Some thoughts on speed and its effects

[NB Alnwick]

 

Full throttle - Alnwick travelling at 7.2 mph on the Gloucester & Sharpness Canal

Note how the hull seems to have sunk into the trough of its own bow wave!

 

While boating back from the recent Saul Festival, single-handed for much of the journey, I had time to consider the question of speed through the water and its effects. Armed with my Garmin GPS which calculates speed precisely, an electronic revolution counter to measure the engine/prop shaft speed and by carefully measuring fuel usage, I have reached the following conclusions:

 

Naturally, these observations, are based on the characteristics of Alnwick which has a deep draughted hull (approximately three feet for most of its length) and is just under sixty two feet long with quite long 'swims'.

 

Even on shallow canals where there is very little depth of water under the skeg, it is possible to achieve a speed of 2.4 mph at little more than a tick-over (200 rpm) and at this rate there is minimal disturbance to the surface of the water and very little power/fuel is consumed - the fuel consumption works out at approximately 1.5 litres per hour which is equivalent to about seven miles per gallon.

 

Similarly, on shallow canals, a steady speed of 3.0 mph is achievable with the engine running at 280 rpm and although this is too fast to pass moored boats without creating a slight disturbance, there is no significant 'wash' and fuel consumption remains at approximately 1.5 litres an hour which is equivalent to about nine miles per gallon. At this speed it is usually possible to stop the boat within its own length by engaging reverse and increasing the engine speed to about 500 rpm.

 

The best speed that I can maintain on shallow canals (like the summit level of the Oxford Canal) varies between 3.3 and 3.5 mph and this requires an engine speed of 300 to 320 rpm. Any attempt to move along faster simply digs the skeg into the bottom and increases the 'wash' as well as the power/fuel consumption without significantly increasing speed (in fact if I open the engine regulator right up, the prop seems to 'dig-in' and the boat actually slows down). It is not possible to maintain this rate continuously because it takes more time to stop the boat from this speed when approaching blind bridges and sharp corners - also when the canal narrows (such as when approaching a bridge) this rate of progress starts to cause a 'breaking wash'. At this speed (an average of 3.4 mph) the fuel consumption works out at approximately 1.7 litres per hour which is still equivalent to about nine miles per gallon.

 

On deeper canals such as the Oxford Canal between Napton and Braunston and sections of the Grand Union Canal it is possible to push the boat along at a rate of four miles an hour with an engine speed of 350 rpm. This rate of progress produces a significant 'wash' which will turn into a 'breaking wash' where the water is shallow at the edges and fuel consumption works out at 1.9 litres per hour which is equivalent to just over nine and a half miles per gallon.

 

Where there is greater depth of water, such as on the River Avon, it proved possible to travel at four miles an hour against the current with the engine running at around 350 rpm and no significant wash was produced but the fuel consumption was lower than when travelling at the same speed on the canals and worked out at 1.8 litres per hour which is just over ten miles per gallon.

 

On the Gloucester & Sharpness Canal where there is a speed limit of 8 mph and a significant depth of water beneath the hull, it proved possible to achieve a sustained speed of 7.2 mph with the engine running at 550 rpm - at this rate fuel consumption was in the region of 3.5 litres per hour which is equivalent to just over nine miles per gallon. Any attempt to go faster required a significant increase in engine power for no significant increase in speed and, as can be seen from the photograph above, the hull then seemed to sink into the trough created by its own bow wave, to the extent that more power would be required to climb out of that trough but then, if the boat goes faster, the trough gets deeper and so requires even more power etc. etc.

 

My general conclusion is that on most canals the best practical speed to aim for is around 3.4 mph (not the 4 mph that most people seem to quote) and that any attempt to maintain a higher rate of progress will only result in excessive fuel consumption and an annoying 'wash' that could cause damage and inconvenience to others. Even on rivers, nothing is really gained by trying to go much faster than 4 mph.

