Thrust?

[Bernd Ziesche]

Morsie,
I interpret it as "thrust" is a term obviously being used in pretty different ways and yet supporting confusion among instructors. Thus I better avoid using it in my teaching.
For sure it does not define momentum to me. Not even to some degree. Momentum is clearly defined as mass times velocity. Not hard to understand, I think.
Regards
Bernd

[Mangrove Cuckoo]

I should probably stay in the shallows as this subject is definitely over my head... but I like the concept although maybe not the word so much.

To me "thrust" implies linearity. I'm not sure if it is straight ahead or at 180 degrees, but somehow straight. But, maybe that is just my lack of understanding technical stuff. I know that in SciFi movies they always use "thrusters" to alter the direction of stuff in orbit... which makes me think "linear" is likely my problem.

What Al's response lit up for me was the idea of a momentum change that is not linear. In casting, I think I sometimes thrust the rod forward right near the end of rotation to induce the rod tip to tighten the loop, but it seems the rod absorbs most of that input. The line's momentum doesn't seem to visibly change, the tip is just delayed.

For what I am mulling, my guess it that maybe "impulse" would be a better word? An increase in force for a short duration of time and, importantly for me, that is off linear? Force is a vector and momentum is a vector, so the line will deviate from the set path in relation to the magnitude of the impulse?

Doesn't that fit into the propagating wave thing?

The majority of momentum has been imparted to the line by translation and rotation with greater force over a longer time. A small off-linear impulse will have some effect but relatively much smaller. Isn't that what sets a wave?

Please excuse my "thinking on paper" and my infatuation with curve casts. I know the physics of a flying line are beyond me, so how or why a cast curves approaches magic.

[Geenomad]

For what I am mulling, my guess it that maybe "impulse" would be a better word? An increase in force for a short duration of time and, importantly for me, that is off linear? Force is a vector and momentum is a vector, so the line will deviate from the set path in relation to the magnitude of the impulse?


The majority of momentum has been imparted to the line by translation and rotation with greater force over a longer time. A small off-linear impulse will have some effect but relatively much smaller.

Hi Gary
How about if the thrust adds to the Net Force applied during translation and rotation in the intended direction of the cast? I use thrust often and understand it to add to the Work done on the line and to extend the Impulse by delaying the straightening of the rod. Space and time thing.

BTW, personally I struggle to see how during an oval cast tension can be kept constant when the caster completes the back cast, reverses direction and begins the forward cast. Some tension perhaps but constant tension no unless constant tension means any amount of tension, variation ignored. But maybe that's just me.

Cheers
Mark

[Mangrove Cuckoo]

Hi Mark,

How about if the thrust adds to the Net Force applied during translation and rotation in the intended direction of the cast?

My focus is really just about curve casts, so the impulse I am dwelling upon is not in the direction of the cast. But I agree that in your case the thrust would add something to the net force. Where I think differently is that my concept of a thrust is forward and is applied when the rod is almost horizontal... so it cannot be applied "during" translation and rotation - only at the very end when the rod has almost straightened.

I do have a very old and slow rod that bends in a large U shape when fully loaded, and with it I think I can add a noticeable input to the line with thrust. But with more modern rods, the tips are relatively weaker than the bottom and thrusting at the end of rotation doesn't seem to add much to the line. I'm sure it does but it is relatively much smaller to the point of being almost negligible.

I use thrust often and understand it to add to the Work done on the line and to extend the Impulse by delaying the straightening of the rod.

Again, I am with you on adding to the work done. I will have to think about how it might extend the impulse? My inclination is that graphite rods are faster than I am, so they quicken things rather than slow 'em down.

BTW, personally I struggle to see how during an oval cast tension can be kept constant when the caster completes the back cast, reverses direction and begins the forward cast. Some tension perhaps but constant tension no unless constant tension means any amount of tension, variation ignored. But maybe that's just me.

Yeah, once again we are on the same page. But I have not given this any thought. All I can seem to think about lately is curve casts... and I am very narrow minded.

[jarmo]

When Al Buhr uses the term "thrust" what does he mean?

In Buhr's book, in the section on the switch cast, the first page contains 17 occurrences of "thrust." I counted them yesterday.

I am not a native English speaker. I have never thought of "thrust" as a clearly defined technical term in that book. Maybe because of the combination of these I have always had an intuitive view of what the that word refers to. This interpretation has had a significant impact on my casting, so I appreciate the irony if I got it wrong.

I see "thrust" as the (powerful) use of the (strong) butt section of the rod when redirecting the line. If I do a switch cast with a shotgun lift and a dip, there are three stages where "thrust" is prominent during the setup of the D-loop:

• lift
• beginning of sweep
• redirecting line up into D.

The (powerful) use of the (strong) butt section comes from a combination of the angle of the rod and how the rod is moved (translation / rotation).

