43 DC 6-71s - Why am I using so much fuel?

[Will]

vindentc 6-71ti's and jaxfishgyd 6-71ns both report using 6GPH @ 9 kts. while I calculate 10 GPH @ 9kts (1200 RPM for me). I don't know if their readings are by calculation or FloScans. I see no fuel leaks, smoke, soot, etc. I have 1.5:1 gear boxes and both engine WOT at about 2350 RPM under load and 2450 no load. I can't remember what my prop sizes are now but they were spec'd to OEM when balanced by propscan.
Other than calculation errors, what might cause my consumption to be so much higher?
Will

 

[Brian Degulis]

It could be realisim vrs optimisim? 6 GPH on a pair of 671s makes about 96HP maybe less. That doesn't sound like enough power for a 43' to make 9 Kts. 10 GPH makes around 160HP that sounds about right for 9 Kts. I'm just guessing but 6 sounds to low.

Brian

 

[TedZ]

Brian is right on the button for my 43DC.

Ted

 

[Will]

Thank you Gentlemen. Think I'll try to find some power curves. Also, I'll run a few more trials.

Does anyone have Floscan input on this?

Thank - Will

 

[Freestyle]

There is an old post of 43 6-71 fuel curbs that matches my boat exactly.

43's ride very nicely at 9 kts and get 10 gph. Slow down to 8.2 and it runs about 5.5 gph. Slow down to 7.4 and burn only 2.9 gph.

At $5 per gallon, that last 1.6 knots is $35 an hour but it saves about 12 minutes of travel per hour.

Bruce
Freestyle
1976 43 DCMY
Tampa

 

[Bill Allen]

I just this week end looked through my log from my trip to Fl. on my 43. With my 23 sq props I got all but 1.5 mpg at 7-8 mph on the icw. I dinged them on the dismal swamp and changed them in the keys. On the way back I put on my topazs wheels 24x26. The boat did seem to run much nicer and do better on fuel the down side was I neglected to write down what I was getting but it sticks in my mind that is was all but 2 mpg. Very little genset time as I run a 200 amp alt and a 3000 watt inverter. The boat was loaded heavily as we were gone for 5 months. My boat is pretty much finished as a planning hull (.6 mpg) and if I have any real problems with my 6-71's I'm considering repowering with 100 hp JD's.

 

[stormchaser]

It could be realisim vrs optimisim? 6 GPH on a pair of 671s makes about 96HP maybe less. That doesn't sound like enough power for a 43' to make 9 Kts. 10 GPH makes around 160HP that sounds about right for 9 Kts. I'm just guessing but 6 sounds to low.

Brian

Its actually about right. 160 would be more than neexced to reach hulls speed. The formula for figuring out power needed for a given speed when in displacement mode is Shaft Horsepower = (LWL) /((10.665/(SL Ratio))^3)

For a Hatteras 53 with a lwl of 47'3" and a displacement of 55,000#...you'd need 109hp for get her up to 9.2knots, which is the hull speed.

A Hateras 43 (from what I can find) displaces 34,000# and has a lwl of 43.3? If so, she would need only 67.4hp to get up to her hull speed of 8.8.

 

[Vincentc]

My 6 gph is calculated based upon what it took to fill the tanks at various stops in our trip home from Sarasota to Pascagoula including a 70 nm or so run in the open gulf between Tarpon and Carabelle where I switched to the full 90 gal fwd tank then refilled. I was running around 8.5 kts. most all the time except when I had to go slower. All speeds are based upon gps and subject to wind and current. The average for the whole trip was about 1.3 best I recall. The gulf run was around 1.6 nmpg. Distance measurements were generally required guestimates for turns and the gulf run should be the most accurate for both fuel and distance; however we had a bit of a following sea. That works out to 5.2 to 5.9 gph.
All numbers were adjusted to take out generator use assuming .6 gph for an 9kw onan / cummins usually operating 2 Ac's.
The LWL for the 43 Hatt is to my understanding about 38 feet and the hull speed is 8.3 or 1.35 SL ( Speed in Kts to square root of lenght water line ratio. 9kts is well above hull speed and should make a big difference.
I have a boat load of calculations re speed and hp requirements courtesy of Dave Gerr's Propeller handbook and and hours playing with Excel and Quattro Pro but like one of our contributors noted we don't live in Theory.

I've tried to reconcile caluculations with reality. However, while GPS readout indicates a 1/10 kt speed over ground accuracy the indicated speed varies over 1 kt at the same rpm. Facinating as it may be, my wife does not want to run a measured mile in two directions with me when we go for a boat ride and I can buy a lot of fuel or an inverter set up for what a fuel flow meter will cost to buy and install. Expect I will just interpolate from theory since actual speed, distance run and / or fuel consumption numbers are not that reliable.

