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Old 08-10-2019, 03:09 PM   #1
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Is there a database of boat fuel efficiencies?

It is very easy to compare different cars on the basis of fuel efficiency because the government collects the data. Even though the mileage you actually get in real life may be different from the window sticker, you can at least make educated comparisons between different models and brands using this information.

Obviously there is nothing similar for boats (that I am aware of). But I'm wondering if anyone has tried to compile this information and post it publically anywhere. Obviously there are a lot more variables with boats and there are a LOT more possible permutations of yacht models, engine choices, cruising speeds, and sea conditions, than there are with cars where a few big multinational corporations make and sell most of the car models.

I'm trying to do a little bit of that myself as I shop. You basically have to poke around and find what sort of fuel effiency data you can find vessel by vessel and hope it is accurate. I'm wondering if anyone else has already done this ahead of me and has posted it anywhere.
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Old 08-10-2019, 03:30 PM   #2
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Yes, and no. For diesel engines it is fairly straightforward. For gasoline engine, less so.

Every diesel engine manufacturer publishes several curves for their engines:

1. A maximum power output vs rpm curve- worthless
2. A maximum torque vs rpm curve- only useful for selecting an engine that can get you over the hump.
3. A curve of power absorbed by the prop vs rpm- not too useful either
4. A curve of fuel used by the engine to produce the power in #3- very useful, see below.
5. Some also supply fuel at maximum power vs rpm- worthless

Curves for the Cummins 6BTA 370 hp engine are attached as an example. For the Cummins, the prop curves are the lower one for each parameter. Go to the manufacturer's web pages to find them.

So, decide what rpm you want to run your engine, and look up the fuel burn at that rpm. Note that the power and fuel required is for a theoretical prop on a theoretical boat, but it is usually in the ballpark.

Gasoline engines are worse because very few manufacturer's supply those curves. You can create one yourself by doing some math and making some assumptions but I will leave that one alone for now. Boattest.com does a decent job of reporting this data though.

David
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File Type: pdf Cummins 6BTA 370 hp data sheet.pdf (131.7 KB, 20 views)
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Old 08-10-2019, 05:28 PM   #3
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Yep. Nice thing is that the diesels made in the last 10-20yrs burn very close to the same amount of fuel for the work they do in mid range (not super near idle, not super near full power). So which engine does not matter too much, most within 10%.

It depends more on the size of the boat and how fast your want to go. Planing boats at planing speeds burn GOBS of fuel.

Trawler type displacement boats running 1kt under hull speed burn about the same amount compared to another one that displaces the same. Some outliers, but most fit pretty well.

Want to burn less fuel, get a smaller boat or go slower.
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Old 08-11-2019, 10:36 PM   #4
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Quote:
Originally Posted by Ski in NC View Post
Yep. Nice thing is that the diesels made in the last 10-20yrs burn very close to the same amount of fuel for the work they do in mid range (not super near idle, not super near full power). So which engine does not matter too much, most within 10%.

It depends more on the size of the boat and how fast your want to go. Planing boats at planing speeds burn GOBS of fuel.

Trawler type displacement boats running 1kt under hull speed burn about the same amount compared to another one that displaces the same. Some outliers, but most fit pretty well.

Want to burn less fuel, get a smaller boat or go slower.
Interesting points. I can certainly see the pure engine idea - diesels burn the same for the amount of work.

I think what the OP is after is more "in this boat, in these conditions, what is the economy":
Boat: length, heavy/light, narrow/wide, twin/single diesel, high/low windage, etc.
conditions: stern/beam/fronton seas, <1m/1-1.5m/1.5-3m/3+m, high/low frequency swell/waves, etc.

