Just bought a '75 30' Hunter equipped with a 25 HP Universal Diesel. Getting ready to run it up the Chesapeake from the Northern Neck to our home marina in Deale, MD. Curious as to whay kind of range we might expect from the engine - fuel capacity is listed at 12.5 gallons. Weather forecast isn't very attractive so we might be motoring a bit. Any insight into fuel efficiency wold be most appreciated as this is our first diesel engine! Sam
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Rough estimate...
1 gph, or 12 to 15 hours. Depending on load amount of throtle, current and winds. pat McCartin Inland Marine Diesel Buford, Ga imd_ga@hotmail.com
1 gph, or 12 to 15 hours. Depending on load amount of throtle, current and winds. pat McCartin Inland Marine Diesel Buford, Ga imd_ga@hotmail.com
0.5 GPH, Will Run All Day
Should move the boat 6.5 knots for 25 hours until you run out of fuel which will be over 150 miles
Should move the boat 6.5 knots for 25 hours until you run out of fuel which will be over 150 miles
Currents and wind and miles travelled do not
influence or affect burn rate. Fouled bottoms, props, currents and wind will influence how much distance you can cover on a given amount of fuel but the burn rate of the motor in GPH is pretty much a constant dependant on the size and condition of the engine. That is why in a boat we measure fuel consumption in hours instead of miles travelled. Sam, I would suggest you determine what is your specific burn rate which can be easily calculate by doing the following; 1) Know the size of your fuel tank in gallons and fill it up. 2) Maintain a record of your motoring time until you accumulate 10 hours. Determine how many gallons it took to fill the tank back up. 3) Divide the number of Gallons needed to refill tank by the number of hours motored, in this case 10, and the result will be your average burn rate in GPH for the hours motored. The avearge will take into account RPM changes in normal motoring. There is no exact figure but the result of this exercise will give you a number you can work with. Example, .7 GPH burn for an 18 gallon tank will give you approx. 26 hrs of motoring on a single tankful. The distance to be travelled will depend on conditions, but you will know the engine will run for approximately 26 hours. In any instance when planning fuel requirements for any leg of a trip allow yourself at least a 20% reserve (in hours) in case you do run into strong currents and headwinds. If you cannot complete a leg without refueling or dipping into you reserve carry additional fuel in a jerry-can on deck.
influence or affect burn rate. Fouled bottoms, props, currents and wind will influence how much distance you can cover on a given amount of fuel but the burn rate of the motor in GPH is pretty much a constant dependant on the size and condition of the engine. That is why in a boat we measure fuel consumption in hours instead of miles travelled. Sam, I would suggest you determine what is your specific burn rate which can be easily calculate by doing the following; 1) Know the size of your fuel tank in gallons and fill it up. 2) Maintain a record of your motoring time until you accumulate 10 hours. Determine how many gallons it took to fill the tank back up. 3) Divide the number of Gallons needed to refill tank by the number of hours motored, in this case 10, and the result will be your average burn rate in GPH for the hours motored. The avearge will take into account RPM changes in normal motoring. There is no exact figure but the result of this exercise will give you a number you can work with. Example, .7 GPH burn for an 18 gallon tank will give you approx. 26 hrs of motoring on a single tankful. The distance to be travelled will depend on conditions, but you will know the engine will run for approximately 26 hours. In any instance when planning fuel requirements for any leg of a trip allow yourself at least a 20% reserve (in hours) in case you do run into strong currents and headwinds. If you cannot complete a leg without refueling or dipping into you reserve carry additional fuel in a jerry-can on deck.
A five gallon Jerry can
will make all this fuss un necessary. You will likely be able to sail at least some of the way. Re fuel when you stop for a pumpout or for the night if you don't anchor out. Edit to add: The wonder full thing about diesels is that they burn about 1 cupfull of fuel per hour per horse power. So depending on your system you could use 1 1/2 to 3 quarts per hour. Throttle back, a little go slower, burn less fuel, take longer to get there. Speed costs money.
will make all this fuss un necessary. You will likely be able to sail at least some of the way. Re fuel when you stop for a pumpout or for the night if you don't anchor out. Edit to add: The wonder full thing about diesels is that they burn about 1 cupfull of fuel per hour per horse power. So depending on your system you could use 1 1/2 to 3 quarts per hour. Throttle back, a little go slower, burn less fuel, take longer to get there. Speed costs money.
