… and got to wondering ??
Wonder what a canoe or kayak would steer and turn like , if there was a small rudder on both the front and back working together … or even just the front only ??
Easy to get a feel for this yourself
– Last Updated: Dec-19-08 5:52 PM EST –
Use ruddering strokes applied to various positions alongside your boat when paddling and you will see how different ruddering from the back is from ruddering at the front. There are times when it is an advantage to apply ruddering force at the bow, but since those occassions are normally special maneuvers requiring correction which is customized to that exact situation, using the paddle works best - much better than a rudder. For normal steering while cruising along, ruddering from the rear is more efficient because there's less water pressure against the sides of the boat at the rear end than at the front end. For a boat to have its rear end "spin out" is very natural, requiring minimal effort to either make it happen or prevent it from happening on its own. The front of the boat is not as susceptible to side-skidding as the rear, and causing it to happen takes more effort. The resistance you feel during corrective strokes of any kind is an indication of how much energy you are robbing from that available for forward power. For plain old "steering" (not special maneuvers), ruddering from the rear is most efficient from an energy-use standpoint. Ever notice that all typical motorized boats and ships are steered from the tail end?
Here's a related topic. Verlen Kruger put a rudder on his expedition canoes because far less sideways force is required to correct the boat's course when that force is applied at the rear than when applied closer to center, as is done when using corrective paddle strokes to control the boat's heading. The farther toward the rear the rudder is placed, the less sideways force is required to steer the boat (basic leverage principle). Best of all would be to place the rudder very far BEHIND the boat on an extension boom. In that case, just a very tiny amount of deflection angle of the rudder would apply the same steering force as a steeper deflection angle on a rudder mounted right on the stern. Less deflection angle for a given amount of steering effort would translate to less drag. it wouldn't be as good for extremely abrupt maneuvers, but it would be more efficient for maintaining course when cruising. No one would build an expedition canoe or sea kayak this way, because that extension boom would get in the way too much, or be prone to breakage.
2c
A boat will turn around center of lateral resistance. For a moving boat, CLR is typically closer to bow than stern.
The rudder will be most effective if placed further away from the CLR - levers and things. Since ruddering is basically creating resistance, less resistance is better. Hence the further away from CLR the rudder is, the less resistance is required to make corrections to the course.
You Mean a Bow Rudder?
I think we just covered that…
Good terminology
– Last Updated: Dec-19-08 6:02 PM EST –
This "center of lateral resistance" thing is a direct consequence of the greater water pressure against the front portion of the boat than the rear when moving forward, as described in my first post. I'm not telling YOU this, only adding this info to help clarify things for the original poster.
This CLR thing is easily illustrated by paddling a canoe solo and applying a draw or pry while moving forward and noticing that to avoid changing the boat's heading while side-slipping the whole boat, the draw or pry must be applied well forward of center. Do the same thing while stationary, and you'd have to apply the draw or pry at the center of the boat to move sideways while keeping the boat aimed in the same direction (that is, NOT causing the boat to pivot).
Absolutely, and in a tandem canoe
when “the bride” does a cross bow rudder, we can just about make a ninty degree turn.
When she crosses to the left, and I do sweeps on the right from the stern, there is almost no forward speed lost.
cheers,
jackL
my understanding of rudders …
– Last Updated: Dec-19-08 7:09 PM EST –
..... when a rudder is deflected , this creates a "low" pressure on side A of the rudder , which in turn "draws" side A surface towards that low pressure .
This is because the fluid (water/air) increases it's speed around side A , creating a lower pressure than on side B ... side B aids A by aprox. 17% dynamic force (pushes towards the same way Side A is drawling) .
An airplanes wing uses the same principle , it is called lift . Lift is acually the wing being "sucked" upwards because of this low pressure on the airfoils top side , and the higher pressure on it's underside (the dynamic) .
I find the responses very interesting in relation to noes and yaks ... the "center line resistence" of a boat is a factor unto itself , which of course is the boats most major drag to be overcome in a turning manuver , I think .
