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Old 01-03-2022, 08:09 AM   #141
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Originally Posted by Vandeusen View Post
Just a quick question to see if somebody can help me figure this out since I'm learning about gallons per hour versus gallons per mile

If I have 100 gallons on board assuming I only use 80 to leave 10 gallons on the bottom of each tank and I average 1.8 gallons per hour how far would I be able to go on 80 gallons? Thank you

GPH is important because your speed over ground is not constant. This is where true navigation comes in. You have to consider current, winds, sea state, and distance to destination to figure how many hours it will take to get to your destination. Based on that you have to calculate your hourly burn rate, based primarily on RPM to determine the gallons you will consume on the voyage. I have made several trips of 300-400 miles between ports and were then able to figure when or if we had sufficient fuel to increase RPM/speed/burn rate and arrive ahead of sundown/weather/etc. It is actually fun once you get the parameters figured out. A working knowledge of EXCELL helps.
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Old 01-03-2022, 11:08 AM   #142
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I sounds like the methods described give you average GPH, but not so much insight into gph at various rpm increments. I'm just trying to get an idea how accurate the numbers are when handed to someone like the OP. So far, the number passed around have been all over the place, ranging from 2 nmpg to 4 nmpg.


I know a lot of manufacturers publish performance numbers, but wouldn't bank on them being correct. Most I have seen are wildly optimistic, and often times are just calculated estimates and not actual measured numbers.


For what most of us do, it doesn't really matter very much, but when someone is trying to calculate range, and doesn't have fuel stops in between, it suddenly matters quite a bit. We all know that a speed change of just 1/2kt has a significant impact on NMPG. If the numbers throw about are off by 1/2 or a 1/4 kt here, and a 1/2 or 1/4 gph there, is suddenly makes a big difference.


MVWeebles asked how accurate engine-reported fuel burn numbers are. From what I'm told, mostly pretty accurate. How's that for a non-answer? The only person I know who has actually checked was James Hamilton on Dirona. He carefully calibrated the small "burn tank" on his boat. It's about a foot tall and holds about a gallon of fuel, so graduation marks pretty small increments of fuel. He then did runs measuring the burn from the tank compared to what the engine ECU reported. As I recall he found that at higher loads it was spot on, but at light loads is was over-reporting how much was burned, and was off by as much as 10% in the worst case. But like a car speedometer, it's always better to report higher than actual.



What I do believe is that the ECU numbers are probably better than any other method, save for a carefully calibrated burn tank. But even a burn tank you need to be real careful with. A little change of heel, or sighting from a different angle can knock off the results. I suspect this is magnified significantly for something like the sight tubes on a main tank. One here was described as being 20l per inch. That would be really hard to read accurately to anything better than +/-2l or so, I would think. That's +/-10%, or an range of 20% that it could be off.
Our findings would agree with Dirona's. We have engineers who just love to test these things. We find on a normal burn of a tank down that our actual fuel mileage is slightly better than reported by ECU. Now, I do believe the variance for a displacement hull run at slow speed would have the maximum variance and, in this case, much like Dirona found, the actual efficiency is better than the ECU numbers. I attribute this to the return of fuel to the tanks being slightly more than measured.

We've also checked reported loads vs. measured by fuel flow and found very similar.

Now, to those without electronic fuel data, it's our impression, although we can't prove it, that many believe they're getting better fuel mileage than they actually are. Two reasons. First is bragging rights. Second is that they measure on long runs at ideal speeds, but tend to not take into account shorter runs with acceleration or cold engines warming up or runs into strong winds, waves and currents.

As to those numbers reported by manufacturers and by sites like boattest and Power and Motoryacht, we find those numbers very accurate for what they are. We only trust those that are actual runs, not those projected but never trialed. Also, we note the conditions. Here is a typical example:

LOAD 3 persons, 1/3 fuel, 50 lbs. of gear
CLIMATE 79 deg., 95 humid., wind: 5-10 mph, seas: calm

That just happened to be from a Swift Trawler 41. Easy to add 1000-1500 pounds of fuel and gear. 1500 pounds increases the weight by 5-6% and that's significant. Then add 3' seas and a 2 knot current or 15-20 mph winds.

