Below is a first-pass, back-of-the-envelope naval-architecture style estimate based on typical Caribbean solar resource, typical displacement hull scaling, and the basic lift equation for fin stabilizers. Real designs need a naval architect plus a seakeeping/CFD model (and decisions about target comfort, sea state, and operating profile). I’ll show the assumptions so you can swap numbers.
You described “equivalent of 60 by 30 ft worth of solar when all fold-out solar is extended”.
Daily energy (typical):
E_day ≈ PV_kWp × PSH × PR
≈ 33 kW × 5.5 h/day × (0.70 to 0.80)
≈ 127 to 145 kWh/day (typical average)
On very good days you might see 160–190 kWh/day; on cloudy/rainy periods it can be much lower. Your “2 days of batteries” improves continuity, but the long-run 24/7 average is still limited by average solar.
Convert daily energy into continuous electrical power:
P_elec_avg ≈ E_day / 24h ≈ (127 to 145) / 24 ≈ 5.3 to 6.0 kW average electrical
That average electrical power must cover not only propulsion, but also hotel loads (computers, refrigeration, watermaking, lighting, comms) and conversion losses. If you budget, for example:
A plausible long-run average might be ~3–5 kW to the propeller shaft if you want comfortable hotel loads, or ~4–6 kW shaft if you run very lean on hotel loads and prioritize motion.
A commonly used approximation is the Admiralty coefficient relationship:
P_shaft(hp) ≈ ( D^(2/3) × V^3 ) / C
Where:
For D ≈ 24.6, D^(2/3) ≈ 8.4. If we take C ≈ 230 as a mid value, then:
P_shaft(hp) ≈ 8.4 * V^3 / 230 ≈ 0.0365 * V^3
| Speed (kn) | P_shaft (hp) | P_shaft (kW) | Notes |
|---|---|---|---|
| 4.5 | 3.3 | 2.5 | Very “solar-friendly”, but slow passage-making |
| 5.5 | 6.1 | 4.5 | Often a realistic long-run solar target |
| 6.0 | 7.9 | 5.9 | Requires low hotel loads or larger effective daily solar |
| 6.5 | 10.0 | 7.5 | Hard to sustain 24/7 on ~1800 ft² PV unless very optimized |
| 7.0 | 12.5 | 9.3 | Typically beyond average-solar-only continuous budget |
Expectation: with ~1800 ft² of PV and 2 days batteries, a well-optimized 60 ft displacement trawler would likely average about ~5 to 6 knots over 24/7 operation in typical Caribbean conditions, assuming you do not require high hotel loads. Weather, currents, and sea state can push that lower.
A fin produces lift:
L = 0.5 * ρ * V^2 * S * C_L
And that lift creates a roll-correcting moment about the boat’s center of mass:
M ≈ 2 * (L * arm)
Where arm is the lateral distance from the center of mass to the fin’s center of pressure (roughly how far
outboard the fin acts). The factor 2 assumes two fins (port and starboard).
One rough way to estimate “how much moment matters” is to compare to the vessel’s righting moment at a modest roll angle. If the trawler has:
Righting moment RM ≈ Δ * g * GM * sinφ
≈ 25,000 * 9.81 * 1.5 * 0.173
≈ 64,000 N·m
You generally don’t need to “match” full righting moment to improve comfort, but for a comfort-oriented system in active seas, asking for ~20,000 to 40,000 N·m of controllable roll moment is a reasonable sizing band for this kind of boat (still very dependent on sea state and comfort target).
Assume:
Required lift per fin:
L_per_fin = M / (2 * arm) = 40,000 / (2*1.2) ≈ 16,700 N
Required area per fin:
S = 2 * L / (ρ * V^2 * C_L)
| Speed | V (m/s) | Area per fin S (m²) | Area per fin (ft²) |
|---|---|---|---|
| 8 kn | 4.12 | ~2.4 | ~26 |
| 6 kn | 3.09 | ~4.3 | ~46 |
| 5 kn | 2.57 | ~6.2 | ~67 |
| 4 kn | 2.06 | ~9.7 | ~104 |
These numbers are large because lift scales with V². If a “normal” 60 ft yacht fin pair is sized to work well around (say) 8–12 knots, then at 4–6 knots you need multiple times the fin area for the same roll moment.
