DD 6v71 fuel burn rate?

Anyone have Detroit Diesel 6v71 with turbos? We’re looking at a boat that has them and we’re trying to figure out fuel burn rate. We normally run 8 to 8.5 knots. Anyone else have those engines and run at those speeds?

Thanks.

John

You can get fuel burn info from individuals, but I always find that suspect.



Another way is to search for a spec sheet and fuel curves for your engine. Look up the rpm on the prop fuel consumption curve (not the maximum power curve) that you run to reach the speed you want. That fuel burn will probably be more accurate than anything you get from individuals. People are ALWAYS optimistic about their fuel burn.


David

BoatDiesel.com is an excellent resource. You need to know as much as you can what horsepower the motors are rated at, as well as prop and boat displacement/hull design and length to get increasingly accurate estimates. Does the prior owner have any logs?

Fuel Burn - lots of different factors need consideration.

Fuel Burn questions are very individual & boat specific & are also very speed related. Many factors go into the number. Hull shape, displacement, prismatic coefficient, drag due to shaping, depth of the hull, hull design - like - planning hull vs semi displacement hull vs full displacement hull, all effect fuel burn.

You also need to know the waterline length of the boat that the engines are in as the relationship of the speed length ratio has more effect on fuel burn that you may realize.
Take the square root of the water line length multiplied by the Speed length ratio you choose to cruse at.: 1.0; 1.1; 1.2; 1.3; & top hull speed 1.34. to determine the S/L ratio cruising speed.
Please refer to Robert Beebe's book for that info in more detail.
--- the lower S/L ratio gives better fuel economy by a large % factor.

You say 8.0 to 8.5 knots - but if your at the Max hull speed for the water line length of your example boat at 8.0 knots you can have a huge increase in fuel burn just to go that 1/2 knot faster !

A S/L ratio economy example that is good at 1.2 S/L Ratio cruising speed for local scooting around cruising speed & 1.1 or 1.0 S/L would be good for long distance runs.

So be advised other considerations need to be put into the equation. For Instance, There is sometimes more under water drag with a twin engine boat than with a single. Twin engines combined fuel burn will often be more than a single engine vessel at same comparable specs.

However, if your just looking for a fuel burn number example for these engines, I had a DD 6 V 71 turbo on a 46 footer with a 39 foot water line length single & it burned about 5.2 GPH at 8 knots at 1800 RPM. The DD's are a two stroke design & in my experience they definitely are more thirsty than a regular 4 stroke engine of same displacement & power output.
I am guessing for comparison the same equivalent boat with 4 stroke would be around about 3.8 to 4.2 GPH. under same conditions.

I hope that info helps. There are many other factors as well, like are you towing your dink, etc.. I am sure there are more guys here on TF that can give their examples & input as well.

Good Luck.

Alfa Mike

As stated by others, there is a lack of information for making an educated response. If you can live with a gallon a mile at displacement speeds you won’t have a problem with a 671. If you need to make 2 miles to the gallon you probably won’t be happy with a 671.

agree with "tiltrider1 "

I would agree with " tiltrider1 " above.


Those engines are a thirsty bunch with the two stroke design.

Alfa - Mike

alfamike said:

I would agree with " tiltrider1 " above.


Those engines are a thirsty bunch with the two stroke design.

Alfa - Mike

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You guys make it sound like the DD drinks diesel. It is actually as efficient as similar older diesels like the Cummins 855. The older NA engines make about 16 hp per gph. The newer turbocharged and intercooled engines make about 18 hp per gph of fuel. Maybe not as good as a modern Cummins, but not bad.


David

DD are great engines. They were extremely economical by 1930’s standards. They became a tad thirsty by 1980’s standards. Unfortunately they are very thirsty when compared to 2000’s standards.

I would not consider them thirsty if I was comparing them to Cummins 555 or CAT 3208.

Greetings,
Mr. JE. It has been said many times that fuel is one of the lesser costs of boating. I would be much more concerned about the actual condition of the DD than consumption. Are there maintenance records? Being a turbo, has the snot been run out of it? Do you like the rest of the boat?
My $.02...

A DD is close to the same efficiency of a modern four stroke. Maybe 16hp/gph compared to 18-20hp/gph as Dave posted. But your burn rate all depends on the boat. OP, post details of the boat. Size, weight, beam etc.

