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Fuel burn question

Discussion in 'General Yachting Discussion' started by Keysbearfl, Feb 21, 2012.

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  1. Keysbearfl, Feb 21, 2012
    #1
    Keysbearfl

    Keysbearfl New Member

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    I am looking at purchasing a 46' Betram with 8V92's. My question is; what is the fuel burn at cruise, and what would the fuel burn be, loafing to the Bahamas at 16-18 knots? My estimate is 50 GPH cruise, 35-40 gph @16-18knots, but I am unsure.

    Any insight yoiu can provide me is really appreciated.

    Thanks

    Keysbear
  2. Capt Ralph, Feb 21, 2012
    #2
    Capt Ralph

    Capt Ralph Senior Member

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    Need the hp tune or what horsepower you are suppose to be making each main.
    Rough (really rough) figure for burn in 1/10 gallon per hp per hour x .5 each.

    For example, 600 hp motor in the corner = 30 gph. Most boats are twins so 60gph per boat.

    It's not quite linear (told you it was rough) but if you figure it takes 50hp per main in gear at idle, ramp up from there to your hp rating.

    If you look at the boat test in the mags, it's pretty close.

    We drive a 58' Bert. Best slow hull speed is just when the second ripple comes from the bow. This is about 11 knots, 200 hp each, 2 gal per mile.

    Then the hump when you break from displacement to planing. Remember; IT'S a BERTRAM. Around 1600rpm, she may start planing. You just doubled your burn. At wot, and in tune, your hp rating per engine, is your gph. 600 hp each, around 60 gph. Speed is not a part of this.

    600 hp is an example. the 8v92 hp range started around 450 and went to mars depending on the marinizer and tune.
  3. Pascal, Feb 21, 2012
    #3
    Pascal

    Pascal Senior Member

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    "We drive a 58' Bert. Best slow hull speed is just when the second ripple comes from the bow. This is about 11 knots, 200 hp each, 2 gal per mile. "

    Hull speed for the average 58 footer is around 9.4kts. Yr fuel burn at that speed will be almost half what it is at 11kts... That will be yr most economical speed
  4. Mark Woglom, Feb 21, 2012
    #4
    Mark Woglom

    Mark Woglom Senior Member

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    I would have thought that hull speed (on most any 2x engine boat) would achieve much better than 2 gal per NM. I would guess the opposite ... closer to 2 NM per gal.

    My 55' Azimut burns 25 gph per side (50 gph total, 1950 RPM), doing about 25 kts. That equates to exactly 2 gal per NM, same fuel usage/hr as your 58' Bert @ 11 kts. I haven't really investigated my boat at hull speed, but I would presume (hope?) I would do MUCH better than 2 gal per NM.
  5. Pascal, Feb 21, 2012
    #5
    Pascal

    Pascal Senior Member

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    Mark

    I was quoting a previous reply (see "") to explain how 1.5kts above hull speed will result in almost double the fuel burn ...

    My 53 Hatt burns 9 Gph at 9kts...

    The 70 footer I run has digital engine controls and from 9.5 to 11 kts flow goes from 17gph to 35... While the boat is bigger, Basic physics don't change.
  6. dennismc, Feb 21, 2012
    #6
    dennismc

    dennismc Senior Member

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    That's a seriously low fuel burn on the Azimut ??
  7. Capt Ralph, Feb 21, 2012
    #7
    Capt Ralph

    Capt Ralph Senior Member

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    I don't see the Azimut's burn rate. But newer & hight tech may be more than I understand. I would think he has to be burning more than 500 hp to do 25knts

    I'm sure it will be a whole (WHOLE LOT) lot better than our 18' beam and twice the weight (or more) Ole Bert.

    I don't match the factory numbers exactly, but I do have the factory range chart from Bertram on our model. It's +/- thru the range.
  8. Capt J, Feb 21, 2012
    #8
    Capt J

    Capt J Senior Member

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    The 50' Azimut I managed with CAT's burned 45-50gph at 23-25 knots......
  9. Capt Ralph, Feb 21, 2012
    #9
    Capt Ralph

    Capt Ralph Senior Member

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    I wish I could burn only 50 gph at 25 knts (try 74 gph). On the other hand, I don't have to wait for weather windows much.
  10. airship, Feb 22, 2012
    #10
    airship

    airship Senior Member

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    "Fuel consumption" vs. "fuel burn"

    "Uhmmm, Houston (or wherever YF are based)? I've got a problem..."

    I can understand using the terminology "fuel consumption" for most terrestrial vehicles including boats and cars. But in future, could we please encourage YF contributors to reserve usage of the term "fuel burn" to NASA and ESA space rockets, hot-air balloons, aircraft, and (the very few) yachts equipped with gas-turbine plants...?

