GPH vs Miles help??
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psneeld
Guru
Sure, but not burning 1.8 gph.
psneeld
Guru
Still a nope in my experience.
Sure for periods with a lot of current pushing you, but I seriously doubt it will ever average that over 100's of miles.
1800 rpm/ 1.8 gph sounds near but low gph for a Lehman 120 and that hull isn't gonna do over 8 knots without pushing water and burning more fuel. Unless a lot of posters for years have been incorrect.
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Iggy
Guru
If it was me....... I would only plan to use 2/3 or at most 3/4 of my fuel to make the trip. And I would lean on the 2/3. One lesson I learned when I first started boating is that I went out with enough fuel to get he out and back. What I did not plan on is the weather turned hitting wind and waves. I burnt up more fuel and just made it back.
mvweebles
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Quick clarifications. This thread has intermingled MPH with knots. There is roughly a 15% difference. 11 mph is roughly 9-1/2 kts.
Second, nomenclature on sometimes oddball nautical terminology.. Knots are knots, not knots per hour. Nautical miles per hour is correct, but knots per hour is not. This is because the old sailors used to toss a line over the stern with knots tied at regular intervals. They would measure how many knots passed at a defined time.
Your boat is happy at around 7 to 7-1/2 kts and will burn around 2 gph. You could go a bit faster and burn 4-5 gph at wide open throttle
Peter
psneeld
Guru
You seem to be missing the whole formula of as speed increases, mileage goes down (like in a car).
So you can turn for mileage or speed...whatever you like.
The most efficient range/mileage for your boat is going to be somewhere just below this formula
speed of the boat in knots = the square root of the waterline length times 1.34.
My guess is RPMs around 1600-1700 and your boat should burn 1.8-2.0 gph and make somewhere between 6 and 7 knots...some claim 7.5 but I am skeptical.
My 40 with a 34 foot waterline and 120 Lehman... over last 20,000 miles does 6.3 knots at 3.2 NMPG at 1650 RPM.
Benthic2
Guru
Here's a fuel useage graph for a Ford Lehman 135 on a slightly larger boat. Notice that at 7 knots he's buring about 1.8 gallons per hour, but at 9 knots he's burning 5.5 gallons/hour. That's 3 times as much fuel usage for an increase of just 2 knots.
You may want to keep a fuel log where you record your time, distance and refueling amounts. Eventually you will have enough data that you won't have to make assumptions.
( Graph copied from fellow member Larry M)
You may want to keep a fuel log where you record your time, distance and refueling amounts. Eventually you will have enough data that you won't have to make assumptions.
( Graph copied from fellow member Larry M)
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psneeld
Guru
Different engine, different boat (if Larry's KK42...it's also a much longer waterline and full displacement versus SD)
I have the charts for a 120 that is accurate at RPM (except we know that there are different fuel burns at a given RPM depending on prop loading).... Almost always my actual was 0.2-0.3 gph higher than the factory charts.
And yes..... I had no real clue what most efficient fuel burn and/or efficient CRUISING speed was until I logged several thousand hours, a thousand or more gallons of fuel and many thousands of miles.
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PMF1984
Guru
As ps says “Actual milage may vary.”
So you have to figure it out yourself for your boat.
I did on my Westerbeke 100. At 7.2 knots, I burn 2.4 gallons an hour. At 6 knots, I burn 1.4 gallons an hour. You can do the math, but why bother; that’s just for my boat.
So you have to figure it out yourself for your boat.
I did on my Westerbeke 100. At 7.2 knots, I burn 2.4 gallons an hour. At 6 knots, I burn 1.4 gallons an hour. You can do the math, but why bother; that’s just for my boat.
backinblue
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Just curious what prompted this question. Does the OP plan on routinely taking trips that would take him from full tanks to near empty (i.e. 400 miles)? And unless really necessary, I wouldn't want several 5 gallon jugs on board as a regular thing. You generally don't want to have full tanks for short trips because you are carrying extra weight for no reason. I'm with Soo on this one and generally run in the middle 1/3 of capacity unless planning a long trip. A rule of thumb I've heard is that for a given trip, you should think in terms of thirds when it makes sense to do so. That means 1/3 to get there, 1/3 to get back, and 1/3 reserve. So if you are going somewhere that takes 20 gallons to reach, you should leave with 60 gallons of fuel in your tanks. Of course YMMV!
rcsomany
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Combined approach speed of 17 kts means they will meet in 58.82352941176471 hours at which time the slower vessel would have travelled 470.5882352941176 nm and the faster one 529.4117647058824 nm. Since neither vessel is likely to be capable of instantaneously reaching cruising speed or instantaneously stopping, I'd truncate to 470 and 529 NM with the last mile for manoeuvring.
