Fuel consumption at low speed?

I've read on line that sailboats can get 8MPG under power(no sail) ~ at low speed like 5-6knots.

Obviously power boats have very different engine, hull design etc. I wonder in general what kind of MPG can a power boat get at low speed? Or could you tell me for your boat, what max MPG can you extract? At what speed and what kind of engine? Thanks!

greatly depends on the size of the boat

45' semi displacement 40,000 lbs.
John Deere 4045TFM75 135 HP
6 knots at 1,200 RPM 1.2 GPH 5.0 MPG

Not a significant improvement below 6 knotts. Also feel the engine needs to run at or above 1,200 RPM when cruising for hours.

Ted

I have looked at the fuel consumption tables for on a Mercury Verado 200 hp gas outboard and on a Sun Tracker 24' pontoon boat, and at 2.8 knots, it gets 9.3 miles per gallon. At 13 knots, it gets 4.8 MPGs.

38' 15000lb single Cummins 450- consistently gets 1.9gph at 7.5kts, 950rpm, just under 4nmpg.

Not happy running engine at that low of a load, but it does not seem to hurt it. Often run it 2000rpm/20kts/12gph to clean it up.

You can take a look at this thread . Lots of information on there.

Sailboats have a full displacement hull and small engine. They burn very little fuel to get to just below hull speed. Full displacement power boats with smaller engines will do the same. Other hull shapes will not get anywhere near those fuel burn numbers.

So as other have said, "it depends"

Kadey Krogen 42, 44,000 lbs. Lehman SP135

1400 rpm 1.2 gph, 5.8 knots
1600 rpm 1.6 6.8

I can go 1200 to 1300 and keep it at 1 gph, at about 5 knots, but engine stays too cool.

Thank you guys! So actually the numbers are comparable with sailboats. That gives me more confidence. I have seen so many 1MPG, 2MPG for power boats on boattest.com, have been wondering with most boats have only a few hundred gallon fuel, how do these boats go long distance at all? Now I understand those 1MPG, 2MPG are at higher speed, the fuel consumption drops significantly at lower speed, and this applies to power boats too, not just sailboats.

My 32'8", 10,000 lb, pure displacement cruiser (see avatar pic) burns about 0.2-0.25 gph at 5 knots or 20-25 nmpg. My 36', 15,000 lb sailboat burns 0.45 gph at 6 knots for 13.3 nmpg.

I burn .42 gph at 5.5 knots, 1 gph at 7 knots. I usually cruise at 4.5 to 5.5 knots and use 7 knots for bucking an opposing current or hurrying across a large open stretch, since the hull doesn't really like it. I get 14 mpg at my 5.5 knot speed and more than that if I slow down, above 6 knots my hull starts digging a hole in the water and pulls the exhaust under. 50 hp Yanmar 3JH with a 3 bladed prop.

chicagoq said:

Thank you guys! So actually the numbers are comparable with sailboats. That gives me more confidence. I have seen so many 1MPG, 2MPG for power boats on boattest.com, have been wondering with most boats have only a few hundred gallon fuel, how do these boats go long distance at all? Now I understand those 1MPG, 2MPG are at higher speed, the fuel consumption drops significantly at lower speed, and this applies to power boats too, not just sailboats.

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I am one of those 1mpg folks(18kts). Range is not really an issue when you do the type of boating I do. To get more range, I just stop for more fuel. It seems you are in the research mode on powerboat cruising. My biggest advice to you is to be absolutely honest with yourself on how you are gonna use your boat. Sailboaters are the worst about this...no offense and I used to be one(had 3 sailboats). Sailboaters have this impassioned perception of what it is all about. They feel guilty if they are too comfortable...or burning too much fuel. They feel the need to have extremely complex and expensive charging systems..... Heaven forbid they start the generator!!!! Anyway, you have perception and you have reality. Hopefully those two "visions" are very close to one another!!! If not, the boat sits and/or is sold.

We sold her last summer but our 1963 34ft Chris Craft Constellation with original twin 283V8's consistently burned 4 gallons per hour at 8mph when we river cruised. So at a "trawler like" speed we got 2mpg. For fairly antiquated technology I always thought that was reasonable. Of course at 25mph all bets were off...

38' full displacement (34' waterline)
Westerbeke 55A
.75gph at 6.5 nautical miles per hour
1.5gph at 8.0 nautical miles per hour (bow thruster visible)

I haven't had my boat long enough to know what the fuel burn rates are yet but doing research I expect about 2 gpm(2 x 4-236 80 hp Perkins) at 1500 rpm which is about 7mph on the GPS in calm conditions. We'll see how it goes this summer. My little Hunter 23.5 sailboat with an 8 hp Tohatsu 4 stroke got about 14 mpg at 5.5-6 mph unless fighting into a heavy chop. Motorsailing I once got 30 mpg at the same speed.

Kevin

2dboat said:

We sold her last summer but our 1963 34ft Chris Craft Constellation with original twin 283V8's consistently burned 4 gallons per hour at 8mph when we river cruised. So at a "trawler like" speed we got 2mpg. For fairly antiquated technology I always thought that was reasonable. Of course at 25mph all bets were off...

