Composite gurus

[Canotrouge]

Good day all!! I have a question for the composite gurus amongst us!! On my ww paddles I laid carbon fibre Indes S-glass on both face, but I was wondering if I would put carbon fibre only on one face is it would have the same or similar effect in terms of stiffness!! And if so would it be better having the CF on the back or the power face of the blade? You can see on the picture what I use to do.... both faces are the same!

 

[Glenn MacGrady]

I have no practical experience with this but do have theoretical thoughts.

First, I don't understand why you are adding the extra weight of carbon reinforcements, unless your wood blades are so thin that they flex and are merely serving as a core material.

Putting that question aside, stiffness of material is measured by Young's modulus. The higher the Young's modulus, the stiffer the material. The modulus of pine is 7, oak is 11, glass fiber is 40-45, and carbon fiber is 181. Therefore, the carbon is much stiffer than the wood and glass on your paddle.

So, for the same reason that carbon fiber is put on the outside rather than the inside of composite boat hulls, I would think it should be put on the power face of a paddle if it is only going to be put on one face. The power face is the face that will be put under tension by a strong pull through the water and it is usually the face that would take the most impacts from rocks, stumps and the river bottom.

This all assumes there is such a thing as a power face and back face on a perfectly symmetrical paddle. I always treated both sides of such paddles with equal respect and without any face discrimination. But, if you want to put a carbon facing on only one face, I theorize it should be on the power face.

 

[stripperguy]

Carbon fiber cloth is strongest in tension, less so in compression, and it abrades like a #2 pencil.
The load applied side of a simply supported beam (beam, truss, canoe hull, etc) is most often in compression, the far side in tension.
A canoe paddle is not a simply supported beam, rather it is a cantilevered beam...the load applied side is under tension.

So you should put the higher strength lay up on the tension side, and the lesser strength material on the back, or compression, side. All this assumes that the core has sufficient shear strength to survive the applied loads and resulting deflections.

Glenn forgot to include the units for the Young's modulus values above...in his examples it's GPa (Giga Pascals)...I can't think in Pascals, I'm a lbs/in^2 kinda guy.
Also, be careful about strength generalizations with respect to modulus, remember there is a dimensional aspect too, so 11 oz glass may be half the strength of 5.5 oz carbon rather than 1/4, due to the thickness disparity and the way the materials are sold and defined (weight/yd^2)
Regardless of the extreme high modulus of the carbon fiber, it's useless if the shear stresses exceed the shear strength of the epoxy holding it in place, or as mentioned above, if the core fails in shear.

Why then do canoe manufacturers put the carbon fiber on the outside of a hull???
Two reasons, first, it looks cool as hell, and cool sells boats.
Second, the designers don't design highly stressed components for a living! With the exception of high hertzian contact stresses (as from a very pointy rock), most loads that a canoe hull will see could be handled by an epoxy soaked T-shirt!! Of course there are other factors, but strictly from a strength point of view, even 4 oz E glass is sufficient.

 

[Glenn MacGrady]

Carbon fiber cloth is strongest in tension, less so in compression . . . .

That's one reason why I think we both agree that the carbon reinforcement should go on the power face. But when we get to this irresistible tangent . . .

Why then do canoe manufacturers put the carbon fiber on the outside of a hull???

. . . the carbon tension/compression differential doesn't answer the question for me. Most lightweight canoes have carbon outside and Kevlar inside. So, if the canoe hits a rock, the outside of the carbon cloth will be in compression and the inside will be in tension. But that wouldn't change if the carbon cloth was on the inside of the canoe rather than the outside, would it?

I don't think the placement of carbon on the outside has anything to do with carbon's resistance to compression vs. tension forces. It has to do with aesthetics, as you say, because carbon looks sexier on the outside than Kevlar would. Second, I think Kevlar on the outside would start to fuzz up in the bottom scratches. Third, Kevlar is very susceptible to UV degradation and carbon isn't. Finally, while carbon may be somewhat weaker in compression than in tension, it's still much stronger in compression than Kevlar. It's thus logical that the overall stiffer material in both tension and compression, carbon, goes on the outside, while the more puncture (fatigue) proof material, Kevlar, backs it up on the inside.

Disclaimer: I've never built a boat, but I've banged (and felt) up many.

 

[Canotrouge]

So I think I will try this, full carbon layer on the front/face/power face of the blade topped with a full layer of S-glass and 2 full layers of S-glass on the back, my paddle blades are not symmetric front to back but they are side to side... see picture! The blade is not super thin, but thin enough to match the Dynel cord I put all around the blade perimeter!

 

[Guest]

I have glassed the blades on some manufactured wood paddles, and put on very light weight edging.

I don’t know how much those additions weighed; 6oz (may have been 4oz) S-glass and carbon fiber tow, so not very much. Maybe an ounce or two in epoxy and near nothing in fabric, but the additional weight was all in the blade.

I was surprised at how different the paddle weight, and especially the swing weight & balance, felt with that minor weight change. Of course I was accustomed to the OEM weight/balance, so the change was more noticeable.

Experienced paddle makers (I am not) can probably design blade and shaft for their desired weight and balance with fabric reinforcements.