[Keeping Up]

I'll concur with most of your musings - they fit in well with my own thoughts which are inevitably based on a shallower draught and higher-revving engine. It's always surprised me that the fuel consumption, in miles per gallon, is fairly constant as long as I am not trying to go too fast, so that deliberately dawdling does not seem to save diesel.

 

Your comment about the hull sitting in the trough of its own bow wave intrigues me. Often when travelling at speed on deep water I have thought that Keeping Up was adopting a relatively bow-down aspect, where looking along the roof line seems to lead straight to the horizon; your thoughts have provided a probable explanation.

 

I'm just a little puzzled by your last sentence though. On big rivers like the Severn or Trent, I usually travel at between 5 and 6 mph and I wonder why you suggest that there is little to be gained there by aiming for more than 4mph?

 

Love the picture BTW

[alan_fincher]

Naturally, these observations, are based on the characteristics of Alnwick which has a deep draughted hull (approximately three feet for most of its length) and is just under sixty two feet long with quite long 'swims'.

 

 

................................................................................

.................................

 

 

My general conclusion is that on most canals the best practical speed to aim for is around 3.4 mph (not the 4 mph that most people seem to quote) and that any attempt to maintain a higher rate of progress will only result in excessive fuel consumption and an annoying 'wash' that could cause damage and inconvenience to others. Even on rivers, nothing is really gained by trying to go much faster than 4 mph.

 

Graham,

 

I think you have probably very accurately summed up what the situation is for Alnwick.

 

Obviously if you then took a very different boat you would likely get a very different set of results.

 

That's probably about all you can say, without making such an in depth investigation of another boat.

 

Out of curiosity, how are you able to measure fuel use with any great accuracy ?

[NB Alnwick]

I'm just a little puzzled by your last sentence though. On big rivers like the Severn or Trent, I usually travel at between 5 and 6 mph and I wonder why you suggest that there is little to be gained there by aiming for more than 4mph?

 

My conclusions related mainly to the economical aspects. I think that there is an optimum economical cruising speed on most rivers which in our case is around 4 mph - at which point going slower or trying to go faster both appear to use more fuel in terms of miles per gallon. On larger rivers like the Trent and Severn, it is obviously worth sacrificing some economy in order to enjoy a more frequent change of scenery! We went from Stourport to 'The Boat' at Ashleworth in a single day and, apart from the occasional boat (including gravel barges), a few pubs and the odd bridge, we saw little else but trees . . .

 

Sorry - forgot to mention the locks!

 

Out of curiosity, how are you able to measure fuel use with any great accuracy ?

 

The engine is gravity fed from a rectangular day tank that holds 17 litres of fuel and can be 'dipped' with an extending metal tape measure. I 'dipped' the tank close to the start and end of periods spent running at constant throttle settings, calculated the difference each time and then calculated average figures.

Edited July 17, 2008 by NB Alnwick

[alan_fincher]

The engine is gravity fed from a rectangular day tank that holds 17 litres of fuel and can be 'dipped' with an extending metal tape measure. I 'dipped' the tank close to the start and end of periods spent running at constant throttle settings, calculated the difference each time and then calculated average figures.

Ah, of course!

 

The day tank.

 

Yes, that does make it a lot easier to be accurate, particularly if it's genuinely regular in section.

 

Being a more run of the mill boat, we only have the inevitable tank being the closed off end of the counter.

 

You might reasonably expect that to be fairly linear, in terms of so many millimetres on the dip-stick per litre, but it isn't at all.

 

Plus, because it's dipped at the filler hole, near the back, you can change the reading dramatically, not by using any fuel, but by filling or emptying the huge bow fresh water tank.

 

I long ago came to the conclusion that knowing how much fuel is in the tank, with any accuracy, is a total impossibility. Hence making estimates of usage is also pretty hopeless.

 

Best I can do really is "fill to fill".

 

And yes it does come out at between 1.3 and 1.5 litres per hour, (BMC 1800, 50 foot boat, typically passing through at least as many locks as it does miles).

[Machpoint005]

You might reasonably expect that to be fairly linear, in terms of so many millimetres on the dip-stick per litre, but it isn't at all.

