Ducts, Fins and Bulbs: Propeller and Hull ESDs

7 min read

What you'll take away Match each energy-saving device to the specific propeller loss it recovers, and sanity-check a claimed gain against the ship's hull form and propeller loading.

A propeller converts perhaps two-thirds of the power delivered to it into useful thrust. The rest is left behind in the water, and, usefully for the retrofit market, it is left behind in identifiable ways: the slipstream spins when it only needs to go backwards, a concentrated vortex snakes off the hub, and the wake the hull feeds the propeller is so uneven that the blades work in a different flow at every point of each revolution. Energy-saving devices, ESDs, are point solutions: each one is a piece of fixed geometry aimed at one of these named losses.

Before the propeller: conditioning the inflow

Pre-swirl stators are fixed vanes on the sternframe ahead of the propeller, angled to give the inflow a rotation opposite to the propeller's. The propeller then spends its own rotation cancelling the pre-swirl: the slipstream leaves straighter, and energy that would have been abandoned as spin stays with the ship. The stator generates no thrust itself; its work is preparation.

Wake-equalising and accelerating ducts sit ahead of or around the upper part of the propeller disc, where a full hull's wake is slowest and most uneven. By accelerating and smoothing that flow they hand the blades a more uniform field to work in: better efficiency, and less of the blade-by-blade load fluctuation that drives vibration and cavitation. Integrated designs combine a duct with pre-swirl fins in one casting, attacking wake and swirl together.

seen from astern, looking forward: the only view that shows the arrangement propeller turns this way swirl put in the other way port stbd pre-swirl stator about three fins, one side only slow water sits here propeller disc faster water fed into the slow patch port stbd wake-equalising duct two half rings above the shaft
Both sit ahead of the propeller and neither produces thrust of its own; the gain arrives through the propeller. Two arrangements are worth getting right, because illustrations often get them wrong. The stator is a cluster on one side, not a ring of vanes all round the shaft, because the hull's own upflow already supplies the turn on the other side. The classic duct is two half rings above the shaft, not one ring encircling the propeller, which would be a Kort nozzle and a different device.

Behind the propeller: recovering what escaped

Boss cap fins are small blades on the propeller's hub cap that break up the hub vortex: the concentrated corkscrew of energy spinning off the boss. Rudder bulbs continue the hub's shape onto the rudder, filling the gap where the flow otherwise separates and swirls. Post-swirl fins and thrust fins on the rudder recover rotational energy from the slipstream as a small forward force. Each is a streamlining fix applied at the exact spot the propeller leaves its mess.

boss cap fins hub vortex, broken up fins shown enlarged they turn with the propeller rudder bulb narrow gap without it the flow separates behind a blunt hub thrust fins, seen from astern slipstream still spinning mirrored, not symmetric: the port fin is the longer one One rides on the propeller; two belong to the rudder and swing with it.
Three fixes at the exact places the propeller leaves energy behind. Two details are easy to draw wrongly: boss cap fins rotate with the propeller rather than against it, and a rudder bulb belongs to the rudder, so it swings with the helm and its gap to the hub cap is shaped to stay constant through the rudder's travel. The fins are drawn enlarged because at true scale each projects only a few per cent of a blade's length.
stern, seen from the side flow 2 4 5 1 3 propeller rudder Before the propeller: conditioning the inflow Behind it: recovering what escaped 1 pre-swirl stator: pre-rotates the inflow 2 wake-equalising duct: evens the upper wake 3 boss cap fins: break up the hub vortex 4 rudder bulb: fills the hub-gap flow 5 thrust fins: recover slipstream spin Not to scale; devices shown together for comparison.
Each device is a point solution at a named place: two of them condition the water before it reaches the blades, three recover what the propeller left behind. Few ships carry all five, and the whole set is drawn together here only to show which loss each one is aimed at.

Honest magnitudes, and who benefits

Individual devices honestly deliver on the order of 1 to 5%, with boss cap fins at the modest end, typically 1 to 2%, and well-matched ducted pre-swirl systems at the top. The economics work because the price of the saving is fixed steel with no moving parts and no crew workload: 3% on a ship burning 25 tonnes a day is 0.75 tonnes daily, every day, for the life of the device.

The gains concentrate on full-form, slow ships with heavily loaded propellers, tankers and bulkers, because that is where swirl and wake losses are largest. The same duct on a fast, fine-form hull can return nothing, or add net drag. This makes ESD selection a hydrodynamic matching exercise, not a catalogue purchase: a serious retrofit starts from the ship's own wake field, measured or computed, and is model-tested or CFD-verified for that hull before any steel is cut, with installation timed for a scheduled dry-docking. The Korean Register's selection guide organises the device families by ship type and loading exactly for this purpose (the guide is open). Claims well above the honest band, or quoted without a hull form attached, get the lesson-1 checklist applied hard.

The quiet dividend

There is a second benefit that costs nothing extra. Much of a ship's underwater radiated noise comes from the same disorder that wastes energy: the hub vortex cavitates and sings; blades passing through an uneven wake cavitate cyclically. Devices that even out the wake and dissolve the hub vortex therefore cut noise and fuel with the same steel. ABS documents this dual efficiency-and-noise benefit across hull forms, propellers and ESDs in a dedicated whitepaper (the whitepaper is an open PDF), worth citing when an ESD business case needs a second leg, as underwater noise draws growing regulatory attention.

One caution to carry into lesson 8: ESDs, wind assistance and air lubrication all change what reaches the propeller. Their claimed savings were each measured alone, and they will not simply add.

Check yourself

1. Boss cap fins target which loss?
2. A pre-swirl stator saves energy by…
3. Which ships gain most from ducted pre-swirl devices?
4. A rudder bulb saves energy by…
5. A typical honest gain from a single energy-saving device is…
6. Which of these are propeller or hull energy-saving devices?

Select all that apply.

7. Devices that recover propeller losses can also reduce underwater radiated noise.
8. A device recovers 3% on a ship burning 25 tonnes per day. How many tonnes per day is that?
t/day