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Figuring fuel burn?

Discussion in 'Engines' started by CTdave, Aug 16, 2010.

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  1. CTdave

    CTdave Senior Member

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    I have my 50' Bertram up for sale & was just asked about the fuel burn rate. I quickly responded 52 gph @ 1900 rpm (21 kts).
    He said he needs to know how many gallons per MILE at roughly 8 kts.
    Huh? I've never figured it this way. How the heck can I give him an approximate gpm estimate?

    '88 50' Bertram with 8v92's, 52,000 lbs.

    Thanks
    Dave
  2. jhall767

    jhall767 Senior Member

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    Gallons per mile is just the reverse calculation

    52 gph / 21 kn = 2.48 Gallons per mile.

    I'm not sure where you got the 52 gph but you'll need the gph at 8 knots to give him the answer he wants. Floscans or the equivalent is probably the only accurate way to get this. However 8 knots is under your hull speed and you will probably be burning 4-6 gph total at that speed with DD. Or .5 to .75 gallons per mile. It's very hard to estimate without a very long run.
  3. Pascal

    Pascal Senior Member

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    8kts may be a little too low to keep the engines warm enough. hull speed is likely around 9kts (it's 9.3 on my 53 hatt). at that speed, you're likely burning 9 to 10gph so figure about 1 NMPG. if he's an inland boater, he's probalby more used to statute miles not nautical so that woudl be around 1.1 MPG

    btw, the beam of the boat affects hull speed fuel burn more than weight... wider = more fuel, 10% or so.

    water depth also affects it, not an issue up north with deeper, but on my 53, in less than 9' of water (lower biscayne bay, part of the keys) speed goes down by as much as 10% for the same fuel burn.
  4. CTdave

    CTdave Senior Member

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    Thanks guys! I've never had to calculate lower speeds. I came up with the 52gph (cruise speed) number by actual consumption numbers on my CT/FL runs.
    Fill up, run for the day offshore & fill up again divided by actual time spent at cruise.

    Thanks again!

    David
  5. Capt J

    Capt J Senior Member

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    8v92's should burn 9-10 gph for both at 1000rpm's. At 800 rpm's your looking at 7gph probably.
  6. 54' Bertram

    54' Bertram Member

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    you assume the potential purchaser is looking to run the boat as a cruiser. If I was asking Dave his fuel burn, I would want to know it at his 19 knot cruise. why would you all assume this person is interested in running a sportfishing boat at 8 knots? I've been looking at moving up for the last 18 months and never wondered what the boat burns at 8 to 10 knots, since I either run it at trolling speed, cruise or WOT to run in from weather.

    some of us have something more important than a stateroom hanging off the aft of our boat!
  7. 54' Bertram

    54' Bertram Member

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    btw- I troll at a fast idle 7 knots burning about 4 gal an hour, 1 gal for the gen set, so that would be about 1.4 gal per hour. hard to figure someone who would run their boat with it in gear with no fuel. the engines would load up in a matter of hours. the potential purchaser needs to find a boat meant to cruise at that hull speed, not a convertible meant to cruise at 2.5 times what they want.
  8. Capt J

    Capt J Senior Member

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    Because if you are buying the boat and have to take it long distances to get it to where you live, it's a huge factor. For example from Key West to Cancun it's 345 NM and there are no stops. So you do 8-10 knots through the night to conserve fuel, safety and it increase range and at first light you hook up and run at cruise. Also 8 to 10 knots is trolling speed if you're trolling artificials for Marlin.

    Also you can run DD's at 800rpms-1000 rpms for 4-6 hours, then run them up to cruise for 1/2 an hour, then back down for another 4-6 hours. If you're delivering a yacht north and have to run the ICW the whole way, there are some days you might not even get a chance to run it up to cruise at all. If the boat travels a lot, and you have a crew, why not run it like this and burn 1/5th of what you'd burn at cruise to get it there.
  9. 54' Bertram

    54' Bertram Member

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    oops.... .7 gal per NM roughly
  10. 54' Bertram

    54' Bertram Member

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    I troll marlin at 7 knots here in the GOM. thats idle and it seems to work for the fisherman down here.
  11. 54' Bertram