I'm not sure that a paddled boat could gain benefit from fore and aft deflection rudders , because I believe more speed would be required to extract maximum efficiency with only minor deflections to both rudders ... I also think that the rudders would nessasarily need to be syncronized so that the aft one has greater deflection than the fore one . Also the fore rudder may need to have smaller surface area than the aft one ??
But the thought is a curious one to me !!
Consider the extreme efficiencies of the Canard design aircraft .
that’s cool jackl , would your lady …
… be interested in showing my lady how to accomplish this , lol … pleeeeeeeeease !!
Point of Order!
While 'Yaks is acceptable shorthand, there is no way to abbreviate “canoes”. The canoe is such an honourable and venerable craft that its name must always be said in full.
As to the original post, I suspected bow rudders weren’t a great idea because most boats don’t use them - it is interesting to read why, though.
Finally, not to hijack, but are rudders allowed on marathon racing canoes? If they are, I suppose they are not as efficient and that’s why they aren’t used? Sprint canoes have a sort of skeg, do they not?
A question
– Last Updated: Dec-19-08 10:45 PM EST –
What do you mean by "centerline resistance"? I've never heard of that and can't figure out what this term might mean. I don't think this could be a typo for "center of lateral resistance" (CLR) because what you said about it is in no way related to that. CLR is not a "form of drag" and it need not be "overcome" during turning, so I've been puzzling over what you were trying to say. CLR is a location on the boat (a discrete "place" on the hull who's location varies depending on the speed of travel through the water); it is not a force or source of resistance to motion.
yes there is , at least for me …
… sorry it bothers you .
it’s not like you didn’t understand what I was calling a canoe , right ?? … I don’t idolize objects , especially noes or yaks … respect is one thing , but I see no disrespect in using noe for canoe … only write it that way occassionally when combined in same sentence as yak … don’t take it too hard , it’s nothing personal or impersonal .
you gonna be OK with that ??
oh ?? …
– Last Updated: Dec-20-08 12:33 AM EST –
..... I read what was called CLR here , and gave it some thought as to what was being discribed .
so when I mentioned it as "center line resistence" , I was just thinking about resistence to turning (you know , longitudinal) , and equated what was being talked about to the center line of the hull , thinking all resistence radiates outward from there . If I had written it exactly as it was originally written "center of lateral resistence" , I would have still been thinking the same thoughts and responded the same way , because center line resistence is what I had in my mind .
So if as you say , CLR "center of lateral resistence" , "is not a force or resistence to motion" ... why is called resistence ??
all drag is a form of resistence , I think , and the center line of a hull is at the center of that resistence in this case (longitudinal), and lateral changes (as in turning) to the center lines track seems it would intiate some more drag/resistence ... hence my misunderstanding of whatever you were meaning by CLR .
I am certain of what I am saying and what I mean by it , only hope my explanations are sufficient communications ... but I guess I don't yet understand what you are saying about CLR ??
ao , did you understand the bulk (95%) of my response regarding rudders , low pressure , different fluid flow speeds on each side of a rudder (elevator, aileron, stabilitor, airfoil, rudder) , lifting forces (lateral or vertical) and the turning force associated ??
Resistance is part of the way you FIND…
… the pivot point. The resistance is not part of the turning process but a way of identifying a characteristic of the boat. My description of drawing and prying should make it really clear, but I’ll try again. Not right now though. I’m busy with something. Check back later, and I’ll provide a better description of CLR.
oh ??
… pivot point ??
right now my thoughts turn directly to “vertical axis” , as in the one of the three axis of gyroscopic motion , lateral , longitudinal , vertical .
pitches around “lateral” ,
rolls around “longitudinal” ,
yaws around "vertical
am I even close to what is meant by “pivot point” in this CLR characteristic you are talking about ??
Okay, here’s another try.
