Flat bottom+"V" hull

Discussion in 'Boat Design' started by Bulut, Aug 11, 2007.

  1. Bulut
    Joined: Apr 2007
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    Bulut Junior Member

    is it posible to combine a "V" hull design with flat bottomed hull design? for good handling in rough waters and good planing when posible? with high performance of course :) thank you.
     
  2. USCGRET/E8
    Joined: Jul 2007
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    USCGRET/E8 Senior Chief

    I have seen many hulls that have a V forward and are flat back at the transom.
     
  3. Loveofsea
    Joined: Jan 2007
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    Loveofsea New Member

    A 'V' bow entry on a flatbottom hull eliminates one of the greatest advantage to having a flatbottom. For a flatbottom to work well in seas, you must induce exaggerated transom lift, you must press the bow on the surface to the point where it fully recovers on the second cycle on otherwise flat water.

    If the bottom is tapered to a 'V', exaggerated transom lift will cause bow-steering in following seas and other undesireable handling characteristics. If there is insufficient transom lift the boat will not work well in seas due to a loose bow. There is no compromise. If you are going to have a flatbottom, which i highly :) recommend, please don't spoil it by having a V in the bow.

    When angling over a swell, nothing grips the face better than a hard chine going all the way to the bow. i use an outboard with a tiller so when the seas are up, i'm always working that forward edge. Think of how much less grip a surfboard would have if the forward end was tapered to a V.

    Remember, in a flatbottom you are not cutting thru the seas you are skimming over them, always looking for the best path...
     
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  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yes, this is a common configuration (V entry with flat aft sections) and has been referred to by several names over the years, like double wedge, modified V, etc. Since we're talking about a high performance hull, this hull form will be on plane 95% of the time and is quite efficient at such, until the chop picks up enough, to get stuffed under the V sections and pound on the flattening midship sections. This hull form is usually limited to protected waters craft, though the old, down east lobster boat hull styles, could be considered a double wedge hull and were often taken well past the protection of sheltered water. They were often used in displacement mode in green water and only kept at pace with the sea, which is always a wise course if you have to make a living doing such daily.
     
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  5. charmc
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    charmc Senior Member

    Loveofsea,

    Good to see you back. I admit, when I first read some of your early posts, I was skeptical. I did a lot of research, however, and then I began to recall my youth and time spent in a flat bottom skiff with a small tiller steer outboard. I was in sheltered waters, but had to deal with frequent large wakes, and I never felt less than fully in control regardless of conditions. I doubt that a full flat bottom would work on larger vessels, but it does work well on light skiffs
     
  6. Bulut
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    Bulut Junior Member

    i have something like this:) this is why i asked. i like to hear some critics from you :)
     

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  7. cudashark
    Joined: Jul 2006
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    cudashark Senior Member

    Hi Bulut,

    Hows about this. She is almost flat at the transom.

    She is just in dry fit stage.
     

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  8. USCGRET/E8
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    USCGRET/E8 Senior Chief


    Not sure what I'm seeing. Is that a flat bottom with a big V-like keel going down the bottom? Describe what you are trying to do.
     
  9. dick stave
    Joined: Dec 2004
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    dick stave Senior Member

    Flat bottom questions

    Is a Flat Iron skiff better in rough conditions because it is more prone to keeping its bow in contact with the water, or is a flat bottom with some rocker forward (pacific city dory) the better performer? Also, is the pointy bow style of flat bottom skiff more succeptable to chine trip than its jon boat cousins?
     

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  10. bahamabay
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    bahamabay Junior Member

    Aquasport from 69-70 thru 79 in the 222 model had a flat bottom with a V entry and is now one of the most popular boats used by guides in the SW Florida region. This is a very nice riding boat. It became so popular that a lot of builders are now building boats off of the same model. Guase Built boats, and Dorado boats are off the mid 70's Aquasport.
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Bulut, what you have modeled is a weird variation of a box keel boat. Typically the box portion (the V part hanging down) ends in a point, rather then being cut flush with the transom as you have. There are a few plane mode box keel designs, but most are limited in top speed before they begin to have bad manners.

    The flat bottom hull form is the most efficient planning shape. It provide maximum lift, but pounds because of the flat sections amidship. This can be accommodated by using a fine, sharp entry wedge and moving considerable displacement aft, but it will still pound, just not as quick.

    The frames shown in Cudashark's photos are typical of the "warped bottom" hull form. The aft sections are quite flat, but transition into a V. This provides a fine entry, self leveling properties (typical of V's) can knock down chop and flat plane areas aft. These shapes are generally used in protected waters, as they haven't enough deadrise aft to maintain comfort, at plane speeds in rough conditions.

