From the Drawing Board
Occasional ramblings from
a Small Craft Designer

by John Welsford


Stability. Need it? Yes! How much? Most would say that our boats need as much as possible. There have been several threads of late on small boat forums and discussion groups, some of the postings indicate that the subject is not well understood by some and sitting here wracking my brains to find a subject for my next diatribe that seemed as good a topic as any.

Stability is a tricky thing, I use a graph known as a “Stability Curve” to give me a visual representation of a boats stability, and in fact when starting out to draw a new design will have a pretty fair idea what that stability curve will look like for that design and the intended use of that design.

Yes, it varies. Different boats and different uses need to have the peak of that curve earlier or later as the boat heels. A canoe intended for beginner users who have no intention of doing “Eskimo rolls” , an open recreational sailing dinghy or a smooth water ferryboat will have stability curves that show that the boats maximum resistance to heeling occurs very early as she heels. An Americas cupper, a very long range ocean cruising yacht or a sailing Dory will have the peak of that curve much later.

Now, for the sake of simplicity, and to keep this below several hundred pages, I am going to leave out a lot of possibilities, will give you a simplistic explanation of “Form Stability” and “Ballast Stability”.

Imagine that you have in your hand a piece of wood, a plank 12 in wide, 1 in thick, and say, 3ft long. It is of nice light wood that floats well, and we are going to imagine that it represents the hull of a boat that we are going to draw a stability curve for.

Toss it into the water!. “Flop!”. Flat on its side! Press down on the outside edge, the edge goes down very slightly. Put more pressure on, same, and more until the edge goes under water, once this happens the edge will keep going down until the plank is floating on its edge. If we graph the amount of force needed to increase the heel at each point we will see the high point of the graph that indicates the resistance to heeling has a big peak very early in the heeling of the plank. So the graph goes from zero to max with only a couple of degrees of heel.

Sounds good so far, just what we need to stop us getting our butts wet? Have a look further on in that graph, as the plank heels it requires less and less weight to heel it further and past 90 deg the thing wants to come back up upside down! In our hypothetical ocean cruiser that would mean among other things that the galley and the heads are unuseable! Not to mention the risk to life and limb and the fact that the boat does not sail well in that position.
So the graph, the stability curve, shows a rapid reduction in stability with the increase in heeling angle around to 90 degrees of heel, and the graph line dips below zero and shows that the boat has a NEGATIVE righting moment and a wide range of stability UPSIDE DOWN!!!

Now for a small open boat this is not an issue, as soon as the gunwale goes under the stability curve is a bit academic anyway and the boat should have enough righting moment at small angles of heel to enable the boat to carry its sail without putting the rail under and swamping her.

However in bigger boats a hull with this type of stability curve will tend to have an uncomfortable short sharp roll period, and not enough ultimate stability to be safe in a really big seaway. So consider the same piece of wood, 12 in x 1in x 3ft, with a lump of lead clamped to one edge. Not enough to sink it mind, but enough to make it float on edge.

We can graph the amount of force needed to heel that just a little, and it is not much so the stability curve will start out very gently, and as the weight on the edge of the plank swings out from under the centre of buoyancy (the centre of the underwater part of the wood that is doing the floating) it gains leverage and starts to try and pull that lower edge back down vertically under the centre of buoyancy again so the graph line on the stability curve trends strongly upward with the increase in heeling angle.

Now here's the really interesting part, the righting moment is strongest at 90 degrees, but unlike the unweighted plank the righting moment does not disappear, it gradually reduces as the plank is rotated further and further and the weight comes closer to being vertically above the centre of buoyancy until a point of equilibrium is reached when the weight is directly above the centre of buoyancy and zero righting moment is achieved.

But this inverted stability has a really narrow range of a degree or two rather than the 180 deg of the unballasted plank. Ideal for an ocean going yacht? Nope! She will sail on her ear all the time, and life at 30 deg of heel is not that desirable.

So what to we designers do?

The above “Thought Experiment” with the flat plank gives us the two extremes, and it is up to the designer to work in aspects of both the former, known as “form stability”, that is “resistance to heeling generated by the shape of the hull” and the latter, known as “ballast stability” where stability is generated by the lever arm created by swinging the weight of the ballast out from under the centre of buoyancy, and the effect of gravity pulling that weight back in under the buoyancy that is stopping it from plummeting to the bottom of the ocean.

A good designer will have a fair idea of the type of stability required for a given type of boat and its usage, the environment where it will be used and all of the other considerations required of the design. By working in a combination of “form stability” to control the early stages of heeling, and ballast stability to control the larger angles of heel a boat can be designed that will suit its use.

This is a very very simplistic picture, of course, and there is a lot more to it than just the issues above. There are issues of moveable weight in smaller boats, having the crew on the rail or out on a trapeze is one example, and the boat with very high ends all decked over like an old fashioned British Inshore Lifeboat that uses a modified form of form stability and the weight of the hull itself to create high angle righting moment is another. ( There are an awful lot more but you get the idea) But unless the designer starts off with those two basic first principles then the rest is irrelevant.

As I write, New Zealand is well into spring. The Daffodils are showing up swathes of yellow in the fields, the lawns seem to need mowing a couple of times a week , the weather goes from clear and fine to howling rain and back again a couple of times a day and the Huffboat needs painting so I will have her ready for the Trad Small craft rally at Rotoiti on November 29 and 30th (that’s an advertisment! See you there Kiwis). It has been a warmish and dryer than normal winter, and looks like being a warm, dry and windy summer. Good for sailing but its out with the paintbrush and sandpaper.

It’s a pity that elbow grease cant be bought in pots like the other kinds of grease.