Some time ago I started collecting fuel consumption data from fellow boaters. The intriguing fact to come out was that many engines up to 40's HP actually can burn significantly less fuel by care on throttle than expected - with similar figures.

EG - My Perkins 42HP 4-107 burnt 30ltrs over an 18hr passage set at approx. 1/3rd throttle .... RPM ? dunno as I don't have a Rev counter. But the setting was the level that all settles and minimum vibration above idle. I would estimate that we are looking at 1500 - 1800 RPM.

If I open her up more - then consumption increases logarithmically ... to an alarming 6 - 8 ltrs an hour.

If anyone is interested in a copy of the data .... I'm still interested in boaters passing on figures to me :

Boat type / Make / Size.

Engine make / model / HP

Tank size / consumption fig's cruise + RPM / max experienced consumption found at ? RPM.....

Speed per consumption / RPM etc.

The more data that can be passed the better ...

On to another matter - the change of diesel that is on market now and spreading across EU. The sulphur levels are drastically reducing for ALL diesels - Red and White. But compensating levels of lubricity are catered for so the fears of many out there of damage to fuel pumps etc. is really not a factor. One area that may be found - as happened to me - is that the cleaner fuel can lift the crud that has happily sat at bottom of your tank and pass to filters etc. Use of "of the shelf cleaning agents" is not advised as this can increase the problem. This "crud lifting" is a personal opinion based on my experience, but the lubricity issue and sulphur is from my blending work for EU market of all diesel grades.

Only problem with demand for low sulphur fuel is I am told  that there is now a great shortage of heavy oil-thats the stuff thicker than diesel that all the big ships used to use-so what!-well many of the Whisky Distillerys still use it for processing the whisky!!

Phil

My 35 year old cruiser has a Ford D seies diesel engine fitted . I think it was originaly designed for the Fordson Major Tractor . with a 14 inch prop and borg warner 1 to 1 drive i get about 10 knots at 1500 rpm which is about the hull speed any increase in throttle does not increase speed . At 1500 rpm she burns 1 gallon of diesel per hour . I think that this is reasonably economical . My previous 2 boats were both sailing vessels one a corribee and the other a Wareham cat  so 10 knots is quite fast . I keep promising to write the history of my boat but when she is back in the water after an extensive rebuild I will tell the story

Heavy Oil .... you are talking Fuel Oil which comes in many different forms / grades. Ranges from what appears to be a dirty Gas Oil through to near Bitumen types.

Common Ships grade is "Cracked M 100 Fuel Oil". Cracked means that all spirit / light components have been cracked from the oil leaving a basic Fuel Oil.

The subject is an extensive one and part of my biz !! Shortage of Fuel Oil is due to many factors with Diesel being a very small part if at all of the equation ! One Trader friend of mine reckons it's better selling FO onto Local Markets at moment due to pricing. I'm sure he's not alone !

My local supplier was commenting that they were having a problem sourcing "heavy oil" locally which older large scale central heating systems and whisky distilleries use as did ships once upon a time to fire up boilers for steam turbines and that they were having to source from E. Europe

This is an up and coming Irish based independant who recently bought out Scottish Fuels and CPL(my supplier for many years-CPL was owned by Texaco who sold it off last year.

.Certainly what ships used to use to fire up boilers post coal was referred to as heavy oil-always assumed this to be something with greater viscosity than diesel itself often referred to as light oil. Also understood that some older big compression ignition engines use this rather than "diesel/"gasoil"

Oh yes and bulk price of gasoil/marine grade/red is now 70p/litre compared to 35p/litre last June(well 69.95)

Phil

Ships burn MDO (Marine Diesel Oil - a diesel fuel that is not suitable for your VP / Perkins etc., it is a little heavier than your usual Diesel). But this is onl;y burnt when manoeuvring into / out of Port. Once Pilot is away and ship sets on passage she changes over to Heavy Fuel Oil - the M 100 as I stated above. This HFO needs to be heated to be used / stored.

The Steam ship comment is only part correct, as the HFO is used in Motor Ships as well as steamers.

There has been experiments to run Ships engines on coal slurry.

