Solar‑Electric Trawler – Active Outrigger Glider Design

This page summarises a quick engineering feasibility study for the proposed “active paravane / flopper‑stopper” system. It answers the questions you asked and gives order‑of‑magnitude numbers, hardware options and a practical test plan.

1. What forces do we need from each glider?

A typical 10‑12 m cruising trawler displaces ≈ 5–10 t. For a roll‑righting moment we can approximate a desired righting torque of about 10 kNm at a 5° heel (a comfortable sea‑state). With outriggers placed ~5 m outboard from the centre‑line, each glider must generate roughly 2 kN of lift (≈ 200 kgf) to cancel the roll moment. In practice you would design for a safety factor of 1.5–2, so target ≈ 3 kN per side.

Target lift per glider ≈ 2 – 3 kN (≈ 200‑300 kgf)

2. How big must the glider be?

Assuming the glider behaves like a simple wing towed at a small angle of attack, the lift equation is:

L = ½ ρ V² A C_L

ρ = 1025 kg/m³ (seawater)
V = 4 kt ≈ 2.06 m/s
C_L ≈ 0.5 – 0.8 (depending on angle of attack, 5‑10°)

Solving for area A gives:

A ≈ 0.7 – 1.5 m² per glider (for 2–3 kN lift)

That translates to a wing about 1.5 m span × 0.5 m chord. The whole glider (including fuselage, tail and fins) would be roughly the size of a small radio‑controlled model airplane – about 1.2 m long and 1.5 kg in air (≈ 2 kg in water when neutrally buoyant).

Resulting drag

Using a drag coefficient of about 0.1 for a streamlined shape:

D = ½ ρ V² A C_D ≈ 160 N per glider.

Total drag for both outriggers ≈ 320 N.

3. Power needed to tow the gliders at 4 kt

Power = drag × velocity:

P ≈ 320 N × 2.06 m/s ≈ 660 W (total for both gliders)

This is the hydrodynamic towing power. The actual power drawn from the boat’s battery will be a bit higher because of losses in the tether cable and actuator efficiency – expect ≈ 0.8 – 1 kW at full corrective force. For a 4‑knot solar‑electric cruiser that is a very small fraction of the available solar‑array power (typically 2–5 kW on a 10‑m boat).

4. Underwater‑rated actuators for the tail fins

You need a compact, sealed actuator that can move a small control surface (the glider’s tail) with a few newton‑metres of torque. Options:

Type	Typical specs	Size (mm)	Weight (kg)	Cost (USD)
Water‑proof RC servo (e.g. KST X12, Blue Robotics “BRS‑500”)	Torque 12 kg·cm (≈ 1.2 Nm), 180° travel, 6‑8 V	≈ 40 × 20 × 35	≈ 0.07	$80 – $150
Sealed brush‑less linear actuator (e.g. Thomson Actuator “Sealed Lead Screw”)	Force 150 N, stroke 50 mm, 12‑24 V	≈ 80 × 30 × 30	≈ 0.4	$200 – $350
Miniature hydraulic cylinder (custom marine‑grade)	Up to 500 N, 20‑mm bore, 100 mm stroke	≈ 120 × 25 × 25	≈ 0.3 (excluding pump)	$300 – $500 (pump & control extra)

For a “glider” that will be towed at low speed, a high‑quality waterproof RC servo is the simplest and cheapest solution – you can get models that are rated to 200 m depth. If you need more force (e.g., larger fins) a sealed linear actuator is the next step.

Cable / tether for power + data + strong load

The line that connects the boat to each glider must simultaneously:

Carry the mechanical load (≈ 3 kN breaking strength)
Transmit power (e.g., 24 V @ 5 A = 120 W)
Transmit data (CAN‑bus, RS‑485 or even simple serial)

A practical “composite tether” can be built like this:

Component	Spec	Typical Cost (per metre)
Strength member	braided Kevlar or Spectra, 6 mm Ø, 3 kN breaking load	$1‑$2
Power conductors	2 × 18 AWG (≈ 1 mm²) – 24 V, 5 A	$0.5
Data pair	2 × 24 AWG twisted‑pair, shielded	$0.3
Outer jacket	Polyurethane, 8 mm Ø, UV‑stable, abrasion‑resistant	$0.8

Total cost ≈ $3 – $5 per metre. For a 10 m tether (the maximum you would need for a 5 m outrigger reach) that is $30‑$50. You can also buy ready‑made “ROV tethers” (e.g., from Blue Robotics) that include all of the above for about $15‑$20 per metre – a good choice for a prototype.

