What is variable deadrise?

Discussion in 'Boat Design' started by Arlon, Jun 15, 2006.

  1. Arlon
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    Arlon "Got Panga?"

    I have a panga that has a 60deg entry and tapers evenly to 37 deg at about midboat, 24deg about 3/4 of the way to the transom and 17deg (avg) at the transom. The hull also changes a few degrees of angle from top to bottom between the chines but just forget that for the moment. I'm calling this rather severe continous change in deadrise down the full length of the boat "variable deadrise". I was informed it was not variable that variable means it changes from keel to chine only. If not variable, what is it?

    Another boat has a 60 deg entry too but 2 ft back it flattens out VERY quickly to 18 deg and stays at 18 deg at the middle and 18 deg at the transom. To me that is NOT variable deadrise, just a sharp entry/forefoot?

    I know it's just a word but I'm curious if there is any standard or if "variable deadrise" has a defination in some nautical engineering sense?
     
  2. lewisboats
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    lewisboats Obsessed Member

    Sounds like variable to me!

    Steve
     
  3. Guillermo
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    Guillermo Ingeniero Naval

    That's not 'variable deadrise' but 'variable geometry'. Have a look at:
    http://www.fjordboats.no/english/fjord38.html
     
  4. yipster
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    yipster designer

    Guillermo, where to read on variable deadrise and / or variable geometry at fjordboats site?
     
  5. tom28571
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    tom28571 Senior Member

    I guess I don't get the distinction. If the deadrise varies, then it must be "variable deadrise". Almost all boats fit somewhere in this category. Except for flat bottom and some deep V's, don't most boats have variable deadrise? There are monohedron, warped V, lobster hulls, displacement, semi displacement and planing hulls, but most of these have some degree of variablity in deadrise.

    Anyway, its just words. I hear boats called dory, panga, lobsterboat, trawler, pinky, etc., etc., that don't have much in common with the original other than the name.
     
  6. yipster
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    yipster designer

    i figger the variable deadrise in the running surface all the way to the stern...
    yet other or better distinctions, actually the whole intriguing variable deadrise story i like to learn more about
     
  7. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    I think that "variable deadrise" is a bottom having variable deadrise angle all long his length.
     
  8. im412
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    im412 Junior Member

    arlon ..

    i know we talk deadrise
    but as i understand it, a deadrise is from the centerline to the chine [i think]
    but on your panga, you might call it chine angle because of the flatish center section
    it could have zero dead rise and a 17 deg variable chine angle

    might be better to post a pic of your hull
    i dont really know, which is why i posted pics of my panga keel lines on this board
    i havent had any replys yet, for what the flatish center section would be called, so i bumped it back up
    http://boatdesign.net/forums/showthread.php?t=12107
    hopefully we will all work it out together
    cheers jack
     
  9. tc42
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    tc42 New Member

    Almost all boats that have a deadrise will have the deadrise vary along their length -think about a what the boat will look like if it does not.


    The common usage for variable deadrise that I am aware of is:

    In a section of the hull (usually referred to at the transom), the angle of deadrise varies from the keel to chine, ie if you placed an angle finder on the underside of the hull and moved it from the keel to the chine it would indicate a reduction in deadrise angle as you move out from the keel.

    In effect this means that the deadrise is curved so that the keel has a sharp cutting angle which reduces as you move from the keel towards the chines. On most boats that I have seen this is done with a series of strakes that are at progressively smaller angles rather than having a curved surface.


    The reason for using a variable deadrise in smaller boats is that it allows you to have a sharp angle at the keel for cutting through chop (as per deep V hulls) without having the lower stability associated with a conventional deep V hull (less displacement down low).

    So you in theory this gives the stability of a shallow V at rest with the rough weather ride of a deep V hull -in practice it is a compromise
     
  10. Guillermo
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    Guillermo Ingeniero Naval

    That's my understanding, too, mainly used to describe the afterwards half of the hull (as bows always have variable deadrise). If the deadrise varies from keel to chine (be it continuosly or by intermediate chines or steps) then that's called variable geometry, as far as I understand the term. Variable geometry can be convex or concave, or even a combination (tulip bow sections). A given hull can present variable deadrise (this being along the hull) and variable geometry (this being sidewards the sections) at the same time.

    Interesting for me to clarify this, as maybe we are understanding different things when we talk variable deadrise in these forums. Native english NA's and designers, please....?
     
  11. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    Here you have what is for me a variable deadrise angle.
     

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  12. Guillermo
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    Guillermo Ingeniero Naval

  13. Willallison
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    Willallison Senior Member

    Ranchi - this is indeed a variable dearise hull - from the rather lovely and famous pen of Uffa Fox if I'm not mistaken....

    Variable deadrise refers to a hull - such as the pange described - where the deadrise (or average angle from keel to chine) lessens from about amidships to the transom. It is sometimes also known as a warped plane.
    Tom is correct in suggesting that all hulls of normal form exhibit at least some degree of variation, but in a monoderon hullform this only occurs from the bow back to about amidships. from here to the transom, deadrise remains essentially constant - the buttock lines are parallel.

    A Hull where the angle varies as you move from the keel to the chine is NOT necessarily a variable deadrise hull. It's sections are simply either convex or concave in nature. It may still be variable deadrise - but only if the deadrise angle varies in the manner I've described above.

    It was common for hulls earlier this century to exhibit some concavity in their sections. Indeed it appears that the one Ranchi has posted does this. But it has since been found that slight convexity is a far more desirable traight. It is both marginally more efficient (though I can't recall why) and significantly stronger than either concave or straight sections
     
  14. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    I use slight convexity bottom on aluminum boats because by welding the metal withdraw itself...
     

  15. jamesflett
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    jamesflett Mech Engineer

    I agree that Willallison is correct. A variable deadrise hull is one that has a varying 'angle' of deadrise from the "transom". For reason of standardising of various hull forms all deadrises are measured from the transom as far as I know. Plain deep vee hulls usually have a deadrise of 20-22 degrees and are of triangular section.

    A variable deadrise hull (Haines Signature boats and Quintrex Boats in Australia) both use a Variable deadrise formation on the models with a deadrise range of around 20-33 degrees at the transom, and is exaggerated along to the bow as like most hulls, giving it a concave section - as already described by a couple of you.

    Advantages of the Variable Deadrise (Haines Signature and Quintrex Boats Millenium Hull) are:
    - softer riding through rough waters - incredibly more comfortable compared to deep vee hulls
    - constant lift of boat when travelling at speeds of 15-25 knots as some boats tend to lift the bow rather high out of the water.
    - easy handling due to the geometry of the hull which radiates the water transversly giving sharper turning.

    The biggest advantage from a design point of view is the Variable deadrise design allows the boat to plane at siginificant lower speeds compared to standard deadrise - as little as 5 knots as I have experienced. This means less power required to propel the boat, meaning less hP required, or less fuel used during the lower to mid rev ranges = LESS MONEY AND COSTS. This characteristic is commonly described as planing efficiency.

    The other advantage which will become more and more beneficial in the next few years with the emphasis on fuel emissions is that you can power your boat with a 4 stroke engine and still maintain about 85% of acceleration compared to regular 2 strokes which very is comparative and very good for people who require the extra acceleration if they water ski or wakeboard. Personally I prefer Direct Fuel Injection engines such as the Mercury Opitmax which is 15% more fuel efficient that 4 strokes and 45% more than standard carburrettored engines.
     
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