Fin planform depends on aspect ratio (AR). If AR ≈ 2.5 (not crazy for a compact fin), then:
Span ≈ sqrt(AR * S) Chord ≈ S / Span
Example at 6 kn using S ≈ 4.3 m²:
That’s an enormous appendage for a 60 ft trawler: structural loads, actuator torque, grounding risk, marina practicality, and added drag become major issues. Also, big fins add drag, which reduces the already-limited solar speed.
“Normal fins need ~6 knots” is shorthand for: below that speed, practical fin sizes and actuator power stop being attractive for the level of comfort most owners expect. The math above shows the scaling: if a fin system is “nice” at 8–10 kn, it becomes ~(10/5)² = 4× bigger at 5 kn, and ~(10/4)² = 6.25× bigger at 4 kn.
Your idea: amas normally ~5 ft above water (so not providing constant buoyant stability), but each ama carries a wing/fin on a beam extending down into the water ~10 ft. The key advantage is much larger lever arm (outboard moment arm), so you can get the same roll moment with much less lift and therefore smaller fins.
Suppose the stabilizer center of pressure is ~4.0 m from the craft center of mass (you can plug your actual geometry). Keep the same target roll moment M = 40,000 N·m.
L_per_fin = M / (2 * arm) = 40,000 / (2*4.0) = 5,000 N
Now compute area per fin:
| Speed | V (m/s) | Area per fin S (m²) | Area per fin (ft²) |
|---|---|---|---|
| 6 kn | 3.09 | ~1.28 | ~14 |
| 5 kn | 2.57 | ~1.85 | ~20 |
| 4 kn | 2.06 | ~2.90 | ~31 |
Interpretation: with a ~4 m arm, fin areas drop by roughly 1.2/4.0 ≈ 0.30× compared to the monohull case. That moves you from “absurdly huge” toward “maybe buildable,” though still substantial for true comfort in open-water roll.
Rough order-of-magnitude (varies widely by builder, displacement, and expectations):
When you push to 4–6 kn continuous operation, the V² scaling drives you toward fins that can easily be several times bigger than typical yacht installations if you want “office-like” steadiness in real Caribbean sea states.
Pricing depends massively on interior finish, classification, welding QA, corrosion engineering, wiring standards, and how “productionized” the design is. The ranges below are only conceptual.
| Subsystem | Low | High | Comments |
|---|---|---|---|
| Aluminum hull + deck fabrication | $350k | $800k | Depends on scantlings, QA, welding standard, paint, corrosion isolation |
| Interior + windows + outfit | $250k | $900k | Huge spread depending on finish level |
| PV modules (~33 kWp) + marine mounting | $40k | $200k | Deployable/folding storm-rated structures can dominate cost |
| Batteries (example: 200–500 kWh installed) | $120k | $450k | Marine packaging, BMS, cooling, fire protection, enclosures matter |
| Electric propulsion (motor(s), drives, props, shafts) | $80k | $250k | Redundancy and quieting can add cost |
| Stabilizers (fin system sized for low-speed) | $150k | $500k+ | If you truly need multi-m² fins and strong actuators, it climbs fast |
| Systems (plumbing, HVAC, watermaker, nav, comms) | $120k | $400k | Depends on “liveaboard office” expectations |
Conceptual total (finished vessel): ~$1.1M to $3.0M+.
The lower end implies modest interior finish, disciplined systems, and not-overbuilt deployable solar. The upper end
is where “real yacht expectations” + heavy batteries + serious stabilization push you.
If the core product requirement is “stable enough to work at a computer” more than “fast passage-making,” then the winning designs tend to be those that reduce wave-induced motions by geometry, not by speed-dependent fins.
If your benchmark is “work on a laptop comfortably,” then a semi-submersible / SWATH-like geometry often beats a conventional monohull + fins at the low speeds that solar realistically supports.
If you provide any of the following, I can redo the stabilizer sizing and speed prediction more tightly: