bbilly
Well-Known Member
I'm trying to get an idea of, at an economical cruise speed (?), how much 2 x 485hp GM DD 6-71s burn per hour, pushing a Hatt 41 at 30,000 lbs. Any ideas?
TIA
Billy
TIA
Billy
GM 6-71 TIs pushing 30,000 lbs - burnrate
- Thread starter bbilly
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Divemaster1
Well-Known Member
oooh yes !!! need to sober up a bit more first ... then get back to the calculations .... give me 24 hrs please.!... but if straight six, they will be stressed at that level...
MapisM
Well-Known Member
If a rough idea is all you're looking for, 8 to 10 gallons.
But mind, "economical cruise speed" on a 41' means 8 knots or so.
But mind, "economical cruise speed" on a 41' means 8 knots or so.
bbilly
Well-Known Member
Yes, they are straight sixes with the turbo that gets 485hp out of them. Weight is 32,000 lbs. 8 knots! Might aswell get a sailing boat and burn 3l/hr at that speed. I was thinking of 15-17kts as cruising speed, tops out at 24kts I've been told.
Thanks for the info and im intrestd in your calcs Diver and you recommended DDs which is the guidance Im following.
MapisM im in Italy (Roma centro) tu dovè sei???
Thanks for the info and im intrestd in your calcs Diver and you recommended DDs which is the guidance Im following.
MapisM im in Italy (Roma centro) tu dovè sei???
MapisM
Well-Known Member
Well, you asked about economical cruise speed, and that's always the displacement speed, regardless of the cruise speed the vessel can handle.
15-17 kts is possibly the worse speed in terms of fuel efficiency, for a 41 footer.
I don't want to completely invent numbers, but rest assured that at such speed you'll have a much lower MPG (which is what really matter at the end, more than the GPH) compared to what you can get at displacement speed.
I'm rather far from Rome - much nearer to Milan.
15-17 kts is possibly the worse speed in terms of fuel efficiency, for a 41 footer.
I don't want to completely invent numbers, but rest assured that at such speed you'll have a much lower MPG (which is what really matter at the end, more than the GPH) compared to what you can get at displacement speed.
I'm rather far from Rome - much nearer to Milan.
Divemaster1
Well-Known Member
% Load| 1x| 2x| g/hk hr| l/hk hr| l / hr| RPM|
20| 97| 194| 200| 0.23| 45.12| 1350|
30| 145.5| 291| 200| 0.23| 67.67| 1500|
40| 194| 388| 200| 0.23| 90.23| 1700|
50| 242.5| 485| 200| 0.23| 112.79| 1800|
60| 291| 582| 200| 0.23| 135.35| 1925|
70| 339.5| 679| 200| 0.23| 157.91| 2025|
80| 388| 776| 200| 0.23| 180.47| 2125|
90| 436.5| 873| 200| 0.23| 203.02| 2250|
100| 485| 970| 200| 0.23| 225.58| 2300|
Please do not take these as correct for your boat but they should give you an indication... 1x = hp taken out of one engine 2x obviously for two....
Have taken my figures & propellor load curve, and changed 100% load to be 485 hp. Assuming engines in-tune and that JT Towers (Presume they are JT, series B with bypass valve to get the 485 HP ... you'll probably rev to 2600 as well), operate with same diesel weight per hp/hr as GM Detroit.
When I spoke with DD, they recommended 1750 RPM as general rule for loger runs (10 hrs +)... For us that translates to an awkward 15-16 knots .... whilst we do 18 knot at 1800 RPM, which is where we have operated most of the time. Turbo's kick in at about 1200 RPM, so if you want economic, displacement speeds you need to keep below that. Remember that the two stroke DD have a lot of mechanical parts, so friction losses at those low rpm's will be higher than the four strokes ... and as such at lower RPM's they will consume more than your equivialent four stroke..(hence not calculated below 1350 rpm). In-tune and at anything above 50% load they should be quite similar to a four stroke ... High torque engines, so your propellor laod curve will be virtually straight ... moving with the RPM ... as the calcs show........