5. Control system – sensors & software

Load cell (strain‑gauge) installed in the tether‑winch or directly on the glider fitting – gives real‑time tension reading (resolution ≈ 1 N).
IMU on the boat (e.g., Adafruit BNO055 or SparkFun 9‑DOF) – supplies roll/pitch acceleration.
Controller – a small embedded computer (e.g., Raspberry Pi 4 or an Arduino Due) running a PID or LQR control loop.
Communication – CAN‑bus is robust, or simply a serial line inside the tether.

The control algorithm would look roughly like this:

error = desired_roll - measured_roll
correction = Kp*error + Ki*integral(error) + Kd*(error - prev_error)
target_lift = baseline_lift + correction    // baseline = weight of glider + static lift
tail_angle = map(target_lift, min_lift, max_lift, -15°, +15°)
send_command_to_servo(tail_angle)

You would also incorporate the tension feedback – if the tension drops (glider is being “over‑lifted”) you can reduce the tail angle to avoid excessive drag.

6. Plausibility & next steps

Overall verdict: Yes, the concept is plausible. The required forces, sizes and power draw are modest for a 10‑m solar‑electric trawler. The biggest challenges are:

Making the tether robust and reliable (it will be dragged through waves for many hours).
Ensuring the glider can survive accidental impacts (fishing lines, debris) – a quick‑release mechanism is advisable.
Implementing a fail‑safe (if power is lost the glider should default to a neutral or “dive‑down” position so the boat does not lose stability).

All of these are engineering problems that have been solved in similar systems (e.g., passive paravanes, ROVs, autonomous underwater gliders).

Recommended prototype test

Buy a small (≈ 1 m) “glider” – e.g., a foam‑wing sailplane or a cheap RC flying wing.
Fit a waterproof servo to the tail and attach a 5‑m tether (the composite cable described above).
Mount a load cell on the tether and an IMU on a small inflatable test boat (or on the parent trawler if you have one).
Run the boat at 3‑5 kt, letting the glider tow behind, and manually command different tail angles to see the resulting lift and drag.
Collect data: lift vs. tail angle, drag vs. speed, tension spikes, etc.
Implement the PID loop on a Raspberry Pi and repeat – you should see a measurable reduction in roll amplitude.

If the test confirms the lift‑versus‑tail‑angle relationship predicted above, you can confidently scale the design up for the final solar‑electric trawler.

7. Quick reference numbers

Parameter	Value	Comment
Target lift per glider	2 – 3 kN	≈ 200‑300 kgf
Glider wing area	0.7 – 1.5 m²	≈ 1.5 m span × 0.5 m chord
Glider drag at 4 kt	≈ 160 N	≈ 16 kgf per glider
Total towing power	≈ 0.8 kW	Including cable losses
Servo torque needed	≈ 1 Nm	Typical waterproof RC servo
Actuator size	≈ 40 × 20 × 35 mm	RC servo
Actuator cost	$80 – $150	Each glider
Tether cost (per metre)	$3 – $5	Composite power+data+strength

8. References & further reading

Bertram, Practical Naval Architecture, 2020 – for righting‑moment & stability calculations.
Blue Robotics “Tether Technical Guide” – excellent reference for composite underwater cables.
“Active Paravane Stabiliser” – US Patent US‑2018/0257335 (shows a commercial implementation).
“RC Servo waterproofing guide” – many hobby forums (e.g., RC Groups) discuss sealing servos with O‑rings and epoxy.
“Solar‑electric trawler design” – boat‑design.net forums have several build threads that list typical solar‑array powers (2‑5 kW).

All numbers above are order‑of‑magnitude estimates. Detailed design should involve CFD / model‑testing and a full safety‑factor analysis. Feel free to adapt the layout for your own site.