Stability One

Staying upright or at least afloat

Here's the real deal on stability in small boats, modern sail boat design http://www.wavetrain.net/boats-a-gear/471-modern-sailboat-design-quantifying-stability. Which is perfect for larger boats with decks and a mostly fixed centre of gravity. But what about small open boats such as the one we've just been working on. Well here is the stability curve for RMSQ&D assuming a fixed centre of gravity.

So degrees of heel are on the X axis and righting arm, in inches, on the y axis. The reason we've only gone to 45 degrees is because beyond that water is coming in over the gunwale and you're going down.

You can see that this boat has a pretty good level of stability up to 45 degrees which is great. However the fact is that you, the person in the boat, has a huge influence on the stability through your ability to move the centre of gravity by moving yourself about.

The question is how did I calculate the data for this curve? It is mind numbing work involving drawing and redrawing waterlines at various degrees of heel and then calculating the centre of buoyancy using stations and Simpson's rule. It is not for the faint of heart. Information on the process is here, www.mi.mun.ca/media/mi/boatrace/files/shipcalculations2.pdf , and here, http://koti.kapsi.fi/hvartial/stab/stab.htm.

The one thing to remember is when you draw in the new waterline at a different angle of heel the displacement must remain the same. With the boat dead level the displacement of station 5 is 109.118 cubic inches, or .7578 cu ft or 48 lbs However when you heel the boat 10 degrees without altering the waterline the displacement is 136.706 cu in, or .9493 cu ft. So we must reduce that displacement by .1915 cu ft, so the waterline must go down but by how much?

If we measure the new waterline it is 3.4 ft, 3.4 into .1915 is .056 ft or .675 inches so we draw in the new waterline .675 inches below the old water line and measure the difference in volume which works out to 28.642 cu in which brings our displaced volume down to 108.064 which is close enough.

We then divide the new waterline into 10 sections, making sure one station line passes through the centre of gravity, giving us the measures for applying Simpsons rule and calculate the transverse centre of bouyancy for station 5.

And then we do it all again for different angles of heel.

Westlawn recommends using the trapezoidal rule instead of Simpson's I don't think there is much difference in the end result.

The thing to remember is that beam is directly proportional to initial stability. But too much beam can create problems with dynamic stability.

We'll talk more about stability next time and about a discovery I have made whilst working on this.