Stability tests. Any engineers or mathematician here?

deandavis1 · #1 (**permalink**) 07-18-2010

Hello,

I want to do a practical stability test myself. This is required for survey in NSW Australia. The test is relatively easy but I need some help with calculations (and maybe advice).

I was hoping there might be an engineer, mathematician or physician in the forum who could help me with the following four calculation:

The following steps:

1. Calculate the profile area of boat (with sails and clear sides up) and work out the height of the center of area (ie the centroid of an irregular shape).

2. Calculate the effective force of a 20kn wind on this area.

3. Calculate the weight required to hang off the boom at right angles to simulate this force.

4. Measure the freeboard to calculate the degree of incline.

Thanks in advance.

Dean

cormeum · #2 (**permalink**) 07-19-2010

Quote:

Originally Posted by deandavis1

Hello,

I want to do a practical stability test myself. This is required for survey in NSW Australia. The test is relatively easy but I need some help with calculations (and maybe advice).

I was hoping there might be an engineer, mathematician or physician in the forum who could help me with the following four calculation:

The following steps:

1. Calculate the profile area of boat (with sails and clear sides up) and work out the height of the center of area (ie the centroid of an irregular shape).

Easiest is to just approximate using a triangle constructed from the masthead to the waterline having a foot equal to LOA

Quote:

2. Calculate the effective force of a 20kn wind on this area.

that's available on Google

Quote:

3. Calculate the weight required to hang off the boom at right angles to simulate this force.

F=Length of boom x weight

Quote:

4. Measure the freeboard to calculate the degree of incline.

Measure height upright and measure height inclined (toerail to water) construct triangle from centerline to toerail (you now have two sides)
You'll calculate the tangent and invert to get the angle. do it again with the boat heeled and subtract to get the angle difference (that's your incline).
see here:
Trigonometric functions - Wikipedia, the free encyclopedia

Quote:

Thanks in advance.

Dean

Boasun · #3 (**permalink**) 07-19-2010

For GM all you need is the rolling period. Time of going from one full inclination back to the same inclination.
.44 x Beam / time(seconds) = Sqrt of GM.
This is a quick and dirty way of determining the initial stability of your vessel. But it works.

deandavis1 · #4 (**permalink**) 07-19-2010

Thanks Cormeum.

re point 2, If I have worked out my centroid is 'a' high and my boom is 'b' height and 'c' long, can you point me in the right direction on google to work out the weight needed.

Boasun, I agree with your approach but they are making me do it this way

(. The rule says the yacht has to tilt no more tha 25deg in a 20kn wind.

Cheers

cormeum · #5 (**permalink**) 07-20-2010

If you know the area of the triangle and the centroid. figure the force on the area in kg/m^2. Multiply by the area to get the total force. The heeling moment will be that force times the height from the water line to the centroid. Add the height of the boom from the wl and go out the boom until the total distance is equal to the height of the centroid. Apply a weight so that g (gravity- 9.8 m/s^2) times the weight equals the previously figured total wind force.