So at 3000 RPM, that boat shows 18 knots and 21 gph or 0.86 nmpg, but I would expect at 3000 RPM in real life, they'll get more like 16 knots and 21 gph or 0.76 nmpg. That is a drop of 11-12%. This is where the problem with tests and builder supplied numbers, not their accuracy, but their misinterpretation or misuse.

Now, I must say, that all our builders numbers have been conservative. In actual usage, we get better fuel mileage than their data they gave us indicated. It's only a small amount but it is better. I attribute most of that to engines just getting properly broken in. After break in, we typically see improvements.

Now, I must give a caveat on builders. Reputable vs. questionable. Reputable builders don't want to give misleading numbers and their graphs are very accurate. However, there are many builders who toss numbers around freely without checking them. Most are smaller builders but then a couple of production builders do the same. They typical trawler builder gives fairly accurate numbers but do not trust the numbers they give prior to ever delivering a boat in a new model. I'm seen some wild sloppiness in those numbers.

One other warning. Don't forget to consider your fuel usage of your generator.
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Old 01-03-2022, 11:30 AM   #143
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Now, to those without electronic fuel data, it's our impression, although we can't prove it, that many believe they're getting better fuel mileage than they actually are. Two reasons. First is bragging rights. Second is that they measure on long runs at ideal speeds, but tend to not take into account shorter runs with acceleration or cold engines warming up or runs into strong winds, waves and currents.
Amen. When I was delivering, I stopped asking what fuel burn was. Owners were always wildly optimistic. Had I adhered to their guidance, I'd still be floating around the Pacific waiting for fuel.

Peter
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Old 01-03-2022, 01:12 PM   #144
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For those with engines that don't directly report their fuel burn, how are you measuring it? Do you have a calibration tank of some sort?
Quote from Weebles:
For most of us, the only practical method is what Psneeld reported towards the beginning of this thread. Measurement over many fill-ups with accurate record keeping to give a single average. Even then, it's his average so you'd have to know something about how he uses his boat - long runs, generator time, etc.


That's how I have done it. Accurate logging of NM traveled each day, compiled by Coastal Explorer set to detailed tracking of distance traveled over ground. Gallons of diesel used in tank after tank over thousands of NM. NMPG calculated every fillup and at the end of every summer's cruise.

Over the last six summers poking some 21,000 NM around the Inside Passage, almost always at 7-7.5 knots (speed over water), our NT 37's filllups ranged from 3.4 to 4.4 NMPG. Whole summers 3.7 - 3.8. The genset and heater used some of that diesel, but I have no guess as to how much.

Depending on current and sea conditions, GPH goes up and down, but I don't pay attention to it. Over the long haul these effects seem to average out. I can pretty much count on 3.5 NMPG or better. Seems to me that's what I need to know.
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Old 01-03-2022, 01:17 PM   #145
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Originally Posted by mvweebles View Post
Amen. When I was delivering, I stopped asking what fuel burn was. Owners were always wildly optimistic. Had I adhered to their guidance, I'd still be floating around the Pacific waiting for fuel.

Peter
The delivery captains I know will absolutely not trust owners when it comes to fuel and range. I remember one recently making a trip. He added four 55 gallon drums. The boat had fuel around 900 gallons. The owner insisted the drums weren't necessary. Well, used the four drums and when they reached the marina added 750 gallons of fuel, so used 970. Of course many running south in the spring offshore on the east coast, forget to consider the Gulf Stream.
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Old 01-03-2022, 01:24 PM   #146
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The delivery captains I know will absolutely not trust owners when it comes to fuel and range. I remember one recently making a trip. He added four 55 gallon drums. The boat had fuel around 900 gallons. The owner insisted the drums weren't necessary. Well, used the four drums and when they reached the marina added 750 gallons of fuel, so used 970. Of course many running south in the spring offshore on the east coast, forget to consider the Gulf Stream.
You are correct about the Gulf Stream. If you have decent info you can play the eddies. We once picked up 4 knots in an eddy.
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Old 01-03-2022, 04:23 PM   #147
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You are correct about the Gulf Stream. If you have decent info you can play the eddies. We once picked up 4 knots in an eddy.
Wifey B: Hit 58.7 knots under my helmsmanship as I found the stream and rode it long and hard the other day. Fastest run we'd had in the boat we ran in Europe all summer. When you're talking about a planing hull with very little draft, you pick up all the fastest moving water from the top.

