Dory - Flat Bottom - J. Spira - Claims

Discussion in 'Boat Design' started by Standpipe, Jun 10, 2015.

  1. gonzo
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    gonzo Senior Member

  2. Easy Rider
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    Easy Rider Senior Member

    Thanks Gonzo,
    I enjoyed the pics very much. A big chunk of them weren't Dories though and the Dories w bottoms anything but flat were wonderful. Needing ballast obviously but I'd wondered if such a boat would be practical for years.. It is. Thanks
     
  3. sharpii2
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    sharpii2 Senior Member

    I think he confuses initial stability with ultimate stability, but in general terms he's mostly right.

    I think he projects small boat stability onto ships, which though flat bottomed, have much deeper sections.

    VCG location seems less important on boats with high Meta Center heights, which are not likely to experience high angles of heel. Your typical runabout is a pretty good example of such a craft. And this is where I think his thinking comes from.

    When the Meta Center height gets lower, such as boats with deep sections for their width, VCG placement becomes more and more important.

    Attached is a drawing from an experiment I once did, using a very simple flat bottomed boat, but making the sections ever deeper.

    Nothing else is changed. The design follows two rules:

    1.) Minimum free board must equal one quarter the Beam or half the draft, which ever is greater, and

    2.) VCG placement is one quarter the WL Beam, or three eighths the draft above the bottom, which ever is greater.

    The hull has a double arrow point plan view, so this effected its behavior quite a bit. I suppose this is because the average Beam was much smaller than the actual Beam.

    Notice how wildly the Meta Center heights varied.
     

    Attached Files:

  4. Oceannavagator
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    Oceannavagator Junior Member

    Well, I just had a heated conversation with Mr. Spira himself on FB. I insinuated that flat bottomed boats would pound in waves and he promptly got wrapped around the axle over the statement, responding with videos of pacific power dories running in what only could be called ripples. He maintains that his designs are totally seaworthy offshore. I'll take him at his word for that but will be picking some other hull form for work in the gulf stream thank you.
     
  5. Easy Rider
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    Easy Rider Senior Member

    It was said early on that a GB dory has a lot of rocker. Not so IMO. All the GB Dories and pics of GB Dories I've seen have almost no rocker. You could even say it's a unique feature of the full displacement GB dory as most dories have quite a bit of rocker.
    Also I question that a skipjack is a dory .. related but not a dory IMO.
     
  6. messabout
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    messabout Senior Member

    A Skipjack of the Chesapeake variety has a vee bottom whereas the Chessy Sharpie was traditionally flat bottomed. Along the way the names of those boats became mixed so that one type was accorded the name of the other type. Neither of these to be confused with the Chesapeake Bugeye which had rounded chines and arced bottom. Of course there was the New Haven Sharpies, and the Key west Sharpies.

    Ruel Parker Had, or still has, sharpie type designs with nearly flat, but slightly cambered, bottoms. Bolger was also a proponent of Sharpie like boats. One of the most practical and pleasing boats I have ever sailed was a Bolger Black Skimmer. And yes, it would pound in a chop, if she was standing up, but when heeled over a bit it was pleasingly smooth.

    As for dories there are a whole conglomeration of boat types who wear the name. Most of the time we are thinking of the Grand Banks three plank type with a narrow bottom, thirty degrees or more of flare, long forward stem rake and tombstone transom. The Chamberlaine Dory was not much like the GB dory because it had rounded sides, often lap straked. Still labeled Dory. Then there is the Buhler, so called power dories, from the northwest. Not a dory at all. ....so we need to be specific when calling a boat a dory. In any case they are not at all like a sharpie just because they have a flat bottom.

    The Banks dory came into popularity at about the same time that we figured out a way to apply power to a large bucksaw in such a way that we could rip wide boards from big logs. A three plank boat was cheap, quick, and easy to build. No telling who dreamed up the wonderful "pit saw". Probably some clever New England Yankee but the Portuguese builders were not far behind with almost identical GB dory like boats.

    GB type small dories are the stuff of legend, most of the legend tales of excellence unfounded. A real GB is a cranky boat and damned uncomfortable, and tiddly unless heavily loaded. There are all sorts of boats with that general layout. Mike O'briens' Six Hour Canoe is almost a dory. It is definitely cranky, but fun to paddle in still water. I still have a How to Build 20 Boats magazine with plans for a lightweight 16 foot, plywood, Banks type dory claimed to be a nice rowing boat.

    Oops! I have gotten carried away with nostalgia. Forgive me because I am almost as old as the discovery of fire so I claim the right to some degree of senile behavior. :eek:
     
  7. Easy Rider
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    Easy Rider Senior Member

    Fire ..... what's that?
     
