A direct-current Lorentz-force drive that pushes conductive seawater with no moving parts — and an honest look at why that elegance costs so much power.
Readout recomputes from first principles as you move the controls below. The numbers are not flattering — that is the point of an honest twin.
| Symbol | Quantity | SI value | U.S. customary |
|---|
| Material | Rec. Jmax (A/cm²) | Wear (mm/yr per A/cm²) | Character |
|---|---|---|---|
| Platinised titanium (Pt/Ti) | 0.50 | 0.02 | Low wear, high cost — premium anode |
| MMO-coated Ti (RuO₂/IrO₂) | 0.40 | 0.03 | Industry-standard “DSA” |
| Graphite | 0.10 | 0.90 | Cheap, but consumed |
| Bare titanium | 0.05 | 0.15 | Passivates; poor anode |
| Mild steel | 0.02 | 6.0 | Corrodes fast — illustrative only |
The magnetohydrodynamic thruster has no propeller, no piston, and no moving part of any kind. It accelerates a working fluid — here, ordinary seawater — directly, by passing an electric current through it while it sits in a magnetic field. Where the current and the field cross, every cubic metre of fluid feels a body force, and the fluid is pushed along the duct. Reaction to that push is thrust.
Because the current crosses a channel of width d while the field is uniform, the total thrust collapses to the same expression as the force on a current-carrying wire. It depends only on the field, the current, and the gap:
This is why thrust rises in proportion to current. The difficulty is that seawater is a poor conductor — a few siemens per metre — so most of the supplied power is spent simply forcing current across the gap as resistive heat, not on propelling the boat. The channel resistance and the electrical bill follow:
The only fraction that does useful work is F·u, the thrust times the boat's speed. Dividing the two gives an efficiency that, at sensible field strengths, sits in the low single-digit percent or worse. Strong superconducting magnets improve it because useful power rises with B while resistive loss does not — which is exactly why every serious demonstrator, from the 1992 Yamato 1 onward, reached for the strongest field it could carry.
The instrument panel above is built to show this plainly rather than hide it: push the current up and watch thrust climb a little while power climbs a lot.
You cannot jump a rung: each step needs the result of the one before it. This is the order this plate's own model is solved in.
This is the single sound idea worth keeping from any "coupling-first" framing: dependency dictates order. The panel enforces it implicitly — change B and every downstream number moves; change a tolerance and nothing upstream does.
| Rev | Description | Date | By |
|---|---|---|---|
| A | Initial issue — parametric | — | twin |
| MHD SEAWATER THRUSTER SINGLE-CHANNEL DUCT ASSEMBLY | |
| Drawing No.MHD-DWG-001 | RevA |
| ScaleNTS | Unitsmm |
| MaterialSEE NOTE 2 | Sheet1 OF 1 |
| DrawnTWIN | Date— |
| ProjectionTHIRD ANGLE ⦶ | |
The drawing is parametric: move any control or load the Yamato datum and the dimension callouts (d, h, L, overall width/height/length) update live. Wall thickness (12 mm), electrode thickness (8 mm) and entry/exit length (100 mm) are fixed build constants here — say the word and I'll promote any of them to controlled parameters.