    Sorry, couldn't resist. Hat, coat, door... :eek:
  11. AMG, Feb 22, 2012
    #11
    AMG

    AMG YF Moderator

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    Do you have combustion engines that are not burning fuel?
  12. Loren Schweizer, Feb 22, 2012
    #12
    Loren Schweizer

    Loren Schweizer YF Associate Writer

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    keysbearfl:

    Not sure if the 8V-92s in your 46 are of the fuel-consuming version or the fuel-burn type, but the Official Bertram Yacht Specifications for a 1987 model equipped with the 600HP motors (or engines to avoid a quibble) are thus....

    [Full fuel & water, 8 people, 1200 lbs. of gear--no dink, no 25-foot tower, no 20 cases of beer, no 850-pound blue flopping around in the cockpit, etc....also, spare shafts , props, prep centers are weighty options that will increase fuel, uh, usage.]

    1300 RPM ---------- 28 GPH
    1500 --------------- 35
    1700 --------------- 42
    1900 --------------- 51
    2100 --------------- 59
    2300 (WOT) ---------68

    Interestingly, the local DDA tech guru once told me that these numbers were higher than the (theoretical) propeller-load figures, so YMMV.
  13. airship, Feb 22, 2012
    #13
    airship

    airship Senior Member

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    Nope. I just wanted to somehow differentiate between combustion engines (eg. on boats which usually just belch out white / grey / black smoke when running) compared to the other more (often spectacular) "combustion engines" which belch out huge flames and smoke off Cape Caneveral on occasions...

    PS. Unsure if the "combustion engines" and fuels used at Cape Caneveral rely on oxygen and can readily be compared to diesel engines.

    PPS. As a previously self-acknowledged "macho-man" myself, I can understand the confusion in using the proper terminology... :D
  14. AMG, Feb 22, 2012
    #14
    AMG

    AMG YF Moderator

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    I think they call it Rocket Science...:)
  15. Capt Ralph, Feb 22, 2012
    #15
    Capt Ralph

    Capt Ralph Senior Member

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    Loren's numbers should be dead on. Do you have any fast idle rpm / consumption numbers?
    These Ole Bert's rode great with some speed in their day. Fuel consumption was not of to great concern. Todays fuel cost keep the manatees happy, all we can do is idle around with a short dash thru the inlets.

    I am not a Rocket Scientist, I would like to play one on T V.
    We are boaters and not necessarily great communicators. Burn / consume, like calories, it all goes out an exhaust pipe of some kind somewhere.

    But for our fine rocket enthusiast out there, I'm sorry I used the wrong terminology and will try to do better next time.
    However, at these con$umption number$, we could post consumption by pound of fuel / hp. I have those numbers around here somewhere also.
  16. Capt Ralph, Feb 22, 2012
    #16
    Capt Ralph

    Capt Ralph Senior Member

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    I guess it only takes 500 hp to go fast on these newer boats. I have learned something new, again.

    Thx
  17. K1W1, Feb 22, 2012
    #17
    K1W1

    K1W1 Senior Member

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    Hi,

    What it needs to get where it's going will depend a lot on the displacement as well as the underwater shape and overall efficiency of the power plant and drive-line
  18. Mark Woglom, Feb 22, 2012
    #18
    Mark Woglom

    Mark Woglom Senior Member

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    Well, that's what it is. I have digital fuel flow gauges, and I was able to verify the flows on a recent calm water trip of about 6 hours. In that instance, I ran the boat at 2,000 RPM and cruised at about 27 kts. Fuel flow guages showed 28 gallons per side, and I took on 348 gallons upon arrival, after a 15 minute idle up the ICW.

    Engines are CAT C-15 Acerts, 715 HP per side.
  19. Mark Woglom, Feb 22, 2012
    #19
    Mark Woglom

    Mark Woglom Senior Member

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    No doubt. I've only been in rough seas with it for about 15 minutes. Came out of the Key West Northwest Channel headed for Ft Meyers, and found what I would guess were 6' to 7' head on seas. On plane, things were flying everywhere. I slowed down to about 10 knots, which was tolerable, but not fun.

    Had my wife and another couple on board. Figured 10 hours of those seas was going to destroy my boat, my marriage, or both. Turned around, flew home, and hired a captain to bring it north when things calmed down.

    Not a rough water boat ... but not a rough water family.
  20. AMG, Feb 22, 2012
    #20
    AMG

    AMG YF Moderator

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    My darling is taking just about half of that, or 5 l/NM at 30 knots... :)

    Delta 54.mov - YouTube
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