BTW, personally I struggle to see how during an oval cast tension can be kept constant when the caster completes the back cast, reverses direction and begins the forward cast. Some tension perhaps but constant tension no unless constant tension means any amount of tension, variation ignored.

In my mind it is the latter: some tension. And, IIRC, when Buhr writes about "constant tension" D-loop (as opposed to a V-loop), power is applied in the early part of the circle up, and then eased.

I find the feel of a "constant tension" switch cast pretty amazing.

[Paul Arden]

Wow quite interesting, guys! We do indeed appear to have differing ideas on this. Thrust for me has always been something at the very end of the forward cast, when the hand “thrusts forwards” in the second part of the stroke. A bit like when sword fighting... That’s maybe the best analogy I can give, thrusting the sword forward into an opponent (maybe Lars can clarify on this point).

I spent a couple of years playing with Thrust in my distance cast. I think there probably is still an element of thrust in my cast but it’s not the Swooping motion that many of us more readily identify as thrust, more a subtle movement.

Where I have found thrust to be a distinct advantage is for distance casting into the wind. I remember coming together with Lasse in Montana and coming to this conclusion. But the timing is tight and I find it difficult to replicate casts even with practise.

Cheers, Paul

[Morsie]

Bernd its not a term I have ever used in teaching either, I think its too indistinct or subtle or I'm too thick.

In the case of Al's book I have always struggled to understand what he means by "thrust" (why I asked). Sexyloops has for years been a battleground for definitions and the words that clarify them. I was listening to a non-fishing podcast the other day, an interview with a mathematician, an Aussie and the youngest ever member of the Royal Society, and they talked about how a single word can open up an idea (theory in this case) and that until someone hit on that word this particular exercise remained unclear. I hoped someone might be able to clarify it. Certainly some really interesting stuff to digest.

I also suspect it applies very much to the Spey casts and turning curves into straights, although in the reverse (straights into curves) it is clearly also applicable, all kinds of subtle and not so subtle thrusts going on there. Thanks.

[Geenomad]

My focus is really just about curve casts, so the impulse I am dwelling upon is not in the direction of the cast. But I agree that in your case the thrust would add something to the net force. Where I think differently is that my concept of a thrust is forward and is applied when the rod is almost horizontal... so it cannot be applied "during" translation and rotation - only at the very end when the rod has almost straightened.

Hi Gary
Not seeking to distract you from curve casts. Two things briefly. I don't have the video evidence but my strong suspicion is that the rod doesn't straighten until after the thrust and any associated haul are finished. It feels like i am pushing into the bend, especially when a bit of TLT influence gets mixed in.

Not sure how exactly you are throwing curves but if it includes a quick in to out or out to in shift in rod plane during delivery (something I play around with) then I think a bit of wave theory might help explain what's going on - or not. I get the idea of the momentum as the cause of the shape but more perhaps from the conceptual rather than analytical perspective.

Cheers
Mark

[Paul Arden]

I agree with both points, Mark. I would add to the first that I think Thrust angled slightly upwards will help straighten the tip path to RSP.

I’ll certainly go with wave and not momentum for curves mends. Overpowered casts on the other hand involve rebounding waves (the Boing ) and well as overpowering to fly the fly around. Underpowered curves are a combination of loop plane and trajectory (for me anyway).

Different momentums I find confusing and I’m not following the thinking in this regards yet.

Cheers, Paul

[Mangrove Cuckoo]

Mark

Not seeking to distract you from curve casts. Two things briefly. I don't have the video evidence but my strong suspicion is that the rod doesn't straighten until after the thrust and any associated haul are finished. It feels like i am pushing into the bend, especially when a bit of TLT influence gets mixed in.

I agree. Pushing into the bend is how I see thrust also. I think Lefty's use of the word "stab" was pretty good, BTW. As Paul stated above, it can help with focusing the loop for casting into the wind... and it can poke your fly back under the mangroves too. I do not see a great increase in line speed though, some but not that much, so I suspect the rod absorbs most of the input or the tip just does not have as much influence on the line compared to what the lower section of the rod has already imparted. I think it effects loop shape much more than line speed.

Not sure how exactly you are throwing curves but if it includes a quick in to out or out to in shift in rod plane during delivery (something I play around with) then I think a bit of wave theory might help explain what's going on - or not. I get the idea of the momentum as the cause of the shape but more perhaps from the conceptual rather than analytical perspective.

The thing is, off my head I can think of about 4 common ways to make a curve with acceleration. So, we are leaving out under-powered ones for now. You mentioned two, Paul's "boing", and a simple deviation from 180 are the others. All of those 4, I see as being applied during the major part of translation/rotation.

And, additionally, the one I use most commonly involves a thrust (the topic) coupled with an abrupt supination or pronation of the wrist. Some day we should have a dedicated thread on curves so I can make weird claims like this curve being caused by a form of pull-back.