Another Fuel Burn Question Thread

[DWJensen]

Curious if anyone on the board has a similar rig and can share (or guesstimate) fuel burn rate. I've done a bit of searching and can't seem to find charts for this setup (Boatdiesel.com apparently has some but I haven't been able to find them with just my guest membership.)

Basically I'm trying to get a rough idea of expected fuel burn running both engines at slow cruise speed (hull speed) on a boat of interest. It's not local to me and having a rough idea of the burn will help in the decision as to whether or not we make a multi-day drive to see her in person.


Engines (twins): 1995 3208T (375hp)

45' LOA, 39.4' LWL, ~15' beam, 3' draft

Weight: 28,000lbs not including ~1,000 gals water/fuel and assorted "stuff"

Thanks in advance,

Daz

 

[mike66]

Define slow cruise please. 7-9 kts vs 12-15 kts probably a big difference. Displacement speeds should be around 3gal/ hrs or less. Over that probably starts uphill pretty fast. If I remember my old Blackfin 32 with 3208 was about 20 gal/hr at 20 kts or so.

 

[mike66]

I see the question has probably been answered a few posts down the list.

 

[DWJensen]

I see the question has probably been answered a few posts down the list.

Apparently I missed it. Link please

 

[tiltrider1]

At 8kts you will be very efficient. After 8kts your fuel burn will go up exponentially until the boat gets on plane. I am assuming a planning or semi planning hull given the HP to boat size. At 8kts you could very likely get 2 nmpg. As your speed goes up that number will fall dramatically by the time you hit 10kts I expect you will be down to 1nmpg. I would not be surprised if you could travel at 18kts for .75 to 1 nmpg. However at 12kts you will probably be getting .4 nmpg.

 

[O C Diver]

A boat of that size with 750 HP would most likely have a planning or semi planning hull. While it can be run at slow speeds, the engines and drivetrains will require fuel just to run. A single smaller engine in that boat could likely cruise 7 knots at <2 gallons per hour and 8 knots at <4 GPH. With the twins I would expect 3 to 4 GPH at 7 knots and 5 to 6 GPH at 8 knots.

That's my SWAG anyway.

Ted

 

[rgano]

I like Ted's numbers and explanation. And don't forget the generator likely installed at maybe a gallon per hour with a heavy load of air conditioning etc. I never run a generator underway except for short periods to accomplish something specific, unless heat/humidity is truly oppressive requiring AC>

 

[Portage_Bay]

OP,

You won't find the info you're looking for on boatdiesel.com until you purchase a membership. I think the basic is about $29 / yr. Well worth it now and in the future as you learn your engines.

I think Tildtider has you in the ball park. A bit optomistic maybe, you won't know till you get some time and hours underway with the boat. He is absolutely correct about the dramatic changes in fuel burn above 8 kts until / if you can get her up on plane.

A question for you. 1,000 gal water/fuel on a 45' boat? What was she designed for?

 

[ranger58sb]

I've done a bit of searching and can't seem to find charts for this setup (Boatdiesel.com apparently has some but I haven't been able to find them with just my guest membership.)

Basically I'm trying to get a rough idea of expected fuel burn running both engines at slow cruise speed (hull speed) on a boat of interest.

Engines (twins): 1995 3208T (375hp)

45' LOA, 39.4' LWL, ~15' beam, 3' draft

Weight: 28,000lbs not including ~1,000 gals water/fuel and assorted "stuff"

I'd have thought CAT might have the fuel curves on their site...

Anyway, our previous boat -- similar in size and weight, Cummins 450Cs...

We'd run at approx 7-8.5 kts at somewhere between 800-1200 RPM depending on wind, tide, current... and fuel consumption (per chart, not flow meters) was in the neighborhood of 2.2-5.8 GPH total.

When conditions were most favorable, that could be as much as 2.35 NMPG (900 RPM, 7.05 kts... that day... although these were from averages of two way test runs).

-Chris

 

[C lectric]

It would help if you said what the boat and model is. There may be others with very similar boats. As it is now we are guessing. Fuel economy is often very vessel specific. The info you gave is good but not quite there.

What you have been given is likely close though.

 

[DWJensen]

The boat is a Tollycraft 45 CPMY, more of a semi-displacement hull, but trying to figure if it's something that I should be looking at or not. She's got two 400 gallon fuel tanks, 140 gallons of fresh water and a 45 gallon black water tank. I understand that fuel is a relatively minor part of cruising expense, but if we're going to do Maine to the Bahamas every year, then it will play a role in the budget.

