novice rudder question

Hi, I'm new here.

Here's something that I've never had explained to me. Why is the rudder always at the stern of a boat? Wouldn't it work at the bow just as well, without all that propwash stuff to worry about? I'm building a pontoon boat, and in my particular case it would make the build of it alot simpler.

Thanks.

 

If the rudder were at the bow, it would be the first thing to hit a submerged object, and therefore is very susceptible to damage. At the stern, it is well-protected. Whether at the bow or the stern, the rudder should work about the same, although the feel of the boat when steering will be different with the rudder in different locations.

 

Most if not all boats experience more pitching motion at the bow than at the stern. This will aeriate and stall the rudder far more easily at the bow than at the stern. Also the mechanics of controlling the rudder from a normal steering position are more complicated at the bow. Boats also have sharper hull sections at the bow than at the stern so I would argue with Eric's thought the they would be equally effective at steering the boat. The stern of the boat actually slides sideways in a turn which is easier to make a wide fuller hull section do than the sharp sections of the bow which would tend to dig in and resist the turn.

Some AC boats have been built woth both bow and stern rudders but were not very successful.

A bow rudder in all but full displacement powerboats would not work at all as the bow lifts out on plane and otherwise faces the same problems listed above.

 

A rudder at the stern induces yawing moments on the hull as well as the direct force produced by the rudder itself. Traditionally, boats and ships had long keels and the rudder was really just a flap that controlled the moments on the hull. For example, if you look at airfoil data, you'll see that a positive flap deflection produces a powerful negative moment on the airfoil. And a hull acts like an airfoil with a very short span. So the rudder has traditionally been located at the stern, and remained there as it developed greater depth for more efficiency, progressing through skeg+rudder to isolated spade rudder.

A modern spade rudder that produces most of its effectiveness as a direct force could be just as well located at the bow. And some boats do have forward rudders, such as the canting bulb-twin foil (CBTF) concept that really has two rudders. But as Mr. Sponberg pointed out, there are practical reasons working against forward rudders. For example, it's easy to control an aft rudder from an aft cockpit, where the crew is more protected and there's less interruption of the interior volume of the boat.

For a keel boat, you need to locate the ballast near the center of the boat. This means the keel has to be near the center of the boat, too. In order to have a reasonable location for the mast, the center of lateral resistance has to shift aft, rather than forward of the keel to match the center of area of the the rig with the proper lead. This requires the rudder be aft in order to balance the boat. You could in principle fix the aft surface and rotate the keel as the rudder, but that has obvious difficulties.

If you search the aeronautical literature, you'll find that in every study comparing the trimmed drag of canard (small surface forward) vs aft tail configurations (small surface aft), the aft tail has less drag than putting the small surface forward. Despite the fact that the trim lift on the aft tail may be going the wrong way, forcing the lift on the wing to be greater. ( http://aero.stanford.edu/Reports/MultOp/multop.html http://aero.stanford.edu/Reports/MultOp/multop6.gif ) You can approach the efficiency of the aft tail by using a three-surface design with fore and aft control surfaces, but the aft tail still has a small edge. So while I've not seen comparable studies for forward vs aft rudders, I suspect the aft rudder configuration has less drag than a comparable forward rudder. The three-surface results may have some relevance to CBTF-like configurations, such as those being used on some Mini-Transat boats.

Finally, you'd want to consider which way the boat goes when the rudder stalls. Stalling an aft rudder makes the boat round up. Stalling a forward rudder makes the boat head down. Although a boat with a forward rudder will probably lie a-hull pointing more into the waves than a boat with an aft rudder. So there are conflicting reasons here.

Bottom line is you have hydrodynamics, structural design, boat balance, damage tolerance, and crew comfort & safety all driving toward an aft rudder as being the most practical choice.

 

And here I was thinking the rudder was on the back end so that we'd crack our shins when we walked around behind the boat when on a trailer.

 

There is also another reason for the rudder to be at the stern.
When the rudder is aligned with the propeller the flow velocity increases, increasing the rudder force.

Furthermore, if you consider manoeuvring theory, for a ship to be directionaly stable the angular moment due to pressure of the water when the ship sways must be clockwise (positive yaw), this is the area aft (stern direction) the yaw centre, i.e. turning axis, has to be larger than the area forward (bow direction), this is why a ship is considered to be more directionaly stable when it has trim by the stern. So placing the rudder at the stern you are increasing the area aft the yaw centre and therefore increasing directional stability.

(Sorry for my english) : )