Our actual fuel burn (Lehman 120s)

Well we just completed a 33 hour run yesterday from Long Island Sound to the C&D Canal. Sea conditions were mild except for beating into 3-5' head seas for 7 hours in the Delaware Bay. Our boat is a 30T full-displacement DeFever 48. Twin Lehman 120s turning four blade 23x20 props with Velvet Drive transmissions and 2.5-1 reduction ratios. Engines were run at 1400 rpm with no genset time. One engine is fit with a 70A alternator the other has two 108A alternators. The alternators were used many times for cooking, coffee, water heater. Bottom paint is 6 months old. Bottom is free of growth but does have a little bit of fuss on it. Rudders have a bit of fuss and the wheels are clean. A crab pot line was picked up and brought along for the ride for the last 8 hours. Tanks are 80% full. Boat has full liveaboard stores. Winds were following or abeam accept for the 7 hours in the Delaware with 20knot winds on the nose.
These are our numbers for the trip:

33 hours
225 nautical miles
258 statute miles
70 gallons diesel used
3.21 nautical miles per gallon
3.68 miles per gallon
6.82 average knots per hour
7.82 average miles per hour
2.12 gallons per hour

I use the same running gear, with a significantly smaller boat, and don't get anywhere near those consumption figures. The only difference is I run the engines at 1600rpm.

My average consumption seems to be around 3.5 gallons(US) per hour.

Food for thought.

How was the fuel consumption measured with the tanks 80% full? If you have a calibrated fuel consummation monitor, I'd say positive current for the trip. The numbers seem low for the speed and displacement.

Thanks for the useful and detailed information, especially helpful since we also have Lehman 120's. Not to pick nits, but "knots per hour" is incorrect. The word "knots" is defined as nautical miles per hour.

Captian K,
What's the matter w nautical miles per hour?

Andy's burn being way different may have more to do w how he's propped that what rpm he's running. Anytime you compare one vessel to another they have to be propped the same. Re fuel burn anyway.

Daddyo,
Your boat is not full disp. But it does not matter re the post.

manyboats said:

Captian K,
What's the matter w nautical miles per hour?

Andy's burn being way different may have more to do w how he's propped that what rpm he's running. Anytime you compare one vessel to another they have to be propped the same. Re fuel burn anyway.

Daddyo,
Your boat is not full disp. But it does not matter re the post.

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I thought I should look it up.I run a three bladed prop, 24" diameter,pitch 16" Not sure about the pro's and con's of three bladed verses four.

His numbers seem pretty accurate to me...and at that speed his boat is in full displacement mode unless someone can show me a definition to dispute it.

Too many people on this forum have sources of calculations that are WAY misapplied...

His numbers seem accurate to what may actual owners that I trust their posts have posted here and elsewhere,

We have a 30,000 pound (fully loaded) boat with a semi-displacement hull, two FL120s, BW Velvet Drive couter rotating transmissions, and 23x16 four bladed props. At 1650 rpm we get about eight knots with a combined fuel burn of between five and six gph.

Similar running gear in our 42. We're 37.4K on our last haul-out. We average just a tad over 2.5GPH on our longer runs (actual gallons used) and typically run at 1650-1675 RPM and make about 7-8 knots. Nothing is getting pushed hard and the mileage is very close to his numbers.

Here is his numbers...

3.21 nautical miles per gallon
3.68 miles per gallon
6.82 average knots per hour
7.82 average miles per hour
2.12 gallons per hour

Note the 6.82 average knots...that is HUGE in these sized boats compared to 8 knots.

My boat will burn almost 30%-50% more fuel trying to stay at that 8 knot lark compared to a 6.5 knot burn rate. So anyone comparing an 8 knot burn rate may be a tad different...

Larry M said:

How was the fuel consumption measured with the tanks 80% full? If you have a calibrated fuel consummation monitor, I'd say positive current for the trip. The numbers seem low for the speed and displacement.

