Semi displacement hull

Discussion in 'Powerboats' started by Steph357, Nov 15, 2009.

  1. fcfc
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    fcfc Senior Member

  2. Easy Rider
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    Easy Rider Senior Member

    I once thought a boat achieved hull speed as soon as it's bow started to rise. I once thought a boat With any emersed transom at rest was not a full displacement hull. I once thought a boat was not planing if her transom was not dry. I once thought any boat that was not FD or FP was a SP or SD hull the SP being slower than the SP. Semi being "partly" would seem to me to mean less than 50%. Then I was told the quarter beam buttock line expressed as a number made everything black and white. These four concepts D, SD, SP and P are clearly very elusive and subject to much discussion/argument as long as there are boats. What I would like to learn is what in-specific elements of boat design produce a SD boat that is graceful, efficient, well mannered in rough seas and whose basic dimensions are not extreme in any way. Boats that perform really well in the lower end of the SD range are rare. I think some designs by Hand and Atkin are excellent and perform well at SD speeds. What are the design features that make them shine.

    Easy Rider
  3. TollyWally
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    TollyWally Senior Member

    I've pondered in great detail over some of Atkin's boats. I inherited an old 1st edition book of his from my old man who bought it when he was a kid. After reading it dozens of times I realized some of what my father taught me came from that book.

    I believe Atkin was very good at optimising his boats. He seemed to be always tweaking the underbody to get the prop in the best water possible. He would splay the shaft and stick the prop farther back away from the underbody in less turbulent water. He was very particular about matching motor, gear, and prop. In a way much the same as optimising a racing machine except his "high performance target" was smooth cruising efficiency not top speed.
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    FAST FRED Senior Member

    I have always thought many of the old wooden boat designers had it GREAT!

    The same design could be tweaked (mostly every wooden boat is a one of) so the designer could experiment and widen the buttocks an inch and see what it does.

    Our 90/90 is a Maurice Griffins hull design .
    During WWII he sold his boat and worked for the Navy.

    Lone Gull II was the result of years of doodeling FOR HIMSELF , and remains one of his finer vessels.

    The ride is sweet , the decks stay dry in most blows , and she cruises well.


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  5. pasty63
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    pasty63 Junior Member

    semi disappointment

    Well, I don't know much, but I currently have a boat that has a hull form labeled "semi displacement". My experiences with semi displacement have led me to a keen interest in displacement and planing hulls :)
  6. TollyWally
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    TollyWally Senior Member

    Wisdom from perceptive observation.
  7. breschau
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    breschau Junior Member

    Why is it called semi-planing and not semi-displacement if its a middle ground, compromise solution?
  8. daiquiri
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    daiquiri Engineering and Design

    It's the same stuff, imho, just a matter of personal choice.
    "Semi" means "half" - so if 50% of the weight is sustained by the hydrostatic buoyancy force (which is due to water displacement), the other 50% has to be sustained by some other force (hydrodynamic planing, in this case). Of course, the vice-versa is also true. ;)
    Some folks use the two terms to distinguish the situation when one of the two forces predominate over the other, but in my opinion it is an unnecessary linguistic complication. Even what we call "planing hull" is sustained partly by a hydrostatic force, for example. And, on the other side, a hull moving at "displacement" speeds is always subject to more or less small hydrodynamic forces.
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Semi-Displacement or Semi-Planing are technically incorrect terms if " semi " is strictly meaning half. In practice, any attempt at measurement of the split between hydrostatic and hydrodynamic would be migraine-inducing.
  10. Easy Rider
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    Easy Rider Senior Member

    I see you are right the D says "half". I thought semi meant partly .....thinking maybe 10-30%. So I had my own personal concept that semi-planing was "partly planing" like a GB36 going 11 knots. And I thought semi-displacement was a boat that did 15-18 knots gracefully but would be bow high and lacking control at 35 knots. But now that you've posted I must put all boats not full planing or full disp into an extremely wide range of
    "semi displacement". Oh well.

    Easy Rider
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Makes the term "semi-conscious" a bit doubtful doesn't it, we are presumably 100, 50, or zero % conscious, and nothing in between ? Sounds like rubbish to me, words mean what people more or less collectively understand them to mean, I think the term "semi-planing" would be generally accepted by boaters to allow a bit of percentage leeway either side of half.
  12. gonzo
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    gonzo Senior Member

    The term is used loosely and means different things among different groups. Dinghy sailors call planing as soon as the bow lifts and start getting clear water off the stern. Speed boaters may call that semidisplacement. "Generally accepted"(vox populi) is not a definition. Even if we agree to a bit of percentage leeway either side of half, first someone has to define what half is. Boat operate on a changing boundary between mediums. It is not as if you pass the speed limit while driving.
  13. tom28571
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    tom28571 Senior Member

    All discussion of definition of displacement speed, semi displacement and planing speed ranges is not ever going to produce a definition. It's a transition range which, by definition is not specifically defined at any one point and never will be. As a boat starts to move forward, the vertical force always divides between dynamic lift and buoyancy, near zero lift and nearly all buoyancy at first and dominated by dynamic lift when fully planing. The beginning and end of the transition range as well as the width of the transition speed range vary from one boat to another.

    A semi displacement hull form can be looked at as a full displacement hull that has some of the underbody shape flattened out toward the stern. A full planing hull can also be modified by adding rocker and lifting aft buttock lines toward the water surface. Both wind up with the same shape and both are called semi displacement or semi planing, whichever you desire.

    Whether or not a semi hull of either designation is suitable for you depends largely on whether you opt for a light weight boat with its limitations or a heavy one with its own set of restrictions. If you can be satisfied with the limitations imposed on accommodation by light weight, then a low speed planing hull may be the best for you. If you demand to include the kitchen sink and all that it implies and still want to run at speeds beyond hull speed, then the modified displacement hullform is likely what you need. Power requirements and fuel consumption also figures importantly into the choice.

    By lightweight planing hull, I don't mean actual weight but rather lightweight in the sense that the waterplane is loaded lightly per square unit of lifting area. Such a boat, with proper design, can easily run in what is considered the transition speed range for most boats and achieve power requirement and fuel use numbers equal to the full displacement hull. This boat will require much less power and fuel than the semi displacement hull at the same speed.

    If you demand a heavy boat, it makes no difference which direction you start from, the result will be the same.

    Demands for definition will never result in the best designs. Talent and experience of the designer is more important than the tools of formulas or spreadsheets that the designer may make use of.
  14. Landlubber
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    Landlubber Senior Member understanding of a planing vessel is when the water breaks free of the transom, at that speed the vessel is it possible to be planing without this free surface, i do not think so, it is still in displacement mode (maybe semi displacement as it reaches the required speed.

    ...from my use of so called, semi displacement hulls, every one of them can make the water break free of the transom at a particular speed, maybe it is this phenomenon that describes the semi as it is being referred is a vessel that can run comfortable at either mode.

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Semi-displacement boats go well in semi-(there's that word again ! ) sheltered harbour or river situations, where wave action can't work up to the point where you are chasing the backs of sizeable crests, when the boats become slugs, unfortunately.
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