20m Planing Hull trim issue.

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

  1. fallguy
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    fallguy Senior Member

    Isn't there some certainty to trim troubles or trim mitigation for such a vessel?

    In the Eliminator above, do I see a step?

    I don't profess to be an expert here, but if you want expert advice; you would need to provide a drawing, in my opinion. Otherwise, too much guessing.
     
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  2. gonzo
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    gonzo Senior Member

    Hulls with a deep vee forward and flat aft sections normally float down by the bow. Constant deadrise hulls will usually float level or down by the stern. What are the characteristics of this hull?
     
  3. Siviconta
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    Siviconta Junior Member

    Just uploaded the trimmed and fixed trim hydrostatic data screenshots please check the first message. Thanks.
     
  4. gonzo
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    gonzo Senior Member

    Can you post the hull lines?
     
  5. Siviconta
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    Siviconta Junior Member

    Sure Adsız.png
     
  6. TANSL
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    TANSL Senior Member

  7. Barry
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    Barry Senior Member

    After re-reading my own post I believe that I have made an error and so put this question out to others:
    At rest the LCG and the LCB should coincide is this not correct?

    But Siviconta does not like the trim appearance when it happens.

    But when planing, then the COL, a combination of static and to a lesser extent buoyancy, will move rearward
     
  8. Siviconta
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    Siviconta Junior Member

    ıt would be better to LCB and the LCG to coincide at rest.

    since its a speed boat and it will eventualy trim aftwards it could be acceptable to trim a bit aft at rest but trimming to bow seems odd to me.
     
  9. Yellowjacket
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    Yellowjacket Senior Member

    There are several issues here. First you need to move all weight aft and use outdrives or V drives to get more weight aft. To get any 20 meter hull up to 60 mph is no mean feat and you're going to need as much weight aft as you can get. The second and probably even more importantly is that your keel line is kicking up from the baseline way too soon. That is, the lift you're creating is aft of the CG . You're essentially removing the area where you need lift and this is causing the bow to drop. This makes the hull essentially have rocker in it and that is not good for this type of hull. You need to look carefully at some of the high speed "Cigarette" boats and their hull form, they don't look like your hull for a very good reason. Too much weight forward is going to give you too low a trim angle, but if you don't have enough area forward, the bow is going to drop and you're creating negative lift aft.

    A step would help, but that also requires a longer flat surface than what you have here. Remember that steps are only efficient in a limited speed range, but that is one way to do this. I would suggest that you talk to someone who has experience in this type of design. You're going to spend hundreds of thousands of dollars on this and you should be getting the advice of a professional who has experience in this area as you're wasting a lot of money on a poor design.
     
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  10. Ad Hoc
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    Ad Hoc Naval Architect

    Hmmm... ok, but this is slightly ambiguous. So, to establish some facts can you confirm you have:

    1 - a General Arrangement (GA) of the vessel?
    2 - a detailed weight & centres of the GA?
    3 - a set of hydrostatics for the hull?

    If the answer to the above is yes, is it possible to post them, in some form to check the accuracy?

    That still needs to be established. Thus once the facts are presented, it is easier to comment and advise.

    Like most things, knowledge and experience of design these hull forms (as well as any technical paper on these types of hull) and supported by tank testing data.

    For example, look at the difference with LCG at 7.5% aft at 40 knots
    upload_2020-3-13_9-1-51.png

    compared to LCG at 20% aft at 40 knots:
    upload_2020-3-13_9-2-48.png

    Much smoother flow and much less resistance, when you look at the resistance data.

    And it continues up to 60 knots too:
    upload_2020-3-13_9-4-4.png

    Smooth flow the running trim is correct and the resistance is better. At 60 knots the resistance of the hull with LCG farther aft is more than 20% less than when farther fwd.
    Since you design the layout for your target speed not when stationary!

    Because even when running slow:
    upload_2020-3-13_9-5-5.png

    Same hull, at 20% aft LCG but at 20 knots, the trim gets even worse at slower speed and of course when stationary.

    So, without the data noted in 1 - 3, it is hard to establish whether your design, as it currently is, is correct or not.
     
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    And picking up Barry's note.

    When stationary you "hide" the effects of the trim by clever use of the sheer line, such as this:

    [​IMG]

    Slow/stationary she exhibits larger aft trim - but so what? She is designed for high speed not loitering!
    But the sheer line is reversed so not to accentuate the trim to the point you don't really notice it.

    And another example:

    [​IMG]
     
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  12. Barry
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    Barry Senior Member

    That was the point of the question before,
    At REST the LCB and the LCG are the same, or the boat would be rotating.

    An example of this would be this: take a physical model of your boat and determine the center of gravity, then put a pencil under the boat and roll the boat until it balances, ie no rotation, the center of lift is the point of interface between
    the hull and the pencil and will be at the center of gravity. Add weight to the model boat to "trim it" as you are considering, mark the new LCG, put it on the pencil, the only time that the boat will balance will be when the pencil is underneath the LCG
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Need to be very careful with terminology here Barry.
    What you are describing is the centre of vertical upthrust from/of the displaced volume in a static, a hydrostatic sense. It is not the dynamic centre.

    upload_2020-3-13_12-43-33.png

    Thus, the LCB is not the same as Centre of Lift.
     
    Last edited: Mar 13, 2020
  14. fallguy
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    fallguy Senior Member

    Sometimes wisdom is not absolutely the ends, but the means.

    Once the OP posted hull lines, YJ was quickly able to see problems. I basically disqualified, but still recommended a means for the OP to get better results. Even disqualifying is professional; despite it not producing the desired results.
     
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  15. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm not sure that is just a stylistic device. It gives much better forward vision from a steering position well aft, where the vertical accelerations are friendlier, better aerodynamic penetration, and less unwanted bow lift heading upwind. It has the added bonus of putting the centre of the lateral plane of the above-water part of the boat, further rearward, reducing the affect of crosswinds that can cause leaning into the breeze, from the need to apply helm to keep the boat running straight. Possible downsides are the increased chance of shipping water over the bow, and especially the phenomenon of "stuffing" in big following seas, at high speed.
     
    Last edited: Mar 13, 2020
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