most economical cruising speed

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homer

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I have seen various discussions about fuel usage in terms of litres per hour but what really interests me is litres per mile. In other words, if (like me) you are not often in a hurry, what cruising speed will get you the furthest for a given amount of fuel? I do know that at 1250 rpm I get 4K; at 1500, 4.6K; at 1750, 5.3K and 2000, 5.9. However, it is much harder to find out how much fuel is used at these various speeds. Has anyone ever taken the bother to find out? (Easy enough if you have a flow meter on the fuel line I guess.)
 
homer said:
I have seen various discussions about fuel usage in terms of litres per hour but what really interests me is litres per mile. In other words, if (like me) you are not often in a hurry, what cruising speed will get you the furthest for a given amount of fuel? I do know that at 1250 rpm I get 4K; at 1500, 4.6K; at 1750, 5.3K and 2000, 5.9. However, it is much harder to find out how much fuel is used at these various speeds. Has anyone ever taken the bother to find out? (Easy enough if you have a flow meter on the fuel line I guess.)
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A simple computation can be found in fluid dynamics literature and the response is quite disappointing: the litres per miles consumption goes down as the velocity, i.e. asymptotically to zero as the speed also goes to zero.
For this reason such figure is not practical and other parameters like time-to-go should also be considered.

Daniel
 
Very simple. Look at the power curves of your engine and you will see the specific fuel consumption - that is the amount of fuel used (usually in grammes) per unit of power produced (usually in KW). That curve is usually flat between 2000 and 3000 revs where max power is around 3400-3600. Therefore every unit of power produced uses the same additional amount of fuel. Specific fuel consumption varies little from engine to engine - that is they are all broadly equally efficient.

In old money (hp) the approximate amount is 1l fuel an hour for every 10hp produced. So if your engine is producing 30 hp at 2400 revs you will be using approx 3 litres an hour. To give you a practical illustration, my 2030 produces 21hp at 2400 and uses typically 2.2l an hour. That gives me 5.6knots in flat water, but less into any significant wind or sea.

It is impossible to give any firm rule for miles per litre as this is dependent on conditions. If you want to use less fuel, use less power (ie lower revs) and go slower. So, if I drop to 2000 my fuel consumption drops to about 1.7 an hour, speed to 5 knots and I average 2.94 miles a litre rather than 2.54 - but my passage time increases by about 10%

This of course is in flat water with no tides or currents.

Hope this helps
 
20 hp volvo......40 litres to motor from Newlyn to Kilmore Quay in Ireland...say 130 NM at 2000rpm...about 4.5 kts. At 5.5 kts and 2750 rpm 80 litres and at 3000 rpm 6.5Kts would need 120 litres.
 
I cant offer any fact based answers I am sorry to say. Previous posters have focussed on engine efficiency which is clearly important.

Of equal importance is what is often erroneously called maximum hull speed. Actually this is the speed at which very significant increases in power is required to gain a modest increase in speed (for a displacement hull that can't go on the plane). There are various calculations offered but 1.4 X square root of LWL seems to be commonly used as a rule of thumb.

My boat has an LWL of around 36ft so the max hull speed is 1.4 X 6= 8.4 knots. In practice this seems to be there or thereabouts.

Up to 6 knots the boat is easily driven by the engine and seems to cause little in the way of bow or stern waves. At this speed the engine is running at around 2000 revs. Above this speed I have to apply proportionately more power for little speed gain and much bigger bow and stern waves start to appear (remember those waves are wasted energy created by your fuel!).

So! when all is said and done you will find that you get pretty close to your optimum cruising speed if you do it by ear and by observation.
Progressively increase the boat speed until the engine is under load and you are making a fuss in the water at near full speed and then back off the revs in steps, say 100 at time, allowing the boat to adjust to the new throttle setting each time and you will find a point at which you can drop 100 or 200 or even 300 revs with little measurable decrease in boat speed. I have always regarded this as the optimum cruising speed.

On all three boats I have had this occurred between 1800 -2200 revs.

If anyone has any maths to support or contradict this, I would love to see it.
 
silver-fox said:
I cant offer any fact based answers I am sorry to say. Previous posters have focussed on engine efficiency which is clearly important.

Of equal importance is what is often erroneously called maximum hull speed. Actually this is the speed at which very significant increases in power is required to gain a modest increase in speed (for a displacement hull that can't go on the plane). There are various calculations offered but 1.4 X square root of LWL seems to be commonly used as a rule of thumb.

My boat has an LWL of around 36ft so the max hull speed is 1.4 X 6= 8.4 knots. In practice this seems to be there or thereabouts.

Up to 6 knots the boat is easily driven by the engine and seems to cause little in the way of bow or stern waves. At this speed the engine is running at around 2000 revs. Above this speed I have to apply proportionately more power for little speed gain and much bigger bow and stern waves start to appear (remember those waves are wasted energy created by your fuel!).

So! when all is said and done you will find that you get pretty close to your optimum cruising speed if you do it by ear and by observation.
Progressively increase the boat speed until the engine is under load and you are making a fuss in the water at near full speed and then back off the revs in steps, say 100 at time, allowing the boat to adjust to the new throttle setting each time and you will find a point at which you can drop 100 or 200 or even 300 revs with little measurable decrease in boat speed. I have always regarded this as the optimum cruising speed.

