[A-PEN68 Development Log] Episode 6: The "Difference" Brought About by A-PEN Sound

A-PEN68 clear color sample. The internal structure is clearly visible. This is what produces the distinctive one-knock sound.

── Acoustic Domination: Unlocking the Power of the One-Knock Sound

Our recent session on the Yodo River with Motoki-san from Tsunami Lures turned into an absolute numbers day—a textbook "blowup fest." But looking back, what stuck with me most wasn't just the sheer volume of fish; it was the acoustics. I want to dive deep into how a lure's sound signature functions as a critical performance pillar for the A-PEN series.

That day, Motoki-san was throwing the Frank Sum, a killer collaborative pencil bait designed by Sum Lures and Tsunami Lures.

Mr. Motoki caught a lively bass using the Frank Sam, a collaborative pencil lure from Tsunami Lures & Sam Lures.

Measuring 70mm and weighing 11g, it sits about two sizes larger than our A-PEN 65 profile. But the most critical architectural difference was under the hood: his walking bait was completely silent (non-rattled), whereas my A-PEN 65 was loaded with a heavy, distinct one-knock rattle system.

The A-PEN65, a one-knock sound type lure, attracted bites from fish of this size one after another.

The Technical Edge: Skill vs. Sound

Let’s establish some context: Motoki-san is a legendary topwater veteran. His understanding of the Yodo River is unparalleled, and his boat positioning, casting accuracy, and presentation timing are, frankly, lightyears ahead of mine. In fact, after watching how the fish were reacting to my line, he perfectly matched my cadence, retrieve speed, and casting angles.

Yet, despite his superior execution, the discrepancy in blowups was staggering.

It wasn't a matter of skill, boat positioning, or cast placement. When you strip away all the variables, the undeniable truth came down to a single engineering factor: The A-PEN 65 had a one-knock rattle, and his pencil bait was dead silent.

Looking closely at the environmental factors that day, the Yodo River wasn't running clear. Due to upstream construction, the water had a distinct milky, stained tint. While there was some baseline visibility, the fish could not lock onto the lure’s profile using sight alone.

In low-visibility conditions like this, a lure needs a strong, structural acoustic footprint to help fish track it from a distance. Our field testing proved this theory beyond a shadow of a doubt.

Due to construction work upstream, many areas of the Yodo River were muddy.

From Reservoirs to Rivers: The Acoustics of Stained Water

This exact scenario triggered a massive wave of déjà vu. Years ago at Lake Ikehara, Satan Shimada pinned a monstrous 65cm, 4kg+ (9lb+) giant bass on our Silent Blaster. The reservoir had taken on heavy mud and stain due to torrential rain, decimating the underwater visibility. Shimada-san leaned on the Silent Blaster precisely because it features a violent one-knock resonance and immense water displacement. That acoustic signature triggered a flurry of giant bites, yielding a 50cm+ backup, the 65cm monster, and another quality fish in rapid succession.

Satan Shimada caught a fish over 65cm and 4kg using a Silent Blaster.

While the settings were completely different, the core variable between that day at Ikehara and our day on the Yodo River was identical: the fish could not rely solely on their vision.

When water clarity drops and silhouettes blur, a heavy, single-knocking frequency functions as a beacon that triggers a predator's lateral line and flips their aggression switch. To verify this on the Yodo, I actually switched over to a silent, non-rattled prototype of the A-PEN 65. The result? The bite completely died. The second I clipped the one-knock model back on, the blowups resumed.

I tried various lures, including one-knock sound and silent lures, on my fishing trip to the Yodo River.

Silent vs. Rattling: Finding the Tactical Fit

I want to be completely clear here to avoid any misunderstanding: This does not mean rattling baits are "good" and silent baits are "bad." This isn't a competition about which design is superior.

Both a heavy one-knock signature and a completely silent profile are specific performance tools designed for distinct scenarios. As a builder, it’s all about finding the optimal situational solution.

On that particular day on the Yodo River, the conditions heavily favored a loud presentation. In stained water where fish cannot visually track a topwater plug cleanly, a deep, singular thud cuts through the environment far better than high-pitch micro-rattles or an entirely silent approach.

Internal structure of a 3D printed sample of the A-PEN65. At this time, the structure was a scaled-down version of the A-PEN90's.

Seeing such a definitive difference in a compact profile gave me immense confidence. It proved that a heavy one-knock rattle inside a downsized walking bait is an absolute lethal weapon, validating the entire R&D roadmap for the A-PEN series.

Conversely, there are plenty of scenarios where a silent stickbait is mandatory. In ultra-clear water, on high-pressure public fisheries, or when fish become hyper-spooked by noise, a loud rattle will shut down a school instantly. In those conditions, a silent profile is exactly what you need to coax a finicky fish into committing. It’s a lesson I've learned over decades on the water.

The takeaway wasn't about choosing one over the other; it was about strategic application. There are bites you will only trigger with a one-knock sound, and there are fish you will only catch with a silent presentation. The A-PEN’s specific pitch simply unlocks a category of fish that traditional topwater plugs have historically missed.

A-PEN65 samples were being manufactured and tested in multiple types.

The Turning Point: Solving the Casting Equation

While the acoustic data from the Yodo River was a massive win, the marathon testing session exposed our next engineering bottleneck: casting distance and ballistics.

After grinding with the A-PEN 65 from dawn till dusk, I realized the current frame lacked the raw cutting power needed during windy conditions or long-distance bank casting. Don't get me wrong—the fish-catching metrics were incredible, and when launching from a boat, the casting was perfectly manageable. But it wasn't perfect.

If we wanted this bait to dominate for shore-bound anglers (walk-the-bank fishing), we had to elevate its casting distance and wind resistance to the next level.

Strangely enough, this realization perfectly synchronized with the feedback I had been receiving from our saltwater bream (Chining) consultants. They were asking for the exact same thing.

Looking back, that day on the water with Motoki-san was the definitive fork in the road that shaped the destiny of the A-PEN 68. It wasn't built purely on theoretical math or boardroom concepts; it was forged by watching how wild fish responded to physical prototypes on pressured rivers.

Once I proved that this specific acoustic layout could dominate high-pressure bass, I knew the exact same blueprint would transfer flawlessly to the topwater bream market.

Initial 3D print samples after adjusting from A-PEN65 to A-PEN68. The settings were still far from satisfactory.

The A-PEN 65 was our proof-of-concept—the vital first step toward creating a multi-species pencil bait that bridge the gap between fresh and saltwater environments. More importantly, it drew the exact blueprint we needed to transition into the development of the 68mm frame.

In our next entry, we break down the dimensional shift: [Scaling Up: The Evolution from A-PEN 65 to A-PEN 68].

Stay tuned.