Transom Pods - Flotation and Extended Planing Surface

Discussion in 'Boat Design' started by tpenfield, Aug 16, 2022.

  1. Barry
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    Barry Senior Member

    The floatation pods will provide some lift (buoyant) depending on the amount that they are immersed when stationary. Obviously.


    This is where your calculations of available lift due to the extended pods falls down. A look at the pressure distribution on a hull bottom of a planing hull, shows that the pressure peaks at the stagnation point, (within the front 1/3 of a
    hull) and tapers back to almost atmospheric at the transom edge. (Certainly there is a bit of buoyant force here)

    So adding a pure extension will not provide much additional planing lift. ( although the addition of the pods will allow moving the trim tabs back, which will increase the moment arm from the center of lift)

    I have seen trim tabs mounted in a couple of ways. One where the bottom of the tab is coplanar to the bottom of the boat. By coplanar I mean the tabs are parallel to the hull bottom AND the leading edge of the tab is tight against the transom. When you look at the hulls pressure distribution, the tabs offers a slight increase in wetted surface but operate in an area of very low pressure.

    The second means of mounting a tab is to have the leading edge of the tab upwards of about an inch above the height of the lowest edge of the transom so that it is ventilated when planing.
    As you bring the angle down on the tab, a new stagnation point will be created. This stagnation line (the line of highest pressure) will be a "new" planing surface and provide the highest amount of lift.


    Width of the tab, or span, ie side to side.

    Keeping in mind that the tab with the second mounting configuration provides a new surface, it is optimum to increase the width as it provides the new stagnation point. Integrating the pressure curve from the front of the tab to the back of the boat provides say 200 pounds if lift ( numbers are not accurate but merely used for comparison) If you only instead add a few inches of chord (front to back) you are changing the shape of the pressure curve but do not attain the same amount of lift per square inch of wetted surface. ( for illustration, say 150 pounds) Of course if you increase the front to back measurement, the center of lift of the tab does move rearward

    Unfortunately with a high degree of deadrise, some of the lifting force is directed 21 1/2 degrees off vertical.

    If it were my boat and I could live with the static position water line, I would increase the width/span to what will fit. Attach the tabs to a position which would allow ventilation of the transom ahead of the tab, ie move the mounting hinge further up the transom.
    Looking at the back of the starboard tab, on the upper longitudinal edge, I would have a curve added in, say a couple of inches to improve the lift as the water is shedding out due to its 21 1/2 degree orientation to vertical.

    The attached graph shows the standard pressure distribution on a mono hull, as well as a stepped hull.

    The black line illustrates a very low pressure at the trailing edge of the hull.
    The red line, shows a ventilated stepped hull. With some imagination, this would illustrate my comments above. You will see the additional stagnation point and another "area under the curve" lift area.

    With the area ahead of the trim tab mounting hinge ventilated, you would get the benefit of the black line lift as well as the addition of a secondary stagnation point
    to the right of 0.0.
     

    Attached Files:

    Last edited: Aug 21, 2022
  2. tpenfield
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    tpenfield Senior Member

    I think steps at both the pod and the tabs would probably be good.

    I also wonder what the 'pressure' x 'wetted hull area' would look like? I'd assume that the pressure across the wetted area of the hull (i.e. pressure x area) and the resulting vertical component of the forces has to equal the weight of the boat and be centered at the C.G. of the boat.
     
  3. Barry
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    Barry Senior Member

    One could assume this as it appears to be simply opposing forces and in order to not have rotation, the center of lift would be the at the same position as the center of gravity.

    In a stationary displacement hull this is the case.

    In a planing hull at speed, this is not the case. This comment will probably cause some raised eyebrows so let me explain.

    To make things easier, lets consider a two dimensional object (just to get away from a Z axis component.)

    Assume 6 forces acting on the object all acting on the object at different directions and force magnitudes.

    You can resolve the forces by turning the acting forces, direction and force, into the Y, vertical component and the X horizontal component. Obviously, with the vertical component, you can find a single force acting on the object
    that would negate movement of the object either vertically up or vertically down. Applying the same process to the horizontal component would yield the same single opposing force. BUT, there is no reason that these forces act
    at exactly same point. Therefore they will permit the object to rotate. In order then to keep the object INSTANTANOUSLY balanced, ie no angular or directional acceleration, a couple must exist (which is different than a moment)

    So what does this mean for a boat. The net vertical forces will be more or less than the weight of the boat and the horizontal forces will be more or less than the thrust of the drive.

