Interesting Technical Question

[Gludy]

I am trying to find out how a twin shaft propped boat performs running on one engine. If you want to really extend range to a thousand or two miles on a trawler style boat there may be a higher mpg if you run on one engine.

However one engine running without locking the other shaft means that the non-working prop is actually having to be turned and is turning the gearbox etc. hence the dragging prop is a hindrance. There is also a slight rudder offset which increases the drag of the rudder.

I know of one boat that has measured fuel consumption and found that running on one engine used more fuel to achieve the same speed than running on two engines at lower speed - no doubt due to the above to factors. However, I cannot find any source that has actually determined if there is a gain in mpg by running on one engine with the idle shaft locked. In this situation there may be an overall mpg gain- as the only drawback is the rudder offset alone.

Does anyone have any experience or figures on this subject?

 

[rickp]

Isn't this just a different take on the perennial raggie problem - when sailing do you allow the prop to spin or not? I think the conclusion is 'it depends on the boat, prop and gearbox'. Get a floscan, do some trials and work it out.

Another advantage of a single engined boat /forums/images/graemlins/smile.gif

Rick

 

[capsco]

I always understood it was best to let the prop revolve.

 

[ShipsWoofy]

I have tiddly engines on my boat, manoeuvring and drive for windless days. Running on one engine I can not get up to the same speed as two, 4.5kts instead of 6.

The single engine must also be raggin its balls off to get 4.5. To get say 4.5 with both engines I can do this running at 1800 - 2000 rpm as opposed to 3000 odd. This makes a massive difference to fuel use when you look at the consumption curves, although my consumption is probably akin to your boat heater!

Always lock props when not in use, especially when sailing, though mine are 43% blade ratio so drag is much less than a mobo prop.

I agree, that running both is more fuel efficient for me. Though with twin sailing rudders I do not have the other problems you mention.

 

[Gludy]

Not exactly the same as the raggie problem.

The only test on a twin screw I have seen to date shows that the mpg is worse with just the one engine - so the best guess for a single engined boat is try to stop the prop rotating or let it rotae without it rotating the gearbox (Freewheel).

Its simple to measure one engine versus 2 at the same speed - that is not the issue, I am fairly happy with the conclusion that two engines are better). It is difficult to measure one engine only with the other shaft locked - that is the issue. There are different shaft locking systems but its too late when you have had the expense of installing one.

 

[Gludy]

How do you lock the shafts?
I think this is difficult with twin 700hp engines.

 

[ShipsWoofy]

For me it is just a case of putting them into reverse, they have a clutch arrangement (Yanmar) that means they will not lock in forward.

Many yachts achieve locking by adding a disk brake to the drive shaft with the pads locked to the hull, pull a push pull cable like a hand brake and the shaft stops. Though you would seriously have to remember to unlock the brake with 700hp. Ah the smell of burning in the morning!

 

[tcm]

http://www.bruntons-propellers.com/products/variable/autolock/

This confirms what i understood - that one must not freewheel for long distances. I ran the (slightly busted, but not terminally) engine on tickover for 20 miles or so.

I'm afraid i can't report details of fuel economy, in the same way that i haven't a clue how much i have spent on lots of other things!

 

[Gludy]

My gearbox maker states its Ok to free run the box for up to 8 hours - however there is no point in doing this as the combination of prop drag and rudder drag makes it pointless.

What I am after is trying to find if someone has measured the difference between having a shaft lock and not.

 

[jfm]

We have instantaneous fuel flow rate metering, which adds the fuel burn rate data for both engines, and also reads SOG from the GPS so can give a readout of instantaneous MPG. Unquestionably running one engine in gear at idle rpm gives a better mpg figure than running 2 engines at idle revs in gear, despite the rudder offset losses and the fact one prop is being dragged. Indeed, running one engine at idle rpm in gear gives the highest mpg figure ever achievable on the boat. This is all as you'd expect.

But turning to your question about locked vs unlocked shaft, on ours the unpowered prop spins freely. I have never done it with the unpowered shaft locked. I dunno for sure but gut feeling says that locking the prop would increase drag, compared with just letting the prop spin. Surely the components are following the path of least resistance, so allowing the prop to spin as it wants to causes less drag than locking it?

The gearbox manuals however say do not let it spin for long. Summink to do with the gearbox oil circulation pump not powered, so the internal lubrication in the gearbox isn't functioning in "free wheel" mode.

 

[Wiggo]

Am I missing summat here, or just being too simplistic? What happens to the prop/shaft/gearbox if you put it in gear with the offending engine shut down? Water tries to turn prop, prop tries to turn shaft, shaft tries to turn box, box tries to turn over bloody great high compression diesel lump. Like trying to push start a jumbo jet, innit?

 

[rickp]

Is it possible to do that with all singing & dancing leccy controls?

