Constant vs Variable Deadrise

Discussion in 'Boat Design' started by Marathon, Feb 26, 2005.

  1. Marathon
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    Marathon New Member

    What are the pros and cons of constant vs variable deadrise hulls when applied to waterjet powered inboard boats 20' - 24' in lenght. What are the specific applications for each? What effect does it have on handling? Fuel consumption? Ride attitude? Plane time?
     
  2. sharpii2
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    sharpii2 Senior Member

    Hi Marathon.

    Generaly speaking, the variable deadrise is best for getting on a plane quicker and with less hp (about 40 per ton). There was a time when that was the only planing hull form available. And it is still the most popular bottom type for planing, When you count all planing powerboats from aluminum fishing skiffs to sports fishermen and probably for that reason. It does have one vice, however. It doesn't like rough water. It can slam and pound ferociously when driven into head seas.

    And that is where the constant dead rise comes in. When given a steep enough deadrise, it can handle head seas much better. It doesn't slam or pound nearly as much, hence its popularity in personal water craft and off shore racing. It, however, has two vices. One, Its more fuel thirsty because it requires more hp (at least 60 per ton) to get on a plane. And two, it dosen't like non planing speeds that much. Some get less mpg at no wake speed than they get while on a plane because they have to drag a wide (usually as wide as the boat) and deep (usually as deep as the boats hull) transom through the water.

    Hope this helps.

    Bob
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Marathon,

    What Bob says is pretty much the basics that you need to know. On my jet powered Independence Classic 20 (now the Cherubini Classic 20: www.cherubiniyachts.com) I used a constant deadrise bottom for the last 40% of the planing surface--20 deg deadrise. Forward from that point, the bottom warps to a deep V shape with some convex curve below the lifting strake. The topsides forward above the lifting strake are concave to help throw off spray.

    These boats handle remarkably well. They are stable, turn nicely, and slip through waves like a dream.

    Eric
     
  4. Marathon
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    Marathon New Member

    Constant vs. Variable Deadrise

    Thanks for the replies Bob and Eric
    What are your ideas on the following application concerning constant vs. variable deadrise.
    Our company builds heavy gauge welded aluminum boats with their heritage in wihitewater river operation. This type of craft has been a regional design primarily here in the Pacific Northwest. However this being a relatively small market many of us have moved into other areas of use such as wider, deeper rivers (i.e. the Sacramento) lakes and ofcourse the Pacific Ocean. Propulsion types are varied also. We produce inboard jets using gasoline, outboards and sterndrive (Mercruiser) style boats.
    At this time some manufacturers are working hard to maintain a constant deadrise bottoms by utilizing fixed jigs that the bottoms are literally bolted to during forming and assembly. Others (including us) rely on the bottom cutout we have developed so we can form the bottom without jigs. This type of assembly produces a bottom that maintains a constant deadrise for probably a third of the bottom from the transom forward. From there it rolls up and the deadrise angle increases. The deadrise at the transom is 12 degrees. Instead of a vee we have a radius and we use four lifting strakes. Bottom widths are 72" and 84". These boats usually weigh approx 2200 lbs. Speeds of 43 - 52 MPH are normal depending on power type. Bottom line we need to be able to run the rivers, lakes and coastal waters.
    The level of expertise concerning bottom configuration is very limited in my view. This is certainly true in my case (a 20 year industry vet). This is something I want to address in our operation. Even though we have been successful I think we can do much better.
    Brian
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    Brian,

    There are probably as many opinions about bottom configuration as there are designers and builders. Everyone has ideas, most of them successful, on which they build respectable boats. From what you describe, you seem to have found a solution for the types of boats you build. That is all very valid.

    Scientifically, all these ideas work to great degree, and I don't think you can say there is a right way or a wrong way to design and build powerboats--excepting the extremes of course, which we can probably all recognise when we see it, but would be hard pressed to define it. The unfortunate thing is that no one yet has been able to collect all of the design and construction experience that is available and condense it down to a formula for design and construction in general. The field is simply too vast, and it would take some monied resources and considerable writing and investigative talent to come up with such a book. The Society of Naval Architects and Marine Engineers (SNAME) has tried in the past, and so far has not been successful.

    There is nothing wrong with stating that in your experience, you have found such and such to work well. If your boats work the way you intend, and that they are easy and economical for you to build, then that is your proof.

    The one source that I have found that may give some more insight into bottom configuration design is "Seakeeping of Hard Chine Planing Hulls", Technical and Research Report R-42, published by SNAME. You can buy a copy from them. It is written by Daniel Savitsky and Joseph Koelbel, two of the most prominent naval architects/engineers in powerboat design. The book talks a lot about waves, dynamics, model testing, and habitability, but the last chapter, #8, is called "Design Features Which Provide Good Seakeeping." It's a short chapter, but it gives guidance on L/B ratio, deadrise, LCG location, displacement, and metacentric height. It also discusses various hull forms that affect seakeeping--V-bottom, inverted V-bottom, variations on the inverted V-bottom, catamarans, and round bottom boats. This is all of their advice that has been distilled from years of theoretical and practical research. It is the most complete statement that I have found about what makes good hard chine boat designs.

    Eric
     
  6. brian eiland
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    brian eiland Senior Member

    Planning Hull Shapes

    Eric, I'm going to look up that reference. I don't know much on the subject of planning power boats.

    Do you have any knowledge of a book by Peter Payne, "Design of High-Speed Boats, Volume 1:planning" ?? , and/or any of his research carried out in the Chesapeake Bay area, some of it for the navy
     
  7. Eric Sponberg
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    Eric Sponberg Senior Member

    Brian,

    I have not heard of a Peter Payne book, but you might be recalling a book called "High Speed Small Craft" by Peter Du Cane. It is quite a famous book, first published in 1951, then again in 1956, 1964, and finally in 1972 by John De Graff. It has long been out of print. You can sometimes find them in specialty marine book stores and agents.

    Eric
     
  8. brian eiland
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    brian eiland Senior Member

    Peter Payne, "Design of High Speed Boats, Planning"

    Hello Eric, I had forgotten about this subject thread on this forum, and continued with some related discussions to this gentleman's work on another forum. I had also secured a copy of this book from the Naval Academy Library, and looked it over. VERY mathmatical, but just this morning I learned of a software product based on his work.

    BOAT3D -- A Time-Domain Suite of Computer Programs for Planing Craft

    http://www.yachtforums.com/forums/16010-post47.html
    http://www.yachtforums.com/forums/16028-post48.html
    http://www.yachtforums.com/forums/16033-post49.html
     

    Attached Files:

  9. kach22i
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    kach22i Architect

    Sea Knife = Cool
     

  10. im412
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    im412 Junior Member

    i would look at other jet builders that are using with what they are calling the "full length delta pad"
    http://outlaweagle.com/history.htm

    the seaknife also seems to use the 'full length delta pad' with a 90 deg flaring deadrise

    yamaha's hull sold by panga.com have been using the full length pad for 35 yrs with a constant moderate dead rise
    where the mercury's angler panga uses a variable deadrise with their pad
    so there are two proven coastal hulls to compare to
     
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