    The boat does not rotate, keeping a set speed horizontally in the water, instantaneously. So the couple that stabilizes the boat is produced between the vertical component forces down, resolved to one force, with the vertical component
    of the lift resolved to one force up, and the distance between creates the couple.

    Hence, for a given horizonal speed of 30 , the boat does not rotate and the speed is constant.
    (horizontal speed is a vector, ie say horizontal being North and the magnitude of the speed say 30 MPH, Kph, M/s

    Assuming the boat is travelling at a bow up attitude of 6 degrees and the prop is trimmed to 8 degrees up from horizontal. The boat is moving right to left. Still wind

    1) wetted surface friction has a 6 degree down vertical component and 84 right horizontal component
    2) the thrust has a 8 degree down component and 82 degrees to the left
    3) The bow wind resistance is to the right and horizontal but there will be an upward component depending on the shape of the hull at the dry hull surface
    4) The transom will create a slight drag to the right horizontal
    5) The boat will have a vertical component up from the center of lift which will produce an upward and to the right force
    6) the boat will have a vertical down component from the center of gravity
    7) the superstructure will have a possibility of creating multi directional forces.
    8) almost forgot the forces due to buoyancy and trim tabs, adding other parameters

    Obviously some are small and in normal practice ignored. ( unless you are designing Miss Budweiser)

    With all this being said then, can you change the characteristics of a planing hull by moving the CG. Certainly. Move the CG rearward, and the center of lift will move.

    But the couple has to exist. The weight of the boat and the hull pressure are normally the forces with the highest magnitude. So moving the CG is significant.

    What if??????????..........
    The center of lift and center of gravity get close together then. Porpoising will more than likely occur and the easiest way to fix this is with a change in the CG. Increase the couple.

    An aside
    Several month ago there was a thread and someone commented about the fact that the center of lift has to be the same as the center of gravity for a planing hull, accommodating only CL and CG.
    Another learned contributor posted a picture of a rather high speed hydroplane, operating with a very limited amount of the hull in the water. The CG would have been in the area that was way forward of the center of lift, but yet the boat was capable of running at this configuration quite adequately
    Supported significantly by the air and the engine thrust to keep this boat relatively stable.

    Re the 8) items above, I may have some of the directions incorrect but the concept is the point.
    Another element is that these additional forces depending upon where they act on the hull will produce a moment in the hull.
     
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  4. DCockey
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    DCockey Senior Member

    For a boat which is symmetric side to side moving at a constant speed (all accelerations = 0) three scalar conditions must be satisfied. Note that the boat is assumed to be symmetric side to side, and moving in a direction parallel to the center plane of the boat. The general case is similar but with six rather than three scalar conditions:

    a) The sum of all vertical components of forces acting on the boat, including the weight of the boat, must equal zero. This is independent of the horizontal components.

    b) The sum of all horizontal components of forces acting on the boat parallel to the direction of travel of the boat must equal zero. This is independent of the vertical components.

    c) The sum of the moments due to forces acting on the boat (including the weight of the boat, drag of the boat through water, propulsive force, pressure of the water on the hull surface, aerodynamic drag, etc) must equal zero. This condition holds independent of the location chosen to evaluate the moments about. This is how vertical and horizontal components of forces acting on the boat are coupled. ​
     
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  5. tpenfield
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    tpenfield Senior Member

    Thank you for the comments . . .

    I like to think in pictures, so here are a couple of illustrations with the forces at work (I believe). . . 'X' and 'Y' axis components of the forces shown in black.

    F=MA . . . So yes we can assume that when the boat is not accelerating in any direction (i.e. at rest or running at a constant speed) the forces net to zero.

    Planing-forces-1200.png

    The boat is certainly at 6˚ 'up' angle while coming onto plane (probably more like 8-10˚ 'up'), and at 30 mph is probably around the 3˚ angle shown in the second illustration.

    FWIW - the boat likes negative trim on the outdrives (full trim 'in') and tends to slow down with even a slight upward trim from the full 'in' position (which is probably -2˚ from parallel to the keel). The illustrations don't quite show the full trim in position of the outdrives.

    Typical cruising speed is around 30 mph, and top speed is 45-47 mph. The boat 'falls off plane' at about 23 mph.

