# centre of gravity help!

Discussion in 'Stability' started by Gianf1041, Nov 11, 2019.

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### Gianf1041Junior Member

Good evening to all, here it is already evening; I am calculating the center of gravity of my small 8 meter motorsailer with ballasted keel and the doubt that I am doing everything wrong has arisen .......... question: the method of calculating the center of gravity with the various weights of the hull distributed for its length multiplied by the distances from a point determined from bow and stern to obtain the moments or with the anilitic calculation of the moments obtained from the surfaces of each section multiplied by their distance from any point. Who can clarify the problem for me. Thanks.

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### TANSLSenior Member

Take a reference point, the one you want, although it is normal to take the intersection point of the baseline with the after perpendicular. Add the products of each weight by the horizontal (or / and vertical) distance to that point (that is, sum the moments of each weight with respect to that point). Divide the total sum of those moments by the sum of all weights.
The attached table shows a fairly complete example for a runabout of 10.8 m in length. You can see how the calculations are performed.

#### Attached Files:

• ###### Weights Estimation.xlsx
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Last edited: Nov 11, 2019
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Firstly you decide upon the location of your datum. Usually transom, or midships. So if transom each lever (distance) is positive +ve from the transom. If you use midships, then items that are aft of midships are negative " -ve" and those items fwd are positive "+ve"...and then you do the same for VCGs, using the baseline as the datum....and centreline of the vessel for the transverse centres.

You tabulate the items and you end up with something similar to this:

Add up the total of each column and the final LCG/VCG is its moment divided by the total weight.

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### BlueBell"Whatever..."

Gianf1041,

Google the term "weight and balance" in regards to "aviation" for airplanes.
The process is pretty simple once a clear understanding is achieved.

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### Gianf1041Junior Member

The attached spreadsheet refers to the calculation of the center of gravity of the hull with the deckhouse without weights with the static moment method ............ the second annex refers to the method with weights as you suggested me. Hello.

#### Attached Files:

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• ###### Scan calculating the center of gravity .pdf
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### HeimfriedSenior Member

Calculating CG without using weights of the parts is (approximately) only possible if the hull has an allover homogenous density. If you mathematically cut the hull in equidistant cross sections, the area of every section is representative for the weight of the respective slice of the hull. Assumption is, that the number of slices is not too small.
weight1 = area1 * thickness of slice * density * g
weight2 = area2 * thickness of slice * density * g
... and so on.
Because thickness of slice, density and gravity acceleration (g) have allways the same value in this calculation, you can leave it out. Instead of dividing by sum of weights at the end, you must divide by sum of areas. Accordingly it is possible to calculate the CG of a plywood hull by using the area and the postion of the respective plywood part.
BUT: what you will need in the end, is the CG of the real boat, motor, gear, fuel, water etc. included. And a CG of the hull without the weight of the hull will not help to achieve this.

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### Gianf1041Junior Member

weight of the hull including keel and ballast of course ??

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### HeimfriedSenior Member

"allover homogenous density" means for example a model boat hull carved out of massive wood or styrofoam.
How could any ballast material be of eaqual density compared with the other parts of the boat?
Above I tried to explain why it is possible in principle to calculate a CG without weights of the parts. That doesn't mean it should be done for a real boat, because it doesn't fit this case.

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### Gianf1041Junior Member

so if I understand correctly, the excel spreadsheet that I set with the surfaces is fine for calculating the center of thrust or center of gravity of the wet surface ........ I have to opt for calculating the weights distributed on the boat .. ...... I hope I explained myself well, bye.

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### HeimfriedSenior Member

That's right.

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### Gianf1041Junior Member

Obviously the weight of the hull will be that of the material of which it is built, or if built in wood or grp or metal .......... I mean hull and deckhouse dry.

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