A lot of great information already. I guess I should have posted these details at the beginning.
It’s a 54’ Ocean Alexander. Dry weight is 52,000. It holds 800 gallons of fuel and 400 of water so I’m figuring roughly 60,000 lbs, wet. The beam is 15.6. It has fin-type stabilizers.

If it matters, the 6v71s were in-frame rebuilt about 1900 hours ago.

The current owner runs it at trawler speeds. I checked the engines over carefully (not surveyor carefully) and saw no evidence of exhaust leaks. Some green corrosion around the impeller cover plates but nothing surprising. The only oil leak was a little seepage from a sensor. The turbos themselves aren’t leaking any oil or discolored.

Love the layout and so does the wife. Could totally see ourselves living on it after some refurbishment.

Should run pretty well at 8-8.5kts. Engines should be pretty happy there, 1200-1400rpm doing the wild guess thing. Another wild guess would be about 6-8gph total.

A rough rule of thumb as taught years ago for the 71 series DD was one gallon per hour per cylinder while making power. Take it as a very rough estimate.

I know OA 54’s very well. The boat won’t go any faster than 12 knots, fuel consumption at 12 knots is like 2 gallons a mile. At 10 knots you will burn .95 gallons per mile. At 8 kts you will burn .8 gallons per mile. Normally I don’t like high HP DD671’s but with that boat the fuel curve is so severe that I doubt they were operated Hard.

tiltrider1 said:

I know OA 54’s very well. The boat won’t go any faster than 12 knots, fuel consumption at 12 knots is like 2 gallons a mile. At 10 knots you will burn .95 gallons per mile. At 8 kts you will burn .8 gallons per mile. Normally I don’t like high HP DD671’s but with that boat the fuel curve is so severe that I doubt they were operated Hard.

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I guess you would know them very well.

That's exactly the first hand information we were looking for. Thanks!

Ski in NC said:

Should run pretty well at 8-8.5kts. Engines should be pretty happy there, 1200-1400rpm doing the wild guess thing. Another wild guess would be about 6-8gph total.

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That's right in line with what Tiltrider indicated. Perfect. Thanks!

Our ex Navy Utility is quite light perhaps 10-12 tons and at 50 Ft is almost all waterline 49ft?

This relates to few boats because of the light weight , but at SL of 1 (7k ,8mph statute) we burn under 3 GPH .Measured over the loop , 6000miles.

On the flowscan 11K is 10GPH , but we have never operated there , only ran up to 1800 for a look.

The weight and LWL of the boat should give a ballpark fuel burn.

On some installations a simple injector change can increase the fuel efficiency .

If the engine was built for max power and you run at 1200-1500 as we do , smaller injectors 55-70 will be an improvement over the stock 135's.

Thirsty bunch,my 453n at 1500 going 7.5 knts is burning approximately 2 gph and if I pulled her back to 6 would be close to 1.25 gph that is a fact

Lostsailor13 said:

Thirsty bunch,my 453n at 1500 going 7.5 knts is burning approximately 2 gph and if I pulled her back to 6 would be close to 1.25 gph that is a fact

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That’s not a function of your 4-53’s lack of thirst, it’s a function of an efficient hull at its sweet spot speed. At 6 knots you are burning fuel to produce less than 20 hp. On my Perkins propelled W36 I tell people the red knob is the volume control. You can make more noise, and burn more fuel, but not go faster! On your Jimmy the maximum volume and fuel flow is way higher!

Bill

FF said:

Our ex Navy Utility is quite light perhaps 10-12 tons and at 50 Ft is almost all waterline 49ft?

This relates to few boats because of the light weight , but at SL of 1 (7k ,8mph statute) we burn under 3 GPH .Measured over the loop , 6000miles.

On the flowscan 11K is 10GPH , but we have never operated there , only ran up to 1800 for a look.

The weight and LWL of the boat should give a ballpark fuel burn.

On some installations a simple injector change can increase the fuel efficiency .

If the engine was built for max power and you run at 1200-1500 as we do , smaller injectors 55-70 will be an improvement over the stock 135's.

Click to expand...

FF,
What is the power rated rpm?
Is this the 160hp basic old 6-71?

Too funny!!!?