 

Millimetres per litre? Inches per day, surely?

Edited July 17, 2008 by Machpoint005

[Chris Pink]

My general conclusion is that on most canals the best practical speed to aim for is around 3.4 mph (not the 4 mph that most people seem to quote) and that any attempt to maintain a higher rate of progress will only result in excessive fuel consumption and an annoying 'wash' that could cause damage and inconvenience to others. Even on rivers, nothing is really gained by trying to go much faster than 4 mph.

 

As 3.4 mph is a fairly meaningless entity without accurate measuring equipment perhaps a more relevant conclusion is that if one sets the speed to avoid (damaging) wash one is also steering at the highest possible speed in a given channel.

 

I find that there is a 'sweet' spot when the engine seems to be just right.

 

You are absolutely right that the only result of a power increase is increased fuel consumption.

 

The reason for the limiting to Alnwick's speed on the G&S (or any other channel) is that, at the point pictured, you are attempting to go uphill, up the trough behind the bow wave, so any attempt to go faster makes the slope steeper and cancels out any power increase.

[blackrose]

The reason for the limiting to Alnwick's speed on the G&S (or any other channel) is that, at the point pictured, you are attempting to go uphill, up the trough behind the bow wave, so any attempt to go faster makes the slope steeper and cancels out any power increase.

 

Isn't this the boat's hull speed? (the maximum speed the hull can achieve because it's stuck in its own trough). I think the only way a powerful boat can exceed its hull speed is by planing over the water.

 

Edit: Can someone explain to me why a boat can go faster in deeper water? I still haven't understood it properly.

Edited July 17, 2008 by blackrose

[Liam]

More water to the prop and less resistance.

[blackrose]

More water to the prop and less resistance.

 

But the prop is fully immersed in water as long as the boat's afloat. Less resistance to what?

[howardang]

But the prop is fully immersed in water as long as the boat's afloat. Less resistance to what?

The nearer the boat is to the bottome the more the effect of water flow to the prop is felt as resistance to forward motion, because the flow has nowhere else to go and causes drag.

 

Howard

[tidal]

*coughs politely*

Not a hull designer but ignoring some of the more esoteric maths, most displacement vessels are still to some degree restricted to the old waterline length calculations for hull speed.

THEORETICALLY a 52ft(WL) full displacement hull will have a maximum hull speed in flat open water (with sufficient depth) of about 9 to 10kts

 

MOST boats won't actually achive more than 85% of that figure UNLESS they have a very efficient hull shape

Narrowboats, whilst pretty well designed for their functions, do NOT have a particularly efficient hull shape. So yes looking at the pictures I would say that Alnwick (whilst doing very well indeed) is very close to her hull speed. (As Chris P and others have noted )

 

the only way Alnwick will probably go much faster is if you could get her to plane (now that I WOULD like to see) or if she was surfing down the face of a large wave (I wouldn't want to try that............. )

 

An experienced NB builder will probably correct me on this but I believe that for any disp craft the fuel efficiency curve peaks out at around 75% of practical hull speed?

 

The nearer the boat is to the bottom the more the effect of water flow to the prop is felt as resistance to forward motion, because the flow has nowhere else to go and causes drag.

 

I would not argue with Howard (he knows more than me....LOTS more) but isn't the actual VOLUME of water in a channel also a contributory factor?

 

*waits to be shot down in flames......again*

[Liam]

Wouldn't mind seeing a narrowboat on the plane but then it'd be useless as the prop wouldn't be in the water.

 

How's about a narrowboat with one of these HUGE outboards you see?

[howardang]

*coughs politely*

 

 

 

........ but isn't the actual VOLUME of water in a channel also a contributory factor?

 

*waits to be shot down in flames......again*

The main reason why resistance is increased is that the shallower the water, the more the effect of squat, which causes the boat to increase its draught and it therefore needs more energy to push the deeper hull through the water, which is what I was trying to say before but not very well!

