Thanks for the good info SappaCreek & co. (and PaddyWest2012 for the insight about tall ships - that’s interesting).
I’m now looking into the effectiveness of the propeller to see whether it can generate enough power in this ‘turbine’ mode to actually spin the engine. At first guess I’m assuming the power draw to turn the engine over will be around 10% of MCR (incl. friction and compression losses), but I’m far from sure of that.
Something else that could be an issue is condensation of humidity in the cylinders - if heat is lost to the walls during compression then the air may fall below the dew point during expansion. Can scavenge air input be dried or heated using normal equipment, to prevent that?
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You should look into the DynaRig yacht Black Pearl, which uses its propellors to turn electrical generators while on long ocean crossings under sail power.
Related to that, you should read up on the “Dykstra EcoLiner”, which is a cargo ship designed by Dykstra to be powered by the same DynaRig that powers both the Black Pearl as well as the Maltese Falcon.
With the Black Pearl and its power regeneration methods in mind, I think that it’s worth pointing out that diesel-electric probably makes a lot more sense than just straight diesel for auxiliary-sail powered vessels, for a wide variety of different reasons.
Dykstra got a lot of things right with its EcoLiner design. I really think this has a shot, however small that shot might be, at being the future.
Agreed, diesel-electric (ideally with CPPs) is ideal. Or perhaps LNG-electric setup for dual fuel operation with green methanol/ammonia etc. Actually I saw something really interesting the other day about methane pyrolysis, cracking natural gas under heat to produce solid carbon and hydrogen - no CO2 produced and hydrogen available as fuel but stored as LNG. I haven’t seen anything about doing it on ships but I hope somebody looks into it.
Those dyna-rigs are beautifully simple - I think a lot of light cargo shipping e.g. RoRo, turbine blades, aircraft parts etc. will turn to those or rigid wingsail designs over the next few years.
On a low-speed diesel (13,000 kWs, 8 cylinders) when the engine is stopped to do an astern test the prop freewheels until the speed drops below almost exactly 6 kts. I’m always watching because if the prop is still spinning the engine won’t start astern.
Thanks, that’s exactly the sort of info I need to know. What sort of rpm does it freewheel at compared to the rpm when creating forward thrust?
I suppose you could work that out from whatever speed you think the vessel will do under sail and compare that with the pitch of the propeller.
Hi PaddyWest, yes, but I doubt it’s turning at zero slip, especially in this case where it’s a straight through shaft so it’s also turning the engine over… One of my colleagues guessed it would only rotate at 1/4 to 1/3 of the design speed, which seems very slow but could be the case since the flow is going ‘the wrong way’ over the blades.
Dead slow ahead on that ship was 28 rpm, about 6 kts (5.8 kts IIRC).
I don’t recall very well the rpm when freewheeling. After stopping the engine the tach would show about 6-8 rpm if memory serves.
Would that be stopping from dead slow or from a faster speed through the water? It sounds like my colleague might be right that it drops to about 1/4 of the driven speed…
That’d be stopping the engine at speeds over 6 kts.
I was just watching for the tach to drop for zero, I know the 6 kts is very close to correct. I’m a little hazy about the 6-8 rpm, might have been higher.
I’d guess that 1/4 to 1/3 is in the ballpark.