Variable deadrise vs continuois

Discussion in 'Boat Design' started by silentneko, Dec 18, 2014.

  1. silentneko
    Joined: Jan 2014
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    silentneko Junior Member

    I may not be using the correct terminology so please bare with me. I'm working on a smaller v-hull power boat and am trying to decide on the deadrise. I'm trying to see the advantages or disadvantages of having a variable deadrise, starting at maybe 16 degrees at the bow and ending at maybe 4 degrees at the transom. Or a continuous, were starting from about midship the deadrise would be the same, maybe 6-8 degrees all the way back to the transom.

    So what different characteristics will this give me? Will one be faster, or more efficient then the other? Get on plane faster?

    Thanks
     
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  2. HJS
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    HJS Member

    You'll probably give us some more information.
    What kind of boat you're sketching.
    How long is it?
    What a payload do you expect?
    Which economy speed do you want?
    Where to use?
    and so on .....
     
  3. silentneko
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    silentneko Junior Member

    I haven't started sketching just yet, and I'm not a great artist, but this is the direction I'm going:

    [​IMG]

    This is very far from my finished design and I have a lot of tweaks I need to make, but this model was made to test my theory.

    The boat will be around 16' long and have a chine to chine beam of around 5'. It should weigh around 450-550lbs, and carry a load of at least 600lbs. It will be powered by a 40-60hp outboard motor, and I would like a comfortable cruise speed around 25-30mph, with a top speed of at least 33mph. It will be used all around Florida, everything from diving for lobster in the keys, to running the skinny waters of tampa bay. Shallow draft is important, but so is comfort when running a chop.
     
  4. daiquiri
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    daiquiri Engineering and Design

    I see a zero-deadrise transom there. It might give you stability problems at speed and is not a good choice for choppy seas. give it some deadrise.
    The bow area looks very odd, could you give a couple of more pics?
     
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  5. silentneko
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    silentneko Junior Member

    This is not the final design, this was a test of materials I did with a mock up. The final version will have deadrise at the transom and the hard chine will be much farther back. My question was to help me decide on a direction I will take my design.

    So what is the benefits and drawbacks of each type?
     
  6. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    This forum has many discussions on that theme, you just have to learn the use of the search function. Just a couple of examples of existing threads about what you ask:
    http://www.boatdesign.net/forums/boat-design/constant-vs-variable-deadrise-6668.html
    http://www.boatdesign.net/forums/boat-design/monohedron-vs-warped-bottom-1642.html

    You can search it also through the Google search engine. A sample search string which tells Google to look just at this forum could be:
    "variable deadrise hull site:boatdesign.net"

    Hope it helps.
    Cheers
     
  7. PAR
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    PAR Yacht Designer/Builder

    Basically it's a trade off on what you want the boat to do.

    Constant deadrise requires more displacement, has less initial stability and needs more power to get up on plane. In it's advantage it has higher speed potential and a softer ride in a chop.

    A varable deadrise, can usually carry more of a load than constant, simply because the aft portions of the hull are flatish and fairly broad. This makes them initially more stable and they can get up on plane faster. These hulls tend to be more efficient too, at modest speeds. The warp in the bottom will help bring the bow down and if a reasonably made isn't too bad in a modest chop.

    8 degrees or less on a warped bottom is an inshore hull form, good for protected and semi protected and (again) is well shaped can handle mild near shore excursions. Over this deadrise and your going to need higher up on plane speeds and it'll offer a better ride in a chop.

    There's a lot more than these simple concepts (warped verses constant) to consider in a hull form. For example a monohedren hull can be a better choice than a true constant deadrise, particularly if you want to get up on plane quickly, have better top speed potential (within reason) and a softer ride in a chop.

    Ideally, you'll establish the goals for the design (an SOR) and shape the hull accordingly. The hull form you've shown will suffer from a number of issues and without a set of lines to look at, difficult to tell what it might or might not do, but generally, that bluff bow and relatively fat entry will pound unmercifully.
     
