Displacement vs Semidisplacement vs Planing

Discussion in 'Boat Design' started by Fernando, Nov 1, 2001.

  1. Fernando
    Joined: Nov 2001
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10

    Fernando New Member

    I am with a lot of you doubt as the difference among the following Hull:
    - Displacement
    - Semidisplacement
    - Planing
    Which them difference really?
  2. Guest

    Guest Guest

    Displacement hulls are designed to travel in the water at a typical hull speed of 1.34 *(square-root LWL) in knots. This formula applies to the distance between bow and stern wave crests. At faster speeds, wave making resistance increases exponentially because the vessel is trying to climb on top of the bow wave - meanwhile the stern is being sucked down by the dynamic forces from the "hole" created in the water as the vessel moves forward. Displacement hulls tend to have pointed bows and sterns because this form poses the least wave making resistance at "displacement" speeds. It takes a relatively small amount of power to push a displacement hull at its "hull speed."

    Semi-displacement hulls tend to have wide, flat aft sections - like a New England lobster boat. These hulls are designed to partially climb on top of the bow wave and separate the transom from the stern wave. Semi-displacement speeds are usually in the area of 1.5 to 2.5 *(square-root LWL) in knots. It takes a *lot* of power to drive a hull in the semi-displacement speed range. The flat wide stern sections help to provide additional lift in the stern to partially overcome this problem.

    Planing hulls are designed with straight sections aft. A typical deep-V bottom hull has the same angle to the 'V' (the same "deadrise" angle) from midship to transom. They are designed to climb completely out of the water at high speed and "hydroplane" on top of the water. At planing speeds, water is breaking cleanly from the transom and the hull is riding on its straight aft sections. The greatest resistance at planing speeds is frictional resistance. It takes more power to climb out of the water over the bow wave than it does to maintain planing speed once this is achieved. At very high planing speeds (>25kts) any change in deadrise angle in the aft sections of the hull can adversely affect performance. Hulls with a "variable deadrise" angle in the aft sections (where the angle of the 'V' decreases and flattens toward the transom) are a further modification of the semi-displacement hull form. They are more easily driven at speeds < 25kts, but at higher speeds tend to push the bow down due to the higher dynamic lift in the aft sections. Variable deadrise hulls can actually become unsafe at very high speeds >30kts because of this tendency - it is possible for the bow to dig in and cause the boat to broach at high speed.

    The numbers I am referencing are not absolutes, just general approximations.
    Tom Bloomer
    Hartly, DE

    <r.b.b Tom Bloomer>
  3. Guest

    Guest Guest

    Ok so my question is: comparing a semi-disp to a disp hull at or below hull speed how do they fare against one another? The wetted surface area of both should be comparable so then is the wave forming characteristic the chief contributor to efficientcy?

    My semi-disp boat narrows at the stern, she seems pretty darn efficient doing 10 kts without much problem. 40' 100hp continental diesel.

    M/V Brenda Jean (converted WWII Higgins boat)

    <r.b.b. Beepme>
  4. Guest

    Guest Guest

    At those speeds the most important factor is drag. In submerged objects the drag coefficient is most strongly affected by largest submerged cross-section relative to the length of the waterline - this is called the Prismatic Coefficient and abbreviated as CP. It works the same way in air, just requires a lot more speed because air is not as dense as water. When I used to reload my own ammunition for target shooting, I would consult the "ballistic coefficient" of various bullets. You can have a long-skinny bullet with the same surface area as a short-fat one, but the long-skinny one is going to have a lot less drag and therefore a lower ballistic
    Tom Bloomer
    Hartly, DE

    <r.b.b Tom Bloomer>
  5. Guest

    Guest Guest

    Most semi-displacement hulls will have an immersed transom, which offers more 'lift' to the stern above displacement speeds, but also creates turbulence and drag while the vessel is operating in displacement mode. This turbulence occurs until the vessel reaches a speed where the water flow
    breaks cleanly from the bottom surface.

    True displacement hulls rarely have an immersed transom.

    <r.b.b. Stephen Pawlowskis>
  6. Guest

    Guest Guest

    Immersed vs. non-immersed, is that similar as to what is sometimes referred to as a "cruiser stern"? I remember reading something about the comparison of flat vs rounded (crusier) sterns. I'll have to see if I can dig the article up.
    I have observed though that the turbulent wake seems to form a few feet aft of the stern at fairly low speeds (not that 10kts is breakneck or anything)

    Thanks for the input, I find stuff like this quite interesting


    <r.b.b. Beepme>
  7. Guest

    Guest Guest

    when the hull passes through the water it has to part the water and then bring it back together again. in parting the water some is pushed aside and some is pushed down. as speed increases it takes more effort to push it down than to push it aside. if the boat is going to power its way onto the surface and skip over it like a stone the back of the boat is wide and flat. if its going to use low power and stay in the water the back is upswept and often pointed to close the water behind smoothly to avoid drag. for low power boats that stay in the water long and narrow takes least effort but has lower cargo capacity. lighty loaded shallow draft boats like some canoes and punts don't disturb the water enough to conform to the usual rules. its not much of an exaggeration to say they barely break through the surface tension of the water.

