Can Power cost less than Sail?

Jeva

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Everybody knows that a power boat will cost more than a sailing boat to cruise, but in fact it can cost less.
On our trawler yacht it can/does cost less. Here’s how: Although we are less than 14metres LOA we have large capacity fuel tanks (4000+ litres), so we only ever buy duty free fuel - in 7 years cruising we’ve never paid more than €1/ litre and recently just topped up at €0.60/ litre. Our engine is a single John Deere 120HP that pushes us along at 7 knots and consumes about 6 litres/hour so an hour run costs between €3.60 and €6.00. I can put up sails to reduce this further if I wish, but by no stretch of the imagination are we a sailing boat.

Most sailing boats will not have the fuel capacity to purchase duty free and so will buy fuel off the dock at between €1.50 and €1.80 per litre. Assuming a fuel consumption of 4 litres/ hour for an equivalent sized boat it will cost between €6.00 and €7.20 per hour. The obvious response is ‘ Yes, but we sail and only use the engine in and out of harbour’. Well there are purists out there, but looking around the average Mediterranean sailing spots they are few and far between.

By the way ‘Destiny’ is now on the market if anybody is interested.
 

RichardS

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Everybody knows that a power boat will cost more than a sailing boat to cruise, but in fact it can cost less.
On our trawler yacht it can/does cost less. Here’s how: Although we are less than 14metres LOA we have large capacity fuel tanks (4000+ litres), so we only ever buy duty free fuel - in 7 years cruising we’ve never paid more than €1/ litre and recently just topped up at €0.60/ litre. Our engine is a single John Deere 120HP that pushes us along at 7 knots and consumes about 6 litres/hour so an hour run costs between €3.60 and €6.00. I can put up sails to reduce this further if I wish, but by no stretch of the imagination are we a sailing boat.

Most sailing boats will not have the fuel capacity to purchase duty free and so will buy fuel off the dock at between €1.50 and €1.80 per litre. Assuming a fuel consumption of 4 litres/ hour for an equivalent sized boat it will cost between €6.00 and €7.20 per hour. The obvious response is ‘ Yes, but we sail and only use the engine in and out of harbour’. Well there are purists out there, but looking around the average Mediterranean sailing spots they are few and far between.

By the way ‘Destiny’ is now on the market if anybody is interested.

6 litres an hour sounds very low for a 120HP engine unless you are running at idiling speed?

What has fuel tank capacity got to do with tax free? At the fuel stations we use in the Med the question which decides which pump we get is whether we are a commercial operation or not.

Richard
 

npf1

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6 litres an hour sounds very low for a 120HP engine unless you are running at idiling speed? Richard

FWIW - that was about the consumption of a Perkins 6.354 NA at around 1300 RPM on my (now ex) boat, which pushed it along @ approx 6knots except when banging into a head sea.
 

blampied

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Totally agree.
33 Aquastar (10m) with single 370hp Volvo
Tick over 4 knots only 2 ltrs per hour and at displacement speed burns similar to above
We have 300 gal tank,
Having left the Channel Islands the beginning of last year. so far we have burnt only a tank and a quater (in two half tank top ups) down through france, along the French med coast East as far as Marseille and Cassis then East and South into Spain down to Barcellona back up into France and along the midi canal to Toulouse and then back down, were now almost back to getting into the Med. About 2500 miles so far.
 

westernman

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A diesel engine will consume between 0.20 and 0.22 litres/HP/hour over a large part of its operating range and loading conditions.

Modern common rail electronic engines will be closer to the lower figure over a larger part of their operating range and for lighter loads.

Older engines will closer to the higher figure when operated outside of their ideal RPM and load conditions but be close to the lower figure under ideal conditions as well.

If you are using 6 litres of fuel per hour you are pulling 27-30 HP out of the engine.
 

maby

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That puzzles me. I have a 55 hp engine so should use 11 litres an hour. But I only use 2.5 litres per hour.

I was just doing exactly the same mental arithmetic and coming to the same conclusion. We have a 55hp Yanmar to a saildrive in our Beneteau and recently did a 28 hour run along the Channel. We did some of it on sail alone, but well over 20 hours was motor sailing close hauled. By the figures given above, we should have burned well over 200 litres, but our tank is only 160 litres. It was not completely full when we started and we arrived with the gauge showing about 1/3 full. We spent several weeks pottering around before we refilled and even then we could only get about 130 litres in.
 

westernman

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That puzzles me. I have a 55 hp engine so should use 11 litres an hour. But I only use 2.5 litres per hour.

