Quarter Beam buttock line

Discussion in 'Boat Design' started by Sipoka, Jan 26, 2015.

  1. Sipoka
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    Sipoka New Member

    I am looking into boat hull shapes and what makes a boat a displacement or planing and as well as weight this came up i was wondering how exactly do you calculate it and what impact it has on the boats performance
     
  2. Pericles
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    Pericles Senior Member

  3. messabout
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    messabout Senior Member

    The subject is discussed at length in Dave Gerrs Book; The Nature of Boats.

    Buttock line: Slice the boat, front to back, at the quarter beam location. The line of the bottom is revealed at that quarter beam location. That is the buttock.

    A planeing boat will have that line either straight or almost straight in the run of the boat. (the run is that part of the boat that is aft of the lowest part of the keel or boat bottom) The boat at rest. in normally loaded condition. will have the transom somewhat submerged.

    A pure displacement boat will have the buttock line, in the run, curve upwards such that the transom will be above the water surface.

    It is possible to have a light displacement boat that is capable of planeing. The angle of the run, with respect to the lowest part of the bottom, must be very small, generally less than four degrees. A lesser angle is better if the boat is to be capable of spirited planeing performance. The run will also be a nearly straight line. This configuration is seen on hot rod type dinghies and sometimes on high performance big boats.

    The above comments are generalities only.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Learning about the buttocks and what they represent is easy enough, though knowing the subtle differences between a good set, for a specific application and a set that might be less desirable takes some experience.

    For example, viewed in profile, the rise of the aft buttocks can have a profound affect on the boats ability to "carry" herself, press in the rig, the ability to sustain plane, etc. Viewed in plan, the quickness of the turn in the after section can mean a good or bad steering boat or one that's well balanced in a confused sea way, etc.

    There are some generalities in regard to these lines, though (again) studying many sets, from all sorts of boats, will give you an understanding of what shapes are employed and possibly why.
     
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  5. Easy Rider
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    Easy Rider Senior Member

    I think the QBBL carries a lot of importance.

    My boat has a lot of curve in hers and the curve is like a turn in a road with a constantly decreasing radius. So her QBBL is shaped a bit like a hook laying on it's side. I'd like to add to the stern and take a lot of the hook out.

    I can relate to PAR's comment "knowing the subtle differences between a good set, for a specific application and a set that might be less desirable takes some experience."

    The shape of sterns says just about all about a boat IMO and the subtle differences are often not so subtle.

    Had to over expose the pic to show the hull shape aft and the QBBL.
     

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  6. tom kane
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    tom kane Senior Member

    Quarter beam buttock line or Quarter beam buttock angle.. the same or different?
     
  7. Hampus
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    Hampus Junior Member

    What's wrong with her stern? Keep in mind that unless perpendicular to the sections, as it would be in a flat bottomed hull, the buttocks nor the waterlines are representative of the flow along the hull. The QBBL may be more important in a planing or semi planing hull where it actually resembles the flow than it is for your displacement hull.
     
  8. tom28571
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    tom28571 Senior Member

    Easy,
    I can't see even a hint of hook in your boat which probably is a pretty efficient hull as long as its not over driven. Yours is Peter's proverbial banana shape hull. Anything short of a complete new bottom would be wasted or worse than what you have. At that, you would need a huge engine to make it go faster. Some Portuguese displacement boats have large aft flat planing plate attached that is underwater at displacement speeds but will lift the boat onto plane but they need big engines too. Similar deal with the Calkins Bartender.
     
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  9. Easy Rider
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    Easy Rider Senior Member

    Hampus and Tom,
    In this pic you can see what I call "hook" in the QBBL. See how the bottom turns up abruptly aft of the rudder shaft? It's probably fly stuff as I cruise at 6.15 knots but I would imagine it may be unnecessary drag at 6.5 and above and I do realize this hull isn't shaped for speeds very close to hull speed. It;s not a smooth line IMO and would prefer it to be more straight. There is a little bit of extra turbulence in her wake on CL that would not be there but for this "hook".

    But now w a bit more thought that little bit of extra fullness right at the stern could be a benefit in following seas or perhaps offer some extra pitching support and dampning in head seas. She rises up to the sea smartly and never buries her bow. Quite a lady she is.

    You would think that "hook" would suck the stern down more than a straighter run but this boat is amazingly pitch stable .. even when overpowered at 8 knots. Seen pics of a W30 doing that.