    Bulut, you model appears to have constant deadrise along the length of the box. This will substantially increase the amount of displacement and drag your drive assembly will have to overcome. The wide chine flats will probably still receive some pounding in a healthy chop, but the box will help knock some of it down.

    The trick, you should be trying to employ is to get a fine entry and a sweet chine sweep, so the forward sections of the chine flats can help with lift. The extreme amount of deadrise in the box will not offer much lift, so you'll be relying on the flats. Shaping the chines to do this isn't a walk in the park for the most skilled among us, let alone the novice.

    I'm not sure I understand you goals with this model, but you may want to consider more conventional approaches to these. Look at

    [​IMG]

    Which is one of Atkins V bottom SeaBrite skiffs. It has relatively flat aft sections and the box (which is faired closed at the aft end). The flat areas aft, provide the plane surface, the forward V sections lift, fine entry and stabilization. The aft sections could be made flatter, like a skiff, but she'd then be less efficent at displacement speeds.

    Much depends on what you're trying to do, which hasn't been clarified.
     
  12. Loveofsea
    Joined: Jan 2007
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    Loveofsea New Member

    Hello Charmc

    I incorporated sort of a double rocker in the hull of my skiff. The widest part of the hull is 2/3 hull length forward from the transom. That point is 8" wider than the transom. That point is also the 'impact point' and i have 54 gallons of fuel nesting in the fwd 1/3 of the hull to give the impact zone' some weight to do it's thing--crush and throw aside the choppy water in front of the hull (rather than split it).

    I made that rocker like this: The Y datum plane touches the hull 2/3 fwd of the transom, fwd of that point it rises 6" in a parabolic curve, aft of that point it rises 3" to the transom on a flat plane. (the bottom is 17.5') I did that for two reasons, one was to better present the impact point to the water that it was to crush, but more importantly, i wanted to eliminate what i consider to be a major fatal design flaw common to virtually ALL production boats; water accumulates at the transom while at rest. I know that in worst case scenareo, that hull charastic (water rushing to the transom while at rest ) is a major cause for small boat capsizing. Water accumulating in the belly of the skiff is much safer. I use a 2,000gph pump in the belly to protect the nights at anchor, and i use a self-priming water puppy mounted on the transom to pull the water out when i am taking shots of water while under way--works great!

    Instead of using a V entry, i did the opposite--i made the widest point of the hull right smack dab where it is going to meet the chop AND i put the most sunstantial weight (54 gallons of fuel) from that point fwd. Then on top of all of that, i use a lifting strake across the transom. It is made of SST and has a cutout for the engine, it projects out 11" from the transom and the corners are hooked down for lift. The lifting strake gives additional lift to the transom and is 'tuned' to augments the lifting capacity of the trim of the lower unit.

    So there are 3 forces that come into play with the bow, The weight of the fuel, the extra width at the impact point, and the lifting of the transom which presses the bow down.

    I've logged 60,000nm of open Pacific ocean in this skiff and if it didn't work as well as it does, that wouldn't have been possible...

    cudashark, that's a V bottom you got there...And it is a great picture of traditional boatbuilding method using many thwarts~

    Bulut, there is very little new under the sun when it comes to small boat hull configuration. I would suggest that you stick to more traditional designs, The cardboard hull that you constructed will not work well in seas...
     
  13. Pericles
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    Pericles Senior Member

  14. SkipperSki
    Joined: Sep 2007
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    SkipperSki Junior Member

    Bulut,
    Do a search for Surf Dory's, a larger power version, with shallow draft, slight vee-bottom, double ender of the Calkins design.

    see; htpp://bartenderboats.com

    They have been built from 16 footers to 40' in many styles, i.e. Workboat and Rescue, Sportfishers and Weekend cruisers. In Wood or Aluminum. Gas Outboards, and Gas or Diesel Inboards

    The Sportfisher Bartender's, will get to the fishing grounds first in choppy seas and swells when other hull designs have to slow down.
     

  15. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    I'm not an advocate of flat hulls but Loveofsea has found a system that works for him. He regularly drives this style boat at planing speeds on fairly long runs in the open ocean off Southern California.

    PAR, we need to be careful about dismissing the flat bottom in rough seas without looking at his whole proposition. He keeps the bow low in order to prevent the waves from hitting the bottom at a more than a very oblique angle. This helps to prevent the pounding normally associated with flat bottoms. I think I remember that he uses a wide trim plate on the stern and weight distribution to keep the bow low.

    I use the more normal V forward with much flatter sections aft but also use an aft bottom design that lifts the stern and holds the bow down. This also helps reduce pounding and gives a smoother ride in chop.

    I'm not sure what Bulut intends for his hull but it at least partly follows the box, or canoe shape, keel with a flat upper hull that has been discuss much on forums lately. I see a lot of promise there which goes beyond the Atkin shape that you show. The aft end of the wedge does look like a problem though.
     
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