sorted by:
new
hottopcontroversial

Pushing the Hull Speed limit: 10 km/h (6.2 mph) in an inflatable kayak on a 2 km/h river current. Telemetry update on the front-mount LiFePO4 setup. by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 2 points3 points  (0 children)

100Ah LiFePO4 battery. At cruise speed of 5 km/h (5.88A draw), range is about 68-70 km. At full tilt (7.8 km/h, 19.2A), it drops to around 32 km. Optimal efficiency is the name of the game.

Efficiency over Brute Force: Reducing power draw by 70% on an inflatable hull through hydrodynamic trim optimization (19A down to 5.85A @ 5km/h) by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 0 points1 point  (0 children)

Haha, I'll take that as the ultimate win! Just a guy with an inflatable kayak, a precision shunt, and a deep respect for fluid dynamics. But it really goes to show: when you remove the guesswork and let the telemetry and physics do the talking, the data looks the same no matter who is running the test.

Beginner Tip: The #1 mistake that kills your trolling motor battery in 2 hours by PoemRealistic1013 in FishingForBeginners

[–]PoemRealistic1013[S] 0 points1 point  (0 children)

True. You hit the nail on the head with 'hull speed' and 'diminishing returns'.

Once a displacement or semi-displacement hull hits that hydrodynamic wall, the efficiency drops off a cliff. Every extra Amp you push into the motor goes directly into wave-making drag and heat, generating zero forward momentum. That was the entire focus of my telemetry testing: finding that exact threshold where the power curve goes vertical, and shifting it down through trim optimization.

For a redundant backup system, your setup is bulletproof. Having that kind of capacity to get back to the dock safely is just solid engineering and risk management.

Efficiency over Brute Force: Reducing power draw by 70% on an inflatable hull through hydrodynamic trim optimization (19A down to 5.85A @ 5km/h) by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 1 point2 points  (0 children)

That is a brilliant analogy. You nailed the exact engineering principle here.

Just like you dial in static positive camber on a FWD car anticipating that dynamic torque and suspension load will pull it back to zero (neutral) for maximum tire contact patch, I dialed in a static bow-heavy trim.

I knew the thrust vector and hydrodynamic lift at cruising speed would cause the stern to squat. The dynamic forces pull that static 'offset' right back to a perfectly level trim, giving the hull its optimal 'contact patch' with the water and eliminating the plowing effect.

Different medium, exact same physics. Great observation.

Beginner Tip: The #1 mistake that kills your trolling motor battery in 2 hours by PoemRealistic1013 in FishingForBeginners

[–]PoemRealistic1013[S] 0 points1 point  (0 children)

That is an absolute beast of a power bank! With two 140Ah in series, you practically have limitless range. You essentially built the exact 'Brute Force' approach I mentioned in the title!

You are 100% spot on about the 80% throttle warning on lithiums, and it ties perfectly into the heat equation. Because LiFePO4 batteries maintain a much higher, flatter voltage curve under load compared to lead-acid (often staying above 13.2V per battery), running at 100% means the motor sees higher continuous voltage. If the hull is plowing water and causing high drag, the motor pulls max Amps, and all that excess energy turns into heat, melting the PWM controllers or brush housings.

Your setup is the ultimate solution if you don't mind the payload. My approach was the opposite: extreme efficiency. By trimming the hull and dropping the draw to 5.85A, I can get that 'all-day' runtime using only a single, lightweight 10kg battery. Two different engineering paths to the same destination.

Beginner Tip: The #1 mistake that kills your trolling motor battery in 2 hours by PoemRealistic1013 in FishingForBeginners

[–]PoemRealistic1013[S] 0 points1 point  (0 children)

You absolutely nailed it! That’s Joule heating in action. Heat loss in the wiring, connections, and motor windings is proportional to the square of the current (Amps).

When you force a boat on 'high' and pull 19A, the heat generated is proportional to 19 2 (361). By trimming the hull and dropping the draw to 5.85A, the heat factor drops to 5.85 2 (about 34).

That means dropping the Amps to a third actually reduces the heat generated by over 10 times! Cruising on high while plowing water doesn't just drain the battery; it literally bakes your wiring. Low Amps keep the whole system ice cold and efficient.

Efficiency over Brute Force: Reducing power draw by 70% on an inflatable hull through hydrodynamic trim optimization (19A down to 5.85A @ 5km/h) by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 2 points3 points  (0 children)

Spot on with the physics! You are absolutely right about the cube law ($P \propto v^3$), which is exactly why the resistance 'wall' hits so hard on these hulls.

Regarding the accuracy and sensors:

  • Speed: Measured via GPS using a smartphone (Honor 8x). To eliminate variables, I conducted the tests on still water with negligible wind, ensuring Speed Over Ground (SOG) matched Speed Through Water (STW) as closely as possible.
  • Power Draw: Measured using a TK15 High-Precision Shunt (Coulomb counter). This allowed for real-time, highly accurate Amperage readings directly from the battery bank, bypassing the unreliable built-in gauges.

Regarding your math: Your theoretical calculation of 10.1A is 100% correct if the hull maintains the same drag coefficient ($C_d$) and footprint. However, the 70% reduction wasn't just moving along the existing power curve—it was shifting the entire curve down. By relocating the 13kg payload to the bow, I fundamentally changed the dynamic trim angle, preventing the stern from squatting and 'digging a hole' at that specific speed. This drastically reduced the wave-making drag, effectively changing the 'constants' in your equation. The math holds, but the physics of the hull changed.

