It computes how the line tension (the force on the hook point) builds up over time when you sweep the rod, and whether it reaches the force needed to set the hook (Fp). The model includes the rod's large bend and recoil, line stretch, sag and slack, the fish's mass and water resistance, the drag and the limits of the angler's arm.
You can test claims such as “a longer rod sets the hook better”, “PE doesn't stretch, so it penetrates sooner” or “a fast-action rod sets the hook more surely” by changing the conditions yourself. Save conditions you find interesting and discuss them in the comments. No sign-up is needed, and every calculation runs in your browser.
2. Reading the screen
Left: the conditions
They are grouped into Sweep, Tackle, Rod, and Fish, hook and angler. Move any slider or menu and every result and chart on the right is recomputed at once. On a phone the conditions come first and the results below them.
Top right: four numbers
Result
“Penetrates” if the tension reaches Fp, otherwise “Does not penetrate”.
Time to penetrate
Milliseconds from the start of the sweep until the tension reaches Fp.
Sweep angle to penetrate
How far the rod has turned when the tension reaches Fp. Smaller means the hook sets with less movement.
Peak tension
The largest tension during the sweep. Above the drag setting the drag slips, so the tension goes no higher.
Panels (charts)
Simulation
An animation of the rod turning and bending and the line tightening, in step with the tension chart below it. When the tension reaches Fp the line turns orange and “Penetrated” appears. Use Pause and the playback speed (1/4 and 1/10 slow motion) to look closely.
Rod length / sweep speed / tip stiffness
Three tension curves that change just one thing from the current conditions. The dots mark the moment of penetration.
Rod action
Builds fast, regular and slow rods with the same power as yours (the same tip drop under a reference load) and compares their bending and tension.
Rod properties
The stiffness distribution, a 1/3 deflection test (a guide to power and action), the natural frequency, the blank mass and the largest bending strain in this sweep (a guide to breakage). With a production rod selected, it also compares the rod with the maker's published bend chart.
Changing the rod length continuously
Rod length on the horizontal axis, showing the sweep angle and time needed and the peak tension.
Line stretch
How much the line stretches under tension. You can compare PE, nylon and fluorocarbon side by side.
Everyone's saved conditions
Conditions and comments posted by others (section 5).
Drag ⋮⋮ at the top right of a panel to reorder the panels, and use Half / Full to change their width (on wide screens only). The layout is kept in your browser; Reset the layout restores it. Open “Calculation model” at the bottom of the tool for the list of equations.
3. What the conditions mean
Sweep
Rod length L
From your hand (the pivot) to the rod tip.
Sweep angular velocity ω
How fast you swing the rod, in rad/s. 6 rad/s turns the rod about 340° in one second. The arm limits (τmax, αmax) can keep you from reaching this speed.
Sweep angle
How far the rod turns before it stops.
Rod–line angle at the start φ₀
The angle between the rod and the line when the sweep begins. Starting with the rod pointed at the fish (a small angle), the tip can hardly pull the line at first.
Tackle
Line type and size
PE, nylon or fluorocarbon. The size (Japanese gō) sets the diameter and how hard the line is to stretch.
Line out ℓ
Line length from the rod tip to the fish. Longer line means more stretch and sag.
Hook-up depth
How deep the fish is. It sets how much line is under water and how much it sags there.
Line tension at the bite T₀
The tension before the sweep starts. Smaller means a slacker line with more sag.
Slack from wind and current s
Extra slack from wind or current.
Drag setting
Above this tension the drag slips and line goes out.
Rod
Choose a production rod
Sets the stiffness and action read from the bend charts of Daiwa Branzino and Saltiga rods. Moving any value below switches back to Custom.
Butt stiffness EI₀ and tip stiffness EI₁
How stiff the rod is; it softens smoothly from butt to tip. A larger EI₀ is a more powerful rod, a smaller EI₁ a softer tip.
Bend section (tip) length a
The length of the soft part. Short gives a faster action, long a slower one.
Blank material and tip construction
For the same stiffness, higher-modulus material makes a thinner, lighter rod that recovers faster (with less margin before breaking).
Rod damping ratio ζ
How quickly the rod and line stop vibrating after the sweep, for a hand-held rod with a fish on. The default 0.15 is a provisional value.
Fish, hook and angler
Fish mass m
A heavier fish is slower to move, so the tension rises sooner.
Force to set the hook Fp
The force needed for the hook point to penetrate. It varies widely with the species, where the fish is hooked and the hook, so use a measured value if you have one.
Arm's maximum torque τmax and angular acceleration αmax
The limits of the angler's strength. When the rod carries a large force, the swing slows down.
All defaults are rough estimates. The small notes under the sliders also explain the current values (line diameter, amount of sag, rod power and so on).
4. Things to compare
Does a longer rod set the hook better? Look at “Rod length” and “Changing the rod length continuously” to see how the sweep angle and time needed change with rod length.
Line: switch between PE and nylon, and set ℓ to 10 m and 60 m, comparing the result and the time to penetrate. The Line stretch panel shows the difference in stretch.
Slack line: change the slack s and T₀ and watch how much the tension is delayed.
Rod action: in “Rod action”, compare how tension builds with fast and slow rods of the same power. Changing the reference load changes how “the same power” is defined.
Match your own rod: pick a production rod, then adjust EI₀, EI₁ and a step by step while watching the 1/3 deflection load and action in “Rod properties”.
5. Sharing, saving and comments
Share a link (without saving)
Copy a link to these conditions makes a link that sets the current conditions. Anyone who opens it sees the same conditions. Nothing is stored on the server.
Save conditions
Click Save these conditions, enter a title, an optional comment and a nickname, and save. Saved conditions appear under “Everyone's saved conditions”, and anyone can apply them with Load these conditions. Each saved entry also has a link that opens just that entry.
Comment
Click Comment on a saved entry to see the comments and write your own: what the conditions are meant to show, what the results suggest, or questions about the model.
Nickname, # code and deleting
No sign-up is needed. Your nickname (1–20 characters) is followed by a 6-character # code fixed for the browser you post from, which helps tell people with the same nickname apart.
You can delete your own posts with Delete. The key for deleting is kept in that browser, so you cannot delete from another browser or device. Clearing the browser's data also means you can no longer delete earlier posts.
Each post asks for a quick robot check (Cloudflare Turnstile). Posting a lot in a short time blocks posting for a while.
See the terms for how posts are handled and the privacy policy for what is recorded (both in Japanese).
6. Assumptions and limits
The hookset is modelled as a rotation of the rod about your hand only. Pulling the rod back or lifting the pivot is not included.
The rod is a single beam whose stiffness changes smoothly from butt to tip, with large bending and its first vibration mode (recoil). The mass of guides and grip and higher vibration modes are not included.
The fish is a body with mass (including the water it carries along) and water resistance, pulled by the line. Its own swimming and head shaking are not included.
Penetration is judged only by whether the tension reaches Fp.
Results are estimates from a simplified model. They do not guarantee whether a hookset succeeds or how strong a rod or line is. The equations and details are under “Calculation model” at the bottom of the tool.
7. Troubleshooting
It feels slow
Every slider move recomputes many cases. On older phones, move sliders slowly or release them once you have the value.
Show it in Japanese
Use 日本語 at the top right (your choice is remembered).
Can't save or comment
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