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The water tanks were full and the fuel a bit over 50% full. Measured before fueling and departure. Happened to burn exactly what I had put on for the trip. The measurement is dead accurate as is the time and distance.

I used to think she was semi-displacement but after studying her on the hard this year I realized she is 100% full displacement. It sure helped me to understand why her wake is nonexistent. Mileage and speed would have been better if not for the Delaware Bay. Favorable tides were one hour in the East River and one hour in the C&D.

Daddyo said:

The water tanks were full and the fuel a bit over 50% full. Measured before fueling and departure. Happened to burn exactly what I had put on for the trip. The measurement is dead accurate as is the time and distance.

I used to think she was semi-displacement but after studying her on the hard this year I realized she is 100% full displacement. It sure helped me to understand why her wake is nonexistent. Mileage and speed would have been better if not for the Delaware Bay. Favorable tides were one hour in the East River and one hour in the C&D.

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My departure date in December is based on tides...I won't run the Delaware against the tide unless I absolutely have to. My timing has me getting a consistent 1.5-20 knot push all the way from Cape May to the C&D.

Daddyo said:

I used to think she was semi-displacement but after studying her on the hard this year I realized she is 100% full displacement.

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Most all trawlers of that design (and mine) are technically semi-displacement, but simply don't push them to that point. Either for economy or lack of power keeps us from pushing them up and out of the water.

They certainly aren't planing hulls, but if you push them, they begin to displace water less and rise in their track. That explains the way we can get beyond mathematical hull speeds (by a tad).

manyboats said:

Captian K,
What's the matter w nautical miles per hour?

Andy's burn being way different may have more to do w how he's propped that what rpm he's running. Anytime you compare one vessel to another they have to be propped the same. Re fuel burn anyway.

Daddyo,
Your boat is not full disp. But it does not matter re the post.

Click to expand...

Nothing at all wrong with "nautical miles per hour." But he wrote "knots per hour," which of course is redundant as the term "knots" means nautical miles PER HOUR.

psneeld said:

My departure date in December is based on tides...I won't run the Delaware against the tide unless I absolutely have to. My timing has me getting a consistent 1.5-20 knot push all the way from Cape May to the C&D.

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Agree! Unfortunately the tide we had was more then wiped out by the head winds. This was the first time that we had unfavorable conditions in the Delaware after running her a dozen or so times

SomeSailor said:

Most all trawlers of that design (and mine) are technically semi-displacement, but simply don't push them to that point. Either for economy or lack of power keeps us from pushing them up and out of the water.

They certainly aren't planing hulls, but if you push them, they begin to displace water less and rise in their track. That explains the way we can get beyond mathematical hull speeds (by a tad).

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SS,
Yes I understand that. What I'm saying is our DeFever carries her deep forefoot well aft and never achieves the flatter aft sections. Her wetted area at the stern is minor and the deadrise aft is still considerable to the trailing edge. Much more like the Krogen wineglass then the typical TTs.

Daddyo said:

Yes I understand that. What I'm saying is our DeFever carries her deep forefoot well aft and never achieves the flatter aft sections.

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I hear ya and mine has that long forefoot and keel that runs all the way back, but if you do the math, you'll probably see that you are capable of doing speeds that are a couple of percentage points beyond pure "hull speed".

What's your max speed? I'm just at 10kts... maybe a hair more if everything is light. I'm certainly not planing much, but the hull is trying to crawl out of the trough and the water rolls off the bow lines.

What's your width at the transom?

Our max wide open is about 8.5 knots. Transom width is 13'. Beam is 15'. The bilge is deep well aft not just the keel.

Daddyo,
Lets see a good pic of your aft end. Perhaps I'm wrong but every DeFever I've seen is semi-disp without a doubt but there was a smaller (34' ?) DeFever that surprised me re her lines aft. A Krogen 42 has almost no submerged transom and is clearly a full disp boat. I'd rather not go into the Buttock Line stuff as some here don't like it but it is the best way to define a full disp hull in the aft end. Let's see what you've got.

manyboats said:

Daddyo,
Lets see a good pic of your aft end. ...Let's see what you've got.

