Canoe paddle blade with "spine"

I noticed that some (straight) paddles carry the shaft far into the blade while others don’t and let the shaft taper off quickly where the blade begins. The Sawyer venture is an example of the first style, the Bending Branches Espresso of the second style.

What are the pros and cons of either design? My guess is the “extended” shaft would add some weight and strength, all other things being equal.

And the paddles
With the shaft tapering quickly with thin blades work well for in water paddle recovery like the knifing J stroke and the Indian stroke.

I have an ash Nashwaak paddle
made to emulate Omer Stringer’s style of paddling, and the blade is rather thin. It can be that way because the maker (no longer active) chose the ash very, very carefully.

I don’t find that a rib, extending from the shaft down to within a few inches of the tip of the blade, interferes in any way with compound paddle strokes, providing that the paddle blade is otherwise properly designed. In unglassed wood ww paddles, if there weren’t an extended rib on both sides, it would be necessary to thicken the entire blade, like a lens, for sufficient strength. And that would often weigh too much.

There is another way to make light blades that knife in a neutral fashion for sculling and other compound strokes. Make the blade thin, and curve it. Then reinforce it on both sides with a thin layer of glass or carbon. Such blades are very light, very strong, and relatively expensive. I have one that is over 15 years old and in very good condition. Many times I have hooked an unseen rock, and felt that blade bend open before the tip slipped loose.

The curve does not add strength

– Last Updated: Feb-18-14 9:58 AM EST –

The fact that such a blade is curved doesn't do anything to its strength. I have no doubt that the curve has hydrodynamic properties that many find desirable, but for a given material of given dimensions, it changes nothing as far as flexural strength goes.

Consider a flat spring, which has exactly the same flexural strength no matter what shape you make it. The simplest example is a leaf spring, which can be made to do exactly the same thing whether it's flat, arched, or reverse-arched (all three types are used in trucks, with the choice depending on the where the axle must be supported relative to the frame, whether more ground clearance is needed, and often whether the progressive action due to "adding in" the contact of additional leaves with progressively greater flexing is needed, but in the context of any individual leaf in the spring pack, shape is irrelevant).

Imagine one end of the paddle blade is held rigid and a normal force is applied along the entire length of the unsupported surface of the blade. Now envision the stress within smaller and smaller increments of blade length, and I think you can see that curvature has no effect.

I have some paddles with extremely thin blades, but they are straight. When catching the tip on a rock as you describe, I feel the same type of flexing action, but instead of that flexing being the "opening up" of a curve, the flat blade bends into a slight reverse curve. The bending process within the material is exactly the same in both cases (as it is in the different shapes of leaf springs mentioned above).

Reinforcing for flexural strength
You are correct that extending the shaft into the blade area will stiffen the blade (and make it heavier), but running it “far into” the blade area can be wasteful. The greatest stress within the blade is at its junction with the shaft. The stress within the blade itself becomes progressively less toward the tip. Therefore the best use of “extended shaft material” within the blade would be for that shaft to become progressively smaller from the base of the blade toward the tip, and tapering to “nothing” within a fairly short distance since the blade really doesn’t need additional reinforcing well away from the base.

I don’t agree. I’ve had paddles that
broke in the blade, but the only one that ever broke at the throat was a Norse where I had cut the upper blade margin down, not noticing that Norse didn’t wrap glass all the way around the shaft once it entered the blade.

I’ve had blades break where there were stress risers caused either by my poorly distributed glassing, or by the maker not appreciating how the central rib should be blended into the rest of the blade.

Must have been a weak spot there

– Last Updated: Feb-18-14 2:10 PM EST –

Your statement of disagreement is not in conflict with what I wrote (a break located where there is poor blending of shaft to blade is not contradicted by the premise of my post, and I didn't attempt to define the location at which the need for extra reinforcing would end, as that will depend on how thin you try to make the blade). Therefore I am assuming you are talking about the value of curvature (as per your first post) and/or some notion that stress is greater in the middle than at the base.

If the blade broke in the middle, there was a weak point there. Stress can't be greater in the middle than at the base. It's a cantilevered system, and the progression of stress magnitude only goes in one direction.

As another example, consider a free-standing tower. Like a paddle blade, the tower is anchored at the base and a perpendicular force (wind) is applied all along one side. Such towers are always progressively thicker/wider toward the base. The reason for that is because the stress is progressively greater toward the base so greater strength is needed there. Stress becomes progressively less toward the free end. The Eifel Tower is the most well-known example, and its shape clearly illustrates the exponential nature of the rate of stress increase from tip to base, but simpler designs are still wider toward the base. You will never see an engineered cantilever that is wider/thicker/stronger in the middle.

Look at it another way. Try holding a board horizontally out into space, anchored at one end. Add weights to the board. No matter where you put the weights, the location under the most stress is the anchor point, and this is most easily illustrated with the principle of levers (the fulcrum in this case is within the anchor point, along with the end of the lever having the opposing force). Load the whole length of the board with similar weights (this is analogous to water force against a paddle blade) and you should be able to visualize how the weights applied nearest the free end have the greatest affect on stress along the whole length of the board, but that the stress within the material is greatest at the anchor point.

To make use of a curve in providing strength in resistance to a force from one side, BOTH ends of the curve must be anchored to an immovable point, or to a solid structure which is contiguous between the two ends. Paddle blades are rigidly anchored to something at just one end, so they don't fit that scenario, but it may be useful to to continue the description anyway. If a curve of this type bends toward the direction from which force is applied, the curve throws the whole structure into compression. If the curve goes the other way, it throws the whole system into tension. Either way, it reduces or even eliminates the bending stress that otherwise would be endured by the material if it were aligned in a straight line, and can therefore make the system stronger (which method is better depends on the material - stone or concrete makes a good arch, and rope or cable supports well along it's whole length when hanging in a "U"). But again, this has nothing to do with paddle blades or any other cantilevered system. In a cantilevered system, introducing a bit of curvature does nothing to change the fact that one face of the structure is in tension and the other is in compression. In a cantilevered system, the nature of the "bending action" and the structure's resistance to bending is basically the same (I'm ignoring how extreme curvature would make this more complex) whether the structure is straight or curved.

Convenient Method of Adding Dihedral
Angle to blade(s)?

thats what i was thinking