 

HTH

 

Howard

[NB Alnwick]

As 3.4 mph is a fairly meaningless entity without accurate measuring equipment perhaps a more relevant conclusion is that if one sets the speed to avoid (damaging) wash one is also steering at the highest possible speed in a given channel.

 

I find that there is a 'sweet' spot when the engine seems to be just right.

 

You are absolutely right that the only result of a power increase is increased fuel consumption.

 

The reason for the limiting to Alnwick's speed on the G&S (or any other channel) is that, at the point pictured, you are attempting to go uphill, up the trough behind the bow wave, so any attempt to go faster makes the slope steeper and cancels out any power increase.

 

Tried a few experiments with the Garmin on land today - I always thought that 'walking speed' was around 4 mph but it is actually quite difficult and exhausting to achieve this pace when walking on the towpath and I often found myself breaking into a trot to maintain an average of 4 mph. I would estimate that most people walk at around 3 mph when they think they are 'walking fast' and this puts the speed of 3.4 mph into context. Certainly, I agree entirely that there is no advantage whatsoever in trying to go faster than a rate which is just below the speed that would start to cause a breaking wash.

 

Even so, the engine does sound very nice when I open it up for stopping or accelerating

[NB Alnwick]

An experienced NB builder will probably correct me on this but I believe that for any disp craft the fuel efficiency curve peaks out at around 75% of practical hull speed?

 

I think I remember that the builder, Phil Trotter, calculated the theoretical hull speed at around 9 Knots or 10 mph - so, on the above basis, the point at which the fuel efficiency peaks would have been around 7.5 mph. My tests indicated a slightly lower figure of 7.2 mph but against that, we have a flat bottomed boat (not the fastest of hull shapes) and a fair growth of canal vegetation below the waterline which would have increased the drag.

[BEngo]

I think I remember that the builder, Phil Trotter, calculated the theoretical hull speed at around 9 Knots or 10 mph - so, on the above basis, the point at which the fuel efficiency peaks would have been around 7.5 mph. My tests indicated a slightly lower figure of 7.2 mph but against that, we have a flat bottomed boat (not the fastest of hull shapes) and a fair growth of canal vegetation below the waterline which would have increased the drag.

 

 

It's not so much the flat bottom as the sharp transitions from the sides to the base at the stern, and to a lesser extent the bow. The water wants to go under the boat, rather than round the sides, ( which is why deep water helps) but it has to come out again at the back. If there was a curve from the base plate to the sides of the swim the water wouldn't have to go round sharp corners and there would be less drag. The double curved stern swim ( never reproduced by washer josher builders!) on a Saltley josher is probably as good a compromise as any, but also probably is the cause of joshers' reputed ability to 'pick up a tea leaf'.

 

It does seem that you might be slightly too over bladed. If 7.2 mph and ~ 550rpm is bracket open then there's another 200 rpm to be had. Using this might save a little diesel- because you are putting diesel in at practically the max rate achievable but it's not delivering all the power it could ( flat torque curve) so at say 740rpm and 7.2 mph there would be the same need for power but it would be obtained more efficiently. It's all rather academic though- theres only a few stretches where you could get the benefit. Once in that wavelength=boat length trough the only way out is to plane- and I doubt Alnwick would do that even with two TS8's. I've seen a few skippers of lighter modern boats apparently trying though.

 

Regards

N

[Dominic M]

Narrowboats, whilst pretty well designed for their functions, do NOT have a particularly efficient hull shape.

Most these days have a very inefficient hull shape, but Phil Trotter is a different class of builder. It comes as no surprise to me that you can achieve the hull speed he estimated.

 

Then look at the lines of this boat - FMC motor Crane. The curves and length of swim you won't find on most NBs today.

 

[blackrose]

The nearer the boat is to the bottome the more the effect of water flow to the prop is felt as resistance to forward motion, because the flow has nowhere else to go and causes drag.

 

Howard

 

I see, thanks.

