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Old 15-06-2016, 10:04   #1
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A theoretical fuel usage discussion. Please check my logic.

I'm new to all of this so thanks for putting up with the questions. Your answers are helping more than you know.

So, someone provided me with this very interesting link where you can punch in the type/size of engine and get some idea of GPH usage at various RPMs. Boat Fuel Economy | Outboard motors and Boat motors | MerCruiser | Mercury | Evinrude | Yamaha | Suzuki | Honda | Volvo Penta Diesel Marine engines

One of the boats under consideration has the big block, gas Volvos in it so we dialed in the 375 hp 8.1 engine on the website and found that the sweet spot seemed to be around 3000 RPM where it was burning about 11.5 gph. Now, lots of things come into play so this is just a theoretical discussion but, that would be 11.5 per hour, per engine, or 23 gph total. Right? Cruising speed is said to be 22 knots or 25 mph. And with a 280 gal fuel tank, that would give a theoretical running time of 12 hours and a range of about 300 miles. A little over a gallon per mile. Faster cruising speed but not as much range.

Another boat we're looking at has 300 hp diesels that are said to have a cruising speed of 16 knots or about 18.5 mph at 2300 rpm. Using the same calculator, we dialed it in and saw that 2300 rpms is roughly 7.5 gph per engine or 15 gph total. This boat carries 400 gallons of fuel. That's roughly 26 hours of running time for a range of 481 miles. Less than a gallon per mile. Slower cruising speed but a much longer range.

Lots of things come into play. The generator will use some of the fuel, sea conditions may not allow running at full cruising speed, etc.

Am I on track with the logic above or did I miss something really pertinent?

Thanks.

John
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Old 16-06-2016, 08:33   #2
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Re: A theoretical fuel usage discussion. Please check my logic.

Quote:
Originally Posted by JohnEasley View Post
Now, lots of things come into play so this is just a theoretical discussion but, that would be 11.5 per hour, per engine, or 23 gph total. Right? Cruising speed is said to be 22 knots or 25 mph. And with a 280 gal fuel tank, that would give a theoretical running time of 12 hours and a range of about 300 miles. A little over a gallon per mile. Faster cruising speed but not as much range.
Hi John,

You have the general idea right. Keep in mind that you really will never get 300 gallons of fuel out of a 300 gallon tank, so most range calculations are done planning on a 10% reserve. In the case of your 280 gallon tank above that would make 252 gallons of fuel available. Cruising at 22 knots, burning 23 GPH is just shy of 11 hours (10.95), multiplied by your speed would give a range of 241 miles.

Typically, you can extend the range, significantly, by slowing down to 7 knots or so.

Good Luck,
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Old 19-06-2016, 11:58   #3
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Re: A theoretical fuel usage discussion. Please check my logic.

Not all engines are the same. Some, especially medium or heavy duty engines, get better fuel use than typical light duty yacht engines.
Another consideration is time between overhauls. Exhaust gas temperatures have a lot to do with how many hours the rings, sleeves, pistons and valves last. Engines with a turbo used at near full throttle have high EGTs. Running at 80% of power or less can double or triple engine life. Also engines that develop their hp at lower rpm usually use less fuel and go longer between overhauls.
I have a 83' boat of 80 tons. I run non-turbo Detroit Diesel twin engines that use 8.5 total gallons per hour at 10 knots. With the turbo version I could do 20 knots at 60 total gallons an hour. My engines need an overhaul every 20+ years for the 500 hours/yr. I do. The turbo version at 20 knots would have to be overhauled every other year. At 15 knots probably every 5 years. I've been doing my own overhauls and others for about 50 years.
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Old 19-06-2016, 12:23   #4
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Re: A theoretical fuel usage discussion. Please check my logic.

Yes some things are not obvious. While working for a boat builder I collected test results of 250 hp outboards. There really was very little difference among manufacturers. The most fuel efficient was running at cruise rpm , maybe 3500, cant remember for sure.... drinking about 12.5 gallons an hour per engine. Efficient because you were going 30 miles or more an hour and getting further on each gallon of gas!. This excluded just idling along at 5-6 mph hour though.
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Old 19-06-2016, 14:54   #5
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Re: A theoretical fuel usage discussion. Please check my logic.

