Effect of flare of sides??

Discussion in 'Boat Design' started by itskens, Dec 11, 2012.

  1. itskens
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    itskens Junior Member

    Hello,
    I'm wanting to build a 10', flat-bottomed pram. In looking at prams and driftboats, I see that the side flare can vary from 110 degrees and less, up to 120 degrees and more.

    I'm not concerned with the stability factor.. I'm wondering what happens to handling and speed with more flare.

    With more flare, do I effectively have more width?,, or more length? on the water,, probably both, but how does that affect the speed and maneuverability?

    Are speed and maneuverability compromises of each other?

    Will I have a better rowing craft with a 110 degree flare than a 120??

    Many thanks!! Ken
     
  2. gonzo
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    gonzo Senior Member

    Flare is not going to affect speed much, particularly in a lightly loaded boat. The advantages of flare are more stability, less spray or water coming into the boat when it is choppy and easier to work (fish, etc.) off the side. Another use of flare is to have the oarlocks farther out. It makes rowing more efficient without the need for outriggers.
     
  3. itskens
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    itskens Junior Member

    Thanks!

    I was getting into the ratios of L:W and thought that maybe on such a short and wide boat~45/46" bottom, and loaded with me, big lab, dekes ~400#, that a few degrees could make a difference on how the water would go 'round it. Sounds like maybe better to keep more flare for the dog's sake.

    Thanks again!.... Ken
     
  4. El_Guero

    El_Guero Previous Member

    Ken,

    Above or below the water line? Maneuverability and stability can be interconnected.

    Wayne
     
  5. itskens
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    itskens Junior Member

    Below the water line, water flows 'round.
     
  6. Petros
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    Petros Senior Member

    flair, as gonzo stated, helps in rough or choppy waters, and helps keep you dry. It also allows you to load more into the boat, the more it is loaded the flare allows for more displacement so it does not site as low in the water. It also gives more inteiror volumn. these last two will also have less drag of the hull when lightly loaded, since a square sided boat would have to be much wider at the base for the same freeboard with the same load, so flare could have less drag on a boat that will have much different payloads in it. flare is also good for boats intended to go into surf or rough conditions, as the prow plows through a wave you get more lift faster than with less flair. You see surf boats and coast gaurd cutters with lots of flair for this reason.

    flare can also make the hull more stiff, so allowing a ligher build. the side panels would have conic sections rather than flat or single curved sides.

    Many boats have no flair, skiffs, scows, Puddle Ducks, many canoes, these are generally meant to be used on inland waters without a lot waves.

    I think the difference between 110 and 120 deg would hardly be noticable, in most uses it would make little differences. It is up to you and the look you want. Most fair in many kayaks for example are purely cosmetic, most hull designs are done to attract a buyer on the show room floor.
     
  7. philSweet
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    philSweet Senior Member

    On a small boat, flare forces you to step closer to the centerline and puts the floor a bit lower. It is primarily a stabilty issue. You can get a bit more ultimate stability with the same materials chine to chine. Can you post a sketch of the boat, or at least its section at max beam?
     
  8. itskens
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    itskens Junior Member

    Great stuff!!... Thanks all!

    I've always thought/intuited that flare is a good thing. My driftboat hasn't enough and not enough rocker, hence it's slow to turn and knifes into a wave. But, back the the pram: Maybe the straighter-sided prams are for transport in the pickup bed?
    Steve Lewis drew the attached "RiverPram", given a 10' gunwale and 45/46" bottom; he also drew the "Thudpucker" I will probably rake back the wide transom, more like a Dave Z's Drifter. I don't need a real "drifter", as I'm not drifting any waves, though I will use this in the river. I'd rather have a compromise boat that I can do a little river fishing/crossing, pond/lake edge fishing, and duck hunting the edges, that are too deep for boots. I also like the looks of the Hudson Springs Pram, as well as Smith Bros and Sea Dog,, but again, with just a little more rake in the downstream transom.
     

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  9. messabout
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    messabout Senior Member

    One of the positive points of flare is that it has the appearance that most people expect it to have. Flare does contribute some to the stability of the boat, but not until it is heeled over to a considerable extent. The contribution to stability is useful but is over rated. It does keep the boat a little drier but so will some simple splash rails.

    The down side of a flared hull, especially with the dog, is that a plumb sided boat will not allow the dog or the person to get so far off center. It is immediately apparent that the farther off center the weight is distributed then the more the boat will heel.

    Some will argue that plumb sided boats are safer than flared sided ones. If you have an 80 to 90 pound Lab or maybe a Chessapeake then this is worth some consideration. Of course you can have flare with the addition of side decks at the same time which tends to diminish the problem. In any case, a lot of flare will add a bit of weight. (Back in the day, flare was thought to be an outward curvature of the sides and a straight expansion from chine to sheer was called flam, but who cares about that semantic difference. I am dating myself.)