  8. messabout
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    messabout Senior Member

    ^ ER, Fire is really, really old. :D
     
  9. wavepropulsion
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    wavepropulsion Pirate Member

    Vee Bottom or Flat Bottom

    The controversy rages on about which is the better boat. I've had people look at me in disbelief when I
    suggest they can use a flat bottomed boat, such as a Grand Banks dory, for deep water ocean use. They've
    always been taught that flat bottomed boats flip over. I don't know where that idea came from, but on several
    occasions, I’ve heard, "You'd actually take a flat bottomed boat to Catalina?" (an island about 25 miles off the
    coast of Southern California.)
    The secret of boat stability is more a function of the boat’s center of gravity and loading than the shape of
    the hull. In truth, a flat bottom hull is more stable at rest than a vee or round bottom hull, when it comes to
    shifting loads from side to side. That's why big ships like freighters and tankers are all flat bottomed. The flat
    bottom hull tips far less than the vee bottom. This is called “initial stability,” and it's why flat bottom hulls are
    often a better choice for fishermen and people who bring along shifting loads, like children or dogs, who cant
    seem to sit still.
    The one thing left unsaid so far, though, is that once a flat bottom hull passes it's stable range, it does tend
    to become unstable, for instance, when struck abeam by a breaking sea, more quickly than a vee bottomed hull.
    In conditions like these, a round bottom or vee bottom hull with a weighted keel has far more tendency to return
    to upright than a flat bottomed hull"

    Probably he likes to sell his plans, what is not any sin. Anyway he is right and clearly explains why initial stability is and how boat trim is affected by the cargo placement. I'm not a NA neither an engineer, but generally speaking, in any language most of the professionals in this areas manages better maths than speech as far as I experienced. Yes, subjectivity and interpretation is important, specially when trying to read words and not equations.
    Another point is Sain Pierre dorys had much rocker, while Grand Banks had much less. Yes was done for stacking and for being pulled, droped from the clippers deck, but the plans was sstudyed by the french government and the model is derivative from more ancient boat shapes. Also new materials comes at the time, as wider planks.
    One of the better uses has the new skiffs derived from dorys, are the dorys from Oregon dory fleet, or Pacific Dory, with much carrying ability and not only able to manage the surf but also big swells and little chop at low planing speeds.
    The better Pacific dory plans in the web are from Spira, because the flare he puts makes the boat to have the best line at waterlines for planing, wider ahead and narrowing to the stern. Also he uses easy materials and simplify a system of construction using the traditional and the new materials.
    This video is from a Glen L Hunky dory, tell me please wich powerboat will have better performance at 23 feet, about 8' beam and just 10hp, because when discussing about efficiency, people in general tends to go for the more complicated, while technology and knowledge must be used to simplify the life. https://www.youtube.com/watch?v=Xg9oDuy2moY&list=FL4LfL9wQdRZ_lIwZvNE00yg Spira winchester dory will be a bit more efficient than this one. This boats foot by foot, or speed by speed, or weight by weight, are more efficient and practical than any low tech displacement boat.
    P.S.: talking about language, I'm the beheaded laughing at the hanged, sorry.
    P.S.2: To add a keel to any dory is to create a boat different, no more a dory, or this or is a lack of understanding how a dory behaves in waves specailly in a beam sea.
     
  10. wavepropulsion
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    wavepropulsion Pirate Member

    ''Chalana'' in Spain is a different kind of boat, I believe is a V bottomed of the Mediterran area, but for sure the name was borrowed here. I talk with a fisherman here, his boat is the ''Batibote''(batboat), is a 19 feet dory he calls chalana built by a portuguese for his father, probably in the 40's. He still fixing she all the time, actually changing some planks. Ahd he has not engine but oar power.
    Portugueses learnt to build dorys from the cod fisheries in the Grand Banks.
    Thanks for the pics.
     
  11. sharpii2
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    sharpii2 Senior Member

    For a boat that size and with that little horsepower, you want a displacement hull. A planing one will only drag its chopped off stern through the water. It will never plane. You will need at least six times as much engine to get it to plane.
     
  12. wavepropulsion
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    wavepropulsion Pirate Member


    The owner cruised at 60% rpm at ten knots. So, for a similar -never equal- displacement hull you will need 3hp (from an outboard) for going at half the speed. Same consumption, double speed and propeller deficiences makes sense to me, ever talking about outboards.
    Then if you are in a displacement hull you can't go against a stream faster than your hull speed. Or you will drag a keel or spent more hp than is needed most of the time.
    I know I exagerate but I like to know opinions about. A flat bottom planing hull looks as the better choice to cruise at the lower planing speed with fuel efficiency. Is the commom agreement a dory of this kind not pounds at that speed and has a good tracking.
    A 10 hp high thrust ob is supposed to be as efficient as a 25 at lower speed, something about torque I believe.
     
