RM. But isn't there a point when too light becomes dangerous?
     
  12. John Perry
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    John Perry Senior Member

    Leaving hydrofoils aside for a moment, I think there is a lower limit to the weight you would choose to make a sailing boat for optimum performance, even in the hypothetical situation that you have materials of much higher specific strength and stiffness than those currently available.

    Consider a multihull, although similar argument applies to a monohull. As you reduce weight you reduce water drag, but for a given plan form you also reduce the maximum righting moment and hence the propulsive force that can be generated. In any other than light winds that means you have to reduce sail area, either by reefing or by having a smaller maximum sail area by design. However, the air drag of the above water structure (hull topsides, superstructures, rigging etc.) probably does not reduce all that much as you reduce weight and sail area, so, given that all fast boats normally have apparent wind coming from ahead, there will come a point at which air drag limits performance. The simple way round this is to take on water ballast as wind strength increases. Another way is to employ hydrofoils, (or possibly aerofoils) that generate downforce, rather than upwards force as is normal for hydrofoil boats built from todays real materials. Note that the Sailrocket project is already using downforce from a hydrofoil to allow a larger sail force than would be possible without that downforce.
     
  13. Petros
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    Petros Senior Member

    if the hull was lighter than air it would float away, like a blimp (a "lighter than air craft"), and it would no longer be a boat.

    So the answer is, it must be heavy enough to stay on the surface of the water to be a viable boat.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Well, the hull isn't all there is, so a negative weight hull would have something, but John is correct, there is a diminishing point of return for lightness, which is only a part of the set of equations, necessary in any design.
     

  15. viking north
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    viking north VINLAND

    Mmmm -- To the point where both it's form friction and gravity will allow control. Beyond that human input is "Dust In the Wind"
     
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