20m Planing Hull trim issue.

Discussion in 'Boat Design' started by Siviconta, Mar 11, 2020.

  1. Barry
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    Barry Senior Member

    Of course you are correct, thank you. I should have used the term then Center of Flotation, as below then. A question, at rest, would the center of flotation act be vertically below the LCB?

    ". Center of Floatation (LCF):"

    When the ship floats at a particular draft, any trimming moment acting on the ship would act about a particular point on the water plane.
     
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  2. TANSL
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    TANSL Senior Member

    It would be a rare coincidence. There is no need to match and, in fact, it is difficult for them to do so.
    That may unleash a controversy, already ancient and repetitive, in which this time I am going to act only as a spectator. Go ahead then!
     
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  3. DCockey
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    DCockey Senior Member

    LCF by definition is on the surface of the water. LCB is the center of a volume of water which means it is below the surface ofthe water. So the LCF cannot be below the LCB.

    A boat with a fore/aft as well as side to side symmetric hull floating "level" (ie waterline symmetric) will have the LCF directly above the LCB. It is possible for the LCF to be directly above the LCB with non-symmetric hulls but it depends on the shape of the hull and the trim angle.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    DC has given you the quick no nonsense answer....so what he said.

    However, for a bit more clarity, as it does confuse many posters/readers. The CoG, centre of gravity, is always in line with the CoB, centre of buoyancy. This is for the vessel to remain in equilibrium, i.e no rotation.

    If we consider trim or roll, what occurs? Looking at the image in #28 you can see that the CoG is in line with the CoB.

    In this transverse sense it is more correct to state the G or KG (CoG) is in line with the B or KB (CoB). The K = keel.
    So, when the vessel is heeled, not list, the vessel takes up a new waterline, because the KG is no longer in-line with the KB. As shown below:
    upload_2020-3-14_13-2-53.png

    The vessel comes to rest when the KG and KB are again aligned.

    The same is true when we consider this case longitudinally. But this time we use the designation LCG (CoG) and LCB (CoB). L = longitudinal.
    The vessels trim/heels about the point/location you noted, the LCF, Longitudinal Centre of Flotation.

    This is calculated and found from the centroid of the waterplane area (WPA). The centroid or LCF is defined subtly different in different counties, but generally taken as the centre of gravity of a thin plane or in this case, the surface of the water = WPA.

    The LCF does not need to line up with the LCB or LCG. The LCF is only the location of the centre of trim. For conditions of equilibrium LCB and LCG must be coincident.

    All the above refers to a simple static case.
    That being from a condition of equilibrium, the vessel at rest, and then a disturbance that "moves" the vessel to a new state. It is generally considered as a simple "slow static move" and a means in which to describe and calculate the final equilibrium position, or the effects of.

    In terms of transverse, the Roll, the location of M - the metacentre, changes with each new disturbance. And the metacentre is the the centre of curvature of the curve of centres of buoyancy. Thus its location moves with each angle of inclination, shown here as an example. With odd shaped hulls, this becomes more extreme.
    upload_2020-3-14_13-22-28.png

    Many confuse this with the centre of rolling. Rolling is a dynamic behaviour with several variables, notable a time dependent case.

    The roll period of a vessel is given by several factors, but it is proportional to the inverse sqroot of GMT. The key aspect that is important is the G, or CoG.
    All bodies rotate/oscillate about their CoGs, and a vessel is no different in that sense, and shown here, where the centre of roll is through the CoG:

    upload_2020-3-14_13-28-56.png

    With trimming, the same is true. Except that the trim, or correct term pitching is also centred about the CoG and is also proportional to the inverse sqroot of GML

    And thus for every other axis of rotation:
    upload_2020-3-14_13-34-16.png

    So, now going back to your COL, the centre of lift, that is analogous to the LCF. Since the center of lift is the centroid of the running WPA. The WPA being the dynamic supporting area or wetted planing surface - when planing.

