Open Hull vs Sealed Hull?

Discussion in 'Boat Design' started by Xyberz, Jun 2, 2014.

  1. Xyberz
    Joined: May 2014
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    Xyberz Junior Member

    Ok, I'm a little confused on this subject. I'm almost guaranteed incorrectly describing the title too hahaha.

    But I'm wondering how open hull boats like the Porta Bote at can hold so much weight without having any sealed air chambers like typical boat builders have compared to ones that have sealed air chambers. They claim the 14' boat to hold 800lbs. Sorry, not sure what the sealed air chambers are called. I think there's a correct technical term for them, please feel free to correct me at anytime :D With boats like this, does it sit deeper in the water since there's no air chambers to hold it higher out of the water? They list this 14' to draft at only 4" which I'm assuming is pretty darn shallow. The material that the boat itself is made out of a copolymer polypropylene material and also has foam supports just in case but they're nowhere near the waterline. That must be to help ensure that this thing is nearly "unsinkable".

    If anyone from my previous thread reads this, then you know that I'm seeking to build a powered cat since it seems to suit my fishing needs with some buddies and gear aboard and hopefully some decent sized fish afterward!

    I'd like to design the cat with open hulls like the Porta Bote if possible. Cats like the SIG45 have hulls so high that you can stand inside of them, but I'd like to have something a lot smaller than that. What I'm thinking is like possibly having 2 Porta Botes as the hull and connected in the middle to make it a catamaran. It would be nice to have the area inside the hulls usable to either hold things or even have seats inside to have people sitting in and fishing from each side of the cat.

    So to all the cat builders out there, what do you guys think? Also I'd like to leave the option open to using either 1 or 2 outboards to power the cat.
  2. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    The last thing you want as a catamaran is two hulls as wide as the Porta-bote.
    There is not point to that because the width between the two hulls is what gives you stability, not the width of the individual hull.

    Sealed chambers or any kind of flotation does not affect the load the boat can carry. It only affects whether it will float when it is swamped (filled with water).
    If you have two boats that have the same shape but one says it holds more weight, you have to worry about what kind of performance the designer is trying for when it is fully loaded.
    You can also "say" the boat has more displacement by just letting the boat sink more. Without more freeboard the boat will be easier to swamp. Sinking it deeper in the water will slow it down with a given motor.

    You would be much better off using two canoes rather than two Porta-Botes, and the canoes would be wider than needed. Think of two canoes with a max width of 24". That is enough for you to sit in each hull comfortably on a seat. Since it is a catamaran you have much more stability than necessary for a person to standup in the hulls or on the deck between.

    The Porta-Bote will not have enough strength in the hull to be tied together with 2 crossarms. Those points take lots of load in waves.
  3. thudpucker
    Joined: Jul 2007
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    thudpucker Senior Member

    I have been overboard from Open and Closed hull boats.

    It can happen anywhere at any time. Just make sure your PFD is "ON" you and a line you can grab is trailing.
  4. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Xyberz: To understand what makes a boat float, you can look in Wikipedia or similar information site. Look for Archimedes principle.

    A boat or any other floating object pushes some amount of water out of the way. The volume of water displaced, when calculated by weight, is the exact weight of the boat and its contents. You can grasp the general idea by playing around in the kitchen sink. Fill the sink part way with water....fill a drinking glass about half full. gently lower the glass into the sink water and notice that it will float. No air chambers involved.

    The glass has pushed an amount of water out of the way that is equal in weight to the weight of the glass plus the weight of whatever water is in the glass. The glass could have contained fish sinkers or tomato juice. It does not matter because it is just weight.
  5. Westfield 11
    Joined: Apr 2008
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    Westfield 11 Senior Member

    The reason that people close in the tops of hulls is to keep splashing water and waves out. Covering the top of an open boat hull will not effect how it floats in flat water. Buoyancy or air chambers only come into play when the open hull fills with water by providing a small amount of flotation, if the inside of the hull is dry the air chambers or foam, etc. have no effect on how the boat floats.
  6. Xyberz
    Joined: May 2014
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    Xyberz Junior Member

    Thanks for the very easy to understand explanation guys! Since I'm still learning about boat dynamics it greatly helps to understand and remember. I especially love the analogy using the cup of water in the sink. I've done that before but never thought of attaching that experience with boats hahaha

  7. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Splash some water in the sink ... notice how without a top, it fills up and sinks to the bottom :)

    try it when your cover the top of the glass with plastic wrap held on by a rubber band - notice how it keeps floating no matter how much you splash it.

    I have vivid memories of capsizing my homebuilt sailing canoe in the ocean, and have it filled up with water - it was hard to calculate how long it would take to float to land 9 kilometres away with me hanging on .......
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