    Hampus I think the QBBL is very important in all hulls. Of course you can have two boats w the same QBBL and very different hulls. The quarter line is IMO obviously there to consider the average buttock line. A flat bottom would have all the buttock lines the same but I've seen many boats w a steep line at the keel and a straight line at the chine. The CHB 34 trawler is much like that.

    I'm very happy w my Willard and don't need to go faster. Two extra feet of space aft would be golden though. But aren't we all looking for a bigger boat? Not for me w these high moorage rates.
     

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  10. Pericles
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    Pericles Senior Member

    Easy Rider,

    Recently, I was reading about some modifications made to a boat by Eric Sponberg. Granted your vessel is nothing like "Blue Bill", however there might be a Lightbulb moment for you with regard to the lifting strakes. What could possibly go wrong?

    http://www.sponbergyachtdesign.com/Bluebill.htm

    Best,

    Perry
     
  11. Easy Rider
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    Easy Rider Senior Member

    Percles,
    Thanks for the link.
    It was interesting but I fail to see how it relates to the Willard.
    Unless you're saying after a mod one may get results other than what was expected. Not having a lightbulb moment.
    "go wrong"?
    Not going to do anything to the Willard except maintain her.
     
  12. Pericles
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    Pericles Senior Member

    Fair do's; it just was a suggestion about changing a bottom profile that would deliver a more Car Dash Ian performance. You are Ian aren't you:?::?::?:

    Apologies for the terrible attempt at light humour.

    Best,

    Perry
     
  13. Hampus
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    Hampus Junior Member

    Yes, it's a beuatiful boat and she appears to be in very good shape.

    The QBBL is important in determining hull shape but in a displacement boat it doesn't really tell you anything bout the waterflow along the hull, unless the hull is shaped in a way that lets the QBBL run perpendicular to the surface. The flow along the hull, especially near the stern, will be one that causes no or very little separation, or one that causes more or a lot of it. Separation causes turbulence.

    Have look at the picture below. It's a William Garden designed Vega. Also built by Willard? You can see that it has the "hook" in its QBBL that you mention. Now imagine a diagonal run perpendicular to the surface (all the stations) from bow to stern. It would probably be a curve with fairly large radius up until station 3, then decrase in radius until the midship section only to increase again as it continues aft. That would be the flow along the hull, or close to it anyway. As straight a flow as possible would decrease the risk of separtion while abrupt changes in the flow, like a hook in the diagonal, that would be too steep or too sharp for the water to "stick" to the hull would cause separation and thereby turbulence.

    I'm not saying your boat doesn't suffer from excessive turbulence, nor that it does. I'm just saying that with a hull shaped like that you can't determine water flow and therefore not turbulence simply by looking at the shape of the QBBL.

    [​IMG]
     
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  14. frank smith
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    frank smith Senior Member

    So what up with the QBBL?
     

  15. PAR
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    PAR Yacht Designer/Builder

    I don't see a "hook" in the traditional sense on that Vega. A hook would be a reverse of the curve's direction, typically seen aft to act as a speed control element, keeping the bow rise minimized, though also limiting top speed potential.

    Determining any hull form's potential is a whole lot more study then a look at one line. Clearly the goals of the Vega are apparent. She's a hefty puppy, with the volume to accommodate this and well balanced ends, to make her behave herself when the need arises. No surprises here. The WL's, buttocks and diagonals will give a very good idea of what's she's intended to do, couple these with some basic dimensions and spec's and performance predictions wouldn't be terribly difficult, assuming one knows what they're working with.

    Where this sort of thing gets interesting is when you have two similar hulls, the same basic form, dimensions and weights, but ever so slight differences in chine sweep, location, strakes, chine flats, volume distribution, etc. Fitting them to be appropriate, for the SOR or any performance expectations, takes some serious understanding.

    Frank, the quarter beam buttock is simply a line. It's one of several we use as guides, when shaping stuff the way we want (usually need). For example, you can have a desirable buttock in profile, but because it "tucks in" too quickly on a double ender, if heeled over a bit, it'll make for a hard turning beast on some hull forms. On the other hand if the buttock looks good in plan, but has too much rise in profile, you may negate the possibility of getting up on plane. It's a line we cue off of, offering hints at other places to look.
     
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