Lifting power of a fly rod

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Lifting power of a fly rod

Post Number:#91  Postby VGB » Sat Sep 09, 2017 11:35 am

Merlin

Taking it back to the beginning, this is the point I find confusing:

Merlin wrote:What happens if he wants to use a stronger rod, let’s say 15% stiffer (one line size approximately)?


The lifting capability of the rod is usually based on a large scale deflection, casting on a relatively small scale deflection. Also a stiffer rod is not necessarily stronger.

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Post Number:#92  Postby Merlin » Sat Sep 09, 2017 1:34 pm

All right Vince, let's go back to the beginning. Give me some time and I shall be back with an example, figures included. I shall restrict the comparison to saltwater rods, and discuss their so called lifting power by reference to their stiffness, which will bring back somewhere the rod/line rating in the discussion. Then I hope things will clear up.

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Lifting power of a fly rod

Post Number:#93  Postby Lou Bruno » Sat Sep 09, 2017 2:04 pm

Merlin

I for one would welcome your research and input, especially focusing on saltwater rods.

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Post Number:#94  Postby Merlin » Sun Sep 10, 2017 12:19 pm

A few words for introduction: I am using a virtual test with models of saltwater rods. There are two 9 foot rods, one for a number 8 line, one for a number 10 line, and I have added a shorter number ten rod, 8 feet long (commercial rods are in the range 8’3 to 11’). These rods belong to the same family (series); their loaded frequency is similar for a given length of their corresponding line in the 20 to 70 feet range of carry (line outside the guides). So the rods once loaded feel as fast.

Number 10 rods have nearly the same small deflection stiffness (a question of fine tuning, I can do better); the stiffness of the three rods is illustrated below with a horizontal rod and an increasing load. The shorter rod is a slightly harder spring by comparison to its longer counterpart, which is normal. To design the rods I used my own scale, the materials used for each piece are the same, and only the mandrels and cloth patterns are changing.

rod stiffness.JPG
rod stiffness.JPG (31.16 KiB) Viewed 395 times

Now let’s see the deflection for a tough condition, where the rod shaft is tilted 30 degrees upwards from horizontal with an imaginary fish pulling vertically. The torque at wrist is a given and you can see the relative difference in “lifting power” (in %).

rod deflection lift.JPG
rod deflection lift.JPG (23.08 KiB) Viewed 395 times

If an angler changes its 908 for a 910 he will lose on the fighting playground, even if its casting (fly, line) is more adequate. If he can choose a shorter rod, then he will win on the fighting ground but maybe loose something on casting distance, although this is arguable: the swing weight of the 810 is lower than the one of the 908, so you can expect some compensation in terms of maximum rotation speed.

Incidentally, in terms of stresses, there is a price to pay. Under these lifting conditions, the highest stresses are met for the short rod (810), followed by the 908 and then the 910. Maximum stress level is raised by 25 % (810 versus 908) or reduced by 10% (910 versus 908). We are still far from the limit however.

Now you may have a better view of the pro and cons of rod/ line combinations for salwater fishing. Feel free to ask for information.

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Lifting power of a fly rod

Post Number:#95  Postby VGB » Sun Sep 10, 2017 2:31 pm

Merlin wrote:
Incidentally, in terms of stresses, there is a price to pay. Under these lifting conditions, the highest stresses are met for the short rod (810), followed by the 908 and then the 910. Maximum stress level is raised by 25 % (810 versus 908) or reduced by 10% (910 versus 908). We are still far from the limit however.



Thank you Merlin, I appreciate your efforts but I am still stuck with your earlier statement:

What happens if he wants to use a stronger rod, let’s say 15% stiffer (one line size approximately)?


Stiffness is a measure of the rods ability to return to its original form, while strength is a measure of how much stress can be applied before the rod breaks. They are 2 different properties and making a rod stiffer does not necessarily mean the rod is automatically stronger. For example, I have a solid glass rod about #2/3, it is a complete noodle but virtually indestructible (I suspect the metal ferrule is the weakest part) and it is most likely stronger than any of my higher line weight graphite rods. I know that you understand this and that it is why I asked for your assumptions at Post 72.

