671 non Turbo diesel consumption

[Mario Tranquilein]

Hi: first sorry if I writed incorrectly!. I just want to know how much my 43 DC 1974 non flying bridge is drinking in fuel. My boat not have genset for the moment I have a clean bottom tanks at 3/4 but adding fuel everyweekend planning to fill to the top. I'am running it at 1,100 rpm and feels fast for me that I a sailor. I think I'am doing 9.5 or 10 knts I need to check with a GPs. But you guys, how much in total both motor is suppose to consume at that rpm with waves probably 3 to 4 feet? She have power to 2,200 without problem but I don't have my trim taps working and don't want to waste money. I just looking for 10 knts and very economical ride. Thanks for the help! Mario

[Boatsb]

keep the bow down and you will run economically. When you start to run fast enough for the bow to rise you lose the battle between speed and fuel. At 8.5 Kts I think I burn about 5 GPH or so on my 41C. Your boat is about the same length and HP so you should be close to that. The best way to find out is to make a run and measure how much fuel it tales to fill it back up.

[SKYCHENEY]

Best fuel economy will be near hull speed. 10kts will not be a good speed if you want to save on fuel. As stated, if you run more like 8kts, you'll do much better.

[eze2bme]

At 1100 RMP you're probably running about 6-7 knots. I have to get to around 1400 RMP to hit 8.5-9 knots. 1979 43DC with 6-71N DD 310 HP.

[Mario Tranquilein]

Hi: I was thinking at 1,100 doing 10 knots because I pass a friend that was doing 3.5 and I passed him like 5 knots at just 700 rpm!. And when I push the throttle to 1,100 I feel a big big diference in speed and no smoke at all and the bow was down. I think the best way will be meassure with a GPS and hope burn 3 galon for motor at 10 knts,could be possible? My 671 have like 300 hours run incredible smooth. Thanks

[Pascal]

3 gph at 10kts? No way!!

The hull speed on your boat is around 8kts, as soon as you go above that your fuel burn will rise drastically. Typically that first extra knot will mean at least 50% more fuel. So assuming you re doing 5 to 6 gph at 8kts, if you bump it to 9kts you ll be close to 9 to 10gph. At 10kts you re probably burning 12 to 13 gph

Measure your speed with gps to know exactly what rpm you need to run, measure in smooth water and light wind / no current

You d be surprised at how much fuel is wasted by going that extra knot above hull speed

[bobk]

Mario, this link will take you to a magazine article about the 48MY which shows measured fuel consumption for the boat. The boat is longer than yours so will have a slightly higher 'hull speed' but you can get an idea of how fuel burn increases with speed. It is non-linear as Pascal noted. Your best economy comes when making very little wave (that's where the power goes). Your hull speed will be about 8 knots so beyond that point the bow should be starting to lift and you will use proportionally more fuel.

http://www.hatterasowners.com/Brochu...48MYA-0781.htm

Bob Kassal
Chateau de Mer
1981 48MY

[REBrueckner]

As I recently posted here, Bill Allen's 1973 43ft DC Hatt with 310 HP 6-71's got about 1.8 NMPG at about 1300 RPM and about 8.5 to 9 knots, about 2.5 GPH per engine...

In general, fuel consumption increases linearly with an increase in HP, which you can see from DD prop load graph curves, but speed (of a boat) increases more slowly (is it as the square or cube of HP, I forget)...anyway, a 10% or 20% increase in speed almost always means a much larger % increase in fuel consumption.

[Mario Tranquilein]

Hi: thanks all for the help I will connect the gps and know the speed with the rpm exactly in the bay. If a burn 3 galon each at 8 or 9 knots I will be very happy! I need to avoid that my sailors friends pass me in the downwind from the trip that a sailboat regularly made 8 to 9 knots under sail. Thanks