And if a fly flew at 15.7 kts from bridge to bridge, out and back repeatedly, starting when the vessels started and stopping when they met, how far would the fly have flown - assuming that, unlike boats, flies can start, stop, and reverse direction instantaneously.
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friz
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This is a great discussion! Probably helps more of us than are willing to admit.
Exactly as your emoji, Huh?
RedRascal
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OP, I would second this opinion on speed and fuel burn. I think you may have a calculation that is off. 1.8 gallons per hour doesn't seem realistic at 9 mph for that boat unless you were going with a current.
I would calc your range at 320 miles.(80 gallons / 2 gallons per hour = 40 hours of run time X 8 miles per hour) 320 miles is still a pretty good run.
I'd run with a lower estimate than what you calculated until you are able to check your fuel burn and distance again.
OldDan1943
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This thread is fantastic especially since my math gene has died at least 10 years ago.
I do have this boats commissioning chart (gph vs speed for the Cummins 5.9L QSB but, the discussion gives gives me hope when it comes to the real world.
Of course, I do need to make allowances for the running generator of about .8gph.... That is a variable I can control.
I do have this boats commissioning chart (gph vs speed for the Cummins 5.9L QSB but, the discussion gives gives me hope when it comes to the real world.
Of course, I do need to make allowances for the running generator of about .8gph.... That is a variable I can control.
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BonesD
Senior Member
At exactly 40 4444 hours after leaving the dock, check your gps and compare present position compared to leaving position. Then look around and see if the 11.111 hours of fuel you have left will get you where you wanted to go ?
Capt Dan
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Then why not have 40 gallon tanks?
backinblue
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Seems obvious that you wouldn't want to run your tanks dry regardless of size.
backinblue
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BTW, the math problems, including the fly are pretty basic grade-school level problems. No disrespect to anyone, but you're not a math genius even if you carry the answer out to many decimal places.
mvweebles
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I am going to back away from my estimate of 2 gph at 7 to 7.5 kts. Psneeld chimed in afterwards about his long term records with a very similar boat being closer to 2 gph at 6.3 kts. While the speed sounds low to my ears, he has a ton of experience and owners always estimate with optimism - opposite of what I'm thinking Psneeld has done which gives me pause (and I mean always). Plus I think 7.5 kts is too fast for a 34 footer in displacement mode.
Somewhere in the range of 6.5 - 7.0 kts at 2 gph will likely be a damn close overall planning metric. Won't be too far off in either direction. Given the 80 gals of usable fuel, it's what I would plan and adjust as more data is available.
Peter
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psneeld
Guru
I might agree 6.5-7.0 kts at 2 gph is accurate for flat water, no current and a perfectly setup engine/ drive train.....
Good luck averaging that over any decent longer trip....
Every boat is different, but most aren't that different from the many posts in the past, here and elsewhere, on the average Ford 120 fuel consumption and efficient speeds for the 32 to 42 foot Taiwan Trawlers. Single vs twins though is hard to nail down IMO as even a few rpm per engine can drastically change fuel consumption and speed numbers...
Plus I sincerely doubt the accuracy of many tachometer as I had an older one and replaced it and have been chasing it for years with alternator changes...etc...
Good luck averaging that over any decent longer trip....
Every boat is different, but most aren't that different from the many posts in the past, here and elsewhere, on the average Ford 120 fuel consumption and efficient speeds for the 32 to 42 foot Taiwan Trawlers. Single vs twins though is hard to nail down IMO as even a few rpm per engine can drastically change fuel consumption and speed numbers...
Plus I sincerely doubt the accuracy of many tachometer as I had an older one and replaced it and have been chasing it for years with alternator changes...etc...
contingensea
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So, if I understand this correctly, my range is…
600 gal - 10% = 540
540\2 gal hr = 270 @ 10 mph
270*10 = 2700 mile range
Not bad for a couple of 375 hp 3208’s ?
600 gal - 10% = 540
540\2 gal hr = 270 @ 10 mph
270*10 = 2700 mile range
Not bad for a couple of 375 hp 3208’s ?
Roger Long
Senior Member
Pilots have died because the wind was on their beam and they didn't think it would speed them up or slow them down. Whether you fly a constant compass course, end up downwind of the destination and fly upwind to it, or make the navigational correction to proceed on a straight line to it; the wind will blow you a certain distance sideways and you will have to burn the fuel to make it up. Same thing happens to a boat in current but boats usually don't cut their fuel reserves as close so that they notice.
mvweebles
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I assume this is tongue-in-cheek as it is wildly optimistic. 375 hp 3208s aren't exactly the most efficient engines, and they aren't in the same size class as the FL 120 the OP has.
A pair of 3208s pushing a GB42 at 8.5 to 9-kts (10 mph) will burn (combined) closer to 7-8 gph (educated guess). Puts overall range in the 650-ish nm range plus reserve. And I wouldn't plan on over 500 (1 nm per gal) until I had more accurate data.
Peter
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