    And as in the case of LCG and LCB, being in-line in the static condition, in the dynamic condition the COL become important and hull shapes plays a large part of that. Since if the Vee is very deep and warped and extends far fwd it also becomes very fine like a long finger up fwd when in running trim, the COL tends to move fwd more than if the hull shape is a more "conventional" Vee hull shape. Because as the vessel pitches and if the pitch the COL tends to change with the ever changing amount of wetted area, but also if this creates a slight roll motion, the hull will momentarily roll and thus all the lift/wetted surface area becomes the whole of the opposite side as she lies flat. Instead of running on the Vee, she runs on the flat area of the side. Then suddenly there is a large increase in the wetted area for lift and moving fwd. This large shift of the COL moves fwd beyond the LCG, thus oscillating around the fixed LCG, causing instability.
    Thus the further aft you move the LCG as a rule prevents this to ensure the COL does not oscillate around the LCG when such running conditions exist... in a nut shell.
     
    Last edited: Mar 14, 2020
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  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Anyway, not sure why the OP's boat, which has not left the computer drawing board, is mysteriously trimming bow down, it may be in the grip of the electronic Kraken !
     
  6. TANSL
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    TANSL Senior Member

    This is an unquestionable truth, two points are always in a straight line. BUT, when that line is vertical, balance occurs.
     
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  7. gonzo
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    gonzo Senior Member

    I agree with almost all Ad Hoc posted except that boats roll around the COG. Hull shape and appendages have a large influence on the location it rolls around.
     
  8. DCockey
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    DCockey Senior Member

    The center of lift when planing will be ahead of the centroid of the wetted planing because the pressure distribution is not uniform. The pressure on the wetted planing surface will be higher at the forward end of the area (usually approximated as stagnation pressure), decreases moving aft, and will typically be close to atmospheric pressure at the transom. The center of lift would be at the centroid of the wetted planning area if the pressure was uniform.
     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed....
    I started off by noting it "analogous" for ease of clarity... but did mis-type/write later , well spotted!
    Sometimes when I over generalise - for clarity - the baby does get thrown out with the bath water!

    So for completeness one should state that the COL also varies, owing to the speed of the vessel from aft to fwd.
     
  10. DCockey
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    DCockey Senior Member

    As does the extent of the planing wetted surface.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Only affects the period. Since the shape dictates the location of M. The the GM is the key influence in the period of motion, beit roll GMT or pitching GML for example.

    If the appendages are large in the sense that their weight is a significant percentage of the displacement of the vessel then this changes the CoG, so it has an effect.
    Otherwise appendages only provide damping.

    Indeed... which is why it is often difficult to 'generalise' in order to get the basics of message across, as the details do not always bear under scrutiny! But got to start somewhere...:oops:
     
  12. Yellowjacket
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    Yellowjacket Senior Member

    Exactly. While the lift is higher at the leading edge of the planing surface, The LCG has to be over the planing surface and not ahead of it. What is happening here is the planing surface kicks up from the base line too early. Remember that the LCG position relative to the planing surface determines the trim angle. In this case the CG is actually ahead of the flat planing surface that is aft. Just because the OP drew the hull sitting in the horizontal position doesn't mean that the boat would run on those lines. The reality is that this hull essentially has rocker built into it the way it is laid out now. You wouldn't draw up the boat with rocker in the hull as that would create negative lift (suck down) at the transom when planing. But the hull as drawn doesn't know what the proper planing attitude is, and the result is that it would act like a hull with rocker in it. The faster you go the more negative lift would be created, you won't need ballast with this layout, it will create huge amounts of negative lift aft, but that negative lift is also creating huge amounts of additional drag and the hull wouldn't go very fast at all.
     
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  13. gonzo
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    gonzo Senior Member

    I agree. He should start by straightening the buttocks for high speed. Also, as the boat gets up on a plane, the bow comes out of the water. At the same time the dynamic pressure is higher forward and less aft. The pressure difference tends to make the boat squat, and the increased length of boat out of the water tends the opposite, to bring the bow down. In an ideal design, these would cancel out. In real life, trim tabs and/or trimmable drives are used to compensate for the changing trim.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    This is just an electronic sketch, it is not a real boat, and the talk about the COG being too far forward is meaningless, because there is no explanation of why his COG is so far forward, where he assumes it is. He may have made a simple calculational error. Obviously in his sketch the COG is positioned directly above the COB, it can't be anywhere else, but he has got the idea into his head that the actual COG is much further forward, but has not explained the reasoning that puts it there. As for rocker killing speed, well it depends how much, a little can actually result in a faster boat.
     

  15. gonzo
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    gonzo Senior Member

    Quite true, unless he did the weight calculations. However, it looks like a preliminary sketch only.
     
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