  13. sharpii2
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    sharpii2 Senior Member

    You are right, of course. I failed to consider semi-planing which can happeen at lower hp per weight.

    The flat bottom is best for planing, period.

    It's not universally used because it can pound and slam quite severely, while on a full plane. On a semi-plane such would be less severe.

    IMHO, semi planing is when the water breaks free of the immersed transom, but the hull stays mostly in the water.

    Your argument that a boat like this is superior to a displacement hull the same length has considerable merit. Displacement speed is governed by WL length. In a boat under say 25 ft the speed is limited enough to create the practical problems you described.

    Another benefit of a smaller planing powerboat is that its total displacement is low enough that its much higher hp/wt ratio really doesn't matter as much as it would in a larger boat, and it usually goes just as fast.
     
  14. wavepropulsion
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    wavepropulsion Pirate Member

    Thank you Sharpii, sorry I didn'r realized differences in planing and semiplaning modes. Just had a flat bottom and Savitsky predicts a regular increase in drag after the bump in that hull. From there my missconception.
     

  15. noreasterner
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    noreasterner New Member

    Although this is an old thread - I felt compelled to reply in order to leave some record of historical accuracy regarding the seaworthyness of dory style boats like Spira and other designs. Firstly, I recommend the interested reader purchase John Gardner's "The Dory Book" to get a bit of education on the history of this design. The term dory has a wide berth of meaning but suffice it to say that the dory plans sold by Spira and Glen-L and some others can be directly traced to the classic double-ender dories from the east coast. Spira's boats are not new designs - he is trying to offer tried and tested designs, but modify the construction in such a way that is easier for the novice builder to successfully complete.

    The original dory boats were initially rowed and some were used to launch and land through the surf - and others were stacked in bigger vessels to be deployed with hand liners... thus the utility of the flat bottom. A beamy, flat bottomed boat with flared sides also makes for a very stable boat capable of large loads....its not rocket science and you dont have to be a naval architect to figure this out.

    Some are tender when light, but firm up loaded. Moreover the light weight construction and flat bottom allow them to be powered with half or even a quarter the horses of modern fiberglass or aluminum hulls of the same length. this is due to hull weight as much as the flat bottom. Yes plywood hull covered with fiberglass is lighter than pure fiberglass hull due to the amount of epoxy used. A 22' ft long power dory with an 8' beam, can be powered with a 60-80hp motor and achieve 30mph at least...although their purpose is better served cruising at 20. This assumes the boat is not overloaded. I would expect a 20-22' open deck center console power dory, glassed on the outside - to tip the scales around 1,000-1,300 lbs - without the motor.

    Most of these designs are not just seaworthy but extremely so. For much of the same reasons a drift boat is so good in a rough river. With the advent of the power motor, they appeared with a motor well, then later transom....like the modern iteration of the Pacific City Dory. The Dory fleet there was an adaptation to the lack of a bay or port for a fishing fleet and the extreme dangers of bar crossings even if a bay was available. (Bar crossings here are much more dangerous than inland seas of the east coast due to the bigger swells and persistent onshore winds). These boats carry more beam forward than the Carolina types and are more seaworthy in open water.

    The comments above saying pacific city dories are navigating "ripples" is pure absurdity. They launch and land in some very rough conditions and have one of best if not these best safety record of any commercial fleet in the country. They are also used to run after albacore 40 miles offshore.

    The real issue is how fast you want to go in rougher seas. If you live in an area with a lot of high chop then the pounding of a flat bottom might get to you. But if you want to beach launch it is priceless as you can fish areas that have no nearby ports. The Dorymen of pacific city just dont plan on running WOT all day (or at least not the way back when the seas are rougher), but often their seas are more big swells than short chop. Big seas is where the dory hull shines. Their bow stands up proud, and when descending a swell, doesnt dive into the swell and performs well in following seas. In other words, you would rather be in such a dory in the worst seas than most modern recreational factory boats.

    The big offshore recreational fishing boats with deep V fiberglass hulls are not more seaworthy, they can just run to the blue water faster in rough seas. But this is the realm of the rich...who can pay for twin 250hp gas guzzling outboards pushing a 15,000lbs hull through the water.
     
    Last edited: Nov 10, 2018
    wavepropulsion likes this.
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