20| 97| 194| 200| 0.23| 45.12| 1350|
30| 145.5| 291| 200| 0.23| 67.67| 1500|
40| 194| 388| 200| 0.23| 90.23| 1700|
50| 242.5| 485| 200| 0.23| 112.79| 1800|
60| 291| 582| 200| 0.23| 135.35| 1925|
70| 339.5| 679| 200| 0.23| 157.91| 2025|
80| 388| 776| 200| 0.23| 180.47| 2125|
90| 436.5| 873| 200| 0.23| 203.02| 2250|
100| 485| 970| 200| 0.23| 225.58| 2300|
Please do not take these as correct for your boat but they should give you an indication... 1x = hp taken out of one engine 2x obviously for two....
Have taken my figures & propellor load curve, and changed 100% load to be 485 hp. Assuming engines in-tune and that JT Towers (Presume they are JT, series B with bypass valve to get the 485 HP ... you'll probably rev to 2600 as well), operate with same diesel weight per hp/hr as GM Detroit.
When I spoke with DD, they recommended 1750 RPM as general rule for loger runs (10 hrs +)... For us that translates to an awkward 15-16 knots .... whilst we do 18 knot at 1800 RPM, which is where we have operated most of the time. Turbo's kick in at about 1200 RPM, so if you want economic, displacement speeds you need to keep below that. Remember that the two stroke DD have a lot of mechanical parts, so friction losses at those low rpm's will be higher than the four strokes ... and as such at lower RPM's they will consume more than your equivialent four stroke..(hence not calculated below 1350 rpm). In-tune and at anything above 50% load they should be quite similar to a four stroke ... High torque engines, so your propellor laod curve will be virtually straight ... moving with the RPM ... as the calcs show........
MapisM
Well-Known Member
[ QUOTE ]
In-tune and at anything above 50% load they should be quite similar to a four stroke
[/ QUOTE ]Really? I always heard that those DD have a reputation as heavy drinkers...
Does the column "g/hk hr" in your figures actually mean "g/hp-hr"?
If so, most 4 stroke diesels at their optimum range are happy with 160g or so, which actually means 20% less than your 200 for the same power output.
Btw, the 200 remains exactly constant at all rpm in your figures, - must be a typo I guess.
In-tune and at anything above 50% load they should be quite similar to a four stroke
[/ QUOTE ]Really? I always heard that those DD have a reputation as heavy drinkers...
Does the column "g/hk hr" in your figures actually mean "g/hp-hr"?
If so, most 4 stroke diesels at their optimum range are happy with 160g or so, which actually means 20% less than your 200 for the same power output.
Btw, the 200 remains exactly constant at all rpm in your figures, - must be a typo I guess.
Divemaster1
Well-Known Member
Also think you'll get something like 24 knots max out of her (looking at others with 485's) .... and high cruise @ about 20...
Assuming high cruise is at 80% engine load, you'd be burning about 180L / hr ...
but you need to take her on sea trials to deterine the best combinaion of speed and fuel consumption...
Assuming high cruise is at 80% engine load, you'd be burning about 180L / hr ...
but you need to take her on sea trials to deterine the best combinaion of speed and fuel consumption...
Divemaster1
Well-Known Member
"4 stroke of same age" ....
and hk is hp ... sorry my norwegian background coming through...
Using 200 throughout as it includes a nice safety margin ... and when I calculate the range, I allow for further safety margins .... Not entirely accurate, I know ... but surprisingly close to real life figures...
Heavy drinkers, compared to four strokes, they are when operating below 50% ... see note on mechanical friction .... Also as they are all mechanical, with loads of moving parts (each injector is adjusted seperately, as part of a rack) ... it is a bit of an art ... read take your time .... when tuning them up. Any injector out = probable additional burn ... but when in tune ... they are great under proper load ... very smooth at idle ... and the sound is second to none.... /forums/images/graemlins/cool.gif
and hk is hp ... sorry my norwegian background coming through...
Using 200 throughout as it includes a nice safety margin ... and when I calculate the range, I allow for further safety margins .... Not entirely accurate, I know ... but surprisingly close to real life figures...
Heavy drinkers, compared to four strokes, they are when operating below 50% ... see note on mechanical friction .... Also as they are all mechanical, with loads of moving parts (each injector is adjusted seperately, as part of a rack) ... it is a bit of an art ... read take your time .... when tuning them up. Any injector out = probable additional burn ... but when in tune ... they are great under proper load ... very smooth at idle ... and the sound is second to none.... /forums/images/graemlins/cool.gif
bbilly
Well-Known Member
Thanks guys, all very interesting and informative.
Billy
Billy