Of Sharpies, Skipjacks, and Carolina Schooners

Discussion in 'Sailboats' started by Bill PKS, Apr 12, 2010.

  1. Bill PKS
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    Bill PKS Junior Member

    Anyone ever opined on the characteristics of the New Haven Sharpie, Skipjack, and Carolina Schooner hard chine hull forms? With less rocker, it seems the Sharpies and Schooners were close to planing hulls.
    Anyone ever seen a Skipjack bow with Sharpie sections aft of the fore-quarters.
    Bill PKS
     

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  2. messabout
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    messabout Senior Member

    Bill; there are many threads here that are related to the sharpie style hulls. More than a few of us actually own sharpies, flatties, skipjacks, bugeyes and others of that general type.

    A sharpie with the rocker removed from the aft portion of the boat will certainly plane. In fact they will plane more readily than round or vee bottomed boats. The down side is that they can pound your eye teeth out. Some relief from pounding is possible by using a vee front section that transitions to a flat bottom aft. That was the standard way of building small runabouts 40 or 50 years ago. That pattern still makes a very adequate boat if competantly designed and built.
     
  3. Bill PKS
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    Bill PKS Junior Member

    Bill PKS

    Thanks for the comments,
    Adaquate "V" forward would reduce pounding.
    On heel the flat bottom becomes a "V".
    Have you seen any lines that combine the V at the stem , and transition to a low dead rise from fore-quarters to the stern? ( maybe 5˚<> 10˚ ) I'm particularly interested in observations as to how the Stern performs on heeling. I.E. does it have to run out above the WL?
    It wouldn't seem to make any difference in a good wind, as a cut off would reduce wetted surface ,,, but what happens at ghosting speeds? Would a truncated stern create a vacuum, or does the runout lines of a stern extend the WL and wetted surface at very low displacement speeds ?
    Bill PKS
     
  4. messabout
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    messabout Senior Member

    Your local library may have books by Howard Chapelle or Ruel Parker. These books go into great detail about Sharpie types as well as Skipjacks and similar boats. For a very good take on the types, you might try the U.S, Government printing office for a copy of Chappelles; Migration Of An American Boat Type. Bulletin Number 228. This is a paper from the museum of history and technology. Lot of good stuff there.

    A displacement boat like a sharpie keeps its transom above the waterline. If the transom drags it is detrimental. A well designed sharpie (flat Bottom) when heeled produces a lovely waterline. A birds eye view of the WL is shaped like avery streamlined torpedo and the aft end comes to a point. This is a condition that is achievable when heeled to some angle in the region of 12 to 15 degrees. A sharpie sailed flat has an exit that is somewhat chopped into a flat. Whereas the vee bottom of the skipjack does just the opposite, Skippies wanted to be sailed flat. In either case the transom was not allowed to drag by virtue of aft rocker. Some configuations of modern versions of the sharpie or skipjack can be encouraged to plane by having the run of the bottom at the least possible angle when viewed from the side, consistent with having the transom just clear of the water.. The run is also kept as straight as is feasible. Old timers did not look quite like that. To plane or semi plane the sharpie needs to be sailed flat and it planes only when the wind is pretty well up. But plane it does. Take a look at Phil Bolgers Skooner design. (odd spelling not mine) That is a hot rod type that he claims will plane readily...I respect Bolger but I am skeptical of the planing bit. It has a rounded run but it did have a cloud of sail. Fun to research, is the Chesapeake Sandbaggers. The all out, haul ***, boats of the time. They were sharpie or skipjack configurations with waaaay too much canvas and long hiking planks with which the crews tried to keep the beasts upright.
     
  5. Bill PKS
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    Bill PKS Junior Member

    Thanks for your insights. I guess i'm wondering about more of a Skipjack hull form. V Entry, with hard chines, relatively low rocker from the fore-quarters, a high aspect CB, full bow sections above the entry ( so she won't run bow down) , a wide stern, and a relatively light weight hull. Maybe 7˚ to 15˚ deadrise, so the leeward half is like a wind surfer with a fine entry. Seems to me that a Sharpie becomes a displacement hull on heel, while the Skipjack runs on a potential plaining surface heeled, ( if the rocker is low enough, and the hull wide enough.)
    My question is how the stern should finish. Most Skipjacks I've seen had relatively soft sterns to a transom that rose well clear,,, so should the stern arc up from the flat run of the leeward half, or cutoff like a windsurfer tail? I 'spect a cutoff form is fine in a good wind, but at hull speed or below, what happens with the alternatives?
    I have most all Chapelle, but I think the objectives of the hulls he discussed were different from these speculations. Most all those hulls were designed for commercial work with following seas in mind, so considering hull speed only, windsurfer experiences leaves me some of confusion.
    Bill PKS
     
