Question about Disp/Lwl

Discussion in 'Boat Design' started by Chuck Losness, Jan 10, 2009.

  1. Chuck Losness
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    Chuck Losness Senior Member

    Hi everyone. I love this forum because there are so many knowledgeable people here that continually enlighted those of us who are uninformed. It's blowing a full gale in the sea of cortez today and with nothing else to do I thought that I would raise a question that I have pondered for a long time.

    My question concerns the Disp/Lwl calculation and how it is applied to sailboats. It seems to me that there has to be a better way to compare sailboat displacement but I may just be missing something obvious. Let's take a hypothetical sailboat of 35' LOA that has a displacement of 12000 lbs. If this boat had a LWL of 35', the Disp/LWL would be 125, light displacement. If the LWL was 30', Disp/ lwl would be 248, moderate displacement. And if the LWL was 25', Disp/lwl would be 343, heavy displacement. But the actual displacement never changes. Scantlings, sail area, ballast and just about everything would be similar for each LWL. So how can the same boat be light displacement, moderate displacement and heavy displacement?

    I think that this calculation would be more meaningful if it was based on LOA instead of LWL. Using LOA would seem to give a truer indication of whether a boat was light, moderate or heavy displacement.
  2. Tcubed
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    Tcubed Boat Designer

    lwl is used because what is important is how the water sees the displacement. ie how compacted or how stretched is this displacement for the water to flow around.
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    Your excamples=> first would smth like a modern racer, the last like old meter class boat with long overhangs. So there's an obvious difference, allthough with the last one this ratio isn't telling the whole truth..
  4. PAR
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    PAR Yacht Designer/Builder

    As with most published figures about boats, they're really only useful in direct comparison to other craft of similar dimensions and configuration.

    For example your figures suggest three quite different vessels. The first has no over hang the next ~15% and the next ~30% overhang. You really can't fairly compare these hull forms, as they differ so much. It's a bit like comparing a Chevy Suburban with a Honda Accord. They're both cars, but are sufficiently different that they require separate categories for relative comparison.
  5. Chuck Losness
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    Chuck Losness Senior Member

    Thanks for your replies. But they miss the point of what I am raising. The connotation that goes with light displacement is a skinned out, lightly built boat with little or no tolerence for carrying gear and provisions whereas heavy displacement connotates a stout, heavily built boat that you can load up. But in my observations of sailboats over the last 40 years that doesn't seem to be true. And in the example, all 3 boats have the same displacement (hull shapes would be different of course) and would be built to the same scantlings. I would think that the so called light displacement boat with its longer LWL would be able carry more load without adverse affect than the so called heavy displacement boat. This is the opposite of how people normally think about sailboats. Maybe there needs to be a calculation that includes a factor that relates to load carrying abillity to compare boats and forget about light displacement verses heavy displacement.
  6. Grant Nelson
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    Grant Nelson Senior Member

    D/L is a measure of the lenght over which you stretch the displacement part of the boat - even overhangs are not important here at they are above the water- if you shorten your boat by 10 feet (30%) you have to widen and/or deepen the portion of the boat that is traveling through the water, which, you can guess will be a fatter slower boat.

    It works mainly for comparing boats of the same size, not the same displacement. So, if you took your thinking, and calculated the displacement for each of the D/L ratios you mention for the same length boat, you will see an increase in weight, and thus an increase in volume, and thus interior room.
  7. Chuck Losness
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    Chuck Losness Senior Member

    Grant, you are right the formula doesn't work for boats that are the same displacement and LOA but with different LWL. That's my point. If you keep LOA constant and vary displacement, you get a true indication of whether a given boat is light, moderate or heavy displacement. That's why I believe the displacement/length formula needs to have a component that takes into consideration the LOA and not just LWL. Maybe I am just thinking too far outside the box or maybe there is already another formula in use that takes LOA into consideration that I am not awhere of.
  8. Tad
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    Tad Boat Designer

    If the LOA for each of your boats is 35', and displacement is 12000 pounds, then your Disp/LOA ratio would be identical for all three boats? How would this tell us anything useful about them?

    Perhaps you are reading more into the D/L ratio than is really there? A D/L number is just a quick and easily calculated way to compare hull forms. It's used because waterline length and displacement are usually quoted in all sales literature. The fact that one or both the inputs are fictitious doesn't seem to bother people. Load carrying can be compared with PPI, but that's more complex to calculate and so we usually don't mention it in public.

    D/L ratios are an attempt at simplifying a very complex series of relationships. Have a look at a design review from 1965, there is no mention of D/L ratio, rather there are three paragraphs of writing about the nuances and relationships in the hull form. Then in the 1970's Bob Perry started writing for Sailing, he only had a little space to cover this subject, so he resorted to a dumbed-down, single number, description. Bob didn't (AFAIK) invent the D/L ratio, but he helped make it's use popular. Others started using it, now many folks understand it to mean something. You could start a movement to compare published displacement with the length of your Granny's may catch on!
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  9. Chuck Losness
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    Chuck Losness Senior Member

    I think that I'll end this thread with your comments. I always find your comments to be pretty much spot on and really appreaciate your weighing in here. PPI seems like a better indicator of load carrying capacity than artificial claims of light, moderate or heavy displacement. If only the gale blowing down here would stop, then I could get back to heading further south to warmer weather and water. Thanks for stopping by.
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