Of course then a center console with 6 engines goes zooming past me, going much faster.
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Old 01-03-2022, 05:35 PM   #148
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Originally Posted by mvweebles View Post
As I've mentioned many times, a W36 went from LA to Hawaii. His model - a Blaine Sealey adaptation with pilot house and small sail plan - reportedly carries 300 gals (for comparison, my sedan version reportedly carries 500). He also carried a bladder with 100 gallons. In a letter to Willard he firmly states he burned 335 gals. But some of his numbers are off a bit - days at sea is missing, etc. In the end, when you do the simple math he burned 0.9 gph at 6/kts which is amazing - too amazing even for his 75 hp Perkins 4.236.
I don't understand your skepticism. Motor's running at ~15 HP, giving 6 knots. Sounds plausible to me. The trick would have been having the discipline to run at that speed all the way.

You asked about ECU reported numbers. I have a JD4045 M75 that gives fuel numbers. It's not common rail, but the ECU controls the mechanical injection. My reported numbers are 20-25% low. I have both cumulative and instantaneous reporting, and they always agree - if I run for an hour at constant throttle the cumulative increase matches the instantaneous reading, and I've done it at various throttle settings. But at fill up I consistently put in more than the reported consumption. I'm still puzzled and a bit disappointed by that. My hybrid cars have always been spot on.

It's still super useful though. Even if the absolute value is incorrect, it gives lots of insight into speed vs burn (hint: go slow), and I'm numerate enough to be able to apply a correction factor that gets me close to true numbers. I know within 5% how much fuel I'll take at fill-up.
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Old 01-03-2022, 06:15 PM   #149
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I don't understand your skepticism. Motor's running at ~15 HP, giving 6 knots. Sounds plausible to me. The trick would have been having the discipline to run at that speed all the way.
I just don't think a 30k lb boat can sustain 0.9 gph over that type of distance in open water - LA to Honolulu. 1987 - measurements were not as precise as today. 6-kt avg is pretty fast. Good news is a W36 easily made the crossing. No one seems to know how it made the return - she's been in the PNW for years.

Peter
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Old 01-03-2022, 07:17 PM   #150
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I just don't think a 30k lb boat can sustain 0.9 gph over that type of distance in open water - LA to Honolulu. 1987 - measurements were not as precise as today. 6-kt avg is pretty fast. Good news is a W36 easily made the crossing. No one seems to know how it made the return - she's been in the PNW for years.

Peter
Easy Peter, they just kept going west!

Also, with any luck you can pick up almost a knot speed over ground
from the current plus wind from L.A. to Hawaii.
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Old 01-03-2022, 10:27 PM   #151
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Nobody needs to know exactly what their gph is.

But it can be very useful to know approximately what it is.

I used my first trip to Alaska to determine my burn rate and it was loaded w irregularities .. but all of them were small or less. The biggest variable was including time running time (hour meter) at less than cruising speed but it was small too. A much smaller variable than w my typical cruising. So other than using time running at very low throttle settings my computations should be quite accurate. I came up w 1 gph. In the years after I compared my number to gph posted everywhere and was pleased to notice how close to other calculations on the same issue.

Most relative to the above is my cruising style. Much more casual than most here on TF. On a typical day I’m not sure where we’re going to anchor or tie up for the night. Most would be at least a bit uncomfortable w that. But I spend a lot of time underway surfing the GPS plotter for possible places to overnight.

Now that you're privy to my cruising style you can see how knowing my fuel burn accurately would not be valuable in great detail. But re the fuel burn (with 100gallons) and a burn rate of 1 gph very little accuracy is required. Fits my style perfectly.

