Flat Bottom design question from a simpleton

Discussion in 'Boat Design' started by jnwaco, May 7, 2010.

  1. jnwaco
    Joined: May 2009
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    jnwaco Junior Member

    Alright, I belong to a bowfishing forum and we're all flatbottom (give or take a few percent deadrise) freaks. Airboats, fan boats, tunnel hulls, etc...

    A question came up as to what effect on performance the angle of the sides to the bottom has on a boat. We noticed some flatbottom makers have almost 135 degree angle to the bottom and others are almost straight up at 90 degrees. Airboats seem to have cupped (out) or curved sides, etc., etc. Quite a few on the site have had custom flat bottoms made and a few of us are considering it. Truly custom from scratch boats.

    I know that given the same bottom width, a larger angle will make for a larger beam, which we like. Most of the sought after or build hulls have at least a 60" bottom and many have 70-80" bottoms, and 90"+ beams.
     
  2. gonzo
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    gonzo Senior Member

    Flare, the angle you are describing, gives the boat more dinamic staibility. As the boat heels, the flare gives more floatation and the other side's weight pulls down. This makes the boat tend to right itself.
     
  3. jnwaco
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    jnwaco Junior Member

    That makes perfect sense. Thanks gonzo! And thanks for the vocab lesson! The boating terminology is definitely a new language!
     
  4. Yellowjacket
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    Yellowjacket Senior Member

    Airboats don't have ridges or fins on the bottom since they would trip and flip if they caught the bottom in shallow water or in weeds or junk. They have flared sides to insure that they don't trip and flip over when they slide sideways.
     
  5. Ike
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    Ike Senior Member

    Also flat bottom hard chine boats (that is the intersection of the side is a sharp angle and the bottom is flat across) are rated at a lower horsepower than a similar boat with rounded chines or a curved or vee bottom. The reason is simply handling. Flat bottom hard chine boats can be easily made to trip over their chine and flip, by too much power. This rule only applies to boats under twenty feet in length.

    Also giving the sides some angle outward rather than 90 degrees (called dearise) allows them to carry heavier loads. As the boat is loaded the immersed volume (the part below the waterline) increases faster if the sides are sloped than if the sides are straight up and down. So as you load it, the boat sinks less vertically and you maintain more freeboard (the part above the waterline)
     
  6. lewisboats
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    lewisboats Obsessed Member

    Perhaps you misspoke/typed?...deadrise is the angle of the vee of the bottom from horizontal not the flare of the sides from vertical.
     
  7. Ike
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    Ike Senior Member

    You are absolutely right. I've got to stop doing this stuff when I'm too tired to see, let alone think.
     
  8. SamSam
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    SamSam Senior Member

    Umm, you might want to look up 'tumblehome' too. ;)
     
  9. jnwaco
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    jnwaco Junior Member

    Thanks for all of the responses. This helped a lot.
     
  10. lewisboats
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    lewisboats Obsessed Member

    Oh...to be sure. I seem to exist in that condition. 2 jobs...working 7 days a week 80+ hours. Besides my vacation I may see 5 days off a year. Gets OLD after a while. Seeing the light at the end of the tunnel though...just hope it isn't a train :rolleyes:
     

  11. jnwaco
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    jnwaco Junior Member

    One more question - what's the technical or nautical term for the crimp in the side of flat bottom boats? I think it's supposed to divert water away from the hull for a drier ride, and on thinner skinned boats, helps stiffen the aluminum a bit.
     
    Last edited: May 9, 2010
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