Unlike car engines where the load and speed can be varied independently the engine load on a boat is determined by the propeller speed.  The power required is a cube law which means  at 6kts you need 1.8  time the power for 5kts and 3.5 times power for 4 kts.  Diesel engines are most efficient at max load which is measured as specific fuel consumption in g/bhp.hr and they are typically most efficeint at the speed at which max torque occurs.  What this means is that as you go slower you need less power, but less power means moving away from the most efficient load and the specific fuel consumption goes up.  If you had all the fuel consumption curves at a range of loads throughout the speed range you could plot the power demand curve of the boat and find the optimum point. Unfortunately, getting the data is not easy.

A reasonable rule of thumb is to run at max torque speed.  On my Bukh 20 this is 2400rpm and gives us about 5.5 knots, going to max speed and load at 3000 rpm gives 6.5 knots.

Hi Malcolm

Thanks for that. Sounds like the formula I was looking for. The question is: how do the non-technically minded find out what their maximum torgue speed is? Presumably it is affected not just by the engine but also by hull shape and propellor size etc?

Why not use vegetable oil from your local supermarket free of fuel duty & VAT at probably 1/3 of the price

Car diesel engine owners have used it for years. You can find many references on the Web to drivers doing 100's Ki of kilometers with no damage to the engine. Indeed I recall that 5th Gear the TV programme did a feature on it And as of last year you don't even have to report your use to HM Customs & Excise if you use less than 2500 litres per year.

One possible downside is that the exhaust smells like your local chippy!

You might also have to consider waxing if cruising in very cold waters but mixing with mineral diesel should sort this. 

There are a couple of simple rules of thumb:

(1) for a displacement boat (i.e.most sailing yachts) the power required to turn the propeller is proportional to the cube of the shaft speed. So if you halve the shaft speed, you cut the power required to one eighth.

(2) most diesels produce about 20hp for an hour from every gallon of fuel they burn.

So if you have an 80hp engine, it should burn about 4 gallons an hour flat out, or about half a gallon an hour at half revs. It's easy enough to work out the in-between figures with a calculator.

Unfortunately, at low reves, the engine uses a disproportionate amount of power doing boring things like overcoming friction, generating electricity, pumping water, and chuffing air through itself, so these rules of thumb become wildly optimistic at low revs, and you need to download the engine manufacturer's data sheet to find the specific fuel consumption (usually given in some cussed units such as grammes per hour per kilowatt).

But at reasonably normal cruising speeds, the combination of these two rules of thumb should give you a rough idea of what to expect.

Bon voyage

Tim 

Cathy

I just Googled and found a range of engine specs,  http://www.frenchmarine.com/ProductsC.aspx?CID=1 This site has brochures for Volvo, Yanmar, VW, Vetus each has the power, torque and fule consumption curves.  The fuel consumption curves are worked out for the propellor law (ie the cube law I and Tim mentioned above) in l/hr per rpm, the shape of the curve will be pretty similar for most engines.  So if you know your engine rpm for a given cruising speed you can look it up. If you can't find your engine take a similar engine, and scale your known fuel consumption by engine revs.

 On the vegetable oil, having been in the fuels business, no way would I do it on my boat. 1 it is illegal because you are not paying duty, 2 if your car breaks down by side of road no big deal, if your engine stops on a windless trip across the channel in the shipping lane it could be a disaster.  Commercial bio diesel is not straight vegetable oil it is esterified to make it more stable and less prone to gumming and deposit formation around injectors.  Look after your fuel, don't store it for ages, drain any water from tanks and separators, try to buy from a place that has a high throughput so you get fresh fuel.

Malcolm, sounds like you are still in the fuel business. Read the links in my post and you will see that it is not illegal (subject to 2500 litres per year) and motorists with far more complex engines than a basic marine diesel donkey do not report any problems.

Personally, I would rather pay for the convenience (as I have done for years) of pulling up alongside the fuel dock and filling my tanks. Others sail on a budget and will be looking to save a few quid. 

 Personally, I would rather see the entire marine leisure industry taking a green stance and move away from mineral oils. The emissions and spills  from vegatable/bio oils are preferable to the stench of mineral fuel oil.

I thought I was going to offer some very erudite thoughts but others have got tehre before me!

 The power to turn the prop may be a cube law, but the thrust from the prop is used to propel the boat.  A vehicle will cease to accelerate when the forces on it are balanced, broadly when drag equals thrust.  Drag, a combination of form drag and skin friction  is clearly a function of shape and smoothness of surface but whatever constant you apply for this, drag will increase with the square of speed. 

