Efficient Fuel Consumption Whilst Cruising

[Rickent]

Have no idea how much fuel we use an hour but we have a perkins 35 hp and our 45ft boat scoots along quite nicely at about 1100rpm.

[frahkn]

20 hours ago, Machpoint005 said:

That makes sense and is in common with my experience (Beta 43, 48ft boat) - 1150 to 1200 rev/min is plenty on a shallow canal.

I'd suggest the OP tries to correlate his speed over the ground (measured on a satnav) with the engine revs. There will be a critical engine speed above which the rate of progress is hardly increased at all -- if he exceeds those revs he is wasting fuel. 

 

21 hours ago, Bee said:

Piece of string innit. Depends on engine and lots of variables. We have a Beta 43 and cruise at a 'fast tickover', about 12 - 1500 rpm, on bigger commercial canals and rivers we usually do 1600 rpm perhaps a bit more. 2000 rpm is reserved for panic and 'You haven't seen that huge ship have you' events. I doubt if we ever achieve 8 hours for 10 litres unless its a very relaxed drift along with the current with few locks or manoeuvres.

This shows how boats do vary. I rarely need more than 1200 rpm and use less about 50% of the time (rivers excluded) to achieve my, admittedly gentle, rate of progress. This is with a Beta 43 in a deep drafted 70' tug weighing more than 23 tons.

Frank.

[Bee]

So many variables, We are 33` overall, 11, 6" wide, about 12 tons  but being short and fat we have less theoretical top speed. However we make almost no wash until we get up to silly speeds. We do though tend to travel at a reasonable speed and can often beat old ladies on pushbikes on deep waterways (Not in Holland though, even the old folks tear along there)

[cuthound]

-When we had our last shareboat, around about the time white diesel duty was applied to the propulsion element of red diesel, several shareholders began to complain about increased fuel consumption. 

I asked everyone to provide me with lock/miles, hours run and lites of fuel used information and compiled a spreadsheet for a years fuel consumption.

The boat was a 58 foot Graham Reeves hull with BMC 1.8 and PRM 150 gearbox 2:1 ratio.

Fuel consumption varied from 1.1 to 2.1 litres  per hour and was noticbly less on heavily locked routes where more time is spent idling.

[Cheshire cat]

I am obviously missing something here. Why did several shareholders think fuel consumption had increased? Were they insinuating that the BMC had developed some kind of fault?

We've still got a Pat Buckle share. The 2.2 Nanni pootles along nicely at 1200 rpm. Similar speeds on our own boat require 1300 - 1400 rpm but the fuel consumption from the 1305 Beta is definitely better at 1.2 litres per hour 

 

[Robbo]

21 hours ago, Gareth E said:

It's 0.8 litres per hour, around 30% less than many other boats, non?

If most boats, moving through water using different sized engines use the same amount of fuel, why can't we say the same for cars moving along a road? For example; a small car with a small engine will use far less fuel than a bigger, heavier car, that has a bigger engine. Is there something about the physics of moving through water that is different from the physics of moving a lump of metal on wheels along tarmac?

However a car only carries 5 people, where a bus carries 80 odd.  The bus therefore is more efficient even tho it uses a lot more diesel as per person it's a lot less.   A bigger boat has more surface area in the water however for per ton its less surface area than a smaller boat so is more efficient.

[Chewbacka]

11 minutes ago, Robbo said:

However a car only carries 5 people, where a bus carries 80 odd.  The bus therefore is more efficient even tho it uses a lot more diesel as per person it's a lot less.   A bigger boat has more surface area in the water however for per ton its less surface area than a smaller boat so is more efficient.

Pedant alert - On my local bus route, most cars have only 1 or 2 people , but buses are not much better (efficiency wise) as during the day there is often only 10 or so passengers.

[mross]

You could lengthen your boat and make it "more efficient" but you would spend more on fuel!

[WotEver]

3 minutes ago, mross said:

You could lengthen your boat and make it "more efficient" but you would spend more on fuel!

I need to go away into a darkened room and think about this...