    As for speed and manueverability, adding flare or not adding it, is not going to change performance enough to argue about. That is particularly true of a very small boat such as the one you have shown. The one you have shown will be serviceable but I suspect that it has too little forward rocker. Built as shown, the boat will often plow with its forward transom which will stop it in its tracks or at least slow it and throw up a lot of splash.

    More slope or less in the aft transom is not going to change things if you mean to hold it to exactly 10 feet. More slope will only shorten the waterline which is not what you want to do. If you plan to use an outboard motor then set the slope at 14 to 15 degrees and call it good. A bevel of 3 on 12 will get it about right for the motor. Otherwise a plumb transom will be just fine except for the appearance part.

    Decide if you want a pretty boat that will not be derided by on lookers or a much simpler to build, lighter, and perfectly serviceable boat.. Flare or plumb sides???
     
  10. messabout
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    messabout Senior Member

    Post script:.......... Dont sweat the speed component. A ten foot boat of the displacement type is going to be constrained to something less than or equal to, if you are lucky, four MPH no matter what you do. A planing boat is a whole different deal. 10 foot planing boats are great fun but not ideal for fishing or hunting or having your canine partner go along.
     
  11. philSweet
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    philSweet Senior Member

    On boats this size, 22-25 degrees flare is about right for a boat with 25% side height vs chine to chine wrap around distance. This assumes the boat load is right (correct B/T ratio for good stability) and the bottom is dead flat. Keep the seats low. These numbers are for a boat of this size. The flare and the relative size of the sides trend a bit smaller as the size of the boat increases.

    The sketch looks good to my eye.
     
  12. itskens
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    itskens Junior Member

    Wow!... need to find a nautical dictionary.... I really appreciate all the advice y'all are giving!! Mess'bout: Yes, my lab is ~100#. I'm not planning on going anywhere fast in this boat. Let me clarify bow/stern: The narrow transom will be upstream, or in the direction of travel on flat water. I wanted to get the wider, wave/splash-approaching transom up a bit, so as not to bite a rock as I'm drifting downstream. Some of the designs are so flat on the water, that I could see taking out the transom, or at least, a chunk of it. Maybe I could adjust that by moving my mass back a bit,, but would rather design that into the design.
    Phil: Ya,, the driftboats I've measured and read of run 24-30 degrees; I've seen prams with 90/0 degree sides.... So, these prams I'm seeing with steeper, 22-25 degree flare sound better than what I was thinking.
     
  13. Petros
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    Petros Senior Member

    those pram type dinghys are popular, for their length they provide the extra width and stability by chopping off the bow. They also have a lot of nostalgia I think since most people that sail usually sailed in one of those first, or it was their first build. I had always wanted one, in my 7th grade wood shop the "rich" kids had their parents buy them a kit and they would build one in shop class, and go race them at the end of the year. I tell you when I was only 12 years old and I saw kids building those fun looking little boats, I wanted one so bad. But even the kits than were just too expensive for our family and no way I could earn that much delivering news papers and mowing lawns. 40 year later I built one for fun out of stuff I had laying around (from plans I got off the intenet for free). That was after I had built some 17 or 18 small boats, including kayaks, canoes and sailing dinghys, and have been in lots of other boats, and even crewed on racing boats, I was really disappointed with the performance. That flat bow did just what I would have expected, plowed through the water and beat against chop. I even wondered why those boxy little boats are so popular, and more recently the puddle duck racers even look worse.

    Unless the most important thing about it is the small size and short length, I would suggest you build something few feet longer with a proper bow. you will not regret the better performance, nor the little bit of extra capacity.
     
  14. itskens
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    itskens Junior Member

    Thank you, Petros! Actually, I'm not sailing, or moving fast.. I'll be drifting and rowing. But/And, I do want the current-facing transom to be up, out of the water, pointed, or not.
    Those Norwegian prams are nice, but for the river rock and the dog, I need/want a flat-bottomed boat.

    From my little, kayak experience, I've learned that a good lofted/surfing nose is important to greet the oncoming water. I intend to rake both ends and get them both out of the water, especially the upstream... (there must be a term..,, 'cuz drift boats/prams greet the water in the opposite direction of a sailboat, but it's the same principle.)

    So, really, this is a kind of drift pram, which I can also row 'round the lake edges. It won't be great for the big waves and it won't be good for going the distance, but I'm hoping it'll be a good compromise.
     

  15. troy2000
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    troy2000 Senior Member

    For ideas you might want to take a look at Edwin Monk's 9' x 4' pram dinghy, called Stubby. The bottom has a shallow vee instead of being flat, but the angle is constant. That makes it quite easy to do in plywood.

    I came up with a stretched and slightly widened version of Stubby for one of my nephews, who happens to be built like a Samoan. He used plywood for the bottom, and solid lapstrake planking for the sides.

    http://books.google.com/books?id=n0...EIQ6AEwAg#v=onepage&q=edwin monk pram&f=false
     
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