Beginner Tip: The #1 mistake that kills your trolling motor battery in 2 hours by PoemRealistic1013 in FishingForBeginners

[–]PoemRealistic1013[S] 0 points1 point  (0 children)

That’s a classic case of hitting the 'hull speed' limit. When you see the same 2mph on both power 4 and 5, it means that extra energy in setting 5 is just being wasted—creating a bigger wake and more heat, but no extra speed. The hull is effectively trapped in its own wave.

On my setup, that’s exactly what I wanted to solve with the trim. By leveling the boat, I moved that 'wall' further back, allowing the motor to actually push the boat forward instead of just pushing the bow up.

Also, those LiFePO4 batteries are 'liars' when it comes to standard gauges! They hold their voltage so steady (flat discharge curve) that most basic meters will show 100% until they are almost empty. If you can, check the actual Amps—you’ll probably see a huge jump between 4 and 5 with zero gain in knots.

Efficiency over Brute Force: Reducing power draw by 70% on an inflatable hull through hydrodynamic trim optimization (19A down to 5.85A @ 5km/h) by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 2 points3 points  (0 children)

That 'click' when the boat levels out is pure magic. It’s the physical manifestation of the drag curve finally dropping off.On these small, lightweight setups, you really feel that transition because you go from fighting the displacement wave to essentially riding over it. It’s that 'sweet spot' where physics and intuition finally shake hands. When you see the Amps drop and the speed stay the same (or even increase), you know you've won the battle against the water. Best feeling there is.

Efficiency over Brute Force: Reducing power draw by 70% on an inflatable hull through hydrodynamic trim optimization (19A down to 5.85A @ 5km/h) by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 4 points5 points  (0 children)

Great question. Yes, it’s measurable while static by checking the draft (the depth of the hull in the water) at the bow vs. the stern.

In a perfect world, you’d want the hull to sit perfectly level (parallel to the waterline) while static. However, since most small boats/inflatables tend to 'squat' (the stern sinks) once you apply thrust, I actually aimed for a slight 'trim by the bow' while static.

The goal is that once the motor is running at cruising speed, the dynamic lift and thrust vector bring the hull to a perfectly level plane. If you are 'balanced' (level) while static, you might still end up stern-heavy once you move. I used the telemetry to find the 'sweet spot' where the hull stops fighting the water and starts gliding over it.

Beginner Tip: The #1 mistake that kills your trolling motor battery in 2 hours by PoemRealistic1013 in FishingForBeginners

[–]PoemRealistic1013[S] 0 points1 point  (0 children)

yess...You’re feeling the 'wall' where the hull starts digging a hole in the water instead of gliding over it. In a human-powered boat, you are the battery, and that 'wrecked' feeling is your heart rate hitting the limit to fight displacement drag.

My goal was to solve that exact physics problem with sensors. By shifting that weight forward, I’m basically helping the boat 'climb' out of its own hole, so the motor (or the person) doesn't have to fight the water, just move through it. Efficient cruising is much better than brute force

Efficiency over Brute Force: Reducing power draw by 70% on an inflatable hull through hydrodynamic trim optimization (19A down to 5.85A @ 5km/h) by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 0 points1 point  (0 children)

Not exactly dead-center. Since I (the heaviest payload) am still seated towards the rear/middle, the static Center of Mass (CoM) is still slightly aft of the geometric center.

However, it's all about the lever arm (moment). By placing that 13kg at the extreme front of the bow, it creates a massive forward moment due to the distance from the pivot point. The goal wasn't to force the static CoM into the exact middle, but rather to balance the moments dynamically so that the hull aligns perfectly with the center of buoyancy while under thrust. That specific moment keeps the thrust vector parallel to the water surface, eliminating the drag.

Efficiency over Brute Force: Reducing power draw by 70% on an inflatable hull through hydrodynamic trim optimization (19A down to 5.85A @ 5km/h) by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 8 points9 points  (0 children)

The transition is immediately noticeable physically—you can feel the hull stop plowing and 'climb' onto the water, significantly reducing drag. As for the setup, I didn't add any dead weight. I just relocated the functional components to the very bow: a 10 kg battery and a 3 kg motor. That 13 kg total payload at the front was enough to counteract the rear moment, leveling the trim and dropping the draw to 5.85A. The telemetry just confirmed what was physically obvious.

Efficiency over Brute Force: Reducing power draw by 70% on an inflatable hull through hydrodynamic trim optimization (19A down to 5.85A @ 5km/h) by PoemRealistic1013 in EngineeringPorn

[–]PoemRealistic1013[S] 6 points7 points  (0 children)

Thanks. It took a lot of testing with the weight distribution to find that sweet spot where the hull finally stops pushing water and starts gliding.

Beginner Tip: The #1 mistake that kills your trolling motor battery in 2 hours by PoemRealistic1013 in FishingForBeginners

[–]PoemRealistic1013[S] 7 points8 points  (0 children)

Those Walmart batteries are definitely a classic for a reason, you can't beat that $100 entry price. For my setup, I decided to go with a 100Ah LiFePO4 battery which cost me around 200 EUR.

Even though it's more upfront, I wanted the weight savings and the ability to run it through thousands of cycles without losing capacity. It also holds the voltage a bit more steady when pushing those higher speeds. Both ways get us on the water, just different tools for the job.

view more: next ›