I took a quick look. I didn't see where the boat details are shown.

We had a 31' flybridge with a pair of Chrysler 360's. It got about 1.5 miles per gallon at 25mph. Any faster really just increased the fuel consumption drastically.

From discussion, by the time you get up to 40', you are probably down to 1.0 MPG.

With any of them, if you slow down to around 80% of hull speed, you get your best MPG but will only be going 6-8 mph. There you might jump up to 2 to 3 MPG.

Unless you get to the extremes, fuel consumption correlates almost exactly with HP generated regardless of engine size.
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Old 19-06-2016, 19:27   #6
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Re: A theoretical fuel usage discussion. Please check my logic.

Quote:
Originally Posted by Lepke View Post
Not all engines are the same. Some, especially medium or heavy duty engines, get better fuel use than typical light duty yacht engines.
Another consideration is time between overhauls. Exhaust gas temperatures have a lot to do with how many hours the rings, sleeves, pistons and valves last. Engines with a turbo used at near full throttle have high EGTs. Running at 80% of power or less can double or triple engine life. Also engines that develop their hp at lower rpm usually use less fuel and go longer between overhauls.
I have a 83' boat of 80 tons. I run non-turbo Detroit Diesel twin engines that use 8.5 total gallons per hour at 10 knots. With the turbo version I could do 20 knots at 60 total gallons an hour. My engines need an overhaul every 20+ years for the 500 hours/yr. I do. The turbo version at 20 knots would have to be overhauled every other year. At 15 knots probably every 5 years. I've been doing my own overhauls and others for about 50 years.
That's great information and a powerful argument for going diesel. Thanks.


Quote:
Originally Posted by Cheechako View Post
Yes some things are not obvious. While working for a boat builder I collected test results of 250 hp outboards. There really was very little difference among manufacturers. The most fuel efficient was running at cruise rpm , maybe 3500, cant remember for sure.... drinking about 12.5 gallons an hour per engine. Efficient because you were going 30 miles or more an hour and getting further on each gallon of gas!. This excluded just idling along at 5-6 mph hour though.
Interesting info about outboards. Thanks.
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Old 19-06-2016, 20:13   #7
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Re: A theoretical fuel usage discussion. Please check my logic.

Theoretically, you get about twice the energy from a gallon of diesel as from a gallon of gasoline. That's due to the chemistry of breaking the molecules in combustion. There are lots of other advantages to diesel, a lively topic in this forum, unless you need a high power to weight ratio, in which case outboards win. Once you top hull speed (1.34 times the square root of water line length, answer in knots, or 8 knots for a 36' waterline boat) you need a lot of fuel for a small gain in speed because you are climbing your bow wave. With a 60,000 pound diesel boat, I get two nautical miles to the gallon at 7.5 knots, and I doubt that I will live long enough to need an overhaul.
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Old 19-06-2016, 21:26   #8
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Re: A theoretical fuel usage discussion. Please check my logic.

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Originally Posted by tkeithlu View Post
Theoretically, you get about twice the energy from a gallon of diesel as from a gallon of gasoline. That's due to the chemistry of breaking the molecules in combustion. There are lots of other advantages to diesel, a lively topic in this forum, unless you need a high power to weight ratio, in which case outboards win. Once you top hull speed (1.34 times the square root of water line length, answer in knots, or 8 knots for a 36' waterline boat) you need a lot of fuel for a small gain in speed because you are climbing your bow wave. With a 60,000 pound diesel boat, I get two nautical miles to the gallon at 7.5 knots, and I doubt that I will live long enough to need an overhaul.
Wow. There is SO much about this that I still don't know. I do love the physics aspects of just about any though. Really good information. Thanks. Definitely diesel.
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Old 20-06-2016, 04:50   #9
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Re: A theoretical fuel usage discussion. Please check my logic.

