IS USING JUST ONE ENGINE FALSE ECONOMY

robyonfrome

robyonfrome

Well-Known Member
Joined
12 Jun 2008
Messages
280
Location
Wareham river Frome
Visit site
Hi all, this is my first attempt, so please be patient.
I have a 30' boat with twin 6 cylinder 200 hp diesels moored on the Frome. To get any where I have to travel 20 mins at 4 knots out of the river, then because of the 10 knot Poole harbour speed limit I travel at 6 knots through the harbour (in the summer) as 10 knots is totally impractical as far as economy goes and I feel I am just pushing water and making twice as much wash. My question. Is it more economical to use one engine at 1800 revs to attain 6 knots, or to use two engines at around 1000 revs. At present I use one engine and believe it must be more economical to feed 6 cylinders rather than 12, anyone know diferent.
 
Perfect timing....

I for one am looking forward to the response to this post, the other question you need to raise is will it damage the gearbox / drive of the stationary engine.

I think this will grow to be quite a large post.......Gludy?
 
I do this all the time on the river, difference in econonmy is unmeasurable, maybe 5.5mpg instead of 6mpg at 5knts 'ish. However I do it for the engines sake, with both running they are under no load at all, with one it has more load and less likely to cause sooting of the turbo or bore glazing.
 
I will not travel on one engine unless I have to as it costs more on one than 2.

I had a cornich 31 ft on 200hp out drives, cruise speed 24knts, river hull speed 8knots.
I was 7 miles from the sea and I also covered a lot of miles inland to Paris and Amsterdam along with one time when my prop fell off and I had to travel 80 nm on one engine.

My results were there is no saving in fuel.
On one engine your props are pitched wrong and inefficient.

I was on out drives and I would raise one leg to reduce drag.
By this time I had already established there wasnt a fuel saving but I could keep my engine hours down.

Unfortunately while entering a lock on one engine I put the steering in full lock and the 290 out drive was crushed against the bell housing causing a small hole to the top of the gearbox, water did ingress but I found it within 500nm and no real damage was done.
Cost to the top of the 290 cover was minimal but any possible fuel savings would have been wiped out especially if I had not found it!

The fuel saving when running on shafts is even less likely as the shaft and prop can not be raised to reduce drag.

Another consideration is you need both your engines warm once you reach the sea.

Answered in relation to your 30ft boat with 400hp which I presume planes.

A slow displacement boat may not be affected by the incorrect prop pitch ?
 
[ QUOTE ]
I do this all the time on the river, difference in econonmy is unmeasurable, maybe 5.5mpg instead of 6mpg at 5knts 'ish. However I do it for the engines sake, with both running they are under no load at all, with one it has more load and less likely to cause sooting of the turbo or bore glazing.

[/ QUOTE ]

Thats a valid point at tick over say 3 knots /forums/images/graemlins/smile.gif

In my opinion once you exceed 6 knots the engine will be under stress as the prop pitch is wrong and could actually cause sooting up .
 
[ QUOTE ]
I will not travel on one engine unless I have to as it costs more on one than 2.

[/ QUOTE ]

Sorry Pete. Got to disagree with you on this one. The more cylinders = more fuel.

I'm sure JezBanks checked this out some time back on his Bavaria, as he'd fitted fuel flow meters. I reckon it was more economical on one engine but not hugely - 20% ish.

I'm sure he'll be along soon....

R
 
The boat is going to need the same amount of power to push it through the water whether you're using 1 or 2 engines so I can't see how running on just one engine can save a significant amount of fuel and, as DAKA has pointed out, the prop of the working engine will be incorrectly pitched for the application
There is another issue as well. I don't know about sterndrives but with shaftdrives there is a potential risk of damaging the gearbox of the shut down engine as the stationary prop loads the gearbox as it is dragged through the water
 
[ QUOTE ]
The more cylinders = more fuel

[/ QUOTE ]

Don't agree with that. When both engines are driving the boat, they will be operating at lower rpm and consuming less fuel 'per cylinder' than a single engine working at higher rpm
 
Prop pitch will have no adverse effect on the engine. Imagine a truck running loaded or unloaded, same situation.
DAKA, surely the same thing would have happened with both engines running when put on full lock?
One engine will always be more economical at slow speed due to the losses inherent in every running engine.
 
