centre of gravity

[hbr]

want to build a scale model of an open60 length approx 1.60 m, master the calcutation of the volume displacement, the upright center of buyancy
defenition of the Center of gravity is an issue for me because no idea about weight caclulation of the original
any guidance, is it possible to retrieve this info with a reasonable accuracy ?

[marshmat]

Hi hbr, welcome aboard
I am aware of two ways to find the centre of gravity.
The first is to build it, then suspend it from load cells to find the actual CG. There are other, similar ways that involve balancing the model on a pivot point, etc.
To find the centre of gravity prior to construction involves a lot of math (a spreadsheet can help a lot here). Pick a reference point, and sum the moments (mass * distance from reference) of every individual winch, fitting, structural member and component of the boat, in the longitudinal and vertical axes. (You can skip the transverse axis if the boat and contents are perfectly symmetrical on either side of the centreplane.) Divide by the total mass to get the co-ordinates of the CG relative to the reference. Tedious, but not a hard calculation to do in Excel or Quattro.

[hbr]

appreciated the guidance, seems that i will have to start another excel in order to calculate it the way you suggested.
want to know already an estimation before the starting to build it.

thanks a lot

[jehardiman]

Is this to be a sailing model?

If it is, then the orginial VCG will be of no use to you. The TCG will be on centerline of course and the LCG will be directly under the LCB at load waterline. The VCG of the model will be much lower that the original though due to scale effects on waterplane inertia and righting moment. To find where you need to put the VCG, you'll need to work out the heeling moment for the scale rig, waterplane inertia for the scale size, and place VCG accordingly to get the neccessary righting moment.

[messabout]

If you are building an RC sailing model then you do not really need to know the CG location. When the model is finished use a test tank to find the appropriate location for ballast, batteries, rx, servos, and such. The tank which can be no more than a crude plywood box is a valuable resource for the modeler.

Trim is important of course. Many of the RC sailors will trim aft down by one or two degrees when at rest. All that depends on a bunch of variables like height of the CE, configuration of entry and exit, and much more. Pre calculation is a worthy pursuit but in the end you will be obliged to rely on empirical evidence more so than calculated stuff. If you are to build a true scale model then the CB is where it is and you will have to work around that reality. You can get some good information from the EC12 website. The EC12 is similar in size (about 1.5m) Other classes with similar sizes include the Marblehead class. See AMYA.org for inspiration. Keep your hull deck and superstructure as light as possible and then do the add ons.