– Last Updated: Dec-20-08 10:19 PM EST –
First off, regarding your statement regarding pressure-differential stuff, yeah, I understand the prinicple behind Bernouli's equation. It's based on two of the most fundemental laws of physics, the law of conservation of mass, and the law of conservation of energy. However, rather than use terms like an object being "sucked" in one direction, it's much more accurate to refer only to the pressure differential across the object, as the force on neither side can be independant of the other when determining what happens. That 17-percent figure for the dynamic portion of the force is by no means an average though. For a propeller, each blade of which is simply a high-speed wing, I would expect the dynamic fraction of the force to be much greater than that. You can't accelerate huge volumes of air from dead-still to extremely high speed in a constant direction without F=MA becoming very significant.
That aside, let's get on to the subject at hand. I see by what you have said so far that you are NOT talking about the same thing as I am regarding the center of lateral reisistance, so I will try to lay it out in bare-bones fashion. So, FORGET ANYTHING about steering for the moment. It's NOT relevant when it comes to DEFINING the center of lateral resistance.
Imagine a boat with a symetrical hull, floating in the water. It is stationary, and is aimed directly north. Now, imagine what would happen if you pushed on the side of that boat, for example, with a small, sideways-mounted outboard motor, so that the direction the motor pushed against the boat was straight to the west. If you apply that force to the side of the boat exactly halfway between the two ends, the resistance the boat has to sideways motion will balance around that little outboard motor like a teeter-toter on its fulcrum, like a teeter-toter that is perfectly level. Therefore, the boat will move straight to the west, but as it does so, it will remain pointed straight north. In THIS situation, the force you apply to the boat is directed through the Center of Lateral Resistance. The lateral resistance is the force of water against the hull on the west side of the boat, and that resistance is the same on both sides of the point of your pushing force. If you mounted the little outboard motor a couple feet forward of center and did the same thing, the boat would not "balance like a teeter-toter" but would change it's orientation and aim a little toward the northwest as it moved sideways through the water. This would be applying a force to the boat that "misses" the center of lateral resistance. So far so good?
Now, take that same boat, but this time lets FIRST apply a standard propulsive force causing it to move forward through the water. The boat is aiming straight north so therefore the boat is traveling straight north. Now, once again lets apply a sideways force against the boat with a little outboard motor. If you do so, the boat will start side-slipping while still traveling north, and the direction of travel will be changed toward the northwest. If the sideways-pushing motor is mounted at exactly the right place, the boat will remain aimed straight north, in spite of its side-slipping action which causes the actual direction of travel to be roughly northwest. However, in this case, the motor-mounting location which accomplishes this balancing act is NOT halfway between the front and rear of the boat, it is somewhere forward of center, and the faster the boat moves in the forward direction, the farther foward of center that "balance point" will be. See why this has absolutely nothing to do with the "actual process" of steering? See why it is not a "resistance" that comes into play while operating a rudder or changing your course? Like I already said in my earlier post, it is ONLY a resistance that comes into play while DEFINING one particular characteristic of the hull, nothing more.
Okay now. Here is why the center of lateral resistance shifts to a more-forward location when the boat is moving forward. When the boat is moving forward, there is greater pressure against the sides of the hull on the front half of the boat than there is against the rear half. Think of the front half as "wedging itself into the water" thus creating increased pressure against the hull. Think of the rear half as doing the opposite, and pulling "away" from the water and letting the water "fall back" to its original position. When you push the boat sideways in this situation, the only way to strike a balance so that the boat does not pivot away from its original straight-north aiming point, is to apply that push within the half of the boat which is experiencing greater pressure against the sides of the hull (remember that the the normal resistance to sideways motion gets ADDED to the pressure that's already there), and that's the front half. The faster the boat goes, the greater the pressure increase alongside the front half of the boat and the greater the pressure decrease alongside the rear half. Therefore, the faster the boat moves through the water, the farther forward a sideways force would need to be applied in order for that sideways force to be perfectly balanced and have no effect on the direction in which the boat is aiming.
