Bow vs Stern rudder effectiveness - questionable theory??

So I’m reading The Art of Kayaking by Nigel Foster, and in particular the section on steering. He notes that the bow has a lot of water pressure on it (when underway) and the stern not so much. As such, it is typically the stern that wanders (e.g., during weathercocking). He then states that “neither a rudder at the bow nor a paddle stroke at the bow is sufficient to steer a boat or kayak when it wanders off course” and that “all correction strokes need to be made at the stern, noting also that this is why a rudder is invariably placed at the stern of a boat, not the bow.”

This implied, if not explicit, rationale for this sounds dubious to me. Except in the special case of a bow jam, the paddle blade does not contact the boat at all. With both the bow and the stern rudder, the paddle exerts torque on the paddler who makes contact with the boat and thereby turns it. The torque on your body is the same irrespective of the position of the paddle blade (bow or stern), and so too is the point of contact with the boat (your hips, so more or less the middle of the boat). It seems like the effect on your boat should be the same as well.

As an aside, looking up stuff on the net as to why a rudder is placed at the rear of a boat, I saw several reasons, none of which had to do with pressure differentials at the bow vs stern (although I can’t say I did an exhaustive search). If my logic is flawed, I welcome a counter-argument. I just want to make sure I understand it correctly. As an empirical matter though, I feel I get at least as much turn radius out of a bow rudder as I do with a stern rudder.

Yeah, if I’m going from point A to point B, I maintain course using more sweeping forward strokes on one side, maybe an extra stroke on that side, possibly with a bit of edge/lean. 90% of my rudders, draws, etc. are done just for the fun of it when I’m playing around, trying to get better at stuff. And yeah, if it’s a significant and sustained problem due to wind, I’ll resort to the skeg.

On my next outing, I hope to paddle to a series of navigation points in different directions (hopefully something like a box to take full account of the winds effects) and to then compare the straight line distance between those points with the my distance travelled (determined using my GPS) to get an idea of how much I am not like a crow.

This implied, if not explicit, rationale for this sounds dubious to me. Except in the special case of a bow jam, the paddle blade does not contact the boat at all. With both the bow and the stern rudder, the paddle exerts torque on the paddler who makes contact with the boat and thereby turns it. The torque on your body is the same irrespective of the position of the paddle blade (bow or stern), and so too is the point of contact with the boat (your hips, so more or less the middle of the boat). It seems like the effect on your boat should be the same as well.

The paddler will be transferring both a torque and a force to the boat. The force being the reaction force from the paddle. The torque may be the same, independent of paddle position, as long as the moment arm is the same. But the force transferred to the boat will have different directions and positions.

If you want to transfer a pure torque and not transfer any force, you need a force pair of opposite directions to create the torque.

Anyway, a simple thought experiment should show where you go wrong:
Imagine that the kayak is pivoting around a vertical pole which is placed exactly where you have the paddle blade. Both ends of the kayak are free to move without any resistance around this pivot point. You put force on your paddle blade, so the force points exactly through the centre of the pole. Now there is obviously torque on the paddler. And obviously the paddler is transferring this torque to the kayak. And yet, the kayak obviously isn’t turning.

If you look at force that the paddler transfers to the boat, you will see that creates a torque which exactly cancels out the torque you were describing. The resulting torque is zero.

If you attend one of Nigel’s “Wind and Waves” courses, you’ll learn that the most effective way to turn depends on the direction of the turn relative to the wind and waves. I don’t know if he has the techniques of the course in any of his books, but there are essentially four turning methods that use combinations of edging, sweep strokes, stern rudders and bow rudders. The basic premise is to use edging and paddle strokes to initiate the turn, then anchor the end of the boat that you want to pivot around, and use the wind/waves to move the other end in the desired direction.

For example, if you’re moving into the wind and want to turn downwind, use edging and a bow rudder to start the turn, then pull the paddle back into a stern rudder, which holds the stern in place while the wind/waves, continue to push the bow downwind.

During the course, he had use paddle a box on the water, turning 90 degrees each time using different techniques. Once you get a feel for the principle, it’s not difficult to figure out the optimum technique, but executing it can be another matter.

Nigel states at page 28 of his new book: “The stern rudder is used primarily to steer your kayak straight when it wanders off course. It is not the best way to make a turn. A stern rudder can be added to the end of a forward stroke to adjust your course when your kayak weathercocks.”

I’ve been playing with keeping my kayak on course without using the skeg, but haven’t figured out how to add that stern rudder at the end of a forward stroke without losing cadence.

@Rookie said:
I’ve been playing with keeping my kayak on course without using the skeg, but haven’t figured out how to add that stern rudder at the end of a forward stroke without losing cadence.

