Displacement vs Planning

Discussion in 'Sailboats' started by cmarrero, Aug 20, 2015.

  1. SuperPiper
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    SuperPiper Men With Little Boats . .


    I'm still curious about this. Can someone initiate the discussion please?
     
  2. SuperPiper
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    SuperPiper Men With Little Boats . .

    Are there any examples of spray rails adding to the lifting area of a semi-displacement hull?

    The rails would be above DWL at low speeds. Then as speed increased and the boat started to crouch, the rails would provide additional surface area for planning? They would be low-drag at displacement speeds, but would add to the waterplane area at semi-displacement speeds.

    Yes? No?
     

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  3. sharpii2
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    sharpii2 Senior Member

    They would almost certainly add to the lifting area. But they'd add more friction drag too. One may make the other not worth its keep. But it would be a good thing to experiment with.

    But semi-displacement boats don't squat that much, because they are not designed to climb over their bow waves, like full planing boats are. Planing boats tend to have relatively full bows and/or rocker up front and straight runs in the stern. Semi-displacement boats have similar sterns but sharper bows and straighter runs up front.
     
    Last edited: Sep 25, 2015
  4. PAR
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    PAR Yacht Designer/Builder

    Those aren't lifting strakes, but simply splash rails to prevent water from climbing up the aft sections of the boat at speed. They don't have any appreciable dynamic lift associated with them and though they get wet and are experiencing some upward pressure, their removal wouldn't change a thing about the boat's performance one bit. Their primary role is to knock down splash and to offer a rub rail at a point where the hull is vulnerable, because of tumblehome. Modern hulls don't display this trait, so these rails aren't used.

    If they were mounted at the LWL or lower, then yes, they could add to the dynamic lift element and the Bartender uses them effectively, but this approach is a pretty rare thing and mechanically disadvantaged, simply because of how they're oriented (cantilevered away from the hull).

    Semi displacement boats do squat, often quite a bit, if not designed well, but only as they climb past displacement speeds and the bow starts to rise up, on the back of the bow wave. Most use a hook or tabs to control this aspect, which brings the bow down, but also dramatically limits speed potential as well. A lot of designs called semi displacement are really just underpowered full plane boats. These are the ones that'll use tabs, but a well designed semi displacement will use hull shapes (a hook, entry half angles, entry bluffness, rocker, etc.) to control running trim and these seems to do a much better job, again with reduced speed potential.
     
  5. SuperPiper
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    SuperPiper Men With Little Boats . .

    Here is a photo from BartenderBoats.com. It is a double-ender that is apparently capable of planing. It's an unusual beast, for sure.

    So Par, is this boat planing on these strakes? Are they called strakes?
     

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    Last edited: Sep 27, 2015
  6. PAR
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    PAR Yacht Designer/Builder

    Those look like the first generation of the strakes. Many use bigger ones. They do help with planing, but they sure do act as stabilizers too.
     
  7. SuperPiper
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    SuperPiper Men With Little Boats . .

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  8. sharpii2
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    sharpii2 Senior Member

    No. It's actually planing on the straight, level, aft run of its bottom, which is well disguised by its pointed stern. If you could see a profile drawing of its lines, you'd see the aft chine hooks downward as it joins the keel at the center line.

    Looking at the aft end of a section drawing you'd also see the downward hook. You'd also see the the bottom lines of the aft stations lining up with one another, just like they do on most planing powerboats.

    The water rushes by these lined up sections, then breaks from the hull along the sides of the pointed stern, rather than at the transom of a more typical planing boat.
     
  9. Mr Efficiency
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    Sounds right to me, that stern coming to a point is more for appearance than practicality, imo. Slow down to the speed where a big breaking wave caught up on a bar crossing, and it would be in dire trouble, the only way boats of this size can safely cross bars is by having the speed to hold the back of breaking waves.
     
  10. PAR
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    The chine of the Bartender doesn't "hook", though it does look like it in profile. This is a function of the descending chine line, having to swing in, to meet the stern post, which has to follow the deadrise in the after sections of the hull, which makes this line look hooked. Think of the double ended stern as a really well curved transom, where the water shears from the edge of the chine. It's essentially a warped bottom, with a pointy stern.

    There's two Bartender hulls, the origional which is a bit squishy at low speeds, as you'd expect from a full plane hull at these speeds and the new displacement version done by forum member Tad Roberts. It's basically an all new hull that has the Bartender styling clues, but is intended for displacement cruising. Parked along side another original hulled Bartender, you'd see the differences.
     
  11. SuperPiper
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    SuperPiper Men With Little Boats . .

    Here is an International Canoe hull designed by Steve Clark. It too has the pointy transom. I'm sure it planes.

    What's the story with the pointy butt? Does it plane easier or at lower speeds? Does the boat need to heel a bit for this to work? Would a straight transom be better? Are there conditions where a V-transom is superior?
     

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  12. gggGuest
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    gggGuest ...

    A canoe has a point at each end. That's the story.
     
  13. PAR
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    I'm not sure what Clark's idea is, but there's more surface area to the V butt, so more drag and more edge for the stern wave to shear off of. It would make an interesting looking wake, but compared to a straight transom of the same width, slightly slower top speeds, I'd suspect.

    You can run one of the few free resistance packages on a planing hull, utilizing different transom configurations and get a good idea of what the shapes will do.
     
  14. sharpii2
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    sharpii2 Senior Member

    There are two advantages I can see for a pointed stern on a planing powerboat:

    1.) there should be less of tendency to wallow at displacement speeds, and
    2.) the pointed stern would tend to part waves coming from behind.

    The second reason is accentuated by the very design compromises needed to get the pointed stern in the first place.

    First, there is less buoyancy in the stern, which means the heavy engine needs to be moved forward. Having to do this just about kills the idea of having an out drive. This boat will need an inboard engine, complete with with an external prop shaft and a rudder.

    This in turn makes for a lot of drag aft, when hit by a following sea, while at displacement speeds. This gives the boat less of a chance to broach, even without the pointed stern to part the waves.

    I suspect there is a top performance price to be paid for this hull configuration, but in certain circumstances it might be worth it.
     

  15. DCockey
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    DCockey Senior Member

    I believe the International Canoe rules require pointed sterns.
     
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