Deadrise Question

Discussion in 'Boat Design' started by elwood, Feb 4, 2011.

  1. elwood
    Joined: Aug 2010
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    elwood Junior Member

    If you were going to design a 15' outboard runabout with a beam of 5' 6" and the transom at about 5' what would a moderate deadrise be? Also would it be better to have the same deadrise to continue up thru the bow?
    Thanks for anyone input.

    Elwood
     
  2. Wayne Grabow
    Joined: Aug 2003
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    Wayne Grabow Senior Member

    My understanding of boat design is that you generally want the chine to be moderately immersed. Given that you have specified the length and beam, deadrise will be, in large measure, determined by the anticipated displacement of the hull. Whether that same deadrise should be maintained up to the bow depends on esthetics, planned usage, and your underlying design philosophy. Many people would increase the deadrise forward. It provides a finer entrance for the bow angle and helps eliminate a possible "knuckle" in the forefoot. Most important is your vision for form and function. Variation is found in where to begin a transition from constant deadrise to a steeper angle.

    I am only an amateur myself, so you hopefully will get better answers.
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Moderate deadrise is a fairly large range, from say a few degrees at the transom, to 20 degrees by some standards. Deadrise for comparative reasons, it's typically measured at the transom and the vast majority of powerboats have a much steeper entry then the after quarters.

    You would be best served to study powerboat hull forms in general. Assuming a full plane monohull, you have two basic choices, warped bottom or sometimes called double wedge and the monohedron or often called constant deadrise. These two hull forms can have a number of variations, though most can trace their roots back to these two basic types. There are some odd balls like the cathedral hull or the Hickman sea sled hull form, but these are rare and not very common. Stick with the easy stuff first, before you venture out onto the skinniest of design limbs.

    Each has advantages and drawbacks, much like dating choices in high school. After some study and further understanding, you'll select one, based on the design requirements and personal preferences in the expected preformance envelop.

    What are you trying to do?
     
  4. elwood
    Joined: Aug 2010
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    elwood Junior Member

    Basically i am wanting to design a sleek looking woody that want beat you to death but have desent performance. I work around deep vee stern drives and v drive crusiers 10 hours aday but have the desire to build a wooden boat. I am in the process of mocking up a small runabout and was concerned about the deadrise.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Nothing personal Elwood, but your knowledge and understanding of the principles, concepts of planning hull hydrodynamics is so weak, that any reasonable expectation of success can't be considered. This doesn't even consider the engineering attributes of the design. This isn't meant to insult you, but hopefully as a wake up call, before you spend a bunch of money on a project, that you can't trust farther from shore then you can swim back to.

    Do yourself a big favor and buy a set of plans, preferably from a living designer. You can find all sorts of cheap and even free plans on line, but most of these aren't complete (you get what you pay for) and usually are very old (half a century), often calling for materials and products that aren't available any more.

    There are thousands of designs in the general size range you're looking (I'm assuming trailerable) and some are very notable. Why a living designer? Well, we have to eat, but more importantly so you can call them up and say, "damn, I got this problem and since you designed it, maybe you can help me figure it out".
     
  6. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    A contributor to the forum...Alik...concisly described the underwater profile of a typical planing motorboat.


    " Better guideline for bottom area/deadrise on several stations. If you divide the boat into 10 stations: Stn.1, the bow of the boat -is responsible for spray making and sea keeping performance, Stn.3 - for slamming loads; Stn. 5 - planning performance; Stn.10 (Tr.) - directional stability. "


    Attached is the lines drawing of a small, well designed, good seakeeping , skiff that will very perform well at 15 to 20 knts.

    http://www.bateau.com/studyplans/PG20.PDF





    You can see that the section aft, between the middle of the boat and the transom, is the planning surface... flat run , minimum dead rise, like a wateski.

    The section forward of the planning surface from station five to station 1..from middle of the boat to the bow...is used for breaking thru the wave and carries the "vee" , to resist slamming and soften the ride.


    Many different interpretations can be made for how much "vee " you carry for the aft section of the boat... for instance if you use a big heavy powerplant... then more "Vee" must be used to displace, to float, the weight of this engine. How fast will the boat go is also relevant to the shape of the bottom. If your boat is designed to ride on its stern section.. more "vee" is needed aft.


    The classic form of an easily driven, lightweight, vee bottom craft is a
    Flat run, minimium "Vee" aft, sharp " Vee" mid and bow section. When driven on there lines, they have very good handling characteristics in waves.

    In general an amateur should not change these proportions...ie design a faster, more powerful deep vee aft craft.

    Also consider ...Deep Vee hulls are also very energy intensive to break onto a plane and make poor sea boats...wilth bow up, stern sqautted, at slow , non planing speed.
     

    Attached Files:


  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Once again, your assumptions and opinions of (this time) V hulls, isn't even close to reality Michael. You really should stick with what you know, like driving, because construction and design elements are just something you don't have a firm grasp of. The Panga 20 you've listed is a typical, fairly bluff bowed, warped bottom hull form. You can't tell anyone why it's better then any other, nor why it might have good or bad handling qualities Michael, so why do you insist on trying so repeatedly? You wouldn't know the difference between an ill handling set of lines from a sweet set. Nor would you be able to describe these reasons.
     
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