Fuel Consumption

[Hockham Admiral]

I don't know if this will help anyone out there but if you want to make considerable savings then here's how we do just that.

 

I'm afraid that, with advancing years, we just tend to set 1400rpm and take what speed comes (unless it gives me over the speed limit, of course). Typically, in still water that will be about 5mph.

 

If you look at my fuel graph for our Nanni 50 (which is a pretty typical power unit) you'll see that we also make a fair bit of savings!

 

 

If I get Breydon wrong and have to go up to 1800rpm, or Heaven forbid, 2000rpm, the resultant fuel use/cost is horrendous!

On displacement boats you seldom go any faster than the design speed (Between 6 & 7 mph for most Broads motor-cruisers)  but just put up a bigger bow wave and wake.

[Hockham Admiral]

If by which you mean make more use of the tides, Dave, then I entirely agree!  :clap

 

We, nowadays, always leave Bells, (when heading for the Waveney) to get to the New Cut by slack LW. Using 1400rpm I usually have to throttle back to keep within the lower limits! We get the ebb  down the Yare and the flood up the Waveney.

[Guest]

.......On displacement boats you seldom go any faster than the design speed (Between 6 & 7 mph for most Broads motor-cruisers)  but just put up a bigger bow wave and wake.

 

Absolutely right.

 

We're all far more used to cars than boats and it's difficult to fully appreciate the tremendous difference in drag between wheels on hard surface and a boat pushing through water.

 

Cars and bikes start to fight wind resistance from about 15mph upwards, but small displacement boats are resisted far more strongly from around 6mph, depending on waterline length.

 

Once the bow wave extends to the length of the boat, it sits in it's own "trough" and tries to climb over it's own bow wave, which is an impossibility. Power can be increased logarithmically with little or no increase in speed. The shorter the boat, the worse the problem. A 10ft dinghy has a max hull speed of about 4.5 knots, an 18ft dayboat about 6.5 knots, yet a 40ft cruiser can do about 9 knots with less wash.

 

The same physical constraint applies to ships, but their much longer lengths allow good cruising speeds weel within their waterline length, eg an 800ft Liner has a max hull speed of about 35 knots, so it is no longer the main constraint on cruising speed.

 

The formula is  the square root of the waterline length in feet multiplied by the constant of 1.4 , which gives the max hull speed in knots.

 

If the constant of 1.4 is reduced down to around 1, then the drag is proportionally much less, giving surprising fuel savings at what is usually about two thirds of maximum speed.

[Hylander]

Dajen you sound just like us.       We topped up with £60 worth of fuel not so long ago and it should have taken us up about 176 ltrs.      Checked the fuel a few days ago and blow me 110 ltrs.    Yep we will now be a little bit more careful about going with the tide if we can,    the trouble with the Southern rivers is it is quite a stretch before you get anywhere.    Like you we make our minds where we are going before we even look at the tides and just go with the flow, or rather we not as the case may be.        I felt like saying 'ouch' when we saw how much we had used.     A lesson learned the hard way, but there again life is too short not to do what you want to  ,  when you want to,  within reason.      Hubby never tears around the rivers anyway and we often get over taken.        

[donnygeoff]

To be honest John I never really monitor our fuel consumption (is that bad of me) the only thing I know for sure is that we use more Diesel now we are based on the Southern Broads than we ever did when we were "up North" 

 

Must try to be more organised.

 

Dave

i never monitor my fuel consumption either....

Clive tells me off for not using enough....

I just potter about, tootle up and down and always get some fuel deposit back for a meal on the way home.

I must admit though, that using the tides helps...

 tight fisted Yorkie

[jeffbroadslover]

Fuel consumption does not worry me this year as we are making use of the "all inclusive" offer at Ferry Boatyard.

 

Having said that I will probably still be as careful as possible in order to "save fuel"   Old habits die hard !

[Hockham Admiral]

I guess I ought to have said that we are retired and so don't restrict ourselves to a set itinerary when we go out. So we're seldom in a hurry to get anywhere and in the long Summer days I like to be away not too long after dawn. It can be downright magical at times on a misty morn... Mary-Jane frequently wakes up to breakfast in a new location! 

 

Nor do we tend to know when we'll be going home... just when we've had enough for that particular outing... and we do appreciate how lucky we are.  

[riyadhcrew]

John,

 

That sounds absolutely perfect and IMHO, that's the way it should be. Long may it stay so.

[Polly]

According to your formula, Strowager, Brilliant has a hull speed of 7.6knots. She can shift in a good breeze, and logged 6.2mph in a force3 on Wroxham Broad last week, but I had assumed a 5 knot hull speed so the formula is interesting.

Sorry, it's not about diesel.

[Hockham Admiral]

Strow, Hi.