Then you are not using 55 HP. But only 13 HP or so. Your engine is either not running at max RPM or is not loaded 100% or both.

If you are runing at max RPM, then your prop pitch is way off.
 

maby

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Then you are not using 55 HP. But only 13 HP or so. Your engine is either not running at max RPM or is not loaded 100% or both.

If you are runing at max RPM, then your prop pitch is way off.

I can't speak for RAI, but this also seems unlikely in our case. The boat is a beamy 43 footer with deep keel and weighing over 13 tonnes. At full throttle, the engine revs to around 3000 rpm which is pretty much what Yanmar describes as maximum revs. We normally run at around 2100 rpm on long distance passages and this moves the boat at around 6.5 knots through the water provided the sea and wind conditions are not against us. I find it very hard to believe that 13hp can move a boat that size at over 6 knots.
 

RAI

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Then you are not using 55 HP. But only 13 HP or so. Your engine is either not running at max RPM or is not loaded 100% or both.
If you are runing at max RPM, then your prop pitch is way off.
I normally run at 6 knots with a boat with a hull speed of 7 knots, so about 80% or about 44 hp deployed. Are your 0.2/0.22 figures supposed to be for Diesels flat out? What is the use of them when practically nobody runs their Diesels at full power continuously?
 

Tranona

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I normally run at 6 knots with a boat with a hull speed of 7 knots, so about 80% or about 44 hp deployed. Are your 0.2/0.22 figures supposed to be for Diesels flat out? What is the use of them when practically nobody runs their Diesels at full power continuously?

The specific consumption quoted is correct but is related to hp being produced, not revs - which is the point that Michael is making. Just because your power curve shows, say 30hp at 2100 does not mean the engine is producing that because the propeller is demanding less. This is particularly true when motorsailing as in maby's case because the prop is doing very little work as the sails are providing some of the speed so the amount of power required is reduced. These conditions can be bad for an engine as light loading at lowish revs is a cause of bore glazing.

In undemanding conditions the amount of hp required to move the boat is tiny until you get to higher speeds when it rises rapidly to overcome hull friction and wave making. The high consumption when you use high revs to offset wind and waves is because the prop is demanding full power from the engine and therefore using more fuel. If you ran the engine with no load at high revs fuel consumption would be low because the governor would reduce fuel supply to equal what power is being demanded.
 

westernman

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I normally run at 6 knots with a boat with a hull speed of 7 knots, so about 80% or about 44 hp deployed. Are your 0.2/0.22 figures supposed to be for Diesels flat out? What is the use of them when practically nobody runs their Diesels at full power continuously?

The specific consumption figures are valid over a wide range.

If you look up the figures for a four litre four cylinder 107HP John Deere engine (4045 TFM M1) for instance connected to a normal fixed blade prop correctly pitched those figures apply all the way from 1000 rpm where the engine will be producing 8HP up to 2400 rpm (maximum for this particular engine) where it will be producing 107HP.

Of course your prop may well be some what under pitched (to ensure there is no possibility of the engine being overloaded), so the maximum power you ever get out of your engine will never be equal to its rated power output.
 

RAI

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The specific consumption quoted is correct but is related to hp being produced, not revs - which is the point that Michael is making. Just because your power curve shows, say 30hp at 2100 does not mean the engine is producing that because the propeller is demanding less. This is particularly true when motorsailing as in maby's case because the prop is doing very little work as the sails are providing some of the speed so the amount of power required is reduced. These conditions can be bad for an engine as light loading at lowish revs is a cause of bore glazing.

In undemanding conditions the amount of hp required to move the boat is tiny until you get to higher speeds when it rises rapidly to overcome hull friction and wave making. The high consumption when you use high revs to offset wind and waves is because the prop is demanding full power from the engine and therefore using more fuel. If you ran the engine with no load at high revs fuel consumption would be low because the governor would reduce fuel supply to equal what power is being demanded.
So on a test stand would my engine, when new, would be producing 55 hp and consuming 11 litres an hour? However, my boat is not a test stand and the engine never develops is full potential. So what use are the figures 0.20/0.22 per HP per hour when comparing the cost of power with the cost of sail?
 

westernman

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I normally run at 6 knots with a boat with a hull speed of 7 knots, so about 80% or about 44 hp deployed. Are your 0.2/0.22 figures supposed to be for Diesels flat out? What is the use of them when practically nobody runs their Diesels at full power continuously?

Why do you think you are using 80% of the available power?
What is your definition of hull speed?

At more than 0.8 x sqrt (water line length in feet), power required goes up very rapidly and significantly. The shape of the power curve depends also on relative beam vs length and depends critically on hull shape and other parameters.

