Discussion in 'Boat Design' started by We're Here, Jul 3, 2010.

  1. We're Here
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    We're Here Junior Member

    I'm having trouble understanding this term and I'm hoping for some help here. Logically, the term would seem to define those boats having a maximum speed potential falling somewhere north of that of a displacement hull and south of that of a planing hull. Certain designs are described as semi-displacement and I wonder if the image that I have of these - bow up in the air and a gawdawful fuel burn - is an accurate one? Is there some advantage to this? Thank you!
  2. Easy Rider
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    Easy Rider Senior Member

    Look in the archives. I'm sure we've discussed every angle of that question.
    Sounds like you're predetermined to not like them. My favorite type. Most boats operating at those speeds are not good semi-displacement hull designs. Good examples of SD hulled boats are lobster boat/yachts w a full "built down" keel and a Krogen Express. Semi-displacement hulls do not have a straight quarter beam buttock line. SD hulled boats are much more efficient than planing types and considerably more inefficient than full displacement. The most graceful boats on the water.

    Easy Rider
  3. We're Here
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    We're Here Junior Member

    I did look at the archives before posting but came away with no clear answer (that I understood) to my question. I am not a marine designer or architect so some of the terminologies used here are over my head - perhaps there are other members in this same (sorry) boat?

    Let me take another swing at this: The lifting areas of a SD and a planing hull apparently differ. Comparing the two, the width of the lifting surface of the SD narrows somewhat as it moves aft as opposed to a fixed width (as it moves aft) of the planing hull, right? Is there any rocker in the lift area of the SD design?

    Like you, Easy Rider, I think the prettiest power boats are the Maine lobster boat designs though I much favor the (downeast) skeg bottom to the (mid-coast and southern Maine) built-down (design). Thank you.
  4. Ad Hoc
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    Ad Hoc Naval Architect

    I’m not surprised you’re having a problem defining the “semi-displacement” category.

    There is no true definition as such, which is the reason why many find it hard to accept. Pigeon holes for displacement and planning are easy, and can be defined by science/hydrodynamics as such. So it ‘seems’ logical to be able to define something that is half-way. Surely it is simply that half of the displacement is supported dynamically??? theory yes, but….

    The trouble is there are many factors that affects the performance of these vessels, and whilst one may look at a vessel and how it behaves in the water, one could say it is a semi-displacement hull form, defining it in terms of its design parameters, is not so easy. Since the parameters of a boat that “looks” semi-displacement, under power, may not be true for another semi-displacement, yet exhibit similar behaviour. It is a contradiction.

    One of my very good friends Arthur Mursell, produces some of the best semi-displacements one can find anywhere, the Nelson range, yet we both agree, it is nay impossible to “define” exactly what is or is not semi-displacement, in terms of mathematical/scientific parameters. Simply because they are not strictly constant and thus, vary from boat to boat.

    Yet look at a boat on the water under way, one can tell pretty much instantly if a boat is semi-displacement or not, which is counter intuitive to the science.

    A poorly designed semi-D can exhibit high bow trim and poor fuel consumption yes, but one that is well designed does not, and can perform very well indeed.

    My advice is not to get too hung up in obtaining an actual definition, which can be inputted into a computer program or analysed in a similar manner. These boats operate in the “transitional” zone, and as such, a minor change in one parameter can have a major effect on performance, but it is different for each boat in the half-in half-out definition!

    I could provide some basic “rules”…but I can guarantee someone will pop up and say, hey, my boat is a semi-D but is not defined by such rules. Which is the whole point!
  5. messabout
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    messabout Senior Member

    Ad Hoc; can we make a non binding generality that SD boats usually have a rather large L/B ratio? Something on the order of 4.5 and above maybe. Of course long and skinny does not automaticly make the boat an SD and long flat runs do not either but those seem to be some of the observable characteristics.

    Tom, Eric, Daquiri, Leo, et al, what say you?
  6. Easy Rider
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    Easy Rider Senior Member

    Ad Hoc,
    Your comments are very broad ..exactly how this topic should be addressed. I'd like to add that I think the difference is largely in the stern. If one was to modify one to become the other one would usually only need to modify the stern. If one were to establish numbers of definition the QBB line angle would probably be the best approach however it would still be approximate.
    We're Here,
    QBBL = quarter beam buttock line and it is a line along the outer hull surface midway between the keel and the chine ..the chine being where sides meet bottom. The QBBL on a planing hull will be straight (when viewed from the side (or very nearly so)). A full displacement hull will have the steepest angled QBBL usually terminated at or near the water line at the transom or it may be a variation of a double ender. The transitional zone is quite wide but pleasure boat designers avoid the SD types for many good reasons. An over powered FD boat hits a wall of resistance and barely will it go faster. The planing hull just goes faster with more power (generally speaking). The over powered semi-D boat frequently becomes dangerous.
    Ad Hoc .. I welcome comments ..especially critical.