 

Using that formula, the square root of 34 gives me 5.831 and times 1.4 that equals 8.163kts. That's  9.4mph!

 

Well.................. at 1600rpm I get about 6mph and it then needs to be about 1900rpm to get 7mph.

Anything more and we just push up a bigger bow wave/wash! The fuel consumption by then is horrific by our normal standards and at least doubled for one more measly mph...

 

Your suggestion to use 5.83(kts) x 1 = 6.67mph approximates to what we actually do if we're ever in a hurry.

 

Still, it's all quite interesting to us thank you.     

[DaylightGate]

My Bounty Sedan also has a Nanni 50 and I cruise mostly on the North, at 1400 to 1500 revs. which is meant to equate to 4 - 5 mph

 

I only have a dip stick to monitor fuel usage but I estimate that I am doing about 2litres/hour. Using the Admirals graph that seems a bit high.I have had Daylight Gate since March last year and I don't think that she has been out of the water for a few years (other than for survey).

 

I intend to get her anti-fouled at the end of this season and I am hoping that this should improve fuel consumption.

 

Am I right in this?

[BroadScot]

Hing oan here....errrrr are we back at school doing MATHS!  Awe this talk of figures puts me into the 36 26 36 numbers mode, now that is what I call Maths   It is staggering though, the fuel you can save, when you do catch the right tide You can bomb along within the limit at tick over. 

 

Iain.

[ranworthbreeze]

Tan was bombing along nicely at 6mph in a 6mph stretch when she was given a warning by the B.A. it would seem that the we were being pushed along with nearly a 2mph tide, she was clocked at 7.5mph by satellite navigation.

 

She still keeps getting ribbed about it.

 

Regards

Alan

[Guest]

Yes, the "constant" of 1.4 is for maximum theoretical  hull speed, where the bow wave equals the waterline length, so that speed is already inefficient as regards engine thrust for speed attained.

 

It's much more efficient in practical terms to stay within 1.2 or 1 instead.

 

Don't forget also,  it's waterline length (LWL), rather than overall length (LOA).

 

(unlike the BA toll calculation which uses LOA and beam to get the square metre footprint of a hull, including deck overhang.

 

It can make quite a difference too, even with motor cruiser's typically vertical transoms.

 

A typical 27ft cruiser has a waterline length of 24ft, reducing the square root from 5.2 down to 4.9, and the max hull speed calculation from 7.3 knots down to 6.9, nearly half a knot less.

[SPEEDTRIPLE]

Tan was bombing along nicely at 6mph in a 6mph stretch when she was given a warning by the B.A. it would seem that the we were being pushed along with nearly a 2mph tide, she was clocked at 7.5mph by satellite navigation.

 

She still keeps getting ribbed about it.

 

Regards

Alan

 

 

That`s the problem with the way the BA rangers measure speed. Unless you have a GPS speedometre or log, you can never really be sure of your speed. If you go by engine revs like the majority of hire boats, then that speed is over moving water, and NOT static as it would be over land. The very idea of having river speed limits "over land" is in my mind rediculous, as there are far too many variables. Your speed will be dependant on how much drag you get from a clean or dirty hull, wind direction and speed, whether you are towing a dinghy, and even if you drag your fenders in the briny. When we were on Kestrel in June, most of the fenders were dragging in the water, so i picked them up and stowed them on the side decks. The result was we gained an extra .5 mph average, so we could drop the revs a little, and saved the yard some deisel. 

 

With these variables in mind, it`s a bit naughty of the BA using speed guns from "static" positions. The next thing will be fixed penalty notices and a fine, It`s coming.

 

Then when the BA start throwing out hundreds of speeding fines every week, the hire boat industry will die, no matter how much deisel you use (or don`t as the case may be(

[Hockham Admiral]

With these variables in mind, it`s a bit naughty of the BA using speed guns from "static" positions. The next thing will be fixed penalty notices and a fine, It`s coming.

 

Then when the BA start throwing out hundreds of speeding fines every week, the hire boat industry will die, no matter how much deisel you use (or don`t as the case may be(

It's not "coming", Neil, it's probably here! But if peeps will exceed the speed limits I don't have any sympathy.

 

The BA might be over-authoritative at times but they're not stupid. They just cannot afford to cause the "hire boat industry" to die; then so would the Norfolk Broads as we know it. It's a simple matter of revenue.

[Guest]

.... it`s a bit naughty of the BA using speed guns from "static" positions.......

 

There's no other practical and economic way for them to do it Neil, other than tailing suspect craft, which would greatly reduce the number of craft monitored by each ranger.

 

The BA's usage of static radar guns and the now very common GPS and Satnavs used by boat helmsmen gives us all a "level playing field" as it were, whereas rev counter conversions and water impeller speed logs are far less exact.