    54' Bertram Member

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    Capt J, I appreciate your comments and understand where you come from. using your trip of 345 NM, if a boat runs 10 knots and burns 7 gal per hour or 10 gal per hour, the difference is approximately 350 gallons at 3 per gallon or 1 grand extra for the trip. not enough for me to not buy a boat because I can't take it home.

    ssems to me the person asking the question would be asking it for all the time they would be running it other than the 35 hours (using your trip).
  12. 54' Bertram

    54' Bertram Member

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    Dave, tell him more fuel than your Hatteras, but a much better ride in the rough stuff!
  13. Capt J

    Capt J Senior Member

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    On the cancun trip, you have to go slow because you can't carry enough fuel to run at cruise the entire trip, nor do you have enough daylight. So it is what it is.
  14. Pascal

    Pascal Senior Member

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    Nobody is assuming the buyer wants to know the fuel burn at 8 or 9kts, that was the original question!

    and in addition to stretching the range as Capt J mentioned, on a winter round trip from the North East to So FL or the bahamas, the difference between 9kts and 19kts is about $12k worth of fuel... that's worth going slow to some people.
  15. Capt J

    Capt J Senior Member

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    Tell me about it. hehehee..... I did the great loop (5300NM) on a 75' Hatteras MY and burned less then 7500 gallons for the whole trip. Not to mention we anchored 75% of the time on the trip with the generator running all night for simplicity. It took 3-5 minutes to drop the anchor. Versus 45 minutes of securing the boat, putting out fenders, shorepower cords, water hose.....each way. Had I run the boat at cruise everyday when I had the opportunity, we would've burned 25,000+ gallons. I saved the owner $80,000+ on fuel, with an additional crew expense of probably of less then $10,000 (extra days, extra crew, food etc).

    I did a straight shot from Ft. Lauderdale to Beaufort, NC at 10 knots for a little over 2 days and cut out over 250NM by following the coast, and we picked up another 2-3 knots from staying in the gulfstream the entire trip.
  16. CTdave

    CTdave Senior Member

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    HAH!!! Your not kidding!!!
    I just took my family for a four day weekend. We WERE headed to Block Is. the first day but East wind at 20 doesn't work so well on Long Island Sound. The 50Hatt was a bucking bronco! My wife called up to the bridge & asked me if we could go get the Bertram:eek:
    We ended up having to drop in to Northport after 30 or 40 minutes & then off to Greenport the following day when it was much nicer.
  17. CTdave

    CTdave Senior Member

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    Right on the money as usual Pascal.
    The interested party wanted to know the slow burn rate to get it from up here in CT to FL and then he is interested in Bahama runs.
    I've run the east cost in it as well as the Bahamas & I've never had the time to do it the slow way. In this economy, I'm sure plenty of people (including myself) are considering burn rates when thinking about long trips.

    Dave
  18. Pascal

    Pascal Senior Member

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    on the 70 footer that i run, i save about 3600 USG each way by doing most of the run from New England to Miami at 10kts instead of 22kts... that's $12k in fuel savings each way! Since i prefer anchoring for the reasons Capt J mentioned earlier, a few more days dont' make any difference since there are no extra marina nights.

    and it's also a lot less wear and tear on the engines...
  19. Capt J

    Capt J Senior Member

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    It also saves you 2 hours each day at the fuel dock, filling up the 1,000+ gallons you burned each day, and you only have to fuel every 5th day. If you're full time crew it really saves the owner money. A lot of times it also saves having to rinse and chamois the entire boat down.

    On the 103' Johnson I work on from time to time. Sometimes we run at cruise (19 knots) the entire day, then spend 2-3 hours topping off the fuel tanks, depending on the owners schedule. Then rinse/chamois for 2 hours, after getting all of the lines and fenders on and off each day. If we have the time we'll do 12 knots or less, and do 2 days at sea or so before stopping, and cover greater distances if we ran at cruise everyday. It saves us (crew) a lot of physical work as well as the owner a lot of money, so long as you have enough crew to cover the watches. It's easy do do a few watches everyday. I enjoy 10-12knots as most yachts are very stable at this speed and you can walk around, cook, or whatever without hanging onto everything.