I think that John Field covers it well in his book, when he quotes Steve Rajeff:
Modulus is the measurement of resistance to strain, expressed as a ratio. In the context of fishing rod design, it describes the stiffness of the fiber used to construct the rod. It does not by itself determine rod stiffness, weight or strength. High modulus material can be designed into making soft rods and conversely, low modulus material can be designed into making stiff rods. Several other important measurements of rod material to consider include fiber strength, elongation, compression rate, resin content percentage and resin toughness, scrim type and weight. The goal of many rod makers is to optimize these variables, achieve the desired action (combined taper and stiffness), high strength at low weight and reasonable cost.


In reality the instantaneous stresses are much higher than the dead weight condition because as Mike and Sakke mentioned earlier, the fish is pulling down and the fisherman is pulling against it with his whole body. If you look at videos of rod breaks during fish fighting, they tend to occur higher up the rods, compared to when the the load is applied smoothly and slowly during manufacturers demonstrations and the breaks occur near the butt. My ideal rod under these conditions would be a short, strong rod that is only stiff enough to cast the fly I need. It may have glass scrim or even be a glass rod and I recognise that obtaining that strength may incur a weight penalty and impact casting performance but it may be a reasonable trade under those fishing conditions.

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Post Number:#96  Postby Merlin » Mon Sep 11, 2017 7:53 am

Hi Vince

I see now, in fact I used “stronger” in a broad sense: more resistant to bending, and not necessarily more resistant to breakage. It was implicit that an angler would just have one length of SW available at his shop. It is quite rare to find short SW rods on the market. But mechanically speaking, you are right, stiffer does not necessarily imply more resistant to breakage, especially if you consider different brands.

I then rechecked my files and realized I had inverted two numbers in the butt section of the 810 rod. Once this corrected, I draw new graphics, here they are:

SW rod stiffness.JPG
SW rod stiffness.JPG (33.24 KiB) Viewed 340 times

SW given torque.JPG
SW given torque.JPG (25.5 KiB) Viewed 340 times

SW given pull.JPG
SW given pull.JPG (24.06 KiB) Viewed 340 times


I added the "fish's point of view" with the last graphic.

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Post Number:#97  Postby VGB » Mon Sep 11, 2017 8:34 am

Merlin wrote:I see now, in fact I used “stronger” in a broad sense: more resistant to bending, and not necessarily more resistant to breakage. It was implicit that an angler would just have one length of SW available at his shop. It is quite rare to find short SW rods on the market. But mechanically speaking, you are right, stiffer does not necessarily imply more resistant to breakage, especially if you consider different brands.




Thanks Merlin, the stiffness/strength debate does surface in forums and I think that generically there is some truth that rods are stronger as you go up the line weights because additional material tends to be added. However, as you say it is not true across the brands and I have also found that some rods have problems with individual models in a range, there is also a question of technique which James addressed in his FP. This article from David Norwich does make you wonder about some design philosophies:

http://www.davidnorwichrodmaker.co.uk/DNscomments.htm

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Post Number:#98  Postby Graeme H » Mon Sep 11, 2017 9:20 am

Merlin wrote:It is quite rare to find short SW rods on the market.

There are actually quite a few 8' SW rods around, although I don't know what you call saltwater. For me, any rod that has components that are a little bit resistant to corrosion is a SW rod. Mainly, no wood in the reel seat.

Here are a few rods that fit the bill for me. All are 8' or less in length. They may not be marketed as being SW rods, but they don't dissolve just because there is salt in the water ...

TFO Mini Mag
Redington Predator
Sage Peacock Bass II and Largemouth Bass II
G.Loomis ShortStix
Echo BAG Quickshot
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Lifting power of a fly rod

Post Number:#99  Postby Merlin » Mon Sep 11, 2017 10:55 am

Thanks Graeme. I missed the FW/SW models. Some are pretty short (7'6).

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Post Number:#100  Postby Graeme H » Mon Sep 11, 2017 11:20 am

They're great for kayak fishing and I use them more often than not when I'm boat fishing too.

Paul's new 7'6" HT10 would be a very interesting rod for my needs. :) However, I've designed my own 8" glass blanks for my needs. The shorter HT would have o be pretty good to supplant them. Maybe supplement. :D

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