Fuel mileage efficiency

[rslifkin]

The brick wall you face w a typical rec trawler is hull form and weight.
When trawler hulls become much more like FD fish boats and sailboats much will be gained in efficiency. Short of that … not much.

Another road to efficiency would be rebuilding the stern of your rec trawler bringing the transom up and out of the water.

And of course smaller boats and smaller engines.

Along the line of submerged transom, I've found that on my boat, loading a little bit bow heavy reduces fuel consumption (and increases speed at the same rpm) due to less submerged transom. Of course, going too far with that would risk handling problems. Interestingly, when running on plane I notice the opposite. More stern heavy is faster (seems like the boat gets further out of the water as it takes more trim tab deployment to reach optimal running angle).

In general, best slow cruise will typically be further below hull speed on a boat with a lot of submerged transom (and therefore a lot of turbulence in the wake) than on a boat with a slippery FD hull form.

 

[Mambo42]

The brick wall you face w a typical rec trawler is hull form and weight.
When trawler hulls become much more like FD fish boats and sailboats much will be gained in efficiency. Short of that … not much.

Another road to efficiency would be rebuilding the stern of your rec trawler bringing the transom up and out of the water.

And of course smaller boats and smaller engines.

Good part for me is that Art Defever designed his boats already with a relatively steep rise in the transom. Whereas the max depth is about 5 feet, the transom only sticks about 1 to 1,5 feet in the water. The stern thruster is barely underwater. I guess that does help boat performance, but indeed it is a full displacement hull, not built for speed at all.



However, when I make the hull speed calculation I get to about 8.9 kts, which means that at 7 kts I am well below that. The only thing I don't know at this moment is: 'what is the best speed for max range' ?

 

[Moonfish]

Is: "What we get is .9 GPH at 1400 RPMs (and 7 kts)." - Meaning fuel used per engine is .45 GPH at 7 knots ... or ... is that referring to .9 GPH per engine for a total GPH of 1.8 GPH [which is still really thrifty]!

.9 per engine, 1.8 total. In our more than 4,000 miles of cruising we round up to 2 GPH for estimating/planning purposes, which has been quite accurate and gives us a bit of wriggle room should we need it. Which we have yet to need...

 

[Nomad Willy]

Mambo42,
Yes the more your stern looks like your bow the more efficient your boat will be … at the appropriate slow speeds tho.

Speed for max range includes a lot of variables.
Over propped-under propped
High-low prismatic coefficient.
Prop blade design .. low aspect ratio (AR) or high. # of blades (the lower the better). One high AR blade is the best but nobody goes there for good reasons.
And always … weight.

M42 wrote;
“ Good part for me is that Art Defever designed his boats already with a relatively steep rise in the transom. Whereas the max depth is about 5 feet, the transom only sticks about 1 to 1,5 feet in the water. The stern thruster is barely underwater. I guess that does help boat performance, but indeed it is a full displacement hull, not built for speed at all. ”

And yes the lower transom area below the WL the better. In your weight fwd example you’re actually making your SD boat more of a FD boat. But w stern seas you will loose directional control.

 

[psneeld]

Ford Lehman 120. Rebuilt 2009.

Since then run 1650-1700 rpm for 3700 hrs.

Averaged 6.3 knots for around 3.2 mpg over 20, 000 miles and around 3600 hrs.

4 bladed prop.

 

[boomerang]

Ford Lehman 120. Rebuilt 2009.

Since then run 1650-1700 rpm for 3700 hrs.

Averaged 6.3 knots for around 3.2 mpg over 20, 000 miles and around 3600 hrs.

4 bladed prop.

Each time I throttle up to 1650, I look at the GPS. If it says 6.3 knots, I think to myself, "yep, we're at that sweet spot Scott told me about...everything must be good!"

 

[Hippocampus]

Believe what NW is referring to indirectly is prismatic coefficient. This is one factor contributing to efficiency in some hulls. For sailboat design at present it’s a very minor factor with hulls looking like triangles (slice of pizza) when looked at straight down from above. These hulls have much less wetted surface and parasitic drag then the traditional balanced hull NW refers to hence very much more efficient. For power a variation has regained interest. Entry is very sharp and extends a significant distance but again a balanced hull is not seen. Rather there’s an near or total absence of rocker to the hull and the hull is kept extremely narrow with flat runs. A marked departure from the hulls NW I believe is being referred to. Yes, the balanced hull was de rigor for many years when efficiency was the prime concern but has been super planted in both power and sail for some decades now by other hull forms. Of course multis, especially those minimizing wetted surface (narrow hulls and kept light) and maximizing clean runs achieve this extreme efficiency as well.

