Locating LCG on existing hull

Discussion in 'Stability' started by orb353, Jun 22, 2012.

  1. orb353
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    orb353 Junior Member

    I have a 22' aquasport that I have removed floor and fuel tank and all accessories. I am replacing the floor, installing a larger fuel tank, and adding a engine bracket. I do not know the original LCG location for the hull. Aquasport has went out of business, and I have asked around at the classic aquasport website, and no one seems to know where the LCG SHOULD BE located. Everyone has ideas on how to balance weight, but no know where I should start at---the original location of LCG-----to make the boat run at a correct trim.

    As a note. I do know that the boat was set up correctly before I started tearing everything out. So, I could use pictures and old measurements to locate where the old fuel tank was at, but even knowing that may not give an accurate starting place.
    How can I find the original LCG?
    Thanks
    Oliver
     
  2. daiquiri
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    daiquiri Engineering and Design

    One thing that comes to my mind is that you could find another identical boat and ask the owner if you can measure it's LCG... Am I too trivial here? :)
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    If you remember what the draft was at the stern and bow...then just match these values by trial and error when you add items to finish up with the same.
     
  4. TANSL
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    TANSL Senior Member

    The best (and perhaps the one) way to get LCG and VCG is to carry out an inclining test.
    Once known LCG/VCG you can start moving tanks, add or remove weights, etc.. to achieve adequate flotation in the desired loading condition
     
  5. DCockey
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    DCockey Senior Member

    (Disclaimer, I may wrong since I have not been a practicing naval architect with XX years experience.) The inclining part of an inclining test isn't needed to determine LCG (as opposed to VCG) for a boat floating in the water. What is needed is the upright waterline location which is measured as part of the inclining test. But for a boat afloat the postion of the waterline can be measured directly without the other aspects of an inclining test. As Ad Hoc noted this could be obtained in otherways for a boat out of the water. I've used the "scum line" for a boat which spent considerable time at rest in relatively still water. Another method I've seen for a boot with a painted waterline is to look at photos of the boat at rest and estimate where the actual static waterline is relative to the painted waterline. But the use of the waterline to estimate LCG also requires a set of lines for the hull which I'm guessing the OP doesn't have.
     
  6. DCockey
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    DCockey Senior Member

    That's where I'd start.

    If the boat is out of the water the easiest way to determine LCD (assuming the hull is in good condition) may be to find the location where the boat is balanced when supported at a single fore/aft location. I've done it for a 20 foot, 3000 lb boat with a hydraulic jack under the keel.

    Another possibility is to find the LCG of your boat and a good estimate or measurement of it's weight as it exists today. Then estimate or measure the weight and CG location when in the boat of everything you've removed. Then a weight and moment arm calculation can be used to find the LCG with everything in place. Do you need help with how to do the calculations?
     
  7. TANSL
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    TANSL Senior Member

    OK DCockey you are very right, no inclining test is needed to calculate the LCG. I've gone from ready. Simply necessary to know the waterline, the displacement to this depth AND TRIM, and longitudinal position of c. of buoyancy. However, as the VCG is very important, I would advise making an inclining test.
    Moreover, I would place the longituinal c. of g. of tanks as close as possible to long. c. of buoyancy, so trimming the boat does not change as fuel is consumed. So my advice, do not relate the longitudinal position of the tanks with the LCG but with the LCB.
     
  8. DCockey
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    DCockey Senior Member

    Per the original post the boat in question is an Aquasport 22, which is a 6.7 m "walkaround", outboard or I/O powered fishing boat.

    Center of buoyancy requires a set of lines/offsets for the boat which would almost certainly require the boat to be measured and the lines developed from the measurements. So determining LCG/longitudinal position of COB using the static waterline would require a lot of work.

    VCG is nice to know but probably not particularly useful for the owner of a boat of the type the OP has.


    .
     
  9. TANSL
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    TANSL Senior Member

    In my country all new boats, and old boats that are modified, are required to make an inclining test to determine the position of the c. of g. In addition, we are forced to raise a number of loading conditions and verify compliance with all requirements of stability. That is, is not "nice" to know the VCG but is MANDATORY.
    If you can not use the forms to determine the c. of buoyancy and volume, you can suspend the boat with a crane and placing weights until the boat hangs perfectly balanced, trace its c. of g. But, sorry, I think it was sloppy (maybe I'm overly rigorous. It's all about the safety of navigation: a minor issue for the owner, I see).
     
