32 GPH Cummins

Russ Calasant

Active Member
Dec 9, 2009
2,128
East Greenwich, RI
Boat Info
2001 480 DB
Engines
CAT 3196
I ran 7 hours, 3 from Block to MV and then 4 from MV to Mystic. 3 hour trip was flat calm, no generator and 2350 rpm at about 19knots full fuel, half water. 4 hour trip was 8'-10' well spaced, generator running at 2200 rpm at 19knots or a little better, 2/3 fuel and 1/3 water. We averaged 32GPH, does this sound about right?
 
Russ, I have similar hull and engines. Based on my performance curves generated with clean bottom and 3/4 fuel I get 22knots @ 2350 burning 36 gph. That would imply I would have burned ~ 93gal on your first leg (57nm/22 x 36gph = 93gal). On the return leg I get the same 19 knots @ 2200 rpm. I would burn 30gph, or 120gal. So I would have burned ~213 vs your 224 (7 x 32 = 224).

Given all the other variables, it looks right on to me.

Looks like you all had a great time!!!

Art
 
I think you are in the ballpark Russ. The weight of the fuel on the first trip will slow you down and the swells and generator on the second leg will make a difference.
 
Russ,
That's about what we are getting too. We just finished a trip up to Mackinac Island (did 750 miles) and burned around 31, 32 gph. Speed was showing 25 mph most of the way but the paddlewheel is a tad optimistic. We ran into heavy seas three days of the trip, especially coming out of East Tawas and the boat handled it better than the crew. We don't normally run the gennie while underway.
James
 
I've done my math and something is way off. I really hope it's not my numbers. Maybe we can get to the bottom of this difference. Anyway, here's what I have:

SOT: p793h/s792h full fuel, Genny ~1135h, 8382NM
EOT: P865h/s864h, 1/3 fuel, Genny ~1370h, 9284NM
--------------------------------------------------
Mains 72h , Genny ~ 235h, 902NM

Overall Stats:
Mains 72hrs, Genny ~235hrs, total 902NM. Burned 1553gal. Now I'll add 200g to make the tanks go to full for easier calculation. 1553g+200g=1753gal. This means I left with full tanks and came back with full tanks.


Deducting Genny usage (AVG 0.5GPH): 235hrs/0.5gph=117g, 1753g-117g=1636g is the number what was used by mains.

Stats for Mains Only:
Mains 72hrs, total 902NM. Burned 1636gal. 902NM/1636g=0.55MPG, 1636g/72hrs=22.7GPH.

Thus, my final numbers are .55MPG and 22.7GPH.

Am I doing something wrong with my calculation or is there something else missing in the picture? But I can't see how 420DB would have 10GPH difference from 460DA with exact the same engines and similar weight.
 
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Alex....that's the magic of 'Average'. When you only have 'Average', your numbers will be all over the place. The guys with the 480CEs have flow meters. Their numbers are spot on.

I've had tanks that I "averaged' 20 gph and I've had tanks that I've 'averaged' 8 gph. It all depends.
 
Am I doing something wrong with my calculation or is there something else missing in the picture?

Alex,

I agree with the comment regarding average. How many of these hours during the trip were at idle speed? (Very low fuel burn). When I'm idling out the harbor, I burn only 2.2 GPH
 
I know what you mean, guys. But I have to say that my AVG is pretty consistent for the past two seasons. It's been always within the range of 20-23GPH. I'm cruising at my normal cruise speed (in the range of 20-24kts@2300RPMs) 90% of the time. I would agree with you when AVG is taken for the entire season where I could have idle speed cruises. But, when I'm on a long trip my idle speed is minimum (getting in to/out of the harbor, slip or mooring). I don't do much idling on long trips unless I'm going only few miles away and have plenty of time, which is very rare.
 
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Alex, if you were going 20-22 KTS for 90% of your run time, you would have gone 1296-1425 NM (65Hrs times 20, then 22). Or, if you went 902NM at an average speed of 21 KTS, it would have only taken 43 hours (902 NM/21). I suspect your perception of how much time you actually run at speed is high. To truly get where you want your numbers to take you, there would need to be an accounting of pre- and post-underway idle time, manuevering on departure and arrival and no-wake transit time. I would guess that's more than you are taking into account.
 