    Easy Rider
  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    how about a displacement with vertical transome that moves fast

    enough for the transom to clear of water at speed.

    I'm reminded of the "long ghost tail" effect on a automobile's aerodynamics.

    Sharply truncating the rear of an aerodynamic car is supposed to reduce drag by tricking the air into thinking the air being dragged along behind the car is a long tail.
  8. tom28571
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    tom28571 Senior Member

    Good general comments Easy. A drawing is a much better explanation than words or "I know one when I see it".

    The aft buttock lines of a full planing hull will be straight unless the builder or designer wants to add in some little change for a specific purpose. How's that for a quibble? Sorry, but there are no absolutes here. My lightweight hulls have absolutely straight, parallel and level buttocks with the waterline aft. This works for my light boats but would not work well with heavy ones (I call them overweight) because it would cause the stern to drag and create a big wave that would inhibit planing.

    The aft buttocks of a full displacement hull will be curved up (rockered) to some degree so that the hull exits the water at or near the waterline. Also a quibble but, what are you gonna do? Such a hull shape may be heavy or light but still be a displacement one.

    A semi displacement hull will fit in between these shapes. It will provide enough lift to get the speed above "hull speed" but will not drag its stern so much as to prevent exceeding "hull speed".

    The above is totally unsatisfactory but pictures are hard to describe. I recommend Ted Brewer's design book as a look at these factors.
  9. gonzo
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    gonzo Senior Member

    Hulls can't be displacement, semi or planing. Those are terms, that even though commonly used, can't be applied. A hull is in displacement or planing mode.
  10. Willallison
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    Willallison Senior Member

    A can of worms if ever there was one!;)
    Perhaps a quick read of Tad's article will help WH...

    All of this might make one wonder why bother with a SD hull at all....? One of the SD's greatest assets is that a good one can carry on at higher than displacement speeds pretty much regardless of the weather conditions. The Nelson that AH referred to being as prime example, which is why they enjoy such universal success as pilto boats and the like. A typical planing hull - especially a heavy one that is unable to stay on the plane at low speed - is a pig of a thing once it is forced to slow down.
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I have my own definitions that others will, no doubt, find inadequate...

    (a) A planing vessel is one in which hydrodynamic forces support more than 1/2 of the weight at the design speed.

    (b) A "pure" displacement vessel has very little weight supported by hydrodynamic forces.

    (c) a "semi-displacement" vessel is between (a) and (b), but a little closer to (a).

  12. apex1

    apex1 Guest

    The most finicky area in boatdesign / boat building. Semi displacement.

    This is one:




    You see the hull appears much (or completely) like a full displacement hull.

    The "typical" characteristics of a planing hull are not seen here.

    Sorry for the poor quality, I did not upload better pictures.

    But very clear is the "twist" from plain displ. to "semi" is closer to the former than the latter!

    This boat is a 28 ft. and does 14 kn with a 75hp Diesel.

    And Leo

    (you posted while I typed),

    NO, I would say the opposite, closer to displacement. But there we go....... is the most finicky area we encounter.

  13. jehardiman
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    jehardiman Senior Member

    Yes, your image is all wrong. FWIW, as stated before, all vessels are supported by hydrostatics and hydrodynamics...just how much of each depends on a lot of conditions, length, beam, speed, hull shape, environment,etc. Generally, where "semi-displacemet" boats excell is the condition of high sustaned speed in water too rough for a true "planing" hull. In this case the need for large deadrise fwd and flatish floors aft drives the hull to a high L/B ratio. Do some looking into the German Schnellboot or "s boot" (sometimes refered to by anglo sources as an E(nemy)-boat).
  14. Ad Hoc
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    Ad Hoc Naval Architect

    To really have some idea of what could be defined as SD or otherwise, you really need to understand what cuaes squat and trim. Since squat/trim is related directly to the SD hull form.

    So, lets look at a body moving through the air-water interface. Anything that moves through the air-water interface, (sea level) will cause a pressure wave on the surface. Whether it is a single point or a boat or your hand ..anything. This is the start of the Kelvin wave pattern (theory) from 1888.