I did a little study of power cats, which some people may consider modern trawlers (I do ) - see here. I found enough basic data on 28 boats to do what I wanted, although it's far from canonical.
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Old 08-11-2019, 11:06 PM   #5
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Not sure what I was looking for exactly. But for example, here are 3 boats we are considering:

Ranger Tug R31: 31'2" LOA, 10' Beam, 11,500 displacement, Volvo 300 HP engine
Nordic Tug 32/34: 34'11" LOA, 11'4" Beam, 15. 500 displacement, Volvo 260 HP engine
American Tub 34/365: 36'6" LOA, 13"3" Beam, 18,700 displacement, Cummings 380 HP engine

In terms of fuel consumption I have found the following data for the Ranger Tug R31: https://www.powerandmotoryacht.com/b...nger-tugs-r-31


------------------------
RPM KNOTS GPH
700 3.5 0.3
1000 4.6 0.7
1500 6.2 2.1
2000 7.5 4.2
2500 9.8 8.2
3000 14.0 11.1
3420 18.7 14.8
---------------------------


For the Nordic Tug 32/34 I found no comparable data table but anecdotes like this one in reviews: https://www.boats.com/reviews/modern...nordic-tug-32/

Quote:
Early models feature 100 to 120 hp diesels, but newer 32s are delivered with 220 hp Cummins. Cruise speeds can be around 7 to 8 knots (with enviable fuel consumption of about 1 gallon per hour), and the 220 hp Cummins can push the boat to 15 knots.
For the American Tug 34/365 I found no data tables but this information for the smaller Cummins 330: https://www.passagemaker.com/cruiser...s/american-tug

Quote:
Although the 330 will power the yacht to 18 knots, most skippers I have observed under way probably are running at 9 or 10 knots. Many of them undoubtedly come from other trawlers or sailboats and are comfortable at slower speeds. The high fuel consumption of all-out speed, about 17 gph at 18 knots, probably is the strong argument for slowing down. (That's more than $25 an hour for diesel fuel.)

At 9 knots fuel burn drops to 3.9 gph, according to American Tug underway testing. At 10 knots, fuel use averages 5.1 gph.
Point being, for 3 fairly popular current models of tug/trawlers it is hard to find comparable fuel efficiency data. Based on this information, the Ranger Tug with nearly twice the displacement and larger engine is more fuel efficient than the Ranger Tug at 9 knots. Hard to believe. It is also hard to believe that the Nordic Tug 32 can get 1 GPH at 7-8 knots while the smaller Ranger Tug is buring 4.1 GHP at 7.5 knots.

So it's hard to know what to make of this information. I would have assumed that the Ranger and Nordic Tugs would have comparable fuel consumption rates with the American being substantially higher due to larger displacement and larger engine. But it is hard to find actual data that confirms this.
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Old 08-11-2019, 11:42 PM   #6
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Boats aren't cars like you said.

Too many variations, engines, optional equipment, dinghies added, weight of owners personal property onboard etc etc etc.

Plus there isn't as big of a market for boats so magazine boat test are not as extensive as from car magazines, which is where some of the info originates.

And the laws require auto manufacturers to test and publish specs and EPA milage data for every model car and every variation of the model. There are no similar laws for boats.

Most of the information will come from actual owners. But no two boat owner measure or report fuel burn rates in the same manner. Unless they have FloScan or a newer engine with an ECM providing fuel flow.

And then you have to sift through the exaggerated claims. It always cracks me up. Fast power boat owners boast about how much fuel their boat burns, often exaggerating upwards from the actual by gallons. Sailboaters, if they admit to starting the engine, and trawler owners brag about how little fuel their boat sips. Usually exaggerating downwards by a tenth or sometimes one hundredth of a gallon or so. "You're boat burns 1.9 an hour? mine only burns 1.84 per hour"
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Old 08-12-2019, 12:00 AM   #7
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Unless range is a concern, fuel is one of the less costly expenses when you own a boat.

Unless the price of fuel experience fluctuations, the cost of fuel stays constant.

A blown engine, transmission, generator or electronics failure requiring replacement is a lot bigger, unexpected and probably an unplanned expense.
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Old 08-12-2019, 01:00 AM   #8
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Cama- you need to understand that a small boat at 9kts can be (and usually is) harder to push through the water at 9kts than a larger, longer boat. You need to understand the math regarding hull speed. That's why I put in my previous post "1kt under hull speed" as a criterion for comparison. That 31 at 9kts is trying to climb its bow wave. At 6kts, it is in a happy spot. Bigger boat can be happy at 7.5.
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Old 08-12-2019, 04:04 PM   #9
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Fuel is the cheapest cost in boat ownership.
I never worried much about it nor think about it much in my 30ish years of boating.
We generally went out every weekend many times running hard against the current/ wind so we could make Block I before dark.
Efficiency? Running single diesel who cares?
My opinion
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