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Theoretically, you get about twice the energy from a gallon of diesel as from a gallon of gasoline. That's due to the chemistry of breaking the molecules in combustion. There are lots of other advantages to diesel, a lively topic in this forum, unless you need a high power to weight ratio, in which case outboards win. Once you top hull speed (1.34 times the square root of water line length, answer in knots, or 8 knots for a 36' waterline boat) you need a lot of fuel for a small gain in speed because you are climbing your bow wave. With a 60,000 pound diesel boat, I get two nautical miles to the gallon at 7.5 knots, and I doubt that I will live long enough to need an overhaul.
No diesel gets you 20-40% improvement and much of that is the diesel engine being more efficient at converting fuel to mechanical energy. There is no where close to twice the energy in diesel.

When talking about a sailboat or single engine trawler (small engine) never intended to go fast, diesel is the more efficient and the extra weight of a diesel is not a big deal. When you start talking at larger twin engine boats designed for higher speeds, the weight of those big diesels need to be accounted for.

With modern fuel injected gas engines, the fuel efficiency is still better for diesel but the gap has narrowed significantly.
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Old 20-06-2016, 05:12   #10
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Re: A theoretical fuel usage discussion. Please check my logic.

Oooo...big block Volvos? Any $avings in fuel economy will be quickly eaten up in repairs and maintenance. Do you homework on the engines maintenance records and talk with little cal mechanics.
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Old 20-06-2016, 06:14   #11
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Re: A theoretical fuel usage discussion. Please check my logic.

You are right, Valhalla360. My aged brain mis-remembered my chemistry of long ego (I got a "C"). Diesels are more fuel efficient, but not by that much. They are also more expensive initially, heavier, more reliable, and safer. Some have argued that they are more expensive to repair, but I have not found that to be the case, and they need repair far less often. To me, they mean not having to deal with gasoline in an engine room.
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Old 20-06-2016, 06:29   #12
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Re: A theoretical fuel usage discussion. Please check my logic.

My wife, who is a chemist but not in that area, says that diesel is more efficient in combustion than gasoline, because the molecules are longer and the heat produced is dependent on the number of bonds broken in converting that long molecule into CO2 and water. She can't put a number on it, and there are lots of other factors to consider at the practical level.
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Old 20-06-2016, 06:51   #13
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Re: A theoretical fuel usage discussion. Please check my logic.

Note that fuel consumption curves are dyno derived. Ie they are brake specific fuel consumption curves.

Fuel quality also varies greatly. Old, stale fuel can have 30% or more loss of energy density due to loss of the most volatile elements. Buying fuel from bowsers which have high turnover nearly eliminates this problem.

You will need to calibrate your specific vessel in terms of prop and torque curve match.

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Old 20-06-2016, 06:55   #14
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Re: A theoretical fuel usage discussion. Please check my logic.

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Originally Posted by valhalla360 View Post
No diesel gets you 20-40% improvement and much of that is the diesel engine being more efficient at converting fuel to mechanical energy. There is no where close to twice the energy in diesel.

When talking about a sailboat or single engine trawler (small engine) never intended to go fast, diesel is the more efficient and the extra weight of a diesel is not a big deal. When you start talking at larger twin engine boats designed for higher speeds, the weight of those big diesels need to be accounted for.

With modern fuel injected gas engines, the fuel efficiency is still better for diesel but the gap has narrowed significantly.
Actually the diesel pressure volume curve is less efficient than a petrol engine. This is offset by the higher compression of a diesel which has much better thermal efficiency (in the combustion chamber) than a petrol engine.

Diesels don't use fuel for cooling. They are fuel regulated whereas a petrol engine is air regulated and fuel vaporisation helps with charge cooling. This ensures diesels are more precise in their consumption of fuel leading to more consistent fuel consumption.

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Old 20-06-2016, 10:49   #15
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Re: A theoretical fuel usage discussion. Please check my logic.

Don't forget the old Detroit Diesels like mine - they do cool their injectors with fuel, and circulate far more back to the tank as a result. On long runs, I actually end up with tanks that are hot to touch.

A very complicated question, and it's been chewed over in previous threads. I don't try to exceed hull speed and engine weight is a small contribution to my overall weight (less than 2%), so for me the decrease in fuel consumption and not wanting explosive gases in my engine room means diesel.
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