That doesn't conflict with the cube law for props.  Except for contantly variable pitch props such as you find on more sophisticated aeroplanes, there is only one ideal forward speed for a given rpm - that at which the prop blades angle of attack is producing optimum thrust.   While speed is low and rpm is high, the prop will develop a high angle of attack as it describes a shallow screw, possibly almost stalling at first.  Constant-speed props on aircraft, gradually increasing angle of attack on the blades, permit them to accelerate more quickly, and thus achieve take-off speed in a shorter distance.  It must be similar for boats.  There must be a point, for a prop of a given size/diameter, when it gets closest to that ideal, and elsewhere the props simply can't accelerate the boat sufficiently to reach it.

On another matter, a question which I would very much like the answer to is this:  how much does towing a tender slow you down?  The drag from a 30-50kg tender must surely be minuscule compared with the drag of a 2+ tonne fin keel yacht, but some think the effect is significant.

Best way to save money is to sail more and motor much less ... if there's not the "right" wind wait or go somewhere else.

In the old days the engine was an auxiliary, for convenience in marinas and tidal rivers (please don't reactivate my "night vovage to the football field" history).

In the past rivers were navigated using quant poles on the Broads, and oars ropes and horses on the Thames - I can't imagine Coarse Sailors or Montmorency (yes I know he was the dog) being too concerned about the price of boat fuels!

As far as sail only goes read Schooner Sunset-the final days of coastal schooners.Many of these boats before ww2 still did not have engines so beautiful descriptions of waiting for the wind;kedging up chanel and warping out into mid river channel on a rising tide by setting an anchor in mid channel and letting the current do work-and these were vessels up to about 100ft-anything to avoid paying for a tug out of Mersey et al.

Phil

Never thought the Merchant Navy was romantic-probably thats why I never did it despite there still being the opportunity when I was a teenager.

-even post war on Liberty Ships!-known several people who served time on those in tropics-for those who dont know-steel down to mess tables and chairs/no insulation or cooling systems and many had doubling plates welded mid ships to stop them breaking their backs.A friend told me of one he served on where the boat despite reinforcing was beginning to break in half-he used to amuse himself by putting pencils in the opening crack and watching them get cut in half!Another friend remembers how before making port they would stick tarpaper over the rust holes and paint over it!

Also for anyone interested in Schooners quite a few of the UK based coastal schooners were ex Grand Bankers.Just been reading a book published in Newfoundland on these-most sank with their crews sooner or later;were run down by transatlantic traffic or never came back from salted fish carrying trips to Europe other than those which were adopted in Europe for coastal trade.

Phil

Hi Cathy,

Beta publish torque & fuel consumption data (propellor law) for the 13.5 BHP BZ482 fitted to my Yarmouth 23. Yesterday I noted speed through the water for various RPM settings while maintaining a constant heading on the autopilot (it was about a F4 on the port bow). The torque curve on the BZ482 is fairly 'flat' up to the maximum torque (around 2600 RPM), it then falls slightly at 2800 RPM before quite a steep drop off. These are the figures I obtained:

RPM =  Kts = litres/hr = NM/litre

1600 = 3.7  = 0.5 = 7.4

1800 = 4.1 = 0.6 = 6.8

2000 = 4.5 = 0.7 = 6.4

2200 = 4.9 = 0.8 = 6.1

2400 = 5.2 = 0.9 = 5.7

2600 = 5.5 = 1.0 = 5.5

2800 = 5.8 = 1.2 = 4.8

These figures seem to work reasonably well in the real world when I generally use between 2000 & 2400 RPM and plan for 1.5 litres/hr but generally use less. They seem to indicate that maximum economy is achieved well before maximum torque but close to the peak torque the figures 'flatten out' before dropping sharply once peak torque is passed (also 5.8 kts is my hull speed). In flat conditions I generally manage 4.9 kts @ 2000 RPM so the figures would change a bit for conditions but I expect the relationship would remain more or less the same.

Does this mean I'll be using significantly less revs? Not really as life at 3.7kts is too slow for me but I will spend more time at 2000 RPM rather than 2400.

You asked how to determine your torque without a graph. Looking at my limited figures, it may be worth noting speed/revs and when the gains start to tail off you are probably around an efficient engine speed (I had 0.4Kt gains/200 RPM increase until between 2200 - 2400 where it fell to 0.3).

Richard