  10. PAR
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    PAR Yacht Designer/Builder

    Unfortunately, this poster has his boat ripped apart, so all the easy techniques are out the window. I think the more appropriate question is why do you think you need the LCG location? If the boat trimmed and preformed properly previously, then you'll likely be fine with the addition of a new tank and bracket mounted engine. The LCG will naturally move aft, which isn't a bad thing on your boat. Unless the tank and the bracket/outboard combination are excessively heavier, I wouldn't worry about the LCG, which can be trimmed back (forward) if necessary.
     
  11. DCockey
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    DCockey Senior Member

    I have not seen anything to support or even suggest a claim that safety of navigation is a minor issue for the original poster, nor is it for me. The original poster explicitly asked about the LCG location so that he could "make the boat run at a correct trim." He did not ask anything about or mention stability or VCG.

    Here are some links to photos of Aquasport 22's, the boat in the original post:
    http://www.classicaquasport.com/gallery/showphoto.php?photo=836&cat=501
    http://www.ryansboats.com/aquasport-22.html
    http://www.thehulltruth.com/boats-sale-wanted/368458-restored-1974-aquasport-22-2-a.html

    Now to the question of VCG.

    Procedures and requirements differ in various countries. In your country does "all new boats, and old boats that are modified, are required to make an inclining test to determine the position of the c. of g" literally apply to all boat, even dinghies and canoes? Or are other types of stability tests or measurements used for small boats? Do all modifications require an inclining test, even the addition of a radio or an fabric awning?

    What would the owner of a small recreation boat, particularly one where the occupant weight is likely to be a large fraction of the unladen weight of the boat, do with the knowledge of the VCG? How would it affect the way he used the boat? How many owners of small recreational boats in any country know the VCG of their boat?

    Certainly anyone building, modifying or repairing a boat should ensure that the boat will be safe and suitable for its intended use.
     
  12. philSweet
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    philSweet Senior Member

    perhaps of no use to the OP now that his boat is in pieces, but-

    Drive to the dump with the empty trailer. Weigh the complete rig and then just the trailer axle. Return with the boat on the trailer and weigh the complete rig and then just the trailer axle. You now have enough info to calc weight and LCG. Be sure to note fuel gage if done weeks apart.
     
  13. philSweet
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    philSweet Senior Member

    By the way, you don't want to duplicate the original LCG. If your bracket moves the motor 2.5 feet aft and raises the prop 5 or six inches and you were running a few degrees of overtrim; Your thrust vector has changed by quite a bit. You might have 200 ftlbs less of bow up moment at cruise. So the static condition should be adjusted for this if you are going to this amount of trouble.

    I agree with PAR. Bolt it all together and you should be fine and within the trim control of your setup. Those things were tanks.

    Can you post a pic of the bracket and tell us about the motor?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Unless there is some desire to radically alter the position of the existing weights ( fuel tank, e.g) I don't know what the fuss is about. Can't see why replacing the floor is going to make much difference if it is of comparable material to the old one. As Daiquiri says, you could visit the owner of an existing boat, preferably one on a trailer, just roll it back to where it starts to tilt, there is your LCG, over the rear roller. But I don't know that is going to be vitally important information.
     

  15. TANSL
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    TANSL Senior Member

    There are several changes that require testing. For example because the engine change, the variation in 5% of the total weight, the variation in shapes. The work boats should do every 10 years. To import a vessel you must prepare a dossier with some calculations based, in part, on the inclining test.
    All boats must pass the test. Large vessels according to the procedure indicated by IMO and small in accordance with ISO 12217. And should be undertaken by a qualified competent in the presence of a representative of the Administration
    The Administration of my country has a very demanding standard, which does not mean we do things better than others. For example, anyone can request a building permit for a boat, or start a modification as mentioned above, if you do not have a project signed by a naval architect and certified by the College of Naval Engineers, together with all insurance liability. A company or individual must demonstrate knowledge about certain conditions to be allowed to build ships, even for private use.
    So things are. That's why, in my opinion, the recreational marine industry is so poorly developed in Spain.
    All this does not matter at all to the creator of this thread, but I tell you with the purpose of exchanging information.
     
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