  6. Bill PKS
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    Bill PKS Junior Member

    Which drag is less?

    messabout,
    When much younger, I sailed on Spritsail Skiffs developed for the North Carolina Sounds. We used sand bags, hiking boards, and people to balance against large sails in high winds,, but , as most had a lot of rocker they were displacement hulls, and most pulled a lot of wake. There was a lot of grousing about Flying Scots, and upon later consideration, it seems to me that even though the Scots had soft chines, there wasn't much rocker, they ran higher, and so were faster hulls.
    Seems to me that a hard chine increases resistance to heeling, and low rocker reduces an up-side-down Bernoulli effect thereby reducing wave making of displacement hull forms.
    But, other than riding up to following seas, what is the advantage of sweeping a stern upward ?
    So, does the turbulence seen behind a cut off stern indicate some drag that reduces hull speed? Or, if you smooth that out by sweeping the stern up, does that just extend wetted surface ( drag), and create downward vacuum (more drag) ?
    Maybe the question is, which drag is less at what speed ??
    Bill PKS
     
  7. messabout
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    messabout Senior Member

    There may very well be some Bernoulli effect that influences our boat. For now, try to forget that phenomena. There are way too many other things that determine whether our boat will be a slug or a lively one.

    You have the right question about the back end of the boat: "which drag is less at what speed". That is much too big a question to answer here because there are so many variables that influence our answers. If that sounds like a cop out, it is. The subject is so big and involved that entire books have not addressed all the ramifications.

    I suggest the following books. They get down to the nitty gritty very well. Dave Gerr: The Nature Of Boats. A good basic treatise about the plusses and minuses of various design characteristics. Gerrs other book: The Propellor Handbook has some more good stuff. The book has information that goes well beyond propellors. C.A. Marchaj : Sailing Theory and Practice. An indispensible book for those who would understand the likes of wave drag and a whole gob of other technical boaty stuff. A must have. Gerrs books are easy reads and easy studies. Marchaj is more involved but well within the capabilities of most of us.
     
  8. Stephen Ditmore
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    Stephen Ditmore Senior Member

    In general a boat with less rocker will be good in strong winds (fast sailing), while one with more rocker will be better in light air (and will be more symetric when heeled). If you want some combination of the two, design the boat so you can move weight aft a bit for planing. Most dinghies sail best bow down in light air and transom down in a blow.

    OK, you probably know that stuff. The question is, what speed, on what waterline length, do you want to optimize for?
     
  9. Bill PKS
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    Bill PKS Junior Member

    Wading on along.

    Messabout & Stephen,

    Thanks for the comments,,
    I think I've kind'a got my mind around sails, entry, and bottom, but I have no "feel" for what happens at the stern or why.
    I'll wade into Gerr and see what I see.

    As for symmetry, I'm thinking I like the increasing resistance to heel the immersed volume a hard chine gives to counterbalance the rig (beyond a desired angle of dead rise ). Seems to me that has advantages over only using ballast to hold the rig up,, as more ballast requires more displacement = more wetted surface = more drag.
    The idea is that each hull half (at some comfortable angle of heel) would be like a windsurfer bottom, ( but give it a finer vertical entry).

    On an extraneous course,,,, some years ago I went sailing with a fellow from Hilo on a polynesian cat. He used a paddle over the stern- quarter to steer. Pulled straight up to come up, and pushed straight down to fall off. It was a totally unexpected lesson on balance of CLE/ CLR. (While I "Knew" it and used it in balancing sails, I didn't really "get it" until then.) That amplified the importance of setting a rhumb line course with a movable CB or sail balance and only use a rudder as a trim tab.

    Just in up to my knees.
    Bill PKS.

    PS: Seems to me the Bernoulli effect is especially unappreciated from lower speeds of displacement hulls with a lot of rocker. The faster it goes, the more it is pulled down, and creates more relative drag.
     
  10. Stephen Ditmore
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    Stephen Ditmore Senior Member

    That's correct: curvature in the lines of flow around the hull = suction. The Bernoulli effect is even more pronounced in shallow water.

    But if you're not going fast enough for the water to break clean off the transom, your transom in the water also causes suction. In that case the suction is drag.