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I had a 27' Constellation with the same power. Same year as yours. Loved that boat.

Kevin

tallswede said:

I haven't had my boat long enough to know what the fuel burn rates are yet but doing research I expect about 2 gpm(2 x 4-236 80 hp Perkins) at 1500 rpm which is about 7mph on the GPS in calm conditions. We'll see how it goes this summer. My little Hunter 23.5 sailboat with an 8 hp Tohatsu 4 stroke got about 14 mpg at 5.5-6 mph unless fighting into a heavy chop. Motorsailing I once got 30 mpg at the same speed.

Kevin

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Did you write that correctly?

Whoops,, I meant 2 gph! I think I'll burn 1 gph per engine (conservatively) maybe a tad less at 1500 rpm. I will also test to see how running on just one engine works. My transmissions are supposed to be okay to freewheel in neutral.

Kevin

I did some research on engine fuel consumption, I found at low rpm, bigger/smaller engines' fuel consumption are more or less similar. Take for example:
Yanmar 4JH5E, naturally aspirated, rated 53HP, burns about 3GPH at 3000RPM,
Yanmar 6LY3, turbo charged, 380HP, at 1600RPM it burns roughly 3.5GPH to produce 68HP
I read the data from the manufacture graph, so it's just a rough estimation

Is my above conclusion correct?

Also, I thought naturally aspirated engines are supposed to be less efficient, but from above data, it doesn't seem to have noticable difference? 6LY3 is more efficient by a small margin. I read somewhere turbo can get roughly 30% more power?

I asked a question a while back about using knots and mph. I was surprised when the majority of you said you used mph. And when a question like this is asked the vast majority of you used knots....?

Just an observation....carry on!!!!

chicagoq said:

I did some research on engine fuel consumption, I found at low rpm, bigger/smaller engines' fuel consumption are more or less similar. Take for example:
Yanmar 4JH5E, naturally aspirated, rated 53HP, burns about 3GPH at 3000RPM,
Yanmar 6LY3, turbo charged, 380HP, at 1600RPM it burns roughly 3.5GPH to produce 68HP
I read the data from the manufacture graph, so it's just a rough estimation

Is my above conclusion correct?

Also, I thought naturally aspirated engines are supposed to be less efficient, but from above data, it doesn't seem to have noticable difference? 6LY3 is more efficient by a small margin. I read somewhere turbo can get roughly 30% more power?

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It seems to me that comparing those two engines is an apples and oranges type thing. It would be more helpful if you could compare two naturally aspirated engines. At what RPM does the turbocharger spool up for example? Turbocharged engines can deliver more power per liter displacement, but I don't know that they are inherently more efficient as far as fuel burn goes.

dhays said:

It seems to me that comparing those two engines is an apples and oranges type thing. It would be more helpful if you could compare two naturally aspirated engines. At what RPM does the turbocharger spool up for example? Turbocharged engines can deliver more power per liter displacement, but I don't know that they are inherently more efficient as far as fuel burn goes.

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OK, so if I comapre 4JH80 vs 4JH57, one Turbo, one natual, at 40HP, both seems to consume same amount of fuel, roughly 2.25GPH

I guess I don't fully understand when people say "Turbo charged engine is more efficient", what does that really mean? Does it mean turbo charge can get more max HP on a similar sized block? Or does it mean they can extract more HP on the same amount of fuel? If the answer is latter, then I just don't see it with above data.

chicagoq said:

I did some research on engine fuel consumption, I found at low rpm, bigger/smaller engines' fuel consumption are more or less similar. Take for example:
Yanmar 4JH5E, naturally aspirated, rated 53HP, burns about 3GPH at 3000RPM,
Yanmar 6LY3, turbo charged, 380HP, at 1600RPM it burns roughly 3.5GPH to produce 68HP
I read the data from the manufacture graph, so it's just a rough estimation

Is my above conclusion correct?

Also, I thought naturally aspirated engines are supposed to be less efficient, but from above data, it doesn't seem to have noticable difference? 6LY3 is more efficient by a small margin. I read somewhere turbo can get roughly 30% more power?

Click to expand...

Lots of apples and oranges.

For 2 engines of moderate size difference, you can get similar economy. You're not going to get similar economy if you need 100 HP and you have 450 HP. There is a cost in fuel to spin the much bigger engine. Newer engines tend to be more efficient producing the same HP. The newest engines (tier 3 and up) will suffer very slightly in economy as cleaner burning requires doing things that slightly impact efficiency. Generally the best efficiency comes at between 1/2 and 3/4 of full RPM. So, in a perfect world, having an engine operating in that range giving you the desired speed would get you very close to optimal economy.

In the real world, most of us are buying used and generally repowering won't pay for itself in fuel savings. There are of course exceptions. Unless you plan to do a lot of cruising, fuel is probably less than 20% of yearly operating cost.

Ted

Turbocharging does not make the engine more efficient per se, but does indirectly.