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  8. silentneko
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    silentneko Junior Member

    Thanks gents, when I get further along in the modeling process I'll post up another pic so you can see what I'm looking towards.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Modeling is a later stage of the design process. You "design" a boat around it's needs, requirements and goals. With this in hand (the SOR), you can work up a GA plan and start weight estimates. From there you have a pretty good idea what you can do, with the options/choices available, under this set of criteria.

    If you approach from a model first point of view, you can spend a lot of time on a set of shapes that aren't well suited to the needs and goals of the design, though the model may look good on screen. The only way you can use the model first approach is to have extensive experience in hull shapes and their suitability to your perceptions of an SOR. Judging by your model and questions here, I'd say you haven't this experience, so using a more precise technique would be strongly advised.
     
  10. tom28571
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    tom28571 Senior Member

    Lots of designers have started a new boat with a model first, BUT these were experienced designers who knew where they were headed from the get go. No disrespect, but you apparently are not in that category. Drawing lines on paper or a computer screen is the best way to start because an eraser or a delete key are remarkable devices for purging bad ideas. As you get better, that can also be done in the mind. A model does not offer anything that is capable of variation like that.

    Your model looks much like something that might have been bought off a Sears Roebuck floor 40 years ago and will perform better than a shoe box but not very good. I recommend some reading and study of basic hull forms that can be found in many books starting with Dave Gerr's "Nature of Boats". Just telling what is wrong with the model for your intended use will not give the knowledge or skill to do another and better one.

    Remember that you did ask for criticism.
     
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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Specifying the size and speed of the boat would be useful.
     
  12. silentneko
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    silentneko Junior Member

    Lol, I love how some of you guys preface your responses with no offense, or no disrespect....and then write things most would construe as offensive. Emotion doesn't come through very well on these forums so I assume most are here just to help, so I'll just say thanks again, before continuing.

    As far as designing goes, like I stated this is not a final model, it's a jump off point I through together with scrap OSB in about 20 minutes just to see if I could make the compound curves I envisioned with the material I want to use. This will be the 4th boat I have built, and the other 3 were very successful. I am not well versed in computer sketching or CAD programs and prefer to work with my hands in small scale. Yes I can envision my plans in my head and I will put some of it on paper as I progress, but line drawings do little for me as it all seems a bit obvious until I finalize the dimensions I want. To each their own right.

    Tom, thanks, but I never asked for criticism, I asked for information and got it in kind along with the info.

    Mr. Efficiency, I posted a quick description under the picture I posted.

    Thanks for the info so far, I think I have what I need.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, thanks, I missed it. A 'senior moment' probably ! There are not many boats 16 foot long, of that weight, that will handle chop well, at your preferred speed, but still be tolerably stable at rest. You are pretty well restricted to moderate deadrise, by that weight and power, and I think I would prefer some increase in deadrise moving forward, rather than say 13-15 degree constant deadrise, which would be about the limit.
     
  14. tom28571
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    tom28571 Senior Member

    My apologies for misinterpreting your postings.
     

  15. silentneko
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    silentneko Junior Member

    No need, again I appreciate all the information and help moving forward.

    I know my description is a little on the vague side, but I'm trying to walk a very fine line to get a decent all around boat in the end. I know it will be a jack of all trades and master of none, but I do not have enough room for more then one boat right now.

    Ok, when I say a chop, I mean a bay chop, typically up to about a 1.5'. I'm not looking to scoot through this at 30mph, I'll gladly slow down to just above planing speed in these conditions. The cruise and max speeds I'm hoping for will be in smoother conditions.

    I have been fishing seriously skinny waters for years now, usually .5-3' depths and the skiff I am using now was great when I built her and will draft 4" easily. However as my family grows I need more options which is why I'm going for a larger and more comfortable foot print, but still need to retain the ability to fish shallower waters. I know, it's a tricky issue to work out.

    To give you an Idea, some of the more popular smaller boats around 16' fishing similar waters are Hells bays, Mitzi's, Ankonas, pathfinders, Carolina Skiffs, Boston whalers, ECC's, Beavertails, Hobie power skiffs, even some of the larger gheenoes. All have trade offs but can be dealt with. I really like the hobie's, but I need something with a little more interior room and a livewell.
     
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