    These waves and their profiles are well known, to a certain degree (they beocme more complex the more Qs one asks). In plan view, (given certain conditions), the waves always follow the exact same pattern. As shown here:

    So, any body moving through a fluid creates pressure waves. As such, the front end, fwd, has a region of higher pressure, than would not exist if not moving and as such seen as a difference from the static water line, or datum line.

    Any 'ship' shape will have as a minimum 2 regions of higher pressure, (detla)P, +ve P, and a 3rd region of negative pressure -ve P around midships. (as in the attached scanned images). In addition to these, there are other pressure disturbances below the surface waterline.

    As the body moves fwd, the fluid is free to respond to these changes in pressure, via gravity and inertia. The fwd region, which has +ve pressure, reacts by the fluid surface rising above the static undisturbed watereline (datum). It begins well fwd of the actual bow, because of the pressure field that is created by the body moving through the fluid.

    (If you imagine, in plan view, dropping a pebble into a pond. You see waves radiating out from the centre. This is basically what is occurring at the bow. Think now the bow is the centre, and the waves which radiate out, are the pressure waves. This pressure system, causes the waves to rise or fall owing to differences in pressure filed created by the moving body, or pebble. Just slightly away from the centre of the pebble being dropped, the water moves, it rises in places...even though the pebble is nowhere near it.)

    However, once these said waves have a profile on the side of the boat the water level is higher in some parts (wave crest) and lower on others (wave trough), along the side of the boat.

    This clearly creates regions of 'excess' or 'inadequate' buoyancy. So what happens, the boat must take up the equilibrium that allows the boat to float and obeying Archimedes principal.

    But, to make matters worse, any body moving through water will drag along a small amount of water with it, this we call boundary layer. The amount and hence thickness is determined by many factors. The main ones being surface area and surface roughness (friction -Reynolds numbers). But as the body moves through the water, the shape will end up dictating that the forward end has less boundary layer (or thickness of water moving with it) than the aft end. Since boat shapes are more thin and "pointy" at the front.

    This boundary layer also affects the profile wave height (amplitude) of the wave, along the boats side. Since the thicker boundary layer at the back, aft end, tends to reduce the wave height that would be there. So, since the wave is now reduced, the buoyancy effect is also reduced. See attached sketches I made up for Mat to explain in pictures…not so clear images, sorry!

    So, depending upon the shape of your hull, the amount of buoyancy available at these ends will determine how the vessel squats/trims with increasing speed. This is addressed by the rocker of the hull form.

    The ventilated transom addresses the pressure differential and thus “loss of buoyancy” issues at the stern. There is no loss of buoyancy, per se, just the fact that the wave profile moves the dynamic-LCB of the boat. So, if the hull has buttock lines/rocker, rising at the stern, there is less available buoyancy to prevent the trim and hence squat owing to the wave profile. The solution is to extend the hull aft with almost same x-sectional area as midships. But how to end it??...the simple way is to “cut the end” thus forming the transom.

    In other words, all boats of all shapes will trim and squat by the stern when moving. The degree, or amount/magnitude to which the trim/squat occurs is influenced by the 'body' and hence its form factors. So a solid square block would behave differently to a long slender plank of wood. They both trim and squat, but the long slender plank of wood "appears" significantly less...but it still trims and squats.

    Which brings us back to the SD hull.

    To say ½ of the weight is supported dynamically…some would say, well what about 45%..or 40%...this is possible, depending on how the lines are drawn. It is all about how the hull with its own waves which are generated with increasing speed affects the wave profile/pressure distribution at the stern, a little or a lot?

    Looking in plan view, on the waterlines, this also gives an indication, but again, no 100% hard fast rule.

    Trust this helps??


    Attached Files:

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I should refine what I mean: with a semi-displacement hull there is actually some significant weight supported by hydrodynamic forces rather than the sinkage force vector being directed downwards and increasing the (apparent) displacement with respect to the static waterline.

    I prefer this notion of apparent weight to using Froude number ranges such as:
    (A) Displacement: Fr < 1/sqrt(2*pi)
    (B) Semi-Displacment: 1/sqrt(2*pi) < Fr < 1/sqrt(pi)
    (C) Planing: Fr > 1/sqrt(pi)

    Of course the two "systems" are roughly related because of the forces and moments on the hull, the boundary layer, and the waves generated by the hull.

    @Ad Hoc: Why not 40% or 45%?
    Because 1/2 sounds like a physics-based constant: 0.4 or 0.45 sounds like you just plucked a number out of thin air :p

    All in all, it's a Lilliputian debate.
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