Now, they also call this balance point a "pivot point" because when you DO apply a ruddering action, the boat tends to pivot around that particular location. For example, let's say the boat is traveling at a speed such that the center of lateral resistance is located three feet forward of the centerpoint. If the boat is aimed north and traveling straight north (no side-slipping is happening), and you apply a force to the stern of the boat (the rudder location) in a straight-west direction, what happens? The stern swings toward the west and the bow swings toward the east, and the location within the boat which does NOT move sideways relative to the water is three feet forward of center, the same point at which you can apply a sideways force at this particular travel speed WITHOUT causing the boat's orientation to change.
Now, to get to the point brought up by another poster earlier in this thread, the farther away from the center of lateral resistance (the natural pivot point) you apply a ruddering action, the more leverage the rudder has for pivoting the boat. The more leverage you have, the less force you need for a given amount of steering effort. The less force you need, the less drag will be induced while creating that force (the drag is due to angling the rudder away from the current direction of travel), and the more efficient the rudder will be, from an energy-saving, drag-reducing standpoint. And that's why it is more efficient to use a rudder at the back of a boat than at the front - the center of lateral resistance will always be farther from a rear-mojnted rudder than a front-mounted one. However, I have no doubt that for abrupt maneuvers where it might be an advantage to "waste" propulsive effort during a turn, a design using two rudders might be good. This discussion just relates to the most efficient design for ordinary course corrections.
Remember that when paddling a canoe, you can apply all sorts of forces in all sorts of directions. And THAT is why in my earlier post I asked you to recall what you must do when you sideslip your boat, or to go out and try it if you never have. Changes in your forward speed result in a HUGE change in where that side-slipping force must be applied in order to get the proper effect. This is something every decent paddler sees every day. You can't miss it, and at first it is very confusing as "something weird is going on", but eventually you learn to automatically know how to alter the location of your side-slipping thrust at different travel speeds through the water. Well, the explanation I just gave you is the reason you experience the variation you do when you sideslip a canoe at different travel speeds.
I mentioned a couple of times that the center of lateral resistance has nothing to do with the process of steering in the conventional manner. It should be easy to see, however, that the center of lateral resistance would be VERY important on a sailing ship, since except in the rare case of traveling directly downwind, a sailing ship must endure a sideways force in order to create a forward-directed propulsive force. Obviously, if the effective centerpoint of sideways force which is transferred to the boat (via the sails) is not close to the center of lateral resistance for that hull at the particular speed of travel, drag-inducing correction (ruddering) will be needed to keep the boat on a proper heading. Complicating the matter even further is that the effective cross-section of the hull, as seen from the side, can change as the sailboat heels over under the force of the wind, and so would the hull's hydrodynamic properties as related to water flowing sideways across it. Clearly there can be no "perfect" design that "always" keeps the lateral force from the sails properly lined-up with the center of lateral resistance. I bet naval architects have fun with this.
Could you set one up like…
The old Honda Preludes had 4 wheel steering. At low speeds the front and read tires turned opposite to give you a tighter turning radius. At higher speeds they went the same direction, to give more stability when turning.
Apply for the patent and call it
the “U turn rudder assembly”
comes complete with rear view mirror so you don’t cut off the guy behind you
Cheers,
JackL
Absolutely
I was joking - sorry that didn’t come through. It did take me some small time, though, to figure out what you meant as I hadn’t seen that one before.
guideboatguy , thank you for …
… the time and efforts you have taken to aid my understanding about CLR .
sinus acting up today , so don’t feel like thinking/talking much … just responding at present out of courtesy and to let you know I have read and thought some about your explanation . I want to read it over agaian and give more attention to it soon … it seems to be getting clearer now .
sinus gives me foggy head 
thanks
No Problemo
Sorry I made that post so long, but I think once you wade through it it’ll make sense. Good luck with those sinuses - I know how bad that can be, because when I was a kid my dad would get debilitating sinus headaches at times (lucky for him, he eventually figured out that it was entirely caused by exposure to other people’s cigarette smoke, and nowadays it’s usually easy to avoid that kind of thing).