Propulsion and endurance
A diesel-electric plant sized around two legs: a long transit on the generator, and a quiet endurance leg on the battery.
Two legs, two power sources.
The HD-18 is not a battery boat that occasionally snorkels, nor a diesel boat that occasionally dives. The mission it was sized for has two distinct parts, and the plant reflects that.
Transit. A 150 kWe generator burning compensated diesel from 3,960 L (3,326 kg) drives the vessel 4,528 km at 6 kn — 407 hours, seventeen days — with the mast up. That is how it reaches an operating area under its own power instead of on the deck of a mother ship.
On task. A 750 kWh LFP pack, 600 kWh usable at 80% depth of discharge, gives 486 km at 4 kn — 65.6 hours — with the mast down and the generator stopped.
Sprint. 12 kn for 79 km at 168.6 kW on the shafts, for when a window has to be met or a set has to be cleared.
Energy
The battery is also the ballast.
750 kWh of LFP occupies 6.857 m³ and sits low, beneath the payload deck, in banks either side of a centre walkway on anti-shock mounts. Putting the single heaviest item at the bottom of the hull is what buys 0.609 m of BG without carrying dead lead to get it.
LFP rather than a higher-density chemistry is a deliberate trade: less energy per kilogram, far better thermal behaviour in a sealed hull with no crew aboard to intervene.
Fixed ballast still comes to 19.75 t of cast iron — solved by the model rather than assumed, and re-solved whenever the payload fit changes.
| Battery pack | 750 kWh LFP |
| Usable energy | 600 kWh at 80% depth of discharge |
| Diesel generator | 150 kWe |
| Fuel | 3,960 L · 3,326 kg · pressure-compensated |
| Specific fuel consumption | 0.212 kg/kWh |
| Quasi-propulsive coefficient | 0.68 |
| Propellers | 2 × 0.80 m diameter on 1.90 m centres |
| Control planes | X-form, 1.10 m span, 1.55 m chord |
| Hotel load, submerged | 2.0 kW |
| Hotel load, payload active | 8.0 kW |
| Snorkel duty | 30% at the transit condition |
| Submerged range | 486 km at 4 kn · 65.6 h |
| Diesel transit range | 4,528 km at 6 kn · 407.5 h |
| Declared total range | 5,014 km |
| Sprint speed | 12 kn · 79.1 km · 3.56 h |
| Bus power at sprint | 168.6 kW |
| Service life | 500 h between overhauls |
Machinery
Twin shafts, offset, gearboxes inboard.
Two 75 kW shafts on 0.95 m offsets drive 0.80 m propellers, with the reduction gears and thrust blocks inboard of the pressure boundary and the shaft seals as the only rotating penetrations. Quasi-propulsive coefficient is 0.68.
Four 15 kW thrusters handle low-speed manoeuvring and hovering — coming alongside, holding position over a point, or working in a current while a payload runs.
Fuel is compensated: as diesel burns, sea water replaces it. The vessel does not change trim across a seventeen-day transit and there is no free surface to manage.
Hotel load — read this before you plan a mission
Passive hotel load is 2 kW. With a payload drawing power it is 8 kW. On the submerged leg that is the difference between roughly 486 km and roughly 294 km of endurance. It is usually the payload, not the propulsion, that decides how long a mission can last.
Give us your sensor fit and duty cycle and we will return the endurance curve for that configuration rather than a headline number.
Snorkelling
Most of the declared range is run with the mast up.
That is stated first because it is the thing a range figure most easily hides. The transit leg — 4,528 km at 6 kn over 408 hours — is run at snorkel depth on the generating set, with the masthead 3.81 m above the surface. The vessel is trimmed down and small, but it is not invisible, and we do not describe it as though it were.
The generating set runs at roughly 30% duty at that condition: enough to drive the shafts and put charge back into the pack, not enough to run continuously. What that buys is a boat that can reach an operating area three days away and still arrive with a full battery for the part of the mission that has to be done submerged.
Fuel is 3,960 L, pressure-compensated, at a specific consumption of 0.212 kg/kWh.
Stern group
Everything aft of the payload is what moves it.
The generating set sits on the centreline with the traction motors, reduction gears and thrust blocks inboard of the pressure boundary. The shaft seals are the only rotating penetrations in the hull, and they are the item the maintenance interval is built around.
Propellers are 0.80 m five-blade nickel-aluminium bronze on 1.90 m centres at station 16.85 m, ahead of the X-form planes. Both sit inside the hull envelope in plan, so the vessel can be craned in strops or set down on its three cradles without an appendage taking load.
Bring us the payload and we will size it.
We run your fit through the engineering model and come back with real numbers for it, not a generic figure.