 

Also, speed limits are not just imposed to control wash erosion. They are also a safety measure against boat collision damage.  "Speed over ground" is then much more important than "speed over water" when passing moored boats or riverside structures.

[Polly]

I have a boat speed app on my iPhone which measures speed over ground. It's handy when motoring. I think it was about 50 p but it could gave been free.

[ExUserGone]

 

"Speed over ground" is then much more important than "speed over water" when passing moored boats or riverside structures.

Sorry but I have to disagree on this one, if on a running tide and trying to keep down to a speed over ground limit you have much less steering and general control so are more likely to hit something in the first place, a bit more steerage way and the collision wouldn't happen in the first place, speed over ground limits have no place on tidal waters, it's just for BA's convenience in enforcement, it's wash that counts to moored craft and you shouldn't be mooring with the tide anyway, always moor against it, theres no safety in being pushed along sideways while trying to abide by a speed limit.

[littlesprite]

Sorry but I have to disagree on this one, if on a running tide and trying to keep down to a speed over ground limit you have much less steering and general control so are more likely to hit something in the first place, a bit more steerage way and the collision wouldn't happen in the first place, speed over ground limits have no place on tidal waters, it's just for BA's convenience in enforcement, it's wash that counts to moored craft and you shouldn't be mooring with the tide anyway, always moor against it, theres no safety in being pushed along sideways while trying to abide by a speed limit.

 

I think the BA recognize the control problems of running with the tide that's why you have to give way to them. 

[Guest]

Sorry but I have to disagree on this one, if on a running tide and trying to keep down to a speed over ground limit you have much less steering and general control so are more likely to hit something in the first place, a bit more steerage way and the collision wouldn't happen in the first place, speed over ground limits have no place on tidal waters, it's just for BA's convenience in enforcement, it's wash that counts to moored craft and you shouldn't be mooring with the tide anyway, always moor against it, theres no safety in being pushed along sideways while trying to abide by a speed limit.

 

I agree with you about the loss of steerage way when trying to keep within a 5mph speed limit with a "pushing" current of say 3mph, yes, you do indeed have only 2mph steerage way and are being pushed along twice as fast.

 

Tha fact remains though, that "speed over ground" is directly proportional to how much damage a moving boat will inflict on a moored boat or stationary riverside structure.

 

Radar guns are more than "convenience" for the BA's enforcement, what other practical way could one Ranger check the speed of every passing boat ?

[SPEEDTRIPLE]

There's no other practical and economic way for them to do it Neil, other than tailing suspect craft, which would greatly reduce the number of craft monitored by each ranger.

 

The BA's usage of static radar guns and the now very common GPS and Satnavs used by boat helmsmen gives us all a "level playing field" as it were, whereas rev counter conversions and water impeller speed logs are far less exact.

 

Also, speed limits are not just imposed to control wash erosion. They are also a safety measure against boat collision damage.  "Speed over ground" is then much more important than "speed over water" when passing moored boats or riverside structures.

 

 

Yes, i do agree with a lot you`ve said Strow, but  if a boat is capable of doing 6 against a strong tide, then the wash and bow wave will be much greater, possibly causing problems for other boats, maybe moored or underway in the opposite direction.The one thing all hireboat skippers are told on handovers is "watch your wash".

[Hockham Admiral]

What is it called please Pauline/

 

dave

I've got "Android Speedometer" on both my phone and tablet. It works better on the tablet as it's accurate to 0.1mph, whereas the  phone jumps up or down in 0.5mph steps.

Here's what it looks like:

 

 

It can also be used as a moving map, the little blue arrow in the bottom RH corner is Friday Girl.

 

[Guest]

I used various free Android apps that showed GPS speed and mapping when they first came out, but dedicated devices are now much cheaper than they were, and I'm always worried about running my phone battery down and then not having it ready for emergencies. (Though agreed, you can plug it in to charge on a boat).

 

For walking and cycling I have a Garmin GPS wristwatch that shows speed and records tracks. I also have a Sony HD actioncam with a GPS that records 1080p video with the speed, distance and moving map overlaid in the corner.

 

For the boat though, I found the simplest and best all round use is a GPS/depthsounder/chartplotter.

 

It shows a moving map for the whole of the UK and surrounding waters (inland too), with GPS "over ground" speed, impeller log (through water" speed, and fuel consumption. The chart is on cartridge, so no 3G connection is needed as with the moving sat map on a phone.

 

The photo shows it running just upstream of Horning, though I hadn't noticed that the water impeller was blocked with weed again, so showing zero when the photo was taken.

 

Not a practical solution for hirers, but well worth it for owners.

[MauriceMynah]

Is there an "Hours per gallon" measurer one can get? if so is it hideously expensive?