20m Planing Hull trim issue.

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

  1. Siviconta
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    Siviconta Junior Member

    Hi everyone, i am designing a 20m planing power/speed boat.

    The problem is, there is a 2 meter difference between LCB and LCG of the hull. It causes trim on a still water at rest condition.

    i can easily solve the problem with adding fuel and fresh water tanks. but as you can guess its not a desirable solution to fix trim or stability issues with fuel or fresh water tanks. since the amount of fluid in those tanks is differential all the time.

    Other option is to add ballast tank but since its a power boat ballast tank can not be accepted. We dont want to add weight to our boat cause power to weight ratio is critical for high speed crafts.

    I might try to change the hullform but it doesnt seem like its gonna solve the problem. It might but it will cause many other problems i guess.

    I am open to any kind of ideas please share. :))

    No-trim https://ibb.co/QczjNvB

    Trimmed2 https://ibb.co/mH1Djv6
     

    Attached Files:

    Last edited: Mar 12, 2020
  2. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Siviconta.

    Is she trimming by the stern, such that the LCG is 2 m. aft of the LCB?
    What type of propulsion units are you proposing?
    Is it possible to move the engines further forward, to bring the LCG further forward?

    If it is the other way around, and she is trimming down by the bow, can you move the engines further aft, perhaps using vee-drives or pods?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Hi Siviconta - welcome to the forum.

    Ok..is this trim by the stern or the head?

    And I assume you mean the optimum location of the LCB in its design load case is 2m adrift of where your current weights and centres suggest that the location is for the LCG?

    Ok... but what speed is this vessel designed to achieve?

    Since many power boats have their LCG far aft for very good hydrodynamic reasons and all of them exhibit large trim by the stern at rest.

    Thus without more data, it is not possible to state whether you actually have a problem or not.
     
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  4. TANSL
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    TANSL Senior Member

    Since adding ballast or new tanks is not an option for a boat that must achieve the least possible weight, there are only two solutions, changing the position of some weights or changing the shapes (or both at the same time). Since we do not have more data, no reasons can be given to choose one solution or the other.
     
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  5. fallguy
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    fallguy Senior Member

    Probably fathomable (hehe) that this is also capable of sleeping in or lounging and so the ideal at speed is not ideal at rest.

    The only thing I can think of is a water ballast. The trick, of course, is the operator remembering the thing is full or hole shot is poor and fuel economy and speed. To make it legal; if a water ballast is not, call it a fishing livewell. Tell the buyer if commissioned. Most boat buyers realize what it is for...
     
  6. Siviconta
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    Siviconta Junior Member

    Thank you for your answer.

    She trimming by the bow and the engine compartment is almost at the AP.
    i am planing to use twin propellers.
    Desired displacement is between 23-24 tons. Moving the engines doesnt help that much.
     
  7. Siviconta
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    Siviconta Junior Member

    Thank you for your answer.

    She is trimming to bow. I havent done a general arragement yet so the LCG weight only consist LWT. its about 0.2m to the bow from the midships.

    I am planning to achive 60knots.

    woudnt mind the trim if it was a bit to the aft but trimming bow is not acceptable.
     
  8. Siviconta
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    Siviconta Junior Member

    Thank you for your answer.

    I can change the shape but it will cause my desireble displacement and draft to change.
    Cthanging the position of the weights is an option too but there are no weights on the boat enough to change trim. I am using LWT to calculate CG LWT consist the weight of the hull and machinery only.
     
  9. Siviconta
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    Siviconta Junior Member

    Thank you for your answer.
    I know 20m is a bit long for a speed boat but the main goal is the speed and performence.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Ok, so the LCG is too far fwd.

    Oh, ok. So how have you established the LCG and thus how have you confirmed that the LCG and thus bow trim is correct?
    I assume you have a set of hydrostatics for the vessel?

    That is a high Fn number so you'll need your LCG to be around 15-20% aft of midships.
     
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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Change the design so it doesn't trim by the bow, seems the obvious remedy, If it was an existing, accomplished fact, then you would have a lot less flexibility. As for 60 knots with a 20-odd ton boat, sounds like an ambitious project.
     
    Last edited: Mar 12, 2020
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  12. TANSL
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    TANSL Senior Member

    So it is best that you change the shapes. That doesn't mean you have to change the displacement, you just have to change the frame area curve to change the longitudinal CB position.
     
    Last edited: Mar 12, 2020
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  13. Siviconta
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    Siviconta Junior Member

    I have established the LCG from hull weight and machinery. I ve located the LCG to where the sum of hull area is on. Basic hydrostatic calculation. Trim is correct i am using maxsurf stability module helps with the calculations.

    So the LCG should be around %15 20 aft of midships. It sound logical but i wonder where does this information come from. If there is a source i would like the check for more details.
     
  14. Siviconta
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    Siviconta Junior Member

    There are some examples check out Nortec 80 roadster
     

  15. Barry
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    Barry Senior Member

    Siviconta
    Even though Ad Hock has suggested an LCG 15 - 20% aft of LCB, you challenged his statement and inquired as to where his comment comes from. I wonder why you believe that for a planing hull that the LCG and LCB should coincide
    so that at REST, where there are no performance issues, fuel burn, optimizing drag, handling, the boat should sit with no trim.
    The LCB at rest would equate to the Center of Lift ,COL, when planing would it not? (ignoring for now some thrust, moment, skin friction, possibly air resistance parameters)
    So while I would not want to guess if your hull, 20 meters and 60 knots would porpoise, the issue is that when planing, the COL moves rearward as the boat planes and increases speed.
    When the LCG gets ahead of the COL, the bow will drop forward or at least begin to assume an extremely low trim angle causing performance problems

    So I think that you should be considering to design around the LCG and COL at cruise speed more than coincidental LCB and LCB at rest.

    If you believe that the stern down attitude does not look good. Then merely change the gunwhale lines to accomplish this. The gunwhale lines and the keel lines do not have to be parallel

    The second picture shows a finely tuned fast and efficient hull and it is easy to imagine that at rest the trim angle is high.
    upload_2020-3-12_7-24-41.jpeg
    [​IMG]
     
    Last edited: Mar 12, 2020
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