That said I wonder if fuel consumption curves are very helpful since they are based on assumptions of the load at a given rpm and fuel consumption is directly proportional to the hp load subject to your engine's varying efficiency depending on load. As I appreciate it, hp / gph can vary from 13 to 20 for a 671 based upon my review of several charts for 671's. Best I recall all ti curves start at 1400 rpm, which is of questionable help if you intend to run1200.
I remain interested in data, but finding reliable info is a challenge.
Regards,
Vincent

 

[Brian Degulis]

Its actually about right. 160 would be more than neexced to reach hulls speed. The formula for figuring out power needed for a given speed when in displacement mode is Shaft Horsepower = (LWL) /((10.665/(SL Ratio))^3)

For a Hatteras 53 with a lwl of 47'3" and a displacement of 55,000#...you'd need 109hp for get her up to 9.2knots, which is the hull speed.

A Hateras 43 (from what I can find) displaces 34,000# and has a lwl of 43.3? If so, she would need only 67.4hp to get up to her hull speed of 8.8.

I think the formula your using assumes a displacment hull with a small engine and a deep reduction gear turning a large prop. We are talking about a planning hull with 2 large high output engines running a light reduction gear and a small prop. A boat that's designed to go fast not displacment speed.

You really can't use only weight and LWL there are lots of other considerations like beam and hull form. Also when your talking low speed fuel consumption on our boats you need to remember your grossly overpowered. When your taking 60 or so HP from an engine that's rated at around 400 thru a fixed pitch prop (very lightly loaded for the RPM) Then you waste a lot more fuel it can be as much as a 40% loss.

Brian

 

[Jaxfishgyd]

Let me add my 1/2 cent worth.....
I usually run at 1100 rpm... my speed is "in the 9 kt" range.....
Heck, sometimes it's 9.4, others it's 8.5.Now, throw tide in and sometimes I'm doing 7 kts, other times 11 kts.
My mpg is based on over 15 trips of over 30 miles. 3 over 100 miles.
My mileage is using the gps. and hours run.
6 gph is what I came up with every time (OK, some of it came to 5.7, others were 6.2).
Both my motors were 100% rebuilt when I bought the boat in 2000.
Sad thing is, I do not know what my actual HP is... Some of my paper work says 220 hp... But also have paper work saying 330 hp.

 

[Trojan]

671N's should be rated at 310 HP. At least mine are.

Bill

 

[stormchaser]

I think the formula your using assumes a displacment hull with a small engine and a deep reduction gear turning a large prop. We are talking about a planning hull with 2 large high output engines running a light reduction gear and a small prop. A boat that's designed to go fast not displacment speed.

You really can't use only weight and LWL there are lots of other considerations like beam and hull form. Also when your talking low speed fuel consumption on our boats you need to remember your grossly overpowered. When your taking 60 or so HP from an engine that's rated at around 400 thru a fixed pitch prop (very lightly loaded for the RPM) Then you waste a lot more fuel it can be as much as a 40% loss.

Brian

Oh, I totally agree that these baots aer WAY over powered and under propped for fuel efficient displacement speed powering. But that forumula does indeed accurately show how little power is really needed as long as you don't ever care to get more than displacement speeds out of her. If you really wanted to go slow and efficient, a Hat 43 wowner would repower with twin 75hp diesels, Maybe 85s if you want a lot of reserve power. Hatt 53 owners would use twin 100-125s. If you wanted a decent "reserve" on a Hat 53, go with twin 150s and you'll cruise along effortlessly at 3 gallons per hour at 8-9 knots. In the LONG term, when my original DDs die, I may very well do that. I went with a Hat 53 for the layout and live aboard space...not the speed.

I thought the list might might enjoy knowing where the formulas for
power required and anchor drag forces come from. It is all beginning
physics.

For fluid flow that is not turbulent the basic pressure (i.e. force
per unit area) exerted by the fluid on any surface goes like 1/2 times
the fluid's density times the square of the fluid speed. You may have
seen this called Bernoulli pressure. To get the quantitative number
you need a fudge factor that is determined by the geometry of the
surface. The drag coefficient is typically about 1.

Since pressure is force/area, you get the force by multiplying the
pressure (given by Mr. Bernoulli's term) by the area. Now you know how
to estimate the wind force on your boat when it is blowing. You also
know how to determine the force on your hull due to the water flowing
past it.

Now since the force varies like the speed squared and power is given
by force times speed you can see that the power required to move a
boat at some speed must vary as the speed cubed. That is the essence
of the formula realting hp to speed.