On all three boats I have had this occurred between 1800 -2200 revs.

If anyone has any maths to support or contradict this, I would love to see it.
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Yes, I have done the maths, with a fuel flow meter, and I can truly say that everyone so far seems to have missed the point (IMHO of course).

The OP is asking about RANGE. He wants "what cruising speed will get you the furthest for a given amount of fuel". It is nothing at all to do with economy which is what people have been discussing.

The answer is actually very simple. The longest range is given at the lowest possible power setting that is propelling the boat.

The reason for this is the power required to overcome the drag of a displacement hull. Any increase in power does not provide a proportional increase in speed as some of the power is just used to combat drag.

Attached is a little table that I have done for my boat. It will be of a similar shape for any diesel engine monohull displacement boat. Look at the two graphs.

The slower you go, the further you go.
 
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SolentBoy said:
The answer is actually very simple. The longest range is given at the lowest possible power setting that is propelling the boat.


The slower you go, the further you go.
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Sorry - but that is exactly what I said earlier. If you want your fuel to last longer (but not necessarily travel further) just use less power.

However, this is a generalisation and only holds good in flat water and no tides or currents - if you are concerned with speed over ground and distance covered rather than speed through the water.
 
Tranona said:
Sorry - but that is exactly what I said earlier. If you want your fuel to last longer (but not necessarily travel further) just use less power.

However, this is a generalisation and only holds good in flat water and no tides or currents - if you are concerned with speed over ground and distance covered rather than speed through the water.
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Sorry, missed that part of your post. Yes, you and I agree on this point as we proved in the other thread.
 
SolentBoy said:
Missed one point earlier. Both range and endurance improve if you go slower. The slower you go, the further you go, and the longer you keep going.
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Yes, but without labouring the point, that is only in flat calm conditions with no tide or current - so speed (and distance) through the water, not necessarily over the ground.

You notice the difference when you "sail" in the Med where there is minimal current and passage times under motor are very predictable - compared with say Yarmouth to Studland with or against a spring tide where the tide can be equal to nearly 50% speed through the water of a modest boat so can double your elapsed time - or reduce it by a third.
 
I agree that volume of fuel used p[er mile is a pretty useless indicator of fuel "efficiency." As noted, it works great on a lake, but not useful with currents. The most important figure is volume per hour.

Because our fuel gauge is kaput, I have been using the fuel volume per hour for the past 13 years on our boat. I made up a simple spreadsheet, and over the course of these years have found we use 0.423 gallons per hour of fuel. That's pretty much like 1/2 gallon per hour, so when I motor for 20 hours I add 10 gallons to our 23 gallon tank, pretty much turning over half of the tank at each fill up. We have a Universal (Kubota) tractor engine, M25, in our 1986 Catalina 34.

We motor at 2300 to 2500 rpms with an engine that can do 3000 rpm at WOT. Our average consumption is based on every sort of engine speed: full out, cruising, slow crawls through rivers, you name it. That's why it is called an average (!). :)

More importantly, the condition of your hull and prop are considerations to assure that they are clean, 'cuz a dirty bottom will significantly reduce your litres per mile. :rolleyes:
 
Thanks to all responders but, particularly, SolentBoy for the graphs which provide precisely the information I was after. For the record, I am cruising in the med and so tidal streams are rarely an issue and nor are headwinds - I only motor when there is no wind! (And I do realise that the engine heeds to worked hard from time to time.)
 
So you should just be able to choose your most comfortable cruising speed and be able to predict pretty accurately what your consumption will be. As I suggested earlier that is likely to be around 70% of max power, which is the recommendation from Yanmar, but you may adjust a little either way as engines tend to have a "sweet spot". In my case, when I was doing what you are doing it was 2400 out of 3600. Engine has done 3500 hours, mostly at that speed.
 
SolentBoy said:
Missed one point earlier. Both range and endurance improve if you go slower. The slower you go, the further you go, and the longer you keep going.
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Not picking on you as several have made the same point, just using you as the example as you have stated the above the most succinctly :).

My understanding is that this PBO forum also includes for power boats, in which case it should be noted that many modern designed planing power boats, especially catamarans, will use approximately the same quantity of fuel per nautical mile regardless of their speed (or throttle setting if one prefers to consider it that way), including at maximum speed, once they are on the plane.

My experience is that if ones boat has been designed by a good naval architect then he will be able to tell you quite accurately what fuel consumption will be but otherwise the only way is to meter fuel in a sea trial at various speeds, anything else is just illfounded assumption.
 
In calm conditions, measure the boat speed when running the engine at various RPM up to maximum. Plot a graph of boat speed versus RPM and you will see that it starts off as a straight line and then curvres over into a flatter region. ie extra revs are not giving a significant increase in boat speed.
If you operate in the linear region you will get reasonable fuel consumption for your particular boat.

As is suggest by others,the litres per hour presentation has its problems. The faster you go , the less time it takes to get there and you will find that the miles per litre is surprisingly constant over the mid speed range.
Planing motorboats are surprisingly economical . For example 20litres per hour would be horrendous for an auxiliary yacht travelling at 5knots but if you in a powerboat and are travelling at 20knots you will find that both vessels will use about the same amount of fuel for a twenty mile trip.ie one hour of fuel consumption for the powerboat and four hours for the yacht
 
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