    <r.b.b William R. Watt>
  8. Guest

    Guest Guest

    I haven't run across the term "cruiser stern" yet, but I'll try to explain an immersed transom. A typical canoe does not have an immersed transom, the aft sections gradually taper to a "point". The underwater volume of the hull is gradually reduced to nothing at the stern.

    Some boats that do have immersed transoms: Classic mahogany runabouts, just about any deep-vee hull, a WWII PT boat. The transom is squared off and extends below the waterline. The underwater volume of the hull is carried all the way aft and ends abruptly at the transom.

    I have read that if you had an object in the shape of a solid cone or pyramid, that it would create less drag if it were towed flat side (bottom) first through the water, than if you towed it pointy end first. The turbulence created behind the flat side would create more resistance than pushing it through the water face first, with the "point" acting as a fairing behind it. Picture the classic teardrop shape with a "flattened" nose.

    <r.b.b Stephen Pawlowskis>
  9. Guest

    Guest Guest

    A displacement hull is incapable in practical terms of planing and thus restricted to a maximum speed determined by its waterline length (average 30ft yacht will not get much past 6 or 7 knots. A planing hull is capable of producing stable lift from its passage through the water sufficient to rise out of the water and thus reduce it's wetted surface and so diminish its drag .. and can go mucho speedyquick :) I assume a semi-displacement hull planes ....... errrm..........."A bit" ? ........<giggle>

    <r.b.b. Victorious>
  10. Guest

    Guest Guest

    A small ( 11 feet, 90 Lbs. ) pointy skiff I use for fresh water fishing rows much better backwards with heavier folks in the bow which is usually the case (I'm not a heavyweight like most of my fishin' buddies). The transom is out of the water but there is a flat surface created where the bottom near the transom contacts the water and a nice "exit" off the pointy bow which while rowing backwards is now in the effectively the "rear" of the vessel. The vessel was intentionally set up for a ?guide? to row from the rear seat, but I can not claim to have predicted that it would row better backwards (with a pulling stroke) from there.

    I also noticed similar effects while testing plugs for a small paddling boat I am working on. Never having paddled before, I assumed that leaning forward, putting the pointy entry deeper into the water would make this double ended flat bottom boat more efficient, however this was not the case. I found that leaning backwards, thus presenting a flat surface (like the wide part or your "cone") forward, and a nice pointy exit rear made the boat much more efficient. To take things further, I found that if I shaped the garboard properly and presented it correctly in the area behind the centerline of the vessel, the wake behind and under the garboards seemed to "push" the boat along forward. Paddling harder eventually gets you to a point of diminishing return when you consider speed returned to effort applied.

    The first model of the WarCanoe was thin and flat. It was very fast but not very stable, I got wet within 30 seconds. After drying off I went back to the drawing board and came back with a slightly wider version and then added some rocker to bring the load (me) lower into the water. Version 3 is the final version and it is stable but does not have the speed of version 1. Both of these boats are the same length, one deeper and wider but presumably having the same amount of water displacement, but again version 3 is slower. The tradeoff is not that bad.

    My point is that all of the small human powered boats I have are more restricted because of what is happening behind the centerline as opposed to in front of the centerline when it comes to ultimate hull speed. I have a couple of other good examples including a rowing Brockway and a Payson Skimmer, but the season draws to a close and I got some glue to splatter onto a 14 foot Gardner skiff I am finishing up now, Later, Scotty The Backyard Renegade.

    <r.b.b. Backyard Renegade>

  11. Schoonertack
    Joined: Nov 2002
    Posts: 26
    Likes: 0, Points: 0, Legacy Rep: 10

    Schoonertack Junior Member


    I had the experience of rowing a hard chine displacement Sabot hull. beside a very round double ended pram. The pram was very tippy and very slow dragging both ends, the Sabot would come clear and stop dragging her stern with very little effort, she was then moving relatively fast,compared to the round pram. I would not say she was planing , but rather moving cleanly with a flat wake. I consider this to be a camber effect,like to a flat clean run which I have seen all sorts of refferences to but not quantified other than prizmatic and waterline length. I have sailed Star class sailboats and watched Thunderbirds sail many times . Again I am not talking chies here but deliberatelly flatened panels that obvioulsly flow water well
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.