It’s not totally uncommon for me to change course and go to a different destination. I only really know two speeds for my boat. Cruising speed and high cruising speed. 2300rpm and 2500rpm. Top speed? I really don’t know. I’m sure it’s very close to 7 knots. The only speed I really need to know is cruising speed .. 6.15 knots.

One place I can use a great deal of information w details is in TF posting. Often I go to the computer room to look up information. Like finding the name of places and correct spelling.
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Old 01-03-2022, 11:24 PM   #152
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My GPH average over three years is 10.10L or 2.6 US Gal with twin 120 Lehman. 1.3 GPH per engine appears to be a good burn. Oh that includes about 60 days with diesel heater running. I am pleased with the cost of fuel to operate at 7-8 knPH
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Old 01-04-2022, 06:50 AM   #153
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My GPH average over three years is 10.10L or 2.6 US Gal with twin 120 Lehman. 1.3 GPH per engine appears to be a good burn. Oh that includes about 60 days with diesel heater running. I am pleased with the cost of fuel to operate at 7-8 knPH
Cost of fuel is going to be much higher this year than last. I hope you will still be pleased.
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Old 01-04-2022, 07:24 AM   #154
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Cost of fuel is going to be much higher this year than last. I hope you will still be pleased.
B&B, he will be much happier than some one who burns twice as much.

As we all have said many times before, 'If we can't afford the fuel, we sure can't afford a boat.'
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Old 01-04-2022, 07:30 AM   #155
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B&B, he will be much happier than some one who burns twice as much.

As we all have said many times before, 'If we can't afford the fuel, we sure can't afford a boat.'
I get it and I will still afford the fuel this year, just saying it will be more than last year and quite a bit more than a couple years ago.
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Old 01-04-2022, 07:48 AM   #156
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B&B, way way back when there was a gasoline shortage, some stations were selling 'futures'. I'm not sure how that worked out.
If you feel confident of a significant price increase, negotiate a price with your 'home' fueling dock for some predetermined volume of fuel.
Let us know how that works out for you. Perhaps we shall all go that route.
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Old 01-04-2022, 07:55 AM   #157
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B&B, way way back when there was a gasoline shortage, some stations were selling 'futures'. I'm not sure how that worked out.
If you feel confident of a significant price increase, negotiate a price with your 'home' fueling dock for some predetermined volume of fuel.
Let us know how that works out for you. Perhaps we shall all go that route.
Probably won't do that as I don't use 1000's of gallons of fuel, but I will be happy to launch this spring with tanks already full!
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Old 01-04-2022, 10:10 AM   #158
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My GPH average over three years is 10.10L or 2.6 US Gal with twin 120 Lehman. 1.3 GPH per engine appears to be a good burn. Oh that includes about 60 days with diesel heater running. I am pleased with the cost of fuel to operate at 7-8 knPH
My 29-year average with a 40,000-pound (actual scale weight) Grand Banks 42 equipped with twin FL 120s and a 7.5 KW Onan was 3.25 GPH running at betweek 1600-1700 RPM making 8.4-8.6 knots. So, you can see the diff in boat size with same engines.
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Old 01-04-2022, 10:24 AM   #159
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My 29-year average with a 40,000-pound (actual scale weight) Grand Banks 42 equipped with twin FL 120s and a 7.5 KW Onan was 3.25 GPH running at betweek 1600-1700 RPM making 8.4-8.6 knots. So, you can see the diff in boat size with same engines.

I am encouraged by all your responses. After a life of sailing I bought a GB36 in Nov 2020. It is on the hard for a 2 year refit. To the point, I have no experience with burn rates, but the boat's PO was a day cruiser and he thought 5-5.5 GPH for both engines. I didn't ask specifics of RPM/Speed but I would be thrilled to burn only 3.25 at the speeds indicated here.
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Old 01-04-2022, 10:32 AM   #160
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Nobody needs to know exactly what their gph is. But it can be very useful to know approximately what it is.
After 10 diesel powered boats and cruising the SoCal waters for 25 years, I agree with the above quote.
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