How to reading data on righting moments

[jamoose]

Hello--
I've been playing with a design for a small duck boat using the hulls freeware program and I've trying to figure out how stable the design would be. I looked at one of the views which allows one to fiew the waterline at various degrees of heel and it produced a bunch of data on righting moments. I have no clue how to read this. can anyone here translate this into layman's terms?

Thanks
Jamus http://www.duckboats.net/upload/Polywog_heel.JPG

[P.I. Stazzer-Newt]

The important figure is the "Righting Moment" (158ftLbs in this instance).
Try this in 10 degree increments to 150 degrees.
Put the figures into a spreadsheet and graph the results.

At the point the R.M. figure changes sign (goes negative or the arrow changes direction) the boat would start to capsize.

[mmd]

In the example shown above, it means that when your boat is heeled over by ten degrees, the centre of buoyancy (CB) moves outboard by 6.2". This distance is called the righting arm (RA). The results in a rotational force (a "couple") called the righting moment (RM) acting about the boat's centre of gravity (KG) to try to bring the boat back upright. The force as measured at the heeled CB is 158 ft-lbs.

Put another way, if you weigh 158 lbs and sit with the centre of your mass 6.2" outboard from centreline, the boat will heel ten degrees.

Some quick math: If the RM at ten degrees of heel is 158 ft-lbs, then at one foot the RM is 158 x (6.2 / 12) - 81.6 ft-lbs. So an 82 lb weight place one foot outboard from centreline will heel the boat ten degrees. At two feet out, 41 lbs will cause the same heel. If you calculate the RM in ft-lbs for angles of heel from zero to, say, 90 degrees and plot the results on an X-Y graph and "connect the dots", you will have a righting moment curve which will give a graphic representation of the stability of your boat. The illustration below is a righting moment curve for a 30-ft mussel-farming work barge with a 24-foot hydraulic crane. The righting moment curve stops at 46 degrees which is where the water comes over the bulwarks and the boat sinks.

http://www.imagestation.com/picture/sraid157/p4f76112fef37728ad5a492b2f96e97d8/f52f9f55.jpg

A steep curve starting at zero degrees of heel indicates that the boat is stiff, a low-angle curve indicates tenderness. The higher the maximum value, the more stable. A steep slope downwards from the apex indicates that the boat will roll over quickly, and a shallow slope means a slow, gentle roll-over. If the plot is continued to where the curve crosses the X-axis, the crossing point is the angle at which capsize occurs, presuming that the hull is intact and no water can get in (downflooding).

Your task, Grashopper, is to 1.) calculate where the KG is for you, your gear, and the boat together, then 2.) calculate the downflooding angle of the boat (at what angle the water enters the cockpit and sinks the boat), then 3.) calculate the new CG for when you must lean out to pick up a duck. Then figure out if the shift of CG outboard during the duck pick-up is great enough to heel the boat to the down-flooding angle. Assume that your personal CG is about on your sternum at the bottom of your ribcage.

[jamoose]

wow. thanks for the reply.

Grasshopper must ponder this and respond...