I have fuel flow meters and on outdrives.

If travelling on one engine, I leave the outdrive down so far, but should pull it up really.

Anyway, I save about 1/3 of fuel if I travel at 6nts or below. I push too much water if going any faster and the saving is minimal on one engine.
 
I've been talking to a few manufacturers about exactly this question and they give just this answer. Namely, you need a specific amount of power to push a boat at a specific speed, and even though there's efficiency differences with one engine versus two the bottom line is that you can expect pretty similar fuel economy using one engine or using two.

There's some exhaustive analysis on a few US websites about the virtues of one outboard versus two outboards on smaller craft, and here again the answer is similar in that there's a small economy to be gained from one engine versus two, but it's actually pretty marginal.
 
I imagine consumption is pretty low anyway at under 6 knots, so although 30pct sounds alot, dont you have to do an aweful lot of distance to make any real finanncial difference?
 
Although i always use both, there are many on here that seem to have a fair run at harbour speeds before reaching the 'open sea' and use only one engine.

Someone recently said that they use one engine for harbour steaming alternating between the two thus keeping the hours on each similar - saving in servicing costs.....Over the course of a year i guess you could save quite a few hours this way if thats an issue.

On fuel saving - I'm not convinced.........yet!
 
[ QUOTE ]
.
DAKA, surely the same thing would have happened with both engines running when put on full lock?
.

[/ QUOTE ]

If both engines were running they would have been in the water and not raised.
The owners books contain a small print that says do not move the wheel while leg is raised.
When the leg is raised you run the risk of crushing the leg against the bell housing.

Most the time your wheel will be near straight but if a sail dinghy tacks in front of you then you may have to use full lock and return back to harbour for immediate lift out.

Reason for me never to run on one leg again.

Power steering also doesnt work on one side.
 
Twin 370hp diesels - I genereally use both at harbour speeds but last weekend running at <5kts on Dutch canal switched one off and saw fuel consumption improve to >6mpg compared to ~4-5 mpg for same speed with two engines. This is according to Navman fuel sensor system.
 
What about prop shaft cooling?

I see there seems to be some cooling of some of the joints on my VP 310s. Recently I anchored (not entirely by choice) in a strong tideway and the props were turning on their own. I did wonder if that would cause a problem but it was only fr a short time. However if I were to motore for a long period at waterline speeeds on 1 engine, would the prop turning on the other engine (a) reduce or eliminate any theoretical energy savings (b) damage the prop couplings, gearbox etc over a long time. Of course this sort of useful info should be in the boat's handbook but that is completely useless on my Beneteau (in contrast to the handbook on the Fairline I used to own).

Fuel sensors etc being added in a couple of weeks so I will know more data after that. The trip down the Hamble form Swanwick to the sea is a useful test!
 
Freewheeling depends on the make of gearbox, some OK some not.
At displacement speed there would be no problem with shaft seal cooling or lubrication. Think about sailing boats that allow their props to freewheel!
 
You asked for this Richard!!!

I looked into this matter and yes it as is my destiny created some large heated threads on this forum.

My consulsions were:-

1. Dragging a moving prop is a lot more drag that dragging a fixed prop - a moving props actually sucks energy away in moving it.

2. Some gearboxes can tolerate a moving dead shaft - others cannot.

3. If you wish to brake the shaft then that is a complex thing to do - one of those things that seems simple but in practice is not.

4. If you cannot brake the idel shaft then one engine running is not worth it.

Now there all that seems simple enough - I read up a lot on the subject and spoke to prope designers, boat designers and those are my conclusions ...... I can see no reason why those conclusions can explode into another mega thread Richard..... /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif
 
You must log in or register to reply here.

Share this page

Top