[rusty69]

4 minutes ago, WotEver said:

I need to go away into a darkened room and think about this...

Theres no room, I'm still in ere 

 

[WotEver]

1 minute ago, rusty69 said:

Theres no room, I'm still in ere 

 

Well move over and make some room. 

Edited May 5, 2017 by WotEver
Remove an apostrophe that me and my phone disagreed over

[cuthound]

6 hours ago, Cheshire cat said:

I am obviously missing something here. Why did several shareholders think fuel consumption had increased? Were they insinuating that the BMC had developed some kind of fault?

We've still got a Pat Buckle share. The 2.2 Nanni pootles along nicely at 1200 rpm. Similar speeds on our own boat require 1300 - 1400 rpm but the fuel consumption from the 1305 Beta is definitely better at 1.2 litres per hour 

 

The BMC engine was nearing the end of its life, and it coincided with the huge increase in the cost of diesel, as the DERV  duty rate was applied to the propulsion element.

I hoped to settle the argument by compiling accurate fuel consumption figures, but it only muddied the waters, as the variation was so large, largely due to differing routes and of course how different cruising speeds of different shareholders.

Our first shareboat was the Pat Buckle built Oberon, which utilised a 1.4 Boatserve Mitsubishi engine. The engine was acheduled for replacement after 8000 hours when  we sold our share.

Oberon is now privately owned and moored at Fradley. It brings back happy memories every time we pass her.

[cereal tiller]

50 minutes ago, rusty69 said:

Theres no room, I'm still in ere 

 

Did you evict Captain Oates?

[rusty69]

13 minutes ago, cereal tiller said:

Did you evict Captain Oates?

No, i ran out of Weetabix! 

[Captain Pegg]

On 5/4/2017 at 12:59, Mike Todd said:

. . . and may actually decrease the speed as the extra engine power draws more water out from under the boat and hence decreases the clearance even more.

It's physics Jim, but not as we know it.

Diminishing returns of increased speed tending to zero as power is increased I can buy; creating a bigger resistive force than the one applied can't be right surely? There's laws about that kind of thing,

JP

Edited May 5, 2017 by Captain Pegg

[Scholar Gypsy]

58 minutes ago, Captain Pegg said:

It's physics Jim, but not as we know it.

Diminishing returns of increased speed tending to zero as power is increased I can buy; creating a bigger resistive force than the one applied can't be right surely? There's laws about that kind of thing,

JP

I think the point is that if you drop the boat onto the bottom of the canal, then the extra thrust from the prop is more than offset by the drag from the mud?

[Captain Pegg]

35 minutes ago, Scholar Gypsy said:

I think the point is that if you drop the boat onto the bottom of the canal, then the extra thrust from the prop is more than offset by the drag from the mud?

I did wonder about that but is it really possible? I wouldn't be surprised if the system will find some equilibrium before that happened (assuming of course the shape of the channel doesn't change).

JP

[Cheshire cat]

4 hours ago, cuthound said:

The BMC engine was nearing the end of its life, and it coincided with the huge increase in the cost of diesel, as the DERV  duty rate was applied to the propulsion element.

I hoped to settle the argument by compiling accurate fuel consumption figures, but it only muddied the waters, as the variation was so large, largely due to differing routes and of course how different cruising speeds of different shareholders.

Our first shareboat was the Pat Buckle built Oberon, which utilised a 1.4 Boatserve Mitsubishi engine. The engine was acheduled for replacement after 8000 hours when  we sold our share.

Oberon is now privately owned and moored at Fradley. It brings back happy memories every time we pass her.

I think you'll find that Oberon is no longer there. The owner of the caravan park has bought the dry dock and all the moorings that were part of Swanline. Two weeks ago there were very few of the "regulars" on the on-line moorings. I think they were all served notice. I don't remember seeing Oberon in her usual spot.

There's an overnight mooring fee to be paid to the cafe if you want to moor above The Swan. No one in their right mind would pay which begs the question as to what is going on?

[Scholar Gypsy]

7 hours ago, Captain Pegg said:

I did wonder about that but is it really possible? I wouldn't be surprised if the system will find some equilibrium before that happened (assuming of course the shape of the channel doesn't change).