In my experience it depends on how much of a correction you’re needing/giving, but stroke linking can result in a forward stroke merging into the latter part of a sweep that then finishes with a bit of a stern rudder. You might (more accurately?) call it a “stern draw”, because you’ll be exerting effort to or from the boat while your paddle is in this position. There are actually lots of things you can do with your paddle when it’s back there, combined with the boat edge as well. I sometimes find myself over-correcting on one side because it takes some time for the boat to swing back on course after the correction stroke. Of course this is going to vary widely with the design of a boat, especially those with significant rocker.

Yes, it will mess with the cadence a little but if you’re only going in circles does your cadence really matter anymore?

Here’s some more:

The most important lesson I learned while weaning myself off the skeg for flat water was to reduce stroke power significantly. Only once I could keep the boat straight did I add power back into the mix. I do find it more difficult to keep a straight course with the GP than with a Euro blade for some reason. I’m getting better with this, which may indicate an improvement in my GP technique overall.

You know they put the skeg , or rudder on these boats for a reason. Yeah, I used to think the skeg was a crutch for those who weren’t paddle savvy. Finally I decided the designer knew exactly what he was doing and I’ve been using the skeg as needed (varying amounts) without any feelings of inadequacy since.

The key hole stroke, great for course corrections while paddling. Imo best to stick with propulsion strokes and edging when moving to keep momentum going, and leave the stern rudders for the surf.
https://vimeo.com/90017663

If I’m just messing around I’ll paddle with no skeg, interesting to learn how many corrections the boat needs while traveling in a straight line.

The box exercise is a fun thing to practice, especially on a windy and choppy day.

@magooch said:
You know they put the skeg , or rudder on these boats for a reason. Yeah, I used to think the skeg was a crutch for those who weren’t paddle savvy. Finally I decided the designer knew exactly what he was doing and I’ve been using the skeg as needed (varying amounts) without any feelings of inadequacy since.

You’re absolutely correct and I regularly use the skeg when on Lake Michigan or any of the major inland lakes. Am glad it’s there.

But when I’m on home water just practicing different strokes, I also like to practice boat control without the skeg. It’s been helpful.

On my second outing on Lake Michigan in April I made a short shore stop to adjust some gear. It was a windy day. When I got back in my boat and paddled out, my skeg wouldn’t release. Later saw there were about a dozen small pebbles stuck in the skeg box.

Absolutely. Skegs are more reliable than rudders in my experience, but they are both prone to failure at just the wrong time.

Of course I still use my skeg when paddling, especially in wind, but I know first hand that I can also manage without it.

a little off subject, but since you’re speaking of skegs. Is it normal for the skegbox to gurgle after rolling. After I roll and start paddling again, I always here gurgling sounds. I think I must be getting air trapped in there and it takes some time for it to work out.

Perhaps he was off a bit as to the physics of this, but it is clear that bow rudders are considerably less effective.

Rather than explain what I understand imperfectly, at this point, I’ll point you to a source:

https://www.thenakedscientists.com/forum/index.php?topic=43154.0

“Bow rudders not exceeding the draft of the hull are ineffective in ahead motion because the oblique water flow generated by the turned rudder is redirected longitudinally by the hull. Thus, transverse forces on a bow rudder and on the foward moving hull cancel each other. The same generally applies to stern rudders in backward ship motion. The yaw instability of the backward-moving ship is one example could not be compensated by rudder actions if the draft angle exceeded β=1.5°. To improve the maneuverability of ships which frequently have to move astern, eg. car ferries, bow rudders may be advantageous.”

Hopefully, this clears up a few things.

Rick

Reading through that thread, I would say that they understand it imperfectly as well as it is rife with speculation. But in any event, I don’t think it applies to kayak bow rudders because the bow rudder as used on a kayak is not connected to the hull at the bow. It is connected to the paddler who sits in the middle of the boat. For me in my boat, I don’t think my bow rudder is noticeably less effective than my stern rudder. I won’t be back in my boat for a few weeks, but I will have to take some turn radius measurements with both, when I am.

@rjd9999 said:
Perhaps he was off a bit as to the physics of this, but it is clear that bow rudders are considerably less effective.

Rather than explain what I understand imperfectly, at this point, I’ll point you to a source:

https://www.thenakedscientists.com/forum/index.php?topic=43154.0

“Bow rudders not exceeding the draft of the hull are ineffective in ahead motion because the oblique water flow generated by the turned rudder is redirected longitudinally by the hull. Thus, transverse forces on a bow rudder and on the foward moving hull cancel each other. The same generally applies to stern rudders in backward ship motion. The yaw instability of the backward-moving ship is one example could not be compensated by rudder actions if the draft angle exceeded β=1.5°. To improve the maneuverability of ships which frequently have to move astern, eg. car ferries, bow rudders may be advantageous.”

Hopefully, this clears up a few things.

Rick