Say you need 300hp continuously. A turbo engine around 8-10 liters can make that power and be in the "sweet spot" of the BSFC map. To do this without a turbo, it would take an engine over 15 liters to be in the same sweet spot. Or an engine aroud 15 liters operating near full power. Larger engines have more frictional losses and more thermal losses, so BSFC suffers. And running near full power is no good for BSFC either.

In Chicago's post, if the smaller engine is full power, that equates to 17.7hp/gph, which is typical for a NA engine at full. Note that no engine is particulary efficient at full power. In the second example the turbo engine is making 19.4hp/gph down at low power setting. If you took the turbo off the 380hp engine and got it making 68hp, it would probably burn about the same. But it would not be capable of 380hp.

That's why virtually all modern large commercial/industrial/marine engines are turbocharged. The whole package is more efficient for the job at hand.

Wifey B: Well the boat I was on this morning gets over 10 nmpg at 9 knots and below. Of course it's a RIB with a 125 hp Textron (formerly Weber). At 25 knots we get 7 nmpg and at 35 knots we get 6 nmpg. Main point is that comparing is so freaking hard as every boat is different. The boat who's swim platform the RIB is back on now doesn't do that well. The best it does is at 10 knots it gets like just over 1 nmpg. It does 10 knots at 900 RPM and we haven't tried to check fuel usage any slower.

3000 rpm is definitely not low on our Yanmar 4JH2 100hp engine. The gauge may go up to 5000, but there's no way I'm taking it there!

We cruise at 2700 rpm and the turbo kicks in somewhere over 1000 rpm or so, I think. At that rpm we average 7 knots, and over a year averaged 1.3 US gallons per hour.

Again...I think the source you found is whacked for saying 3000 rpm is low.

Large engines have much more surface area exposed to the heat of combustion and thus have much more heat loss. These engines are heat engines and heat loss is power loss and efficiency loss.

I have a 100 cu in 4cyl NA engine in my 30' boat. She cruises at 50% load. Notice I did not mention rpm. If I put a NA Ford Lehman of 380 cu in my boat I would bet large sums of money my fuel consumption would go up pushing the boat at the same speed of 6 knots.

But it would not be dramatically different even though both engines are making 20hp. Lots of veriables. But this example is very close to apples and apples. Very little difference except for the size of the engines and the extra 1000lbs of the FL may need to be factored in. But that's only about 1/16th of Willy's displacement. So you could "factor" w a stroke or two of the pen.

Bigger engines doing little work is not efficient. If I said engines working hard is very efficient it would be closser to the truth.

This should be more true of the turbo engine ... considerably more true. I mention this as a turbo v/s non-turbo engine comparissom was made. Ideally for efficiency one would have a small engine with a good turbo working at about a 75% load. Notice I didn't mention rpm again for the same reason .. it's not relavant. But of course in my example given the rpm would be high working at a 75% load.

But basically all one has to do is look at the "specific fuel consumption curve" (I think that's what it's called) and see at what rpm the lowest fuel burn is per one hp. Then you get into the variables ... and there are many. So many that if one were to set out to disprove my post here it would be easy to find a comparison showing the opposite to be true. But if you take a little care to compare apples to apples larger engines working less will fail to be efficient.

Efficiency, as stated by several posters, is the amount of power produced by a given amount of fuel. It is generally independent of engine size (displacement). Efficiency depends on compression ratio, friction and, in particular, how the fuel is metered into the engine. Modern electronically controlled injectors meter precisely the correct amount of fuel,
(and no more) producing high efficiency.

Turbocharging simply forces more fuel/air mixture in to the cylinders thereby producing more horsepower for a particular sized engine. It doesn’t have much effect on efficiency except that the effective compression ratio may be increased.

Part of the efficiency equation is pumping losses and friction in an engine. A larger engine has more of both. So if a boat needs 50 HP to go at a certain speed at partial throttle there will also be some power expended on friction and pumping losses. A 200 HP engine has less of these losses than a larger, 400 HP engine, although both are putting out 50 HP. Therefore the smaller engine will get better fuel economy. However, if the 400 HP engine is the same size/displacement as the 200 HP engine and gets its power from turbocharging, its losses will be the about same as the 200 HP engine and both will get the same fuel economy.

A modern automobile trend is to use small, four-cylinder turbocharged engines. They have the advantage of both high power and low losses, thus get good mileage.

Paul

40 ft, 15 tons, full displacement, @ 1500 RPM, 7 knots, 1.5 gl/hr. Fuel is the cheapest part of our cruising summer. Moorage, insurance, maintenance, etc.

jndavidson said:

40 ft, 15 tons, full displacement, @ 1500 RPM, 7 knots, 1.5 gl/hr. Fuel is the cheapest part of our cruising summer. Moorage, insurance, maintenance, etc.

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Hey, no fair! Your avatar photo is too vague...is this your boat?

Kasten's Coaster 40

If so, good choice

Yes that's the Kasten Coaster 40, photo taken during my more artistic period, sunset in the Gulf Islands. This was the first one built, but a Canadian from Pender Harbor built his own, but himself. It's unpainted but virtually identical.