Check the manufacturers figures for your engine. Unless you have something extremely unusual (or are not using Diesel fuel), you will find your figures fall into the 0.20 to 0.22 litres per hp per hour range.
 

westernman

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So on a test stand would my engine, when new, would be producing 55 hp and consuming 11 litres an hour? However, my boat is not a test stand and the engine never develops is full potential. So what use are the figures 0.20/0.22 per HP per hour when comparing the cost of power with the cost of sail?

There are various methods which can be used to compute the power required to move a given boat at a given speed. The calculations range from approximate to very complex.

But very basically, up to about 0.8 x sqrt (water line length in feet), the power required is proportional to displacement. You can look these things up in a table. At higher speeds than that things are more complicated and the different methods start to diverge.
 

Tranona

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So on a test stand would my engine, when new, would be producing 55 hp and consuming 11 litres an hour? However, my boat is not a test stand and the engine never develops is full potential. So what use are the figures 0.20/0.22 per HP per hour when comparing the cost of power with the cost of sail?

Because if you go the other way, and average 2.2 litres an hour as in my old MD 2030 (29hp max) you know you are using little more than 10 hp to move 6 tons of boat at just over 5 knots which is relaxed cruising or motorsailing. If I run it at 3600 to get 7.4 knots it will need most of the 29hp so would use over 6l hour.

If a boat is correctly propped you would expect it to achieve near maximum revs and achieve hull speed using maximum hp.
 

RAI

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Why do you think you are using 80% of the available power?
What is your definition of hull speed?

At more than 0.8 x sqrt (water line length in feet), power required goes up very rapidly and significantly. The shape of the power curve depends also on relative beam vs length and depends critically on hull shape and other parameters.
http://www.oceannavigator.com/March-April-2013/Balancing-speed-with-fuel-consumption/
Check the manufacturers figures for your engine. Unless you have something extremely unusual (or are not using Diesel fuel), you will find your figures fall into the 0.20 to 0.22 litres per hp per hour range.
I see where you are coming from. This article gives one explanation.
http://www.oceannavigator.com/March-April-2013/Balancing-speed-with-fuel-consumption/
However, the OP was talking about 6 litres/hour being good for a mobo, cruising at a speed not much different from a sailing boat under motor, but a sail boat is using half the fuel and the thread is asking "can power cost less than sail?"
 

Bitterend

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Steve Dashew (ww.setsail.com) the designer of the Sundeer, Deerfoot and Beowulf sailing boats who then designed his own motor boat, says its cheaper to run a motor boat when All UP costs are included, ie Building in fittings and mast (s) standing / running rigging, sails, winches and all the other paraphenalia needed to sail a boat.

I had a large, heavy, steel yacht and whatever way I did the calcs, the fuel consumption over 8 years always averaged 7.5 litres per hour for a 120 hp engine This was usually 1500 - 1600 revs for a 2200 rev rated engine. We just purred along....The reason for the frugal consumption was always a clean (ish) bottom and sensible revs giving - 5-6 knots and I'd agree with 7.5 / .22 = 34 hp.

But of course people have individual preferences and these always have positives and negatives. The discussion could be endless....
 

BobnLesley

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Leaving aside the original advert and drifting from the subsequent discussion about hp/hrs per litre, I would return to the threat title and suggest that depending upon where you are, then probably yes; we've spent a lot (too much) of the last year on the US east coast and the longer we've been here, the more certain I am that if it was permanent, I'd be far better off with one of the displacement motor-trawlers that are justifiably popular around here:
Offshore sailing/motoring's not great, mostly shallow water and often an ocean-wide carry to the seas, so by the time the wind's picked-up to a decent 'sailing breeze', it's dog-rough out there, as a result a large proportion of 'sailing' is done with the Intracoastal Waterway (ICW) which tends to be shallow and relatively narrow - better suited to motoring than sailing. We often hear claims of the trawlers burbling along at 2-3 litres/hour and some of the comments above would confirm they might not be too far off, but have the advantage that if the run to the next decent anchorage is a long one, or time's tight to make the next bridge lift's schedule and most especially if the weather starts closing in, they can up their speed to 10-12 knots, just burn a bit more diesel and get to where they want to be, whilst we're still out there.
Whilst here we've bought some new rigging bits, a roller-reefing unit and a head sail and would estimate that the costs of these has been 10-15% less than I'd expect to have paid in the UK (perhaps 25-30% less than mainland Europe?) However the local cost of diesel fuel - if you've a big tank (which the trawler-yachts do) and buy wisely, would be consistently around £0.50/litre.
 
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