Trawler wanna-be: What are fuel burn rates

[rslifkin]

In general, a faster boat will idle faster, assuming engines with the same rpm range. Boats with higher revving engines will move slower at idle, as idle rpm is slower relative to top speed.

For the Nordic Tug vs Ski's boat, I'd bet that speed at 950 RPM is a similar percentage of WOT speed (give or take a bit) on both boats.

 

[TDunn]

Fuel burn depends on three things: speed, displacement and hull design. Displacement and speed are the most important. Fuel burn is more or less proportional to displacement and increases exponentially up to planing speed. My 32'8" boat has a pure displacement hull (no planing) and displaces 9,000 lbs. I have a 40 hp diesel. My fuel burn is 1/4 gallon per hour at 5.5 knots (22 nautical miles per gallon or about 25 land mpg), 0.4 gallon per hour at 6.2 knots (15.5 nmpg), 3/4 gallon per hour at 7 knots (9.3 nmpg) and 2.2 gallons per hour at wide open throttle or 9 knots (4 nmpg).


I use so little fuel that I gerry can fuel to the boat. I don't go to fuel docks because my old (1936) fuel fills are too small for modern diesel nozzels and I have to use a funnel and fill VERY slowly at a fuel dock. My wife likes to cruise at 5.5 knots so a five gallon can of diesel lasts us 20 hours of run time or about 110 nautical miles.

 

[ScottC]

Single VW Marine 165-5 TDI Turbo Diesel
ZipWake Interceptors (which are on in the chart below from 7.2 knots.)
Semi-displacement hull
15,000# (Loaded for extended cruising; 2 people)

I generally run at 7.0 knots, which is close to hull-speed. At this, my burn is about 5.8 l/hr (1.55 gph) and this is while the engine is also charging the hybrid battery.


I achieve a "rooster tail" at just over 9 knots. Not sure I can really call that "plane" but at this speed it just feels better and handles better than in the 8.0-9.0 knot range, where I really feel like I'm pushing water. Regardless, there is no magic that happens after 9.0 knots in the fuel consumption curve :-(


Click on the picture to enlarge it. Use the Y axis to read values (knots & liters per hour) for both curves.

 

[altinr]

Fuel Burn cruising

Our Fathom 40 trawler weighs 30000 pounds and has a single Cummins 500 hp QSB engine.
We burn 3 gallons per hour at 7 knots, about 1.5 gallons per hour at 5 knots.

 

[garychurch]

10 knots - 20 gph or 0.6 mpg.

 

[SteveZ]

Fuel Burn

The attached article might be helpful.

 

[BrianSmith]

52' full displacement steel hull (83,000 lbs), single 838 cu in Isuzu diesel with a tiny turbo (that never engages at the speeds we run), 1300 rpm yields 7.2 kts on avg, burning .67 US gallons per nautical mile.

 

[Zoar]

Traw-------LER.

Say it s-l-o-w-l-y. They go slowly. I burn about 2 gal per hour (gph) with twin Lehmans going 7.3-7.5 KNOTS. I burn closer to 10 gph going 9 knots.

Slow is good.

 

[Jon H]

My dads boat - 110' LOA with twin 8-71's (250 HP each - very early ones)
burns about 12 GPH at 10 KTS.
In its original Navy trim it did about 20 KTS with different engines.

 

[Jon H]

My boat gets 1.4 NMPG at 26 KTS. Single Yamaha F350.
28' Hull, 32' LOA - 10,000# Displacement.

 

[Liveaboarders]

Here is our experience based on a full displacement Nordhavn 47 with a Lugger (Deere) 1066T engine:

The very ordinary diary of some liveaboards

 

[jeffnick]

Houseboat, not a trawler...9000 lbs 28' 2.5mpg @ 7 mph. 350 Chevy gasser with 30 mph potential. Less than 3k to replace entire engine when it 'wears out'.

 

[lwarden]

My dads boat - 110' LOA with twin 8-71's (250 HP each - very early ones)
burns about 12 GPH at 10 KTS.
In its original Navy trim it did about 20 KTS with different engines.

Cool boat, would like to see more pics of it (maybe in another thread).

 

[OldDan1943]

52' full displacement steel hull (83,000 lbs), single 838 cu in Isuzu diesel with a tiny turbo (that never engages at the speeds we run), 1300 rpm yields 7.2 kts on avg, burning .67 US gallons per nautical mile.

Your Kristen 52ft pilot house trawler is pretty

 

[Frosty]

The attached article might be helpful.

The linked article lays it all out very logically, and the various comparisons and charts make it easy to grasp. Thank you!

There is one scenario I'm curious about. You compared a larger single to a smaller pair of twins. That makes perfect sense in that obviously you don't need two of the same size engine that will power a boat singly.

But, at least in my "league" of potential boats, that's often what you get. For example, a Grand Banks 36: Would you like one FL 120 or two FL120's? Etc. The twins are almost always just two of the same engine as the single.