 

[Nomad Willy]

Hippocampus,
Re low prismatic hulls … they are often called “diamond shaped hulls” The most efficient hull at low speeds. Well below hull speed.

 

[rgano]

I ran a pair of FL120s for a few decades in a 42-foot waterline boat, and I believe that at 1300-plus RPMs the engines in a Defever 49 are being babied too much. I would think 1600-plus RPM would see them happier and less likly to gunk up as the owner found them to be.

 

[Nomad Willy]

I agree rgano.
But it depends a lot on how the engine is propped. It’s a function of load .. not engine speed or rpm. There’s only one rpm.
But I’m curious … what’s a “waterline boat”?

 

[RT Firefly]

Greetings,
Mr. NW. 42 foot waterline. Sheesh!

 

[OldDan1943]

You can do the math to determine the optimum hull speed and a projected fuel usage but the reality is based upon how much stuff you/we have on board.

 

[syjos]

You can do the math to determine the optimum hull speed and a projected fuel usage but the reality is based upon how much stuff you/we have on board.

FD hulls become more efficient with additional weight, though infinitesimal.

 

[OldDan1943]

FD hulls become more efficient with additional weight, though infinitesimal.

Then we must consider the effect of ever changing 'fuel loading'

 

[Art]

Then we must consider the effect of ever changing 'fuel loading'

And... any type of growth on bottom.

 

[rgano]

I agree rgano.
But it depends a lot on how the engine is propped. It’s a function of load .. not engine speed or rpm. There’s only one rpm.
But I’m curious … what’s a “waterline boat”?

I guess it's one that's at least afloat.

 

[Art]

I guess it's one that's at least afloat.

Hopefully... With boot strip still above the "waterline"!

 

[Hippocampus]

Physics and science doesn’t care what you believe.

The more water you displace the greater the wetted surface. The greater the wetted surface the greater the parasitic drag. The greater the drag the more energy is required to move the object and even more so for acceleration. This is true for cars, trucks, boats and airplanes. It’s not Einstein but Newton.
For any object moving in a fluid there’s a boundary layer. To the extent that layer is undisturbed and/or flow along it remains laminar drag is the lowest achievable for that object moving at that speed through the fluid.
For boats there’s a displacement and speed for which they are designed to operate at to optimize efficiency. Going above or below that speed to a significant degree is inefficient. Overload a FD or SD hull laminar boundary layer flow becomes non laminar, turbulence and drag increases and efficiency suffers. Before that point wetted surface increases and so does drag. Underload to a significant degree resistance to rotational movement decreases and beyond comfort decreasing but also tracking decreases. This requires more steering input which increases drag. Go slow enough the frictional forces and unproductive heat loss of any internal combustion engine becomes an increasing fraction of total energy with energy spinning the prop a decreasing fraction. Running FD hull or a SD hull or planing hull at idle means less gph but much less mph. Yes there’s a range of speeds as well as displacements to think about.
Some of what’s posted above is counter to the laws of physics. Unfortunately for any hull moved through water there’s a range of displacements and speeds for which it’s designed to operate at and even a “best” speed and displacement. They are interrelated. Usually more weight less speed.
LDL hulls have been around since the ancients. For internal combustion engine powered the commuter boats are a decent example. Basically a light boat where LDL is nearly equal to LOA and is extremely long and lean. Unlike sailboat and FD hulls very low rocker or none. Unlike traditional sailboats not balanced as the entry is very fine and extends for much of the hull but the run for the rest of the hull s flat ending in a transom stern. Artnautica, Arksen, Deepwater, LM, K&N are current examples being built.
For recreational boats and especially A rated boats the other concern is how resistance to heel is generated. Two major contributors are form stability and gravity. Multihulls are the extreme ample of form stability. But FD hulls make much use of gravity to generate their righting arms. Foilers and some planing hulls use lift when exceeding displacement speeds. These techniques have the added advantage of decreasing wetted surface dramatically hence parasitic drag. There is a boundary layer but only for the minute areas still submersed. So if your boat moves in the water (FD) one set of hydrodynamics applies. If it moves on the water another. If out of the water a third. But in all cases the laws of physics aren’t violated.

