WHAT WOULD YOU SAY is the
most unappreciated part of a sailboat, the one thing that’s invariably
overlooked and neglected, but without which the boat simply cannot function?
Yes, that’s right, it’s not the skipper. It’s the rudder.
On larger boats it’s out
of sight (apart from the few stern-hung rudders still around) and it’s largely
out of mind. It’s a truly modest and uncomplaining piece of equipment, one that
doesn’t need to be fed constantly with expensive diesel fuel, one that never
goes flat and needs charging, one that never has to be taken to the sewage
pump-out every week, or varnished every six months.
It is a marvel of
efficiency; and also such a marvel of simplicity that you have to wonder why it
took so many hundreds of years to make an appearance on sea-going ships.
The ancient Phoenicians,
the Romans, and even the Vikings, used steering boards — oars or paddles,
usually hung from the right-hand quarter. (Hence the starboard — steerboard —
side. The other side was the side that rested against the port, so the steerboard
wouldn’t be damaged.)
It wasn’t until 1242 A.D.
that pictorial evidence appeared of a centerline rudder on a ship in Europe. It
was, if you’ll forgive me, a turning point in marine design. Apparently this
new-fangled invention was received with such enthusiasm that it was quickly
adopted by shipbuilders all over the world. Pintles and gudgeons suddenly
became household words.
How does the rudder work?
Well, in the crudest of terms, if you push it to one side, the water flowing
past strikes that side with more force than it strikes the other, so the rudder
tends to be pushed sideways, taking the stern with it. (It is actually a foil
whose shape provides “lift” like an airplane wing does, but never mind that for
the moment.)
I often marvel at the way a
tiny rudder can turn a 250,000-ton oil tanker; although, lurking in the back of
my mind somewhere is something vague I once read about the rudder only
initiating the turn, and the ship’s hull itself acting to reinforce the turning
moment, as might a wedge driven through the water.
It’s interesting to note
that the average sailboat rudder will stall and lose efficiency if it is put
over more than 35 degrees from dead ahead. In fact, it will act as a good brake
if you are approaching a dock too fast and you can put it over to 90 degrees,
first one side and then the other.
While most rudders lurk
quietly and invisibly beneath their mother ships, the importance of their roles
has not been lost on naval architects, physicists, and engineers. The design
and performance of rudders is the subject of countless scientific papers and
even whole books. In this respect, if you should wish to develop your latent
rudder fetish, get hold of a book called The
Development of the Rudder, by Lawrence V. Mott. It’s a fascinating and
lavishly illustrated volume of nautical archaeology that digs deep into the
conception of the modern rudder, starting with its crude but rather interesting
parents in various parts of the world.
Even if you don’t find
yourself entirely enthralled by rudders, it will help while away large parts of
the long, bleak period between the morning cocktail and the evening sundowner.
Today’s
Thought
The
ancient sailor said this to Neptune in a great storm, “O God, thou shalt save
me if thou please, if not, thou shalt lose me; yet will I keep my rudder true.”
— Montaigne, Essays
Tailpiece
“And you, madam, what’s
your husband’s average income?”
“Oh, usually well after
midnight.”
(Drop
by Monday, Wednesday, Friday for another Mainly about Boats column.)
10 comments:
John, for years I've been told to think of the forces involved in sailing as being aerodynamic. I can appreciate the theory of a sail acting like a wing but I have trouble taking the analogy below the waterline.
As I understand it, an airfoil is formed so that air passing over one side travels a longer distance than on the other. It travels faster creating a low pressure area and the higher pressure air on the opposite side provides lift. The airfoil shape is possible in a sail because of the flexibility of the fabric (which also enables the sail to form an airfoil on either tack)...BUT...
Below the waterline the keel is a rigid and symmetrical shape so the flow on either side should be the same. How is lift generated? Moreover, above the waterline the air is compressible so high and low pressure areas can exist. It was my understanding that liquids are non-compressible so in an open system (such as the water that floats your boat) there can be no pressure differential.
My mind tells me that a rudder works by deflecting the flow of water to one side and the opposing force against the rudder pushes it, and the stern, in the opposite direction.
I'm sure the problem is that my feeble mind just can't grasp the concept. Can it be explained so that a dolt like me can comprehend it; or are we just overcomplicating the reality? Sorry if I'm being a pain.
Don, I'm no expert but I know how lift is generated by a keel. Ironically, it's leeway that enables lift. The keel doesn't travel in a straight line through the water when you're going to windward. Like the hull, it's angled about 3 or 5 degrees to the flow of the water and it gains lift just as the wing of a plane does. Same goes for the rudder, basically, which is why a little weather helm is a good thing -- helps drag the boat to windward. But if a sailboat didn't make leeway the keel wouldn't generate lift.
Cheers,
John V.
Don, It may be more helpful to think in terms of fluid-dynamics rather than aerodynamics... Air and water are both fluids, the difference being (as you noted,) air is compressible. Nevertheless, for the purposes of our discussion, compressability has nothing to do with pressure differentials. Nor does a foil necessarily need to be cambered to create that differential. (Most aircraft wings are cambered to create a more dramatic differential, but not all. There are lots of high-performance military airplanes, and civilian aerobatic airplanes with little to no camber in the wing.)
There are actually two forces at work: Newtons third law, and Bernoulli's principle.
Newton tells us that for every action, there is an equal and opposite reaction. Water striking the angled rudder, or leeward side of the keel, tries to push the rudder/keel in the opposite direction.
Bernoulli tells us that a decrease in pressure occurs simultaneously with an increase in the speed of a fluid. If the rudder is pointed to the right, water to the left of the rudder must increase in speed, creating a pressure differential. Likewise with water on the windward side of the keel. The rudder and keel will want to move towards those areas of low pressure.
I would add, if I may, that hydrodynamic is about behavior of moving fluids, and both air and water are indeed fluids: one just lighter than the other.
On pressure: there is no need to compress the fluid to gain lift: it is the difference in fluid speed between the two faces of the foil that generates push on one side and pull on the other.
Well, 57degN, we posted simultanously, but I guess my comment is superluous by now!
Ladies and gentlemen, before we get too carried away with aero- and hydrodynamics, we should all read Arvel Gentry again. His theories on how sails work are very interesting, to say the least. Look up a column in my archives dated October 2, 2010, and be dazzled and amazed.
John V.
Sorry, it's October 3, 2010, entitled Aerodynamic Perfection
Oops, Seems Arvel Gentry gave up aero- and hydrodynamics in favour of cooking rice and veggies. It's probably just as well.
Maybe I'll get a nice little, unscientific rowing skiff and avoid all the pressure!
Just Google Arvel Gentry and you'll find plenty to occupy your grey cells.
John V.
My understanding of wings and lift is that it is not so much a higher pressure under an aerofoil that forces it up, rather it is the decrease in pressure i.e. lower pressure above the wing from the fluid having to travel a greater distance around the curve that effectively 'sucks' the wing upwards - or sideways in the case of a keel/rudder.
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