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Are you sure you didn't mean to post that on another site?

This is the boating site ...

RickB said:

Are you sure you didn't mean to post that on another site?

This is the boating site ...

Click to expand...

:lol:

Rick,
AFT end .. not rear end. What have you been think'in lately?

I run the same engines on a lighter boat and consistently get around 2 gallons per hour at 12-1500 RPM. That includes some generator time so I think my numbers are pretty much the same as Daddyo's

Unless you have some sort of fuel management system, I don't see how one can possibly determine how much fuel was used on a short trip. Fuel gauges are notoriously inaccurate so unless one started with absolutely full tanks and filled them after ther trip or measured with a stick or sight gauges, the fuel use can only be estimated.

Current and wind play a big part in the amount of fuel used to get from point "A" to point "B". RPM plays a big part in GPH.

Fuel use on a single trip is only important if you run out of fuel before the end of the trip. Meaningful fuel use figures can only be obtained over a long period unless there is a fuel management system in use.

Here are my figures: Note that I used a lot of fuel the first couple trips before I learned to slow down and enjoy the ride. This still skews my average fuel burn figures which you can see are about 1.9 GPH. This is typically at 2K RPM which translates to 7 knots over water.

rwidman said:

.... or measured with a stick or sight gauges, the fuel use can only be estimated.

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I have inch marks on my sight tubes, and after every trip I take a measurement. I have a spreadsheet that models my tank dimensions and has been accurate to within a couple of gallons every time I add fuel. Even with the inaccuracies in the readings, the actual re-fuels are added back in as correction factors and the overall accuracy improves. My numbers are very consistent so far.

SomeSailor said:

I have inch marks on my sight tubes, and after every trip I take a measurement. I have a spreadsheet that models my tank dimensions and has been accurate to within a couple of gallons every time I add fuel. Even with the inaccuracies in the readings, the actual re-fuels are added back in as correction factors and the overall accuracy improves. My numbers are very consistent so far.

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I would love to have sight tubes and have considered adding them, but haven't gotten around to it. My tanks are rectangular so the calculations would be pretty simple.

yep...any way to accurately measure fuel such as sight tubes, measuring stick, sounding tape or even just fill it to the same point in the filler neck and see what it takes next time you fuel....easy peezy.

sure there are a few assumptions about possible errors even then.... but after getting to know your boat a lot of the assumptions are quickly eliminated.

psneeld said:

yep...any way to accurately measure fuel such as sight tubes

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Each inch in mine equates to about 8 gallons (actually 8.023 gallons). I have a graduated rule behind each sight gauge so I can get with better than a half inch on each side. So my margin of error might be 4-5 gallons overall (assuming two 200 gallon tanks).

SomeSailor said:

Each inch in mine equates to about 8 gallons (actually 8.023 gallons). I have a graduated rule behind each sight gauge so I can get with better than a half inch on each side. So my margin of error might be 4-5 gallons overall (assuming two 200 gallon tanks).

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Why not remark them to represent gallons, not inches?

I don't think this has been mentioned but measuring fuel consumption w a dip stick or sight tubes is best done by starting w empty tanks and filling 5 or 10 gallons at a time marking or measuring the fuel on the stick/tube. This method takes care of any shape of fuel tank and single gallons can be interpolated between major marks such as 5/8ths" per gallon or whatever. Depending on the sight tube or dip stick location laterally on the tank even variable list could be accounted for but usually tubes and holes will be inboard so the boat should be listing or not listing in the same way when fuel measurements are made. But any such measuring system taking this much effort should only apply to boats going far offshore or doing predicted log races. If normal people need to compute accurately their fuel consumption they may need to find a boat that burns enough less fuel so they wouldn't need to think about such things.