 

[Seevee]

Question: If we really wanted to know fuel economy, why not get a fuel flow meter?
Now, can anyone comment on where do get a good one at a reasonable price?



I believe there's only one transducer manufacturer (Flowscan), is that still true?

Back when I bought one for the plane, the Flowscan transducer was rather cheap, the display and the rest of the stuff was expensive. But accuracy is dead nuts. I know how much fuel was burned, more accurate than the fuel pumps at the FBO. Would like to have that set up for the boat.

 

[rslifkin]

Question: If we really wanted to know fuel economy, why not get a fuel flow meter?
Now, can anyone comment on where do get a good one at a reasonable price?



I believe there's only one transducer manufacturer (Flowscan), is that still true?

Back when I bought one for the plane, the Flowscan transducer was rather cheap, the display and the rest of the stuff was expensive. But accuracy is dead nuts. I know how much fuel was burned, more accurate than the fuel pumps at the FBO. Would like to have that set up for the boat.

Maretron and a few others make NMEA2000 based setups. At this point, I think Floscan is dead and gone. The company went under and was bought up, but I don't think they're actually making product at this point. None of the options are particularly cheap, unfortunately.

 

[syjos]

Efficiency of a FD hull increases with additional weight because as the hull settles lower in the water, waterline length increase. Unless the hull has a perfectly plumb bow.

 

[rslifkin]

Efficiency of a FD hull increases with additional weight because as the hull settles lower in the water, waterline length increase. Unless the hull has a perfectly plumb bow.

That's not an efficiency increase. That means top speed increases and the steep part of the fuel burn curve will come a little later. But extra weight (and therefore extra wetted surface) will still require more power to move.

 

[Art]

I don't mean to put a damper on the fun of constantly making fuel-use calculations...

However, I think there is a little too much xtra-effort too often being employed in constantly trying to attain "EXACT" fuel usage MPG or GPH results in every way possible.

IMO, once I have the basics of how much MPG or GPH my boat [any boat I own] gets at a few differing speeds and in a few different conditions that's enough for me to then go at speed I feel appropriate.

In other words... how much is it really important if I'm getting 2 MPG, 2.5 MPG, 3 MPG... etc... in the long and short of life. Also, wind, tide current, wave types/size, boat bottom's condition and other variables throw any calcs made at premium best conditions into the hat anyway.

Soooo... I cruise and anchor out to enjoy... not so much to worry if my fuel-use calcs are staying the same!

Happy Fuel-Use Daze!! - Art

 

[Nomad Willy]

Efficiency of a FD hull increases with additional weight because as the hull settles lower in the water, waterline length increase. Unless the hull has a perfectly plumb bow.

Also and probably more inflewensive the wetted surface area becomes less per ton of displacement.

SeeVee,
Yes using a fuel flow meter would shed a lot of light … or darkness on the subject. Many wouldn’t like what they’d see.

Hippocampus,
What’s a LDL hull?

 

[Mambo42]

Mambo42,
Yes the more your stern looks like your bow the more efficient your boat will be … at the appropriate slow speeds tho.

Speed for max range includes a lot of variables.
Over propped-under propped
High-low prismatic coefficient.
Prop blade design .. low aspect ratio (AR) or high. # of blades (the lower the better). One high AR blade is the best but nobody goes there for good reasons.
And always … weight.


And yes the lower transom area below the WL the better. In your weight fwd example you’re actually making your SD boat more of a FD boat. But w stern seas you will loose directional control.

@ Nomad Willy


Thanks for your input. As far as I know I have the original engines and props on my boat, in other words, how it was designed by the yard at the time. Boat is a 1979 Defever 49, 43 years old this year, but still running fine.



Moving weight around I don't really see as a viable option. But your tip is valuable, so that I don't move batteries (heavy weight) into the lazarette. Unfortunately I cannot move it to the bow area, I have no space available there.

Next year I want to change to lithiums and they should not be in a hot engine room, but now I understand they should also not be in the lazarette. Great tip.



As a bit of back ground, unfortunately, the former owner died suddenly and he did not write anything down whatsoever. There is absolutely no paperwork or information on this boat. When I bought the boat I had no clue how all the systems worked, what is connected to what, so I started with the engines, the fuel tanks and worked my way up from there. Nobody could tell me anything about cruising rpm, max rpm, cruising speed, max speed, fuel efficiency etc etc, so it is a discovery tour for me. But that is also fun to do.

