casting a longer and a shorter head being equal in all else

[Bernd Ziesche]

Hi everyone,
long time ago I started to dig my way into shooting head adjustment. I soon realized, that using two different head length of the same weight, profile and density would make the longer head feel lighter during casting it.
Therefore I started to add weight when using longer heads.
Ever since I was thinking about the resons why the shorter head feels lighter.

Some time ago I started to put down some notes and had a few discussions about them here and there.

I would love to discuss these notes and find out, if they make any sense to you and to dig up all factors I probably did not yet think of.

My notes:

Comparing the feel of casting two heads only varying in head length on the same fly rod:

fly line (head) 1:
diameter 1,5mm, radius 0,75mm (=0,075cm), length 10m (=10³ cm), density 0,9g/cm³
mass = volume x density = Pi x radius² x length x density = 3,14 x 0,075² x 10³ x 0,9 = 15,9g
surface = 2 x Pi x radius² x length = 2 x 3,14 x 0,075² x 10³ = 35,3cm³

fly line (head) 2:
diameter 0,53mm, radius 0,265mm (=0,0265cm), length 20m (=20³ cm), density 0,9g/cm³
mass = volume x density = Pi x radius² x length x density = 3,14 x 0,0265² x 20³ x 0,9 = 15,9g
surface = 2 x Pi x radius² x length = 2 x 3,14 x 0,0265² x 20³ = 35,3cm³
Comparing both heads in fly casting:

Head 1 & 2 are having the same density (floating line), the same weight (15,9g), the same surface (35,3cm³), but different length (head1 = 10m & head2 = 20m). Friction between surface head (coating) and air will remain the same for both heads.

The shorter head usually means increased tension between the rod tip and the end of the head. Therefore the shorter head feels heavier during acceleration of the rod (based on increased resistance against the rod tip). This is the dominating factor. Think of it as casting with good and bad timing. Bad timing means to start with less rod bend due to less resistance provided by the non straight fly line. You can feel that immediately!

During deceleration of the head (unrolling) each centimeter of it will be stopped one after another when morphing thru the (fast) fly-leg into the (less dynamic) rod-leg. This means that the timeframe between the first centimeter oustide the tip will be stopped (unrolled) until the last centimeter of the head will morph into the rod-leg, is significant bigger for the longer head. Due to that there is a bigger loss in momentum (mass x velocity) for the first part of head outside the tip during unrolling for the longer head. In addition tension between loop front and rod tip decreases proportional to the size of timeframe for unrolling. This leads to less force acting on the tip during unrolling of the longer head.
For the shorter head each centimeter has more weight (compared to the longer head). Due to this the force acting on the tip during unrolling will also be higher for the shorter head.

Another effect has to be taken into account (thanks Aitor): The mass center of the longer head has to be thrown further for unrolling (because the head is longer). This needs more force application. But this is not the dominating factor!

Final conclusion:
The longer head needs increased weight in order to offer a similar feeling (based on the force acting on the rod tip) during fly casting (compared to the shorter head).

Best
Bernd

[James9118]

surface = 2 x Pi x radius² x length = 2 x 3,14 x 0,0265² x 20³ = 35,3cm³

Hi Bernd - you might want to correct this.

[Bernd Ziesche]

Hi James,
let me know how
Thanks
Bernd

[James9118]

Surface area is 2 x Pi x r x length, ignoring any tapers etc.

[James9118]

Do you think the shorter head could feel heavier because you 'hook-up' with it earlier as there is likely to be less sag in it?

[Lasse Karlsson]

Hi Bernd

Pretty cool thoughts!

Usually I just get the point past when saying it's the difference in feel when throwing a 4 weight or a 9 weight. That's the two lines you would have to use to make those heads and a 4 weight isn't very good in carrying a size 1 zonker

I never understood the one weight, different length argument, shorter should be lighter, longer should be heavier in my logic (and that might be the problem )

Cheers
Lasse

[Ben_d]

it's the difference in feel when throwing a 4 weight or a 9 weight. That's the two lines you would have to use to make those heads and a 4 weight isn't very good in carrying a size 1 zonker

That's how I see it. What is interesting is how, in certain conditions two heads of the same weight but different lengths can often perform differently.

I never understood the one weight, different length argument, shorter should be lighter, longer should be heavier in my logic

I'd agree with that, there's a certain set of Spey lines that were made in different lengths but same weight. Out of the three different lengths, one works, the other two aren't so hot.

Cheers

Ben

[Bernd Ziesche]

Surface area is 2 x Pi x r x length, ignoring any tapers etc.

James,
many thanks! I mixed up surface and volume...

Here is the correction:

fly line (head) 1:
diameter 1,5mm, radius 0,75mm (=0,075cm), length 10m (=1000 cm), density 0,9g/cm³
mass = volume x density = Pi x radius² x length x density = 3,14 x 0,075² x 1000 x 0,9 = 15,9g
surface = 2 x Pi x radius x length = 2 x 3,14 x 0,075 x 1000 = 471cm²

fly line (head) 2:
diameter 1,06mm, radius 0,53mm (=0,053cm), length 20m (=2000 cm), density 0,9g/cm³
mass = volume x density = Pi x radius² x length x density = 3,14 x 0,053² x 2000 x 0,9 = 15,9g
surface = 2 x Pi x radius x length = 2 x 3,14 x 0,053 x 2000 = 666cm²

Comparing both heads in fly casting:

Head 1 & 2 are having the same density (floating line) & the same weight (15,9g). Heads vary in Surface (head1 = 471cm² / head2 = 666cm²) & length (head1 = 10m / head2 = 20m). Friction between surface head (coating) and air will remain the same for both heads.

Since though head 2 (same weight, same density, but longer, thinner and more surface) has more surface, I find it amazing that still the shorter head feels heavier.

Hi Lasse,
if I remember correct you like to distance cast 12m head in 16 gram on the TCX6wt.?
The MED 5wt. has 19 gram on 20m. Your max carry was ca. 23,5m outside the tip.
Do you agree, that a 12m head offering the weight of 23,5m 5wt. MED would feel very heavy on that rod?
Greets
Bernd

[Bernd Ziesche]

I'd agree with that, there's a certain set of Spey lines that were made in different lengths but same weight. Out of the three different lengths, one works, the other two aren't so hot.

Hi Ben,
if one works, which one was it and what means the other two aren't so hot? Does that refer to them feeling too light or to heavy?
Greets
Bernd

[Ben_d]

Long one too light, short one too heavy middle one about right for most rods bearing the same rating as the line box Bernd.

Cheers

Ben