DDA 6-71 fuel efficiency

[Doug]

I just purchased an '84 46' Uniflite w/ twin 410hp DDA 6-71 diesels. I'm trying to get a feel for what I might expect with respect to fuel efficiency. Hence, if anyone with comparable engines could respond with the following info, I'd be very grateful:

1. Your engine's "sweet spot" in terms of engine RPM and related speed. IOW, at what RPM/speed do you get the best fuel efficiency?
2. What is your average fuel consumption in gph at this ideal RPM/speed?
3. If not the same as #1, what RPM/speed do you generally cruise?
4. What is your fuel efficiency at that RPM/speed?

For each engine, I'm told this vessel has a 68 gal. tank forward and a 182 gal tank aft. Do I need to switch between these tanks or is that done automatically? How?

Thanks for any and all responses.

Doug

[Larryjack54]

Hi Doug,

I own a 48' Uniflite Yachtfish with twin 410 hp. 6-71's. We normally cruise at 1500 rpms and average about 28 gallons per hour total for both engines (about 14 gph per engine). I never found a "sweet spot". Our tanks have to be switched manually. I recommend studying your fuel system very carefully. Not only do you have fuel supply lines, you also have fuel return lines. I tried running just from our port tank once. After a few hours, fuel started spewing out of the starboard tank.vent. Make sure the proper vent valves are open also or you may be drawing from one tank and returning to a different tank as I did. My boat tends to sit heavy, listing about 5 inches, to the port side and I have never found the reason for it. My wife and I got off the boat and I asked her to watch the waterline as I got back on and stood on the starboard side. My 200 lbs. didn't even phase it. The trim tabs will level it out at cruisng speed, 15 to 16 knots. I feel like if I could find the reason for the list, possibly, my fuel usage would be reduced. We've owned and lived on our Uniflite for 4 years now. I love it but my wife wants to move back into a house. Can't have both so our boat is for sale. Good luck with your boat. The longer you own it, the more you will appreciate just how great a Uniflite really is.

 

Jack Smith

Galveston, Texas

[Frank Spor]

Good morning,  I'm far from an expert on this, but I too own a 46', 1984 Uniflite.  As far as I can tell....and this is an estimate.   1900 rpm is a good speed which gives you about 1 gallon per engine per mile.  Probably not exact, but easy numbers to work with.  The information on my boat indicates two, 50 gal tanks in  engine space and 400 gallons under the bed in the captains quarters. 

 

As far as I can tell there are cross feed valves between tanks.  Mine appear to be open.  Like I said I'm no expert so if someone has better information........

 

Good luck   frank

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[Kevin martin]

You MUST change the valves which are located under the steps that go to the aft stateroom.

the center valve is the crossover that allows the tanks to equilize if you need them.  I would suggest leaving that open.

However.  You must control the flow of fuel from the forward tanks and the aft tanks.

I use the forward tanks until they are low then switch to the aft tanks.  Then when the aft tanks get about 1/4 i fill all of them up again and switch back to the forward tanks to start over.

 

Happy motoring.

I just sold our beloved MC (1984 46') what a great boat she was for us...and the new owners love her.

 

Kevin martin

San Diego