The input all of you are giving is highly appreciated, it helps me build the picture of what I can expect and that is necessary so I can make more accurate passage plans for short and longer trips. And of course, the more fuel efficient I can run the boat, the longer the range will be and that means I can plan longer tours around the islands of Greece !

 

[Hippocampus]

RS has it exactly right. No increase. In fact a decrease in most instances. Above posts seem to repetitively fail to acknowledge what produces drag. Think simple physics gentlemen. Skip a stone across the water. Push a square block in the water on its diagonal. . Push a pencil along its axis floating on the water.
Anything that disturbs the boundary layer increases drag. Through hulls. Bow thrusters. The perturbances in Nordhavns to allow headroom for engine rooms. Fins. Fish. Rotors. Growth. Anything. Anything that increases the amount of boat in the the water as well (displacement and appendages). Anytime you steer you increase drag and slow the boat. Many people here are ex sailors. They know if you want to win buoy races steer as little as possible. Even that should be done as gradually as possible. They understand induced drag.
When thinking about this boat’s should be compared on the basis of displacement. The 58’ Artnautica has about the same displacement as my Nordic Tug 42. It has a 75 to 90hp engine in it. I have 540hp. At 7.2 kts (my sweet spot) it’s using less than half the fuel I am per hour and less than a third per mile traveled if exhaust temps (engine loading) are kept in their respective sweet spots. . It also beats out a KK solo or N40 by similar numbers. So if judged by weight (displacement) both multis and light long thin hulls are way more efficient than FD as well as SD hulls. Problem for both multis and LDL hulls is berthing and storage logistics and expense. It’s expensive to eliminate weight just like for race cars.
You see the same logic in ships. Longer and leaner is more efficient. That’s why non Panama ships were created. For equivalent payload those ships are more efficient. Be it carrying crude or containers.

 

[Mambo42]

@ Art,


One of the main reasons why I want to figure out the fuel usage is that I have zero information on this boat. I bought the boat last year after the former owner had died suddenly.

The boat had been lying in a marina for about half a year, the widow emptied it and then it was offered for sale. The broker could not tell me anything and the widow had no clue at all, her husband had always managed the boat.

The surveyor and the mechanical plus electrical engineers could only tell me a bit about the status of the boat, but nothing about the particulars.


After I got back home after closing the deal (boat is lying in Greece, while I live in the Caribbean) I decided to try to gather as much information as possible on this Defever 49. I had the basic information, call it the sales pitch, but now I had to start digging.

First decision was to start cleaning the engines and make sure they are in tip top condition. Then came electrics, systems, interior etc etc. Also there I will need to make changes, since this boat currently fully depends on the generator to run all the time while on anchor. So that is going to be changed as well.



As for the most efficient speed ? I have no clue at which speed this boat is most comfortable. We are happy at any speed, our goal is to go from one island to the next island in Greece and stay there for about 3 to 4 days before moving to the next island again. We want to use this as our home underway so we can see a large part of Greece and later on, Croatia. Speed is not important to us, but if we can do it at the most fuel efficient rpm setting and speed it will only increase our range, bringing more islands in view for us.

The fuel usage is important for me for a simple reason: I need to be able to make a fuel planning, since marine fuel is not available on all islands. And 1 or 2 mpg difference is going to make a big difference when I would be planning a long trip with a lot of passages to different islands. In the Aegian sea you can have long stretches between the islands, would be a pity if we have to cut islands out because of poor fuel planning.

Also, if I see the fuel usage all of a sudden go up I know that I have a problem with the engines or the fuel supply.



So that is a bit of a back ground about why I would like to know what I can expect while underway. All the input all of you are giving me at least helps me to think in a direction, which I highly appreciate.


But I agree fully with you that in the end it is all about enjoying the time on the water and the places the boat will bring us. After all, that is why we bought the boat in the first place.

