20m Planing Hull trim issue.

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

  1. Jimboat
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    Jimboat Senior Member

    Agree with Barry. Your hull's static LCG is important when boat is at rest. if your hull is going to go 60knots speed, then Dynamic balance (Dynamic CG) will establish trim required - forces generated by all acting Lift, Drag and Thrust forces, including force of hull static weight. Static LCG of hull at speed is only then helpful to the extent that placement can help dynamic balance at each speed through operating velocity range.
     
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  2. DCockey
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    DCockey Senior Member

    What is Dynamic CG? How does it differ from static LCG? Do you mean the mass distribution of the boat changes at speed?
     
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  3. Jimboat
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    Jimboat Senior Member

    Dynamic center of Forces (Dynamic CG)... changes with velocity, trim angle, etc. similar to COL, but must include all moments from all lift, drag and thrust sources.

    More info on static CG and dynamic stablity: Dynamic Stability in Performance Powerboats https://aeromarineresearch.com/dynamic_stability.html

    Also an article on Static CG and Dynamic stability: Let the Forces Be With You (PBR May 2017) https://aeromarineresearch.com/publications/PBR_May2017.html
    excerpt... "Although it's tempting to move weight around in an effort to 'balance' your boat while on the trailer or on a scale, it is not this 'static' location of weight CG that matters most when optimizing the 'dynamic stability' of your performance hull. Here's an explanation of how the static CG really affects your performance boat's 'dynamic stability'..."

    "Dynamic Stability - The combination of all the lift and drag forces while a boat is underway is called the 'dynamic center of forces' or 'dynamic CG'. Lift, drag and thrust forces from planing surfaces, lower-unit drives, propellers etc. all act in different locations and different directions, and all contribute to the 'net' combined forces acting on the hull This represents the delicate dynamic equilibrium of the performance hull. All of these acting forces and the resulting dynamic CG location change throughout the operating velocity range and this is the most important design measure to consider when balancing a performance boat."...
     
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  4. DCockey
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    DCockey Senior Member

    JimBoat, thank you for the reply and references. I assume you are Jim Russell, author of the references you cite. Did you originate the term "dynamic center of gravity" or had you seen it in use previously?
     
  5. Jimboat
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    Jimboat Senior Member

    David - no, didn't invent the term, but use it in my papers consistently. It seems to be mostly used as equivalency to center of forces, but is sometimes easier to relate to CoG terms (eg: static CofG vs CoForces). Always tough when get into more details. thanks for feedback.
     
  6. Barry
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    Barry Senior Member

    The term hit this forum at least as far back as 2013,
     
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  7. Jimboat
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    Jimboat Senior Member

    Sounds good. People have preferences. i know that i've used the term 'Dynamic CofG' since 1980's in my publications. had several discussions with Daniel Savitsky and others re: dynamic stability, porpoising, etc., and used these same terms. So i guess that we're not alone.
     
  8. DCockey
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    DCockey Senior Member

    You are the only person I've seen use the term. I have never seen it during an extensive study of publications on planing boat hydrodynmaics and dynamics. Perhaps it is unique to the performance/racing boat community.

    How is the "dynamic center of gravity" determined? What is included in the determination?
     
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  9. DCockey
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    DCockey Senior Member

    I'd be interested in any references where Savitsky used the specific term "dynamic center of gravity".

    Added: Just searched through seven of Savitsky's publications I have available and found no references to "dynamic center of gravity". There are multiple uses of the word "dynamic" but in connection with lift, pressue, etc.
     
    Last edited: Mar 16, 2020
  10. Jimboat
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    Jimboat Senior Member

    Dynam CofG is location where moment (about static CofG) of all dynamic forces (vertical and horizontal) balance. (also called center of dynamic forces).
     
    Last edited: Mar 16, 2020
  11. gonzo
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    gonzo Senior Member

    Are you considering gravity as a dynamic force?
     
  12. Jimboat
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    Jimboat Senior Member

    No, but gravity is the force associated with the static mass (weight) of the boat, so in that the 'center of gravity' is the average location of the weight of the boat...the 'dynamic CofG' is considered the average location of 'dynamic forces'.

    i'm sorry now for participating in the thread. i'm happy to refer to the balance of dynamic forces as something else, such as 'center of dynamic forces', if it will annoy some of you less.

    ...was just trying to add some help to Siviconta's original question.

    good luck you guys.
     
  13. DCockey
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    DCockey Senior Member

    That would be less confusing. Your concept of determining an equivalent single location for forces other than gravity is interesting though.
     
  14. Jimboat
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    Jimboat Senior Member

    No worries. If anyone wants to talk about any of this stuff further, you can call or PM me. Cheers all (unfollowed).
     
    Last edited: Mar 16, 2020

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.
    It is analogous to that of aircraft and its dynamic stability. The aircraft's CoG does not move (other than slow fuel burn - just like a vessel), only the effects of lift owing to differing AoA and wing area creating the "imbalance" and hence need for control.
     
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