I’m thinking about high levels of wind propulsion on existing ship designs with basic propulsion train, e.g. bulkers, i.e. with fixed pitch prop directly coupled to engine shaft.
If enough wind propulsion is available to require no engine power, what is best to do with the engine? Running at normal speed but low load can’t be good for efficiency and is possibly bad for engine wear too, but stopping the propeller completely adds loads of drag which acts against the wind thrust. Is it feasible to allow the prop to spin and turn the engine over but without injecting fuel, so it turns but doesn’t fire? Seems to me that would be the most efficient solution if it could be achieved…
To do it conveniently you’d have to make some minor modifications, but nothing outside the realm of practicality. For a slow speed diesel I’d think that all you would need is a small level of ongoing cylinder lubrication and a way to let the air out. For the newer electronically controlled engines the exhaust valves could be held open via the computer, so you’d only need to do some software work.
If you were retrofitting the system on an older ship it would be a bit harder. Lowest effort option would be remotely operated cocks so that they could be closed if the engine needed to be started in an emergency. However, the small diameter of those pipes would introduce a huge amount of resistance at anything but the slowest speeds.
Offhand, I’d think the more professional way to do it would be running a hydraulic system that could hold open all of the exhaust valves regardless of camshaft position. Details would vary with engine design, but it wouldn’t be devastatingly expensive.
It definitely would, but the issue isn’t so much about fuel consumption as it is about wear. Prolonged low load operation can cause a lot of nasty things to happen to the engine. Carbon deposits, cold corrosion, acid condensation, and so on. That’s all just from slow steaming, I’d imagine that zero-load operations would make things even worse. If the engine is just spinning with no combustion you’d eliminate a lot of future maintenance headaches.
On a side note, if designing from the ground up it would be better to avoid the engine turning entirely via different design choices. A controllable pitch propeller could eliminate most of the drag, or the prop could freewheel via a diesel-electric or clutched drive. If you can get enough wind power to make this scenario an issue in the first place, then there’s a lot of different options available. If you anticipate a large but variable wind contribution, a CPP and diesel-electric with multiple prime movers seems like the most reasonable choice.
Hi SappaCreek, thanks for your answers. You’re right that for a new build CPPs, clutches etc. would solve the problem straight away. But this is specifically about existing ships, and trying to avoid the need for propulsion train modifications.
Would the exhaust valves need to be held open? Could we not just allow compression but no combustion? I know there’ll be some losses in doing that but we may get most of the compression energy back on the downstroke. I presume cutting off the fuel supply is no great issue - unless perhaps on cam-driven injection engines…?
PS also I’m wondering if temperature is going to be an issue and whether that can be solved by running the jacket water preheater (or preheating scavenge air?). My only concern with things like that is running systems for prolonged periods which are only designed for transient, short duration use. I’m hoping piston motion would be enough to warm up the lube oil to a sufficiently low viscosity, but with the huge thermal mass of the piston, rings and liner, that may not be the case.
I can tell you from firsthand knowledge that standard practice on several of the world’s great tall ships is to lock the shafts in place when running under sail-power only. It definitely has a negative impact on efficiency, but it’s also definitely the better option for the engine itself.
If there was a clutch system involved then it would be possible to let the shaft and prop spin, but without impacting the cylinders in any way. In that case you would still need to provide a steady stream of lube oil to the gear-box (plus cooling and filtering said oil), but at least you wouldn’t have to worry about things like pistons, cylinder liners, etc…
I don’t know why a sailboat transmission would care about freewheeling, I’ve always set feathering props astern to reduce drag, but regardless you’ll never run into a sailboat with a direct coupled engine. But that’s neither here nor there.
For a ship I can’t see a great situation for a direct coupled prop. Adding a clutch would be easier then adding CPP, but still no simple (cheap) endeavor. Those with twin screw and clutches and/or CPP trail a shaft all the time for efficiency with no issue. (All CPP vessels I’ve sailed on have clutches too) So if you are truly talking about freewheeling a direct coupled main propulsion engine I don’t see a great outcome. Sure, you could potentially add modifications to hold open valves…maybe. Only if they are oil actuated. If it’s a cam you are out of luck. And you’d absolutely need oil flow, but (purely conjecture) that doesn’t sound great. You are talking about relatively low rotational speed of large mass, no consideration for harmonics, oil flow but no real heat. Oil flow to cylinders but no thermal expansion of piston rings. This just all sounds like a mess.