[NB Alnwick]

It does seem that you might be slightly too over bladed. If 7.2 mph and ~ 550rpm is bracket open then there's another 200 rpm to be had. Using this might save a little diesel- because you are putting diesel in at practically the max rate achievable but it's not delivering all the power it could ( flat torque curve) so at say 740rpm and 7.2 mph there would be the same need for power but it would be obtained more efficiently. It's all rather academic though- theres only a few stretches where you could get the benefit. Once in that wavelength=boat length trough the only way out is to plane- and I doubt Alnwick would do that even with two TS8's. I've seen a few skippers of lighter modern boats apparently trying though.

 

I have set the top limit on the governor to 550 rpm under load and it comfortably achieves this. We previously had it set to the factory figure of 750 rpm but this meant that it went way over this if you accidentally knocked the regulator when it was in neutral and that was a bit too scary for me! Another reason is that, although I am sure that Phil Trotter correctly calculated the prop size, the cooling capacity of the closed cooling system with a 6ft x 6ft x 2 ins bottom tank, is not adequate for continuous cruising at maximum power - we found this out when we made the trip between Sharpness and Portishead a couple of years ago - when although we did the 20 odd mile trip in two hours flat, after running the engine a 750 RPM for the first 90 minutes or so, the temperature gauge was rapidly approaching boiling point so it was necessary to reduce the engine speed back to 600 rpm - at which point it settled back to about 80 degrees Centigrade - which is still a bit hot - it rarely runs at more than 60 degrees on the canal network.

 

Although these musings are all rather academic, I find the subject quite absorbing and it may even help me to understand how to get the best economy out of the fuel - something which is now a very expensive commodity!

[bjornunda]

No bow wave, no swim but look at the wash ..........

 

[Nickhlx]

It occurs to me that it would be useful to know your boats most economic cruising speed, with fuel prices about to take a serious hike. I for one would like to know that e.g. cruising at say 1400 rpm instead of 1600 rpm or 3.2 MPH instead of 3.8 MPH saves me 25% of my fuel, when in suitably deep water.

 

Naturally, to measure speed on a boat needs a GPS receiver and to measure fuel consumption needs some sort of flow meter giving pulses out for each cu. mm. of fuel consumed ( not sure how they do it on cars ) and a box to process the inputs. Naturally water speed need to be taken into account but generally on canals this will be small compared to forward speed.

 

Are there any people on here who have the necessary skills for the design ? I could make up a PCB and house in a suitable box with a display, and have a GPS that outputs speed to a connector / cable... and with this, all that would be needed would be to interface into a suitable point on the fuel system.

 

It could be then used by interested people (for a suitable nominal fee to be ascertained) to establish "most economical speed for their boat" is ??? rpm or MPH from their own GPS.

 

Anyone - or is all this a bit too technical ?

 

Nick

Edited July 19, 2008 by Nickhlx

[Keeping Up]

That may be true if you spend most of your time on rivers or on the G&S canal but otherwise the optimum speed will be different on every canal, and even on every section of the same canal, because of differences in depth and canal profile. Part of the problem is that you create a significant water speed backwards past the boat all the time; the shallower/narrower the canal, the greater is that speed.

 

Looking at your own wake (or wash) is a much better guide, it changes in real time with you. I find I can even detect the difference in the tiller; it develops a completely different 'feel' if you try to go too fast for your particular stretch of canal, and the handling characteristic changes towards the 'oversteer' too, significantly before the wash starts to build.

[tidal]

Looking at your own wake (or wash) is a much better guide, it changes in real time with you. I find I can even detect the difference in the tiller; it develops a completely different 'feel' if you try to go too fast for your particular stretch of canal, and the handling characteristic changes towards the 'oversteer' too, significantly before the wash starts to build.

 

Agree 100%

The evidence of your eyes, the input through the soles of your feet (or the seat of your pants if you sit), your ears and the feel of the tiller will tell you if your boat is "going right" long before an instrument will register the change.

 

in my humble and totally biased opinion of course

[bottle]

It is quite simple really, an engine is at its most economical, when it is not running.

 

Speed over the water has nothing to do with economy, it is all to do with the revolutions of the engine.

 

Lower revolutions lower fuel usage.

 

Now waits for incoming.