JP

If you run the engine fast enough, and the channel is shallow & narrow (eg going through a bridgehole), then you can end up with a much lower water level around the boat, which will lead to much more drag. So Newton's third law is not contravened.

[cuthound]

6 hours ago, Cheshire cat said:

I think you'll find that Oberon is no longer there. The owner of the caravan park has bought the dry dock and all the moorings that were part of Swanline. Two weeks ago there were very few of the "regulars" on the on-line moorings. I think they were all served notice. I don't remember seeing Oberon in her usual spot.

She was still there on the Thursday after Easter when we last came past.

I live in hope of the owner being on board one day so we can have a chat about her past.

Edited May 6, 2017 by cuthound
To add a missing worm

[mross]

But increasing engine power will never slow you down although you reach a point where the increase is very small.

[Mike Todd]

2 hours ago, mross said:

But increasing engine power will never slow you down although you reach a point where the increase is very small.

Starting from the observation and working back to an explanation:

it is certainly my experience that there are circumstances in which increased power leads to decreased speed. But I have not conducted a thorough analysis so  . . .

I think part of the issue is not to assume that the whole of the forward speed is obtained simply by thrust through the prop shaft onto the boat itself (like it might be with a jet stream) The propellor is pulling water from in front of the boat and forcing it out behind. Part of what this does is to create a 'slope' so that the boat is 'falling' down in a forward direction. If the demand from the prop is increased and the physical context cannot meet that demand then gradually the boat comes closer to the bottom and ground effects take over. Ultimately, and one has seen people, not always novices, doing just this - they end up with the boat 'attached' to the bottom and the more the power is increased the more the boat is pulled to the bottom and stays there. The wave effect no longer propels the boat forward and all of the energy is dissipated into, in effect, churning up the mud.

Another context is to watch closely what happens when passing through a narrow bridge hole. If the power level is kept constant then it can be easily seen that the forward speed drops noticeably. 

Overall, any 'proper' analysis needs to recognise that the changes in power (which usually result in greater thrust but, of course, there is the possibility of causing bearing to begin to seize so that less of the power goes into thrust) alters the overall behaviour pattern so that linear formulae do not necessarily work.

One other item to throw into the ring: on a sea shore, waves break because of the differential ground effect: the top part of the wave continues at its deep(er) water speed whilst the bottom  part is slowed down - ground effect - which allows the top part to overtake and hence create the breaking wave.

 

All of which probably proves that my involvement in hydrodynamics of hull forms is very ancient indeed and that I have forgotten almost all of it! Instead, I simply prefer to treat the canals with a degree of respect and seek to find the way in which they work for me rather than against me. Makes for a simpler life, especially as one's own physical capabilities reduce! (Note: I keep putting more and more into it but life inexorably insists on slowing down . . . )

[Chewbacka]

Put simply, if you can eject water out the back faster than it flows in at the front the boat will sit on the bottom and not move much if at all.

[Grassman]

 

We have a BMC 1.8 in a 62' Colecraft shell, PRM 150 2:1 ratio gearbox and need 1800 rpm to maintain 3-4mph in reasonable depth water.

The engine has done 10,500 hours and we've had it checked and it's in good condition.

It averages about 1.4 litres per hour fuel.

We had a larger prop fitted (from a 16x12 to an 18x12)  and it only made a difference at slower speeds, i.e less revs needed for say 2mph. At a cruising speed of 3-4mph the larger prop has made no difference.

[noddyboater]

Having experienced both, I'd definitely say that a large capacity vintage engine uses less fuel than a modern fast revving one. Maybe that's because most of the hulls they're fitted to are a better shape/more efficient or possibly just the fact they are running at lower rpm. 

My 4.2 litre, 3cyl lump just used a shade over 2 gallons on a 10 hour cruise split between deep river and shallow weedy canal. I'm not sure about the revs, (no tacho) it was that nice "potato potato potato" speed mostly but obviously moving a lot quicker on the river. Sorry if that's not very technical.