Locating LCG on existing hull

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

  1. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You mean to say a 22' pleasure craft (as an example), originally equipped with an outboard, has to have an inclining test, just because a transom bracket mounted outboard is installed? What is the logic behind such over burdensome regulations? I can understand the need on commercial vessels over a certain size and/or capacity, but small pleasure craft? What happens if you buy a new set of paddles for your canoe . . .
     
  2. TANSL
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    TANSL Senior Member

    Things are as they are and should not be out of context. When the Captain cap changes DO NOT NEED any calculations.
    For example, in mass production of pleasure boats, the first unit should be subjected to an inclining experiment, supervised by an entity authorized by the Administration. After the manufacturer shall certify that each unit is exactly like the first. Each model differing in the arrangement of engines, weight, dimensions, etc.. needs its own test.
    I love being able to satisfy your scientific curiosity : Paddles are not considered as lightship weight. Therefore, it is NOT necessary inclining test.
     
  3. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Thanks for the information.
     
  4. orb353
    Joined: Feb 2010
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    orb353 Junior Member

    Thanks for all the replies. Wow, a vast amount of info.

    The original issue is, when replacing a transom mounted outboard with a bracket and a heavier outboard, the LCG will naturally move aft. Excessive weight will effect performance. That is why I am seeking to establish the original LCG location for the hull. I can then adjust location of tanks and such to get correct balance.
    I replaced a transom mounted outboard with a bracket and larger outboard a few years ago on another boat of similar size. We eventually had to add trim tabs to try to improve planing time and trim. The stern was lower in the water, water coming in the scuppers, and rode bow high. The trim tabs helped, but I feel the correct thing to do is to get the boat somewhat balanced over the LCG, then it perform as designed.
    So, now on this new project, I am trying to do it right.

    I do understand the basic principles of balancing a boat, and how to go about balancing weight. I also understand that it is not necessary to try to pin down the LCG exactly on a small boat like this. I am not trying to be too technical with it, just trying to make an adjustment or two that will positively effect the ride of the boat. Most people who add brackets on outboards really do not even consider what it does to the balance of weight of LCG. Everyone sees that the stern sits lower in the water at rest, the scuppers that used to drain the deck, are now,at, or below the water line and that the boat is harder to get on a plane and that it rides with a slightly bow higher trim at cruise. But without repositioning heavy objects( fuel tank and such) it can be difficult to correct the balance of adding the outboard further from the LCG.
    The bracket 16". I have not decided on an outboard yet, but a 4 stroke 150 hp is in the 500 lb range. The picture is of a 20' proline--very similar to the aquasport. See how low the stern sits in the water. These boats were not designed for outboards on brackets.
     

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  5. orb353
    Joined: Feb 2010
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    orb353 Junior Member

    I am planning on trying to establish where the original LCG was in the boat, with original equipment. Then use the 40% rule. The LCG should be somewhere in the neighborhood of 40% of the PLANING surface--measured from the stern. And compare this to the original figure---come up with a good compromise. Then by using the Weight*Arm=Moment...balance the weight moment over the LCG.
    The boat may be still be slightly stern heavy, just because it was not designed for a bracket mounted outboard, and the bracket does not extend the planing surface nor does does it add any floatation. It is a paralift- porta lift bracket.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The easiest way would be to calculate the MT-1" and some simple math to see how much stern down the new equipment will trim her out, then compensate with the tank and/or weights or moving other things around.

    Estimate the PPI (Pounds Per inch Immersed)by multiplying the WL beam by the LWL, then by .8. This will give you the amount you need to sink by 1". Next take the square of the water plane area, multiply by .35 (for sq. ft.) and divide by the WL beam. You'll have to make some hull measurements at the LWL for the water plane area, but this isn't too hard. The calculation requires you figure the cantilever over the CB, this new set of weights, buts the boat out of trim. You can assume the CB is about 67% aft of the beginning of the LWL on your boat. Now with the MT-1" (Moment to Trim 1") in hand you can calculate how much out of trim the new engine and bracket will make the hull, given that you also now know the PPI. Adjust the placement of the tank(s) and other equipment so she's leveled up again, understanding that there's a whole lot more leverage on trim with the outboard so far away from the CB than the tank. This simply means, the tank will need to be move quite a distance to offset the heavier engine/bracket combination,. So, considering moving other things, such as batteries

    In the end, I frankly think you will not have as big an issue as you might think, again unless you're making a huge swing in weight differences. What was the old engine (weight), what is she rated for (max HP), the difference in tank size and the bracket set back?
     

  7. orb353
    Joined: Feb 2010
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    orb353 Junior Member

    Thanks Par for that info. I will use it for sure. The boat had a 150 mercury on it. It was removed before I purchased the boat. The guy said it ran fine with the 150. A 150 weighs about 400 lbs.

    A fourstroke 150 weighs about 500 lbs. The bracket weighs about 50 lbs. The bracket will put the engine 16" aft of the transom.

    The boat had a 60 gallon aluminum fuel tank. I am installing a 75 gallon fuel tank.
    I love to figure stuff out. So, for me, half the fun of rebuilding the old boat is to figure out why everything is the way it is, and work on it accordingly.
     
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