Al,

I don't disagree with you. Perhaps in reality my cruising speed ends up beying less, but I thing that Russ started the thread based on his AVG numbers from the trip. There's no majic and if Cummins table states that we should burn 15gph per side at 2300rpms, then that's the true number and we can't beat that. So I'm just using my trips AVG.
 
Alex - I think your discrepancy is based on "bridge boat vs. Sundancer" syndrome. Your drag is vastly greater than a comparable size/weight 'dancer I see the same thing when I compare my numbers to those of a early 90s 350DA with the same engines. I'll bet the diesel dancer guys are getting around 1MPG -Dom has averaged 1.2MPG since he's owned his 410DA.
 
Alex, if you were going 20-22 KTS for 90% of your run time, you would have gone 1296-1425 NM (65Hrs times 20, then 22). Or, if you went 902NM at an average speed of 21 KTS, it would have only taken 43 hours (902 NM/21). I suspect your perception of how much time you actually run at speed is high. To truly get where you want your numbers to take you, there would need to be an accounting of pre- and post-underway idle time, manuevering on departure and arrival and no-wake transit time. I would guess that's more than you are taking into account.

Agreed...... The time ticks away as you warm the engines up, untie, stop at fuel dock, idle out a harbor, bla bla bla....... It is definitely more than 10% IMO
 
Ron,

Actually my point is quite oppopsite. I was expecting to see much more optimistic numbers from DAs vs DBs and surprized that it's not the case here. At 2200rpms the fuel birn rate should be 27gph, so if 460DAs are reporting 32gph as their AVG, then it's very high.
 
Alex, if you were going 20-22 KTS for 90% of your run time, you would have gone 1296-1425 NM (65Hrs times 20, then 22). Or, if you went 902NM at an average speed of 21 KTS, it would have only taken 43 hours (902 NM/21). I suspect your perception of how much time you actually run at speed is high. To truly get where you want your numbers to take you, there would need to be an accounting of pre- and post-underway idle time, manuevering on departure and arrival and no-wake transit time. I would guess that's more than you are taking into account.

One thing I can add here is that it's impossible to maintaing 21kts AVG speed even on 28kts cruise yacht. The speed avg drops drastically even on the good day.
 
Looking at the GPM figures might be better as it reduces the impact of time. Based on the OP's numbers, he got about .59 GPM against your .55 which is consistent with the theory that the DA is somewhat more efficient than the DB.
 
My primary focus is always MPG and I actually rarely look at the GPH. In the end it all comes down to only one question: how much fuel did I use to get from point A to point B?

Last year when I was just learning about sweet spot for my boat the GPH was about 20 while my MPG barely reached 0.5. Later when I've learned that her sweet spot is 2300-2350RPMs, my MPG improved from being steady 0.55 to 0.6. Apparently, the faster I go the more fuel efficient she is. If I recall, running her at 2400RPMs my MPG gets much closer to 0.6. However, I won't run my engines so hard, especially after my Cummins tech told me that 450Cs love to cruise at 2300 and do it with ease all day long. So, I'm not going hunt for lousy 0.03-0.04MPG improvement, but instead I'll cruise at 2300RPMs to ensure my engines remain very healthy for many years to come.
 
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Not for nothing, but if you get better mpg at a higher rpm (assuming similar conditions) then your motors and your entire system are running more efficiently, and hence under less load. If you run under less load you burn less fuel to get from point A to B. Your time between rebuild is determined by how much fuel you have burned which is directly related to the load you have run them under.
 
Dom,

In theory you might be right, but if I have engines that are rated for 2600RPMs, running them at 2300RPMs is 88% of the load, which IMO is aggressive enough. I'm sure that pushing her to 2350-2400RPMs will slightly improve my MPG, but I just don't feel comfortable running them at 92% of the load.

If I recall, the basic magic number people use is around 80-85%. At 88% I'm crossing that "usual" comfort zone, so I don't want to be too aggressive either.

It's another one of those things, either pay now or pay later. Run them high to save few bucks now could mean pay a lot more later for repairs. I'll do more research about running my engines at slightly higher load and what the result is, but for now I'll stick with the conservative 2300RPMs.
 
Alex
You are confusing percentage of WOT and load. The percent of WOT is only the same as load at two points. Idle and WOT.

To calculate load you MUST know instantaneous fuel burn/GPH.
 

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