 

[O C Diver]

If I were going to run it at hull speed all the time, I would want to know what RPM that will equate to. Then I would want to talk to a mechanic with lots of experience on that engine with that HP rating. I would need to know if multiple trips of that distance will be detrimental to the motors, cruising in that manner.

Ted

 

[tiltrider1]

I bet that boat has 2 tanks for a total of 400 gallons of fuel.

Cat recommends that 3208s be run at 1500 RPMs. I don’t see a problem running them at 1200 and then jumping up to 1800 for the last hour. I have not found much fuel savings between 1000 and 1200 RPMs with 3208s. That said there is always an exception somewhere.

 

[High Wire]

We chartered a GB 42 MY with those engines and the economy was awful. 190 SM at 8 mph used 271 gal diesel with about 20 generator hours.

 

[FF]

"I understand that fuel is a relatively minor part of cruising expense, but if we're going to do Maine to the Bahamas every year, then it will play a role in the budget."

Cheap to do at 6 or 6.5K,, 3x-5x or more expensive if you go faster.\

Think in statute miles and it will be almost 8 MPH . Miles per day is a better yardstick.

The easy way to transit at very low cost is go slow, and anchor out 99.9% of the time.

 

[ranger58sb]

I understand that fuel is a relatively minor part of cruising expense, but if we're going to do Maine to the Bahamas every year, then it will play a role in the budget.

Cheap to do at 6 or 6.5K,, 3x-5x or more expensive if you go faster.\

Think in statute miles and it will be almost 8 MPH . Miles per day is a better yardstick.

The easy way to transit at very low cost is go slow, and anchor out 99.9% of the time.

Yep, if you use marinas mostly... either go faster and and spend more fuel money/less marina money... or go slower and spend less fuel money/more marina money...

Or go slow and anchor out a lot, saves both fuel and marina costs. Cheap trip.

-Chris

 

[DWJensen]

Thanks for the on-topic responses, I appreciate you guys taking the time to help.

After doing more talking we've decided to pass on this one (not because of the engines, but rather some things with the vessel itself).

 

[Seevee]

2 mpg is probably a good wag, and it doesn't make much difference if the boat is being pushed by one or two engines. It take xxxx hp to move the boat at X speed, regardless of how many engines you have.



Fuel may or may not be a big expense. Too many variables. If one doing a 1000 mile trip up and down the ICW every year, the burn has some significance. At $3 a gallon that 2000 mile round trip will cost $3000. Some spend more than that on marinas, boat slips or insurance. But if you're doing a 6000 mile loop trip, fuel IS significant.


For most of us, not a huge deal.

 

[dacap06]

Fuel Efficiency

I can confirm Tiltrider's estimates are spot on. I have an '89 Jefferson 42 Sundeck. It has a semiplaning hull with LOD of 42 ft, LOA of 45 ft, LaW of 37.7 ft, beam of 14' 3", draft of 3' 10" and weight of 30,000 lb. Power comes from twin Cat turbo 3208's making 260 HP each (i.e. much less turbo boost than yours, and no aftercoolers). Props are 4 blade 28" dia x 28" pitch.

At lower RPM, your power curve and fuel burn will be similar, at higher speeds when your turbos are more active, less so.

On my Autumn Cruise in Oct/Nov, I tracked fuel burn vs. speed. I was curious, as it had been in the water a few months and I had always done previous checks in the Spring right after splashing. At 8.1 Kts (just below hull speed) I got 1.91 nmi/gal. At 10 Kts, I got 0.56 nmi/gal. I was heartened that these figures were only about 10% below Spring figures.

Tom

At 8kts you will be very efficient. After 8kts your fuel burn will go up exponentially until the boat gets on plane. I am assuming a planning or semi planning hull given the HP to boat size. At 8kts you could very likely get 2 nmpg. As your speed goes up that number will fall dramatically by the time you hit 10kts I expect you will be down to 1nmpg. I would not be surprised if you could travel at 18kts for .75 to 1 nmpg. However at 12kts you will probably be getting .4 nmpg.

 

[Osprey69]

On our 49' @42,000 displacement at 8.5 knots burn (with 2-3 generator hours per day) 4.5 gals per hour with CAT 3208 TAs (375 Hp) at 1400 RPM. Not same but may help.