Cat 3408 fuel consumption

[kenster]

Greetings...Can anyone share their knowledge concerning the fuel efficiency (gph) of running two 210 hp 3408 Cat diesels pushing a 49' semi displacement hull with a beam of 14.60 ft, draft 3.75 ft & dry weight of 39,000 lbs @ 1200 to 1500 rpm's? Thanks

 

[DNT99611]

Going out on a limb here but I am thinking you mean 3208's and not 3408's.
At 1200-1500 I would think 5 gallons an hour per engine but that is only a guess. I never ran my 3208 that slow for very long. Not saying it is bad on them I just never did it.

 

[Comodave]

Running that slow I would venture a guess of about 7 to 8 GPH for both engines.

 

[tiltrider1]

At 1500 RPMs you will probably go 8 kts and get 1.25 nautical miles to the gallon.

 

[Portage_Bay]

There are too many variables to give anything more than an estimate. I'll agree with DNT99611 that you most likely have 3208s. The 3208 NA is rated at 210 HP. The 3408s are all much higher HP ratings.

If yours are 3208 NAs then I think Tiltrider's estimate is close. I run a pair of 3208 NAs 210 HP in a 42 Californian LRC and see about 1.1 nautical mile per gallon at 1600 - 1800 RPM.

If you already own the boat careful monitoring will help you learn the fuel consumption. If you are considering purchasing the boat then spend some time on boatdiesel.com and look up the info on your engines and use their calculators.

Regarding running 3208s at lower RPM. I have been advised by many who know these engines far better than I not to run them for extended times at low RPM. If my memory serves CAT recommends running the 3208 NAs at 2200 - 2400. I don't do that because I don't like the sound level at that engine speed and I don't like the fuel burn. What I do is run most of the day at 1600 - 1800. 1600 more than 1800 because the boat is much quieter. Then when I'm about 45 min out from where I will end the day I run them up to 2000 for 30 min. I don't go to the recommended 2200 - 2400 because the PO over proped the boat so the 2200 to 2400 could be putting too much stress on these old engines.

Greetings...Can anyone share their knowledge concerning the fuel efficiency (gph) of running two 210 hp 3408 Cat diesels pushing a 49' semi displacement hull with a beam of 14.60 ft, draft 3.75 ft & dry weight of 39,000 lbs @ 1200 to 1500 rpm's? Thanks

 

[tiltrider1]

I assumed OP meant 3208. I do not think it likely that some one shoved an 18 liter engine into a 49’ boat. However, if I am wrong the fuel consumption won’t change much. Fuel consumption is a factor of HP needed and most Diesel engines convert fuel into HP at a rate of 17-22 HP per gallon. Your old Detroit Diesels were in the 17-18 range while your 4 cycle Perkins, Lemans, Cummins and Cats were solidly at 19. It’s only the newer electronic single line engines that can get into the 20’s.

 

[Steve1.0]

I'm not sure I'm reading the chart correctly from Portage Bay, but there is an appreciable difference with the one working Flo-Scan I have and the chart.



Here is what my gauge shows; this is a one time reading for the starboard engine only. Speed is with both engines turning in sync. or as close to as I can get them. Boat is a GB 46'



rpm gph kts.

1200 2.1 6.8

1450 3.2 8.6

1500 3.6 8.0 ?? may be tidal

1600 4.0 8.3

1700 5.2 9.5

2100 8.3 12.1


Are these numbers reasonably close?


P.S. Anyone know much about flow scans?

 

[Portage_Bay]

Steve1.0, the chart is from a CAT document on the 3208s posted on boatdiesel.com. Tiltrider is using a well understood conversion of diesel volume to HP. How much HP it takes to drive a given hull at a certain speed can also be estimated but
will vary from boat to boat. Hull form, FD, SD etc. Running gear, 3, 4 or 5 blades, prop pitch. Cleanliness of the bottom. And I'm sure there are more variables than that. I don't know about flo scans having never used them. I use gallons burned over distance run and average RPM over many days and miles to get my average.

Please keep in mind I'm no expert but your #s for 1600 & 1700 RPM giving nautical miles per gallon of 1.03 and 0.92 respecively look OK to me. 2100 supports the truth that speed costs. You're now down to 0.73 nmpg.

To find your most effecient nmpg others have suggested looking at your wake. I have found that to be very telling. When I try to approach the oft mentioned hull speed of 1.4 X the square root of the waterline length I'm throwing a big wake. That's just wasted energy. My fuel burn shows it. My wake is minimal at 1.0 to 1.25 X the square root of the waterline length.

 

[tiltrider1]

I'm not sure I'm reading the chart correctly from Portage Bay, but there is an appreciable difference with the one working Flo-Scan I have and the chart.



Here is what my gauge shows; this is a one time reading for the starboard engine only. Speed is with both engines turning in sync. or as close to as I can get them. Boat is a GB 46'



rpm gph kts.

