Configuration: 70-ft triangular truss SWATH-style seastead with three 10-ft chord NACA-0030 legs and auxiliary “airplane” stabilizers. Below is a point-by-point analysis of drogue steering, drogue sizing, and the hydrofoil escape alternative.
Paying out one stern winch and hauling the other shifts the drogue attachment point to port or starboard of the centerline. Because the three vertical legs act as enormous combined keels (total underwater lateral area ≈ 85–100 sq ft), the vessel strongly resists crabbing. The offset drogue therefore generates a yaw moment that pivots the bow away from the drogue side.
Yes, for sea-room management—avoiding taking waves directly on the stern quarter, or steering away from a lee shore—but not for precise navigation. You will not be able to claw to windward, but you can bias your drift track 20–40° left or right of the pure downwind line over many hours. That is usually sufficient for storm avoidance.
The table below assumes an all-up weight of 35,000–45,000 lb and an equivalent downwind drag area (CdAair) of ~450–550 sq ft (transom, above-water legs, dinghy, solar edges). “Drogue Drag Area” is the product Cddrogue × Adrogue needed in the water at 6 knots to balance residual wind force after subtracting the leg’s own hydrodynamic drag (~250 lbf at 6 knots). Values are rounded.
| Wind Speed | Apparent Wind (minus 6 kt drift) | Est. Wind Force | Required Drogue Drag Area | Para. Diameter (Cd≈1.3) | Gal. Equiv. (Cd≈0.9) |
|---|---|---|---|---|---|
| 30 mph | ~23 mph | ~650 lbf | ~4 sq ft | ~2.0 ft | ~2.4 ft |
| 40 mph | ~33 mph | ~1,400 lbf | ~11 sq ft | ~3.3 ft | ~4.0 ft |
| 50 mph | ~43 mph | ~2,400 lbf | ~21 sq ft | ~4.5 ft | ~5.5 ft |
| 60 mph | ~53 mph | ~3,600 lbf | ~33 sq ft | ~5.7 ft | ~7.0 ft |
If your actual windage is larger (e.g., more superstructure, side yawing), scale the diameter roughly proportionally to the square root of the force increase. For example, if you estimate your CdAair closer to 1,000 sq ft, multiply the diameters above by about ×1.4.
| System | Adjustability | Best Fit? | Notes |
|---|---|---|---|
| Jordan Series Drogue (JSD) | Poor for this use. | No. | JSDs are designed to bring a vessel to a near-stop (≈1–2 knots) in breaking seas. The dozens of small cones create far more drag than you need for a controlled 6-knot drift. Retrieving or “collapsing” some cones under load is not practical on the fly. |
| Galerider (perforated basket) | Low. | Marginal. | Galeriders top out around 36–48 in for 45–55 ft monohulls. They generate steady, non-snatching drag, but sizes for a 70-ft platform in >50 mph are on the edge of what commercial units offer. You would likely need a custom size or dual deployment. |
| Adjustable Parachute / “Purse-String” Drogue | Excellent. | Best choice. | A heavy-duty parachute or basket drogue (4–10 ft) with a collapse line lets you vary the open diameter continuously from a winch. You can go from “minimal drag” (partially collapsed, 2–3 ft) in 30 mph, to fully open (6–8 ft) in 60 mph, all under load. This is the only option that naturally spans the wide range in your table. |
If you try to escape by partially hydrofoiling, the three “little airplane” wings must carry a substantial fraction of weight. Below is the lift analysis assuming:
Using a maximum continuous hydrofoil lift coefficient of CL ≈ 0.90 (doable with a cambered section or symmetric section with deflected elevator):
Your current stabilizers are 12 ft × 1.5 ft = 18 sq ft each. This is a remarkable match; the area is exactly in the right ballpark. However, note that a symmetric “little airplane” wing with elevator will need significant elevator deflection (or all-wing incidence) to reach CL=0.9, and stall margin becomes tight. A dedicated 16–18% cambered hydrofoil section (e.g., NACA 2415 or similar) would be safer and more efficient.
Each wing root must react roughly 6,700 lbf of lift at the conditions above. For a 12-ft-span cantilever, the approximate root bending moment is:
Using aluminum 6061-T6 with a conservative allowable of 20,000 psi (safety factor ~2), the required section modulus is:
To achieve that inside an 18-in chord, you need roughly a 3 in × 8 in rectangular tube spar (S ≈ 32 in³) or an 8-in diameter round tube (S ≈ 14–20 in³ with proper wall thickness). That is a bulky internal spar for a thin cruise wing. Three practical ways out:
With a 5° wedge over the bottom ~10 ft of each leg, the three legs present roughly 3 ft × 10 ft = 30 sq ft each of low-aspect planing surface. At 12 knots and 5° effective trim, a flat planing surface can generate a lift coefficient of roughly 0.10–0.18. Taking CL ≈ 0.15:
This is a meaningful contribution (≈14% of total vessel weight). Combined with the stabilizers, you could theoretically lift nearly 25,000 lbf at 12 knots—well over half your weight. The concept is therefore physically reasonable; the sloped leg bottoms act as low-efficiency water skis that help reduce hull wetted drag as speed rises.
A two-line power kite or a rigid traction kite can generate thousands of pounds of thrust in 20–30 mph winds. If the storm is still hours away, launching a large kite (e.g., 20–40 m²) from the roof could give you 8–12 knots of boat speed. The three deep kegs let you maintain a broad reach or run, exactly as a keelboat does. The caveats:
Best use case: kite for pre-storm repositioning when winds are 15–25 mph, not as a primary survival tactic in 50+ mph conditions.
All calculations above are order-of-magnitude estimates. Provide your exact displacement, wind tunnel / CFD drag data, or measured thruster bollard pull, and the drogue and foil sizes can be refined precisely.
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