Displacement at DWL vs actual capacity

Discussion in 'Multihulls' started by Tom Makes Things, Oct 2, 2019.

  1. Tom Makes Things
    Joined: Sep 2019
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    Location: Portland, OR, USA

    Tom Makes Things Junior Member

    In searching for the boat I’d like to build, I’ve been running into a problem-

    I’d like to use this boat for sporty day sailing w 2-4.5 adults, or for overnights w my girlfriend and some camping gear- let’s call that 300lbs-700lbs load.

    I currently own a Hobie Getaway (17’ cat) which works for both those uses, but is very wet and points poorly. Im not a racer or a very experienced sailor, so I really don’t even notice the extra weight on the Hobie.

    I’ve also been enamored with the Hobie Tandem Island, though it’s much slower and even more wet (8kts in good wind). But it does have the pedal drives which are really useful when the wind dies.

    In my search, I’ve found many designers who list the empty weight of the boat, and the displacement at DWL. When I subtract, I can calculate the boat capacity when loaded to DWL. For the trimarans under 20’, the best I’ve found are ~400lbs. Clearly there is some wiggle room, as long as the water line is below the cockpit floor... but performance will be compromised. But by how much? One designer told me that I should reef the sails when overloaded, I’m assuming so as not to damage the rig or akas.

    Now when I compare that w my Hobie Getaway or the Tandem Island, I find there is no DWL listed, instead it just says 1000lbs for the Getaway and 600 for the TI. (Weta says 400lbs). So where are those numbers coming from? Does my 17’ Getaway really have that much more buoyancy? Does it mean 1000lbs is the point at which the boat is no longer safe- like the mast will break? How do I find the actual displacement at DWL for those boats?

    Please advise, and thanks for reading.

    -Tom
     
  2. Corley
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    Corley epoxy coated

  3. Ad Hoc
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    Ad Hoc Naval Architect

    Tom
    You're going into an area that few fully understand, because the numbers are generally 'sales' numbers.

    The full load = lightship + payload. ...simple enough.

    But what draft is that??...well, a naval architect would create a hull form that floats level trim on the draft they want for THAT. When the Lines are drawn up...that draft will be called to DWL or Design Waterline.

    If the designer then uses that same hull but modifies it to create a new design, for whatever reason, they will still use the DWL as the basis for all their calculations. BUT, and this is the important part...the draft at which it now floats on and the payload will be different from the original design.
    Thus....not knowing what these are...how do you know what is what?.....and their is your dilemma.

    And on top of that, when ti comes to performance, some yards/designers will always quote the most favourable values, like a displacement that implies it is full load, but actually it is a light load....and with these caveats in very small print....to aid further confusion.
     
  4. Tom Makes Things
    Joined: Sep 2019
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    Location: Portland, OR, USA

    Tom Makes Things Junior Member

    Good points.

    I would assume that naval architect must design for a range of weights, since the boat would sometimes carry more, sometimes less. It would be nice to know that range- there would be an optimum load, but then a +/- and a max, so you would know when you are overloaded or underloaded.

    Seems like an overload situation is when the cockpit no longer drains. And an underload would be when a dinghy doesn’t have enough human ballast. Less of an issue for a multihull.

    Then again it’s probably conditions and sailor dependent.

    I’m just trying to compare apples to apples.
     
  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    You also want to know ppi.

    Ad Hoc may explain it better, but simply, how many pounds load for an inch deeper in the pond. Any boat you purchase will fair better or worse with additional load, but how much?

    Or the Brits use kg/cm.

    I strongly encourage you to email Richard Woods.

    In my search for a vessel; this was also a primary concern. I wanted a certain amount of available loading, but to also function well light.

    Richard will not steer you wrong.
     
  6. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Tom
    Good luck.
    As you are experiencing it is difficult to compare apples to apples when the sales brochures are listing fruit salad.
     
  7. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    I agree with you that information in relation with a variable payload is helpful for the user (inc. the eventual buyer of a model), especially for small boats under 20' where the payload is an important % of the total displacement, the payload in hiking posture an important % of the righting moment RM.
    But I would put these info in 2 categories : the facts, the previsions
    Speeds on flat sea, on given sea state, ... are typically in the second category, with a lot of parameters and uncertainties, inc. the human factor (your proper sailing skills). You cannot not demand to an NA the speed variation with payload, too difficult to answer, can be a source of misunderstandings.
    On the other hand, you can demand evolution of the RM, the wetted surface, the minimum freeboard, hull body draft, ratio ama volume/total weight, ... versus the payload variation (when in a given hiking posture), ... trim variation when 100kg is move forward or backward by one meter, etc....these are facts and quite easy to give with CAD tools.
     
  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Well, to be correct 'we' use TPC, tonnes per centimetre.

    Basically the waterplane area x thickness of draft change = volume and then x water density = weight of water displaced.

    So, if your boat has a TPC of say 0.50, this means for every centimetre of draft change you have added 0.5 tonne or 500kg.
    It is - in a way - a means of measure your carrying capacity.

    BUT...i would not focus on one characteristic of a vessel alone. You will loose the bigger picture. You need to look at the vessel as a whole and not cherry pick parts that you like or dislike.
    It is like picking the cherry's of a cherry tart, saying how much you like cherry's, only not to like the custard and sponge when eating it all in one bite. It all has to work as one..
     
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  9. fallguy
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    fallguy Senior Member

    I have seen in smaller boats pounds per inch, kg per cm.

    I never saw it expressed in tpc.

    Which tons?
     
  10. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    TPC = metric = tonnes per centimetre
    TPI = imperial = tons per inch
     
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  11. Tom Makes Things
    Joined: Sep 2019
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    Location: Portland, OR, USA

    Tom Makes Things Junior Member

    Thanks for the helpful insight, and humor. Apples to fruit salad is apt, and makes me grin.

    I definitely recognize that a single metric does not make for a fair comparison of boats- one could easily design specifically satisfy a single measurement (like TPC/TPI) and come out with a really crappy boat. I just was trying to understand more about buoyancy and DWL displacement figures in marketing materials as it relates to how I'd like to use my next boat. My understanding has grown thanks to this thread. Internet for the win.
     
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