1200 2.1 6.8

1450 3.2 8.6

1500 3.6 8.0 ?? may be tidal

1600 4.0 8.3

1700 5.2 9.5

2100 8.3 12.1


Are these numbers reasonably close?


P.S. Anyone know much about flow scans?

Your numbers look very reasonable with the exception of 8.6kts at 1450 rpm. Is that suppose to be 7.6kts?

 

[kenster]

Gentlemen...My apologies to each one of you that replied! I was incorrect with the model number designation...3208 is correct.

I am interested in making an offer on a trawler. I have received several estimated gph ratings before asking here. The range has been between 3.75 to 9+ per engine...That is such a wide range. Guys, Is there any way to dial it in? Maybe the info I presented is not definitive enough. Would any other info be of value?

Would I be better of replacing the Cats? Or move on to another Trawler with Lehmans or ?? Full displacements are hard to find and would be our first choice. Respectfully...

 

[Portage_Bay]

I think if you keep your speed down to theoretical hull speed you'll see about 1 to 1.2 nautical mile per gallon. If you slow down to 1 kt below hull speed you'll do even better.

I wouldn't consider buying a boat to repower it unless the boat is both a screaming deal and something special enough to be worth the expense and effort. The cost could be well over $100K. If you don't like the 3208s find another boat.

 

[Ski in NC]

Gentlemen...My apologies to each one of you that replied! I was incorrect with the model number designation...3208 is correct.

I am interested in making an offer on a trawler. I have received several estimated gph ratings before asking here. The range has been between 3.75 to 9+ per engine...That is such a wide range. Guys, Is there any way to dial it in? Maybe the info I presented is not definitive enough. Would any other info be of value?

Would I be better of replacing the Cats? Or move on to another Trawler with Lehmans or ?? Full displacements are hard to find and would be our first choice. Respectfully...

All diesels in that hp and age range burn about the same fuel for the hp they create. 16-20hp per gph. So it does not matter too much which engine you are running.

What does matter is what hp you are asking the engine to create. And that depends highly on the boat and highly on the speed. The difference in fuel burn between 7kts and 9kts can be huge. Like double. Again, depends on the boat.

My gut feel is with twin 210hp 3208 running a bit under hull speed is about 2nmpg.

 

[tiltrider1]

At 1500 RPMs you will probably go 8 kts and get 1.25 nautical miles to the gallon.

This is the best estimate you will get until you provide better data.

 

[FF]

"How much HP it takes to drive a given hull at a certain speed can also be estimated"

Yes , the most common ROT (rule of thumb) is each ton (2240lbs) of boat will need about 3 hp at normal (never hull speed) cruise.

Cruse is usually SQ RT LWL x .9 to 1.15 for nautical miles.

SO your 39,000 lb boat with 10,000 lbs of fuel water and living goodies will be close to 50,000 lbs, so only about 65 hp needs to be put into the water.

There are many engine losses and tranny power requirements that add to the fuel use.

But 3.5 GPH would be a good guess.

 

[tiltrider1]

"How much HP it takes to drive a given hull at a certain speed can also be estimated"

Yes , the most common ROT (rule of thumb) is each ton (2240lbs) of boat will need about 3 hp at normal (never hull speed) cruise.

Cruse is usually SQ RT LWL x .9 to 1.15 for nautical miles.

SO your 39,000 lb boat with 10,000 lbs of fuel water and living goodies will be close to 50,000 lbs, so only about 65 hp needs to be put into the water.

There are many engine losses and tranny power requirements that add to the fuel use.

But 3.5 GPH would be a good guess.

This would be 32.5 HP per engine or about 1000 RPMs and the boat would probably be going 6 kts.

Just my professional guess.

 

[kenster]

Gentleman...Every bit of information has been appreciated...Your knowledge is truly admired...Thank you

 

[Southern Boater]

Hi Kenster

Good luck with the trawler search and acquisition.

Don’t get too hung up on fuel burn, because over the average year fuel cost will never be your big expense for the recreational boater. Keep focused on the general condition of all the systems and hardware and rely on recommendations by experienced, independent surveyors, keeping personal emotions out of the equation…….which is harder than you think ;-)

 

[Riff Raff]

2 nmpg

Had similar boat with twin 3208’s. Easy to achieve 2nmpg if willing to go 7.5 to 8 knots depending on bottom cleanliness. As noted, fuel consumption will rise quickly as you get close to theoretical hull speed. Also, in my opinion, running 3208’s at lower rpm’s is fine. Just over prop slightly to work engines slightly harder (allows engine to stay warm which keeps it healthy). Then don’t cruise at wot. Also, run engines up to 1700 once and a while to keep clean ( this is mostly for TA engines but still worth doing for NA I think). 3208’s should last forever. Good luck.