    Have I missed any of the forces? o_O
     
  6. tpenfield
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    tpenfield Senior Member

    As far as the pressure curves, I'm thinking my better approach would be to have the steps in the pod and tab as shown below.

    Screen Shot 2022-08-22 at 11.14.47 AM.png
    .
    I should get the benefit of some additional pressure at each 'step' vs. a continuous surface of hull-pod-tab.
     
  7. tpenfield
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    tpenfield Senior Member

    I found this website/page on a similar modification. The main difference was the modification included an extension of the swim platform.

    https://www.burnnandmarine.co.nz/projects/boarding-platform-hull-extension/

    boarding-platform-after.jpg
    .
    The transom extensions (pods) seem to be a bit large vs. what I am planning, mostly because the boat is single engine. They even have the trim tabs and the under water lights. :D I did notice the New England Patriots look-alike as the boat name. :cool:
     
  8. kapnD
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    kapnD Senior Member

    Have you tried something like this?
    They’re specifically designed to do what you want, and are cheap and easy to install, and you don’t have to build anything!
    Looking at your illustration of the vessel at 6*, and penciling in these tabs, it’s obvious that it will be offering lift in the exact place where it will do the most good.

    59CDA361-E034-4DDB-AECA-ACE664005A90.png
     
  9. tpenfield
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    tpenfield Senior Member

    I see you haven't given up on the 'whale tail' idea. :D

    EDIT: I have not considered whale tails, and probably would not as it only addresses one of my objectives.

    I found this guy who got one and it lasted 3 days before it broke.
    .
    WhaleTail1.png
    .
    This was on a 260 ish HP Alpha One. My boat has twin 350 HP Bravo 3's. More than the HP rating of the device and it does not say it fits or is recommended for dual propeller outdrives.
     
    Last edited: Aug 24, 2022
  10. tpenfield
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    tpenfield Senior Member

    I did a little work with my Sketchup modeling software to 'see' what the pods will look like.

    T-Pod102.png
    . T-Pod103.png
    .
    T-Pod101.png
    .
    T-Pod100.png
    .
    Lots of various angles at play. My plan is to rough the pods out of urethane foam sheets and add some internal structure, keeping the top (or bottom) open. Then fill the pods with the expanding foam. I'll probably use vacuum bagging for the glassing of the pods.
     
  11. tpenfield
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    tpenfield Senior Member

    Another point of interest is that using the Pressure Graph the @Barry posted vs. the wetted surface area is that the area of most pressure is fairly small as compared to the area of least pressure is the largest. Red area being the highest pressure and the aqua area being the lowest pressure.
    .
    running-surface-pods-2B-2.png
    .
    I suppose we could calculate the pressure x area to get the center of force from planing. The illustration also assumes the pods are stepped and raked downward slightly . . .
     
  12. Barry
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    Barry Senior Member

    It is a little more complicated than that as the diagram that I posted was a pressure distribution along the keel. The is also a transverse pressure distribution with the high point close to the keel of the boat and decreasing toward atmospheric
    towards the chines.
     
  13. tpenfield
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    tpenfield Senior Member

    I did find some images of pressure distribution on traditional V hulls and stepped V hulls.

    Some of them shows the pressure a bit more pronounced along the keel as you mentioned.

    Screen Shot 2022-08-27 at 1.42.43 PM.png
    .
    .
    Screen Shot 2022-08-27 at 1.50.08 PM.png
    .
    40430_2019_1866_Fig8_HTML.png
    .
    Screen Shot 2022-08-27 at 2.00.02 PM.png
    .
    Anyway . . . interesting stuff. Probably beyond what I need to be overly concerned about here with the addition of transom pods.
     
  14. wet feet
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    wet feet Senior Member

    I would say that you do need to be a bit concerned because the CFD plots tend to show that the pressure at the stern isn't exactly huge.Which may mean that to generate a significant amount of lift,the pods would need to be quite large and consequently influence static trim quite a lot.
     

  15. tpenfield
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    tpenfield Senior Member

    Yes, I think in order to be reasonably effective, the pods need to be stepped and have a slight downward rake to generate more pressure/lift. Also, I think during the transition to planing, when the boat is at about 8˚ of bow lift, the pods will have their desired effect. Time will tell . . .

    My boat is 'scheduled to come out of the water on/about Sept 27 (2022), so I'll be able to start measuring then and get an idea of the sizing. I'll need to make sure the outdrives have ample clearance at all angles.
     
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