Stability
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.