 

[Hippocampus]

M don’t know how deep your pockets are or how technically sophisticated you are. Given what you say about your program you don’t seem to require trans pacific range but rather 1000-1200nm would be more than enough. So that implies your desire to increase efficiency is driven by expense not range. I don’t know your boat but seems reasonable original specs allowed adequate range but that’s your first order of business. Given for early adapters the expense of rewiring a boat, buying and installing the necessary ancillaries and then the LPF batteries is way overkill imho. The expense of all of this will decrease in time and the technology will improve so any Li based system you put in now will be antiquated in a few years. Early adapters faced this issue with solar, now with EVs and in existing boats with Lithium iron phosphate. For your program with the boat you have would think spending money on a clean bottom, good AGMs and the ancillaries to keep them at 70-100% SOC would give you less headaches and way more bang for your buck. A new build or different boat a different decision matrix. Run your boat at around 70% of hull speed if you want to increase efficiency. Don’t worry-be happy.
Given sea state, current, windage etc. effect fuel burn you can get different readings at different times from flow meters. Pragmatically calculate your hull speed. Decide what’s the slowest you will tolerate and allow landfalls in daylight. As part of your engine survey they should have given you a chart of fuel burn at different rpm, load, speed. Use that to pick the knee of the curve. If you don’t have that run the boat a ~70% of hull speed for a few days then 60% then 80%. Even using sight gauges should be sufficient to get realistic numbers for mpg (or mpl in your metric mind). Those will be real word numbers so useful in planning. I was fortunate. My survey gave me my numbers and turboboust, oil pressure/temp, coolant temp, alternator output at each rpm. I didn’t use those numbers nor the numbers my engine monitor gave me. I had several long runs shortly after starting to use her and got real world numbers using the sight gauges and gallons required at fill ups. So I know 6.8k is most efficient but days work and comfort best at 7.2k with very little penalty. 6. 8k gives me a range of ~1000nm with adequate reserve. So for my program my 600g is adequate. It’s that calculation you want to think about. What’s your longest expected transit. How much fuel is required at the rpm that’s best for efficiency, comfort and engine health to get that distance and allow sufficient reserve. What’s your existing tankage. Is it adequate. If not that’s where your bucks should go before lithiums.

 

[Marco Flamingo]

Faria sold a small fuel management system (for gas) for awhile. Expensive and now discontinued. Here's a NOS on Ebay for $350. Because most diesel have a return line, the flow meter on the Faria unit wouldn't be accurate but . . . the Faria does have the ability to adjust the reading, meaning that if your diesel returned 10% it is possible to adjust the reading down by 10%.

I bought several when they were first discontinued for use on my 16' cruising boat with a 50hp outboard. I had installed an aftermarket 23 gallon underdeck fuel tank (to get a 150 mile range) and I needed an accurate fuel gauge. I installed both a floppy needle gauge and the Faria. The Faria reads gallons per hour (or liters) down to .01. It tallies trip usage, total usage, and has a self-set low fuel alarm. I've found it to be incredibly accurate. If it says I've used 18.3 gallons, that's all I better put in at the pump or it will burp out the vent. At WOT, I can use just over 4 gallons per hour. Of course, if I did that for an hour I would be 30 miles away.

As useful as the Faria is to me with a 23 gallon tank, I can see why it was discontinued. Its impeller can only measures "small" gallons per hour (maybe up to 30gph?). Back when it was first manufactured, and gas/diesel was under $1 a gallon, installing an expensive monitoring system for "small" differences just didn't pencil out. Maybe that will change.

I'm not sure how difficult it would be to use with a diesel that has a return line. If the return went into the feed line instead of the tank, it would slow the impellor and likely give an accurate reading. Some diesel returns cool the injector pump and I don't know if heat buildup in the fuel could be a problem.

Prior to installing the Faria I looked at other fluid flow monitoring systems. They are out there, including fuel rated flow meters. As they are not claimed to be "marine," they tend to be relatively inexpensive (<$100). Digiflow was one company that had meters that looked like they could be repurposed. I actually bought one, but before I could install it I saw a couple cheap Faria units on Ebay.

A monitor system would take all the guess work (and arguing) out of a thread about fuel usage for trawlers, but, and here is the problem with buying and installing a system, once you graph out the fuel usage/rpm, you have all the information you need and the system becomes a superfluity. Well, you can still show it off to guests.

 

[tpbrady]

Data is everything. After 8 years 14,630SM, 1.999GPH, average SOG 7.075MPH. Cummins 6CTA8.3, 420HP, typically run at 1200RPM. Without data look at the manufacturers graphs on HP, Torque, and fuel consumption at various rpms. On the graph on my engine I am operating at the far left so you have to interpolate what the fuel consumption is at 1200 RPM. Since the engine can reach fully rated WOT RPM (2600) with the prop that is installed then this graph should be reasonably accurate. So 2GPH is expected and may be a little less than predicted.

Tom