    The primary variables that effect stability are waterline beam and vertical center of gravity. I know a lot gets said about section shape, and displacement, but mostly these things effect stability to the degree they effect waterline beam and vertical center of gravity. (Of course when the boat is all the way over on its side, waterline beam is no longer helpful. Along with VCG, freeboard and the angle of downflooding become critical.)
     
  11. frank smith
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    frank smith Senior Member

    I have seen sharpies with vee worked into the stern , here is one[​IMG]

    It adds to the complexity of building , but it is done .
     
  12. Bill PKS
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    Bill PKS Junior Member

    Stephen,

    Last > first..
    I see WL Beam is important, but as the hull heels, don't softer chines provide less increase of immersed hull volume ( buoyancy) while harder chines more rapid increase of immersed hull volume, and consequently act more to resist the leaver arm of the rig? The alternative is ballast which results in more wetted surface.
    ( I'm not thinking about rolling to beams ends.. only within the range of nominal angles of heel.)

    To the Stern //
    Your comments are directly to the point of my mind game.

    At Lower speeds//
    If the lines of the stern rise above the WL, it looks like a Stern wave is pulled up ( or builds up), increasing wetted surface on the overhang. Why?
    Alternatively,
    With a Cut off stern bubbling along, I see the water rolling over against the vertical surface. You say that is suction ( a vacume). Why?
    Which is better or worse? More wetted surface (hull friction ) or suction? Why?
    At higher speeds//
    With rising stern lines, the wave just gets larger under the counter the faster the hull goes.
    Seems to me that is the effect of Bernoulli creating waves for a displacement hull form.
    With the Cut off stern, the water runs on back, but builds a Wave some distance aft of the stern. While, that doesn't appear to be pulling the hull down, nonetheless wave-making takes power and can't be good.
    Which is better or worse and why?
    ( I want the hull to sail over the water, not swim thru like a fish.)

    Getting deeper.
    Bill PKS
     
  13. Bill PKS
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    Bill PKS Junior Member

    Sharpie

    Capt. Smith,
    As I look around at Chapelle, it appears to me that the New Haven and Carolina Sharpies were flat in cross section stem to stern,, Some with no skeg at all. When heeled the hull became a displacement V". In low winds, I guess if you held them flat, hulls with lesser rocker were said to be faster. ( I don't think there is a single Carolina Sharpie Schooner left. There is a pdf of a ≈ 44' hull in my first post.)
    Skipjacks seem to have had some dead rise with a skeg (increasing wetted surface, but providing resistance to leeway as the leeward hull section from chine to the skeg ran level on heel.). ( Several Skipjacks have been preserved due to Va law's requiring sailing vessels for oystering.)

    The stern quarter in the picture you posted has some dead-rise at the stern. Does it have a flat section amidships?
    Also it seems to be running down by the Bow.. are there other pictures showing how she sets to windward?
    I don't see much of a stern wave, which is good.
    The rocker shown, which raises the lines to the WL, is the point of my deliberations. What is the trade off between the increased rocker which would seem to increase (Bernoulli) down vacuum on the hull, vs. " Suction" at the stern, if the stern is cut-off and the lines finish under the WL?
    Bill PKS
     
  14. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Most good naval architects can do a pretty good job of optimizing a hull form (this includes locating the longitudinal center of gravity in the right place) IF you can say what your target speed is, on what length boat. Have you answered that question?

    At low speeds, dragging the transom will further slow you down. Britton Chance tried to show otherwise with the 12 meter Mariner. It was a failure.
    http://www.12mrclass.com/cms/index.php?option=com_zoom&Itemid=444&catid=106

    As for stability, on a given waterline beam I agree that a hard chine boat MIGHT have a SLIGHT edge in form stability.
     

  15. Bill PKS
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    Bill PKS Junior Member

    Opinions are??

    Fellows,
    Just found this in, "Resistance , Wave-Making and Wave-Decay of Thin Ships, with Emphasis on the Effects of Viscosity" pg 1-5 Lazauskas - U of Adelaide AU. April, 2009 .
    " ..... recent attempts to characterize wave wakes have met with considerable disagreement within naval architectural community, .... adding to the uncertainty and confusion, there is a lack of data from far-field full-scale measurements that would allow validation of mathematical prediction techniques [63], and the little data that does exist is often very poorly documented."
    Sounds like any opinion is just as good a guess as any other guess.
    Mostly though, the paper did imply that breaking skin friction was of relatively high importance. This is consistent with experience with surf boards and golf balls, where slick surfaces are less slippery than some textured surfaces.
    Beyond that, I don't have any basis for any kind of opinion.
    Help.

    Bill PKS.
     
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