Some transmissions have lube oil pumps and some of them drive off the engine (input) end of the transmission, so freewheeling them will cause damage because there is no oil being pumped when only the output end is turning. In my case my transmission uses an oil bath and doesn’t really care which end spins. It is worth checking the engine manual on an unfamiliar boat to see.
As for direct drive, that isn’t common with small engines but it does exist. IRRC Saab sold a direct drive engine with a CPP a long time ago that actually worked pretty well. I haven’t seen one in a long time, but they were a thing.
From approval of several dead tow of ships I can attest to two methods used:
For short towage of scrap ships it is common to clamp or weld stoppers on the shaft to avoid the propeller from rotating. The rudder is blocked at a slight angle to compensate for the directional effect this has.
For longer scrap ship tows, or for dead towage of ships that is intended for furter trading, a section of the intermediate shaft is removed, allowing the propeller to rotate without turning the engine.
Unless water lubricated it is important to ensure that the stern tube luboil header tank is full.
If necessary additional luboil is carried, with an arrangement whereby it will automatically refill the header tank when it reach a certain level. (I have seen some ingenious arrangement to obtain this)
For large direct-drive diesels, the jacket water preheater would have no issue running continuously. Standard operational practice is to run them continuously in port, as well as during periods of low speed maneuver. My prior comments assumed that jacket water preheat was still running, and I wouldn’t try to operate without it.
The main question would be how far off the timing is between exhaust valve opening and exhaust valve closing. That depends on the engine, but for some quick back of the envelope math, try assuming that you’ll lose all of the pumping work between 150° and 120° BTDC. That’s on the conservative side for the difference, but without a specific engine there’s no way to be more accurate.
You’ll have to assume a cylinder size & count, prop RPM, and so on, but should give you an idea of whether it’s something to be worried about. Figure on adiabatic processes, because any loss into the cylinder walls helps you reduce preheat anyway.
No issue at all, fuel pumps would just run at zero index, similar to during blowdowns (would obviously need some software hacks throughout to convince the automation that everything is ok)
Even camshaft driven exhaust valves on a slow speed are still hydraulic in any modern-ish engine. The cam actuates a high pressure pump. So if you were making modifications you’d tee into that system. There’d be weird engineering challenges associated, but it’s a weird project to start with.
Agree with you on the mess aspect. If you had the wind power side sorted (which seems like a way bigger issue than managing prop drag) and really wanted to do something like this, your cheapest bet would probably be to buy an old piece of sh*t ship and tow it around with your initial systems installed, just to see what exciting failures occur
Sound like a very hypothetical question, since modern sails or rotors are only expected to be able to supply 10-15% of the propulsion power required for a modern bulk carrier or tanker to maintain normal service speed.
Even under ideal conditions it is unlikely that the main engine will be idled, or run at very low MRP for very long periods.
So if in reality it is similar to “slow steaming” than the problems should also be the same. (??)
Slow steaming of tankers around Africa was a common practice on the bad old days and has been practiced on container ships for several years now:
I know all about large CPPs from the movies, Mark Whalberg will drain the oil of it while you are not looking, causing you to have no reverse and hit the dock. While everyone runs around putting the containers back on the ship, Markie Mark will go steal something and hide it on the ship.
most new builds have variable pitch and NO, no one is going to make the engine freewheel at the cost of minimal ‘green’ power. just like a airplane you’d feather the prop.
but then, I never worked on yachts or ‘boats’!!
“Slow steaming”, or “idling in field” , some times for days, should be well know from OSVs.
Standby Boats/ ERRVs would spend weeks in the field doing this.
The older boats with fixed pitch props would make high speed runs around the rig /platform regularly to “blow out soot” as they termed it.
PS> In S.E.Asia some fields had a “Standby Buoy” for the St.by boat to tie up to, with engines being kept warm and ready.
This became less common after a few cases where they didn’t answer VHF calls quickly and one case in a real emergency were the St,by boat took too long to get under way, claiming they needed 15 min. notice before they could respond.
AHTSs with CPPs and big engine power would run one engine on low MRP (Max rated power) and near neutral pitch, alternate between engines.
In the 1980s it became normal to have a drop down bow thruster that was used for this purpose in reasonably good weather, using generator(s) at or near 85% MRP to supply electricity for the thruster.
With modern Diesel/Electric propulsion, large Battery Banks and Power Management System there are many more options to avoid running engines on low MRP for lengthy periods.