I tend to favor a single because I like the prop protection, but in certain boat models that really limits the pool (twins were popular). So it does make me wonder how two *same sized* twins would fit in to your article.

(Not that the difference would probably be a notable percentage of overall expense -- mostly just curious.)

 

[OldDan1943]

Then there is the 'get home' on a separate shaft and folding prop. Had it on my very old N46. I was surprised, the boat would back while on the 'get home' and folding prop. Never found a need to use the 'get home'.

 

[Frosty]

Then there is the 'get home' on a separate shaft and folding prop. Had it on my very old N46.

True, but at that point one probably wouldn't be concerned with fuel burn (at least I wouldn't).

 

[SteveK]

My 45' boat isn't a true displacement hull. With a 135 HP engine, it burns 2 GPH at 7 knots ( 3.5 nautical miles per gallon). 8 knots moves the fuel burn to between 3.5 and 3.7 GPH, so not worth it, IMO.

Ted

Ted, reading this post made me think that you know your boat well. IMO each boat has this character. Up to a certain point fuel consumption is reasonable for the result, followed by a sharp increase for little gain.
Without looking at gauges can you feel/hear the engine as it begins to work harder once the 2 GPH point is exceeded? I think I can.
BTW OP, fuel burn is the least expensive part of owning a boat. IMHO

 

[bayview]

People worry excessively about fuel. In most cases it will one of the least expensive parts of boat ownership.

 

[ScottC]

People worry excessively about fuel. In most cases it will one of the least expensive parts of boat ownership.

I would expect that many on here are, perhaps, more concerned about range, when considering fuel consumption. Even where I am with fuel at $7-$8/gallon, I pay more attention to my fuel consumption for range calculations than for cost.

 

[SteveK]

With big gassers like my boat, don't count on good mileage at low speed. As an example, I get between 0.5 - 0.6 nmpg on plane and in the 1 - 1.3 nmpg range at 6.5 kts (depending on wind / waves, etc.). Gassers lose a ton of efficiency at light load, so the fuel burn difference between fast and slow gets a lot smaller. Other than burning a bunch of fuel the gassers have no complaints about being run slowly though.
............

You know your boat too. Most gassers I have had were planning hull, smaller boats. At troll or on the plane was most economical. I think it is due to these boats plow water until they get on the step to plane. The engine appears to labor and then relax once planning. But of course once on plane more throttle did not proportionally increase the speed above the speed once up on plane. There was a sweet spot.

 

[Oldersalt]

My 1990 (grp) Grand Banks 32 has a Ford-Lehman 135 diesel. Hull speed is right around 7.5 knots and that is also the sweet spot for engine speed, about 1700 or 1800 rpm. Total displacement fully loaded with fuel, water, food, people, toys, etc is probably close to 20K pounds. Fuel burn is about 1.6 or 1.7 gph, which works out to about 4.7 mpg.

Unless you do a LOT of traveling, cost of fuel on a good trawler is pretty low compared to other costs, like slip fees, insurance, maintenance, etc. When looking at boats to buy, don't fixate on fuel burn rate. For used boats, evidence of good maintenance over the years is FAR more important.

Good luck!
Oldersalt

 

[OldDan1943]

I would expect that many on here are, perhaps, more concerned about range, when considering fuel consumption. Even where I am with fuel at $7-$8/gallon, I pay more attention to my fuel consumption for range calculations than for cost.

I agree, range and gph burn rate.
With 400 gallons of fuel and speed of 7knts (1200 rpm), the burn rate is about 1.5gph I should have, in theory, enough fuel to get to Bermuda from Hollywood FL. LOL I love theories.
I am not taking into account of riding the Gulf stream diagonally to Bermuda.
Of course, considering Bermuda is about 750 miles away, 100 hours at sea. I guess I better pack LOTS of food. CHUCKLE
(no generator)
My math gene died about 10 years ago.

 

[NCheaven]

Camano 31: The modified displacement hull with “Keel-Form” features, wide keel, engine set low, and hard chines for planning, has the potential to hurry up for pressing needs, but is far more efficient at and below “hull speed”. Technically that is 1.35 x the square root of the waterline length in salt water, 1.25x in fresh water. For the Camano that is theoretically 7.01 kt, or 8.07 mph. But, in practicality, it works like this (per my flo-scan readings in flat water conditions, minimal tide or current. NO - Not Observable) BTW, idles at 650, but smoother idle is 850:

RPM Kt (SOG) GPH Statute mph N. miles/gal Stat MPG

Idle 850 3.3 NO 3.8
1,000 4.2 NO 4.8
1,200 4.8 slight 5.5
1,400 5.4 .4 6.2 13.5 15.5
1,600 6.0 .7 7.0 8.6 9.9
1,800 6.5 1.2 7.5 5.4 6.2
2,000 6.8 1.5 7.8 4.5 5.2
2,200 7.0 1.9 8.1 3.7 4.3
2,400 7.3 2.7 8.4 2.7 3.1
3,800 WOT 14.5 8.0+
SOG – Speed over the ground. At WOT (wide open throttle) of 3,800 RPM the numbers are 14.5 kt and around 8 GPH. All readings included app. Est. 1,325 pounds people, fuel and water on board. Heavier loads, wind, waves, cleanliness of the hull, and tides and currents change these numbers greatly. Fuel usage pushing a little over hull speed might be 20g/day or less on a well-planned, more leisurely trip. Most days a few minutes above 3,000 RPM are used to cleanse fuel lines, injectors, exhaust, etc. I rarely run 10 hours a day over an average rpm of 2,400, 2,100 average is more common. Specs: 31’ LOA, 10’ 3” beam, 10,000 pounds displacement, 3’3” draft, 200 hp Volvo 6 cyl. Turbo Diesel, max RPM 3,900, 2.4:1 transmission.

Other fuel burn tables including a comparison with and without trim tabs is on the Pacific Camano Owners' website.

But, here is the simple truth: While range is an important consideration of fuel burn, you can burn a little more and "pay" for it by anchoring out instead of docking at a marina. I believe in being kind to our environment by buying a Trawler and staying close to or under hull speed, but you can enjoy the water within or slightly above hull speed and anchoring more of the time. Besides, as Steve Zimmermann noted in a Passagemaker article around a year ago, in terms of engine life, it's the revs and % of rated effort thatdetermines engine life. So, using our trawlers as they were designed has many advantages. Plus, the world, especially Trawler Heaven North Carolina is even more beautiful within hull speed.

 

[NCheaven]

Word document fuel burn table above compressed after I hit send. I apologize for that.

 

[Bilgewater]

35' Chris Craft aft cabin with twin 350Q gas engines. Just above idle, 6 mph, takes 3 gph total, or 2 mpg. 10 mph gets 10 gph, 1 mpg. I don't go faster than that. Readings verified by fuel flow meters on each engine. I find these numbers very acceptable for gas engines.

 

[markpierce]

A boat that idles over three-something knots is something I'd avoid. Too much play needed with the transmission.

 

[SeaMoose]

34’ old heavy fiberglass Trawler single FL120 1.7 gal/hr 6.5 knots
27’ Express, 350 cu in V8 10 gal/hr 22 knots. WFO 14 gal/hr 29 knots.

The trawler needs lots of varnish and time, the gasser needs a constant injection of money, everything shakes loose, it is generally running no more than half the season.

They are both equally fun depending on your goal for the day, I take the gasser on rides to nowhere, the trawler goes places for the weekend or longer.

 

[grandbanksbayfield]

Grand Banks 32 Fuel Use

7.2 knots at 1700 rpm: 3.5 nm per gallon. Lehman 135, non-turbo. We are very pleased. Fuel burn is the least of our costs. Our boat is in very good condition, yet there is always something to upgrade, maintain, etc. Yet, she does not break down, give us a lot of ugly surprises. We feel safe out on long crossings. Bought with 1200 hrs in 2018. Apparently had been a cocktail party boat. Put on 500 hours in two years doing trips to northern Great Lakes from Detroit. Not even a hiccup. These boats are built with good equipment and do not fall apart if maintained.

 

[fractalphreak]

People worry excessively about fuel. In most cases it will one of the least expensive parts of boat ownership.

Very true, but its one aspect of ownership that we can control a bit by boat selection and how the boat is run so its not as fixed as the marina bill.

No speculation about the OP, but I think some of these inquiries are out of a lack of understanding of the total cost of boat ownership, like you point out, or as a secondary cause a lack of appropriate cash flow/vacation savings to go out cruising.

I remember back to my early days of boating, and while our annual expenses (mainly marina and maintenance) FAR outweighed our annual fuel bill, the nature of cash flow for my young family always made our longer summer trips tight due to the cost of fuel combined with the rest of the "extra" costs you tend to have during a vacation-type cruise for a family with 3 young children. Its very likely there are a number of newer boaters finding themselves in a similar place.

With the gasser I worried excessively about fuel burn from the standpoints of logistics of our route planning and the cash expenditures for our longer trips. The boat had (what I think of now as) a resulting short range, so most trips of anything more than a weekend had to include fuel dock stops. When you start talking hundreds of gallons of at $5 a gallon , marina over nights at peak, some eating out, all during peak season back when the kids were young and my income not as robust, it wasn't the bulk of my annual expenses, but made for an expensive few weeks that if we didn't have it planned financially it wouldn't have happened.

With the larger, more fuel efficient diesel boat fueling is done once a year when we feel the price is right, or just to keep the boat heavy. I track it closely to monitor the boats performance (and our own performance in its use), but don't worry about the cost or quantity so much. I spent less on fuel for NWD this year (for a fuel load that will last me until late spring 2021, and probably another 20-25 days of cruising during off season) than we did on our last two week trip on the gas boat.

Worrying more about our fuel burn will take the form being able to accurately predict it and will come into play when we have time (or take the time) to do months long cruising into remote areas in coming years and will need to seriously plan fuel for the summer...