Rick

 

[Deleted User YDKXO]

To my way of thinking, a boat travelling at a certain speed needs a certain amount of power to drive it so the crux of the question is whether it is more eficient to provide this power through 1 engine or 2. I would bet that the mechanical/heat efficiency loss is greater with a 2 engine system than 1 engine even though that one engine is working at higher rpm to deliver the same power. I agree with jfm in that locking the dead propshaft may well cause greater drag than allowing it to spin freely but not so much as to make it less efficient than a 2 engine system
The only circumstance where a single engine might use a lot more fuel is where the helmsman tries to get on the plane with a single engine but on most boats you can't get on the plane with just one engine so you're just wasting fuel and stressing the engine
I believe that delivery skippers do helm boats on one engine or the other to eke out the fuel supply on long trips so I guess it must be accepted wisdom that one engine uses less fuel than two, at least at displacement speed

 

[Gludy]

In fact measured mpg on a 42 foor twin screw power (twin 120 hp) boat doing 7 knots gave a 5% higher fuel consumption than using both engines!

The rudder offset plus the fact that the free prop offers much more drag than a fixed prop (this is an accepted fact) led to the higher fuel consumption.

The case may be different at tickover - accepted but it is not the case at higher speeds.

If the shaft is locked there is some losss due to the water resistance of the fixed prop but this is not as great as that of a prop slowly turning a gear box.

The two solutions are to break the shaft and insert something like a disc brake - the other is to hydraulically freeze the gear box.

 

[Gludy]

I found this on the web

"I installed Floscan fuel consumption meters recently, and there are first sea trial results!

My boat is GB42 Classic woody, with twin Lehman 120, approx. 5200 hours. My test ride has been carried out in very unstable wind & sea conditions, so that result may be accordingly inaccurate. I can provide complete table if someone interested, but real reason for this post is folowing simple experiment:
I choose speed 7 KN, and adjusted 1) only port engine, stbd freewheeling, 2) only starboard engine, port freewheeling, 3) both engines, to reach this same speed.

Speed RPM LPH (port) LPH (stbd) L/NM (MPG)
7 1750 10 0 1.43/2.65
7 1700 0 10 1.43/2.65
7 1400 4.5 5 1.36/2.79

Connclusion: on my boat, fuel efficiency is better when I am running both engines, than any one alone. It makes sense: when I shifted any side to neutral, autopilot needed to correct ruder for 3-5 deg to compensate it, creating non-optimal motion through the water. Plus drag of passive proppeler.

Of course, it doesn't mean that twin engine boat is better efficient then single; it only means, that twin with both active is more efficient than twin with single engine active. "

 

[Gludy]

"I agree with jfm in that locking the dead propshaft may well cause greater drag than allowing it to spin freely but not so much as to make it less efficient than a 2 engine system"

That is wrong - locking decreases drag.

"I believe that delivery skippers do helm boats on one engine or the other to eke out the fuel supply on long trips so I guess it must be accepted wisdom that one engine uses less fuel than two, at least at displacement speed "

That is true. Although they may be wrong to do it unless they lock the free shaft.

There is a difference between locking and unlocking the shaft - its always better to lock the shaft. I am just trrying to find out by how much.

 

[Deleted User YDKXO]

Typical Gludy thread then. Just posted to reinforce what you think you already know
You're right, delivery skippers are wrong

 

[rickp]

Yup - I was wondering why Gludy bothers asking a question and then a few posts later says "accepted fact" and everyone else is wrong. Hey ho.

Rick

 

[Gludy]

Typical stupid reaction.
I had hoped you had grown up a bit but no - the same old personal attack based on your inability to glean the facts and read the clear points..

I am trying to find out by how much the fixed shaft improves the efficiency of the system - I do not know the answer.

It is an accepted fact that a fixed shaft is more efficient - its also an accepted fact that many delivery skippers run on one engine then the other - so it seems that these facts clash- given that clash I wrote that:-

"That is true. Although they may be wrong to do it unless they lock the free shaft."

I did not know so I wrote MAY BE WRONG..

It really is about time you grew up and got a life- the ability to read and actually understand a subject without resorting to personal attack would be a very helpful attribute.

Now if you cannot do this I suggest you simply stay away from the thread.

RikUp

"Yup - I was wondering why Gludy bothers asking a question and then a few posts later says "accepted fact" and everyone else is wrong. Hey ho."

I did not ask that question - I have stated that a fixed shaft is more efficient - if you deny that it is you that is going against what is accepted throughout the marine industry not me. All I have stated is that it is not correct to assume that a rotating prop offers less drag - the opposite is true.

So how come you attack me personally - the opposite is true, I am siding with the industry on this efficiency question - I am also pointing out that letting one haft spin may lead to a situation where one engine running offers poorer fuel efficiency - I actually gave such an example that was measured and then pointed out that delivery skippers may not be getting the efficiency they thought.

if you cannot follow that simple logic without resorting to personal attack then just stay away from my threads or grow up and try reading what is being said.

 

[Talbot]

Only have one prop, and get over the drag problem by lifting it out of the water when not in use!

speed is dependent on state of the bottom and amount of wind opposing passage - 7.5 kts on a good day with no wind.