The M25
at cruising range will burn 0.4 to 0.5 gph. I have measured this for the past 7 3/4 years, since we do not have, nor desire, a working fuel gage. All comments about distance traveled and bottom condition are valid. Stu
at cruising range will burn 0.4 to 0.5 gph. I have measured this for the past 7 3/4 years, since we do not have, nor desire, a working fuel gage. All comments about distance traveled and bottom condition are valid. Stu
Run Hard
Reducing RPM to save fuel is not a good thing to do. Diesels like to run at 80-85% of rated output. The money you save in fuel at lower RPM is not worth the shorter life of the engine that you will cause. Putting along at a low RPM will KILL your diesel and you will still use almost as much fuel because it will pass unburned through the exhaust. This unburned fuel also will coat the inside of the engine with hard enamaled carbon. That is what kills diesels. If your diesels max output is 3500 RPM you could start the engine run it up to 3000 and never turn it off. The engine would most likely run for 15,000-20,000 hours, no problem. If you run your diesel at 2000 RPM for 2-4 hour per month like most sailboaters do you will be luck to get 1000 hours on it before it needs a major rebuild. Run it hard, heat it up, It will be your friend.
Reducing RPM to save fuel is not a good thing to do. Diesels like to run at 80-85% of rated output. The money you save in fuel at lower RPM is not worth the shorter life of the engine that you will cause. Putting along at a low RPM will KILL your diesel and you will still use almost as much fuel because it will pass unburned through the exhaust. This unburned fuel also will coat the inside of the engine with hard enamaled carbon. That is what kills diesels. If your diesels max output is 3500 RPM you could start the engine run it up to 3000 and never turn it off. The engine would most likely run for 15,000-20,000 hours, no problem. If you run your diesel at 2000 RPM for 2-4 hour per month like most sailboaters do you will be luck to get 1000 hours on it before it needs a major rebuild. Run it hard, heat it up, It will be your friend.
run hard??
A diesel squirts just enough fuel to make the engine run at a set RPM. If the engine can't achieve a given RPM it squirts fuel in excess making black soot and possibly overheating. I agree that an engine should be ran at a reasonable speed but hammering down with a dirty hull/prop or trying to go faster than hull speed probably puts the engine into an overload situation. Talking to a diesel mechanic he said that overload or unload can kill a diesel but that overload kills more quickly. We were specifically talking about too much prop. But a perfectly matched prop with a clean hull could be overpropped with a dirty hull.
A diesel squirts just enough fuel to make the engine run at a set RPM. If the engine can't achieve a given RPM it squirts fuel in excess making black soot and possibly overheating. I agree that an engine should be ran at a reasonable speed but hammering down with a dirty hull/prop or trying to go faster than hull speed probably puts the engine into an overload situation. Talking to a diesel mechanic he said that overload or unload can kill a diesel but that overload kills more quickly. We were specifically talking about too much prop. But a perfectly matched prop with a clean hull could be overpropped with a dirty hull.
Should be 1/2 to 2/3 Gal per hour
I had a Catalina 30 with a 25 HP Universal Diesel. Depending on the speed, I used anywhere from 1/2 gph to 2/3 gph. This is running from 2/3 to almost full throttle. The Catalina 30 is a little heaver than the Hunter, but I dont think it will make much of a difference in consumption. If I were to take a wild guess , I would think that your engine will start sucking air and you will lose your prime when you get below about 3 1/2 gallons (depending on tank shape and roughness of the water). That will give you approx approx. 9 usable gallons. Then depending on throttle setting you should get approx. 13 to 18 hours at approx 5.5 to 6 knots for a distance of roughly 70 miles erring on the safe side. (again, depending on wind and seas). BTW, the difference in speed between 2/3 throttle and full throttle is barely 1/2 knot.
I had a Catalina 30 with a 25 HP Universal Diesel. Depending on the speed, I used anywhere from 1/2 gph to 2/3 gph. This is running from 2/3 to almost full throttle. The Catalina 30 is a little heaver than the Hunter, but I dont think it will make much of a difference in consumption. If I were to take a wild guess , I would think that your engine will start sucking air and you will lose your prime when you get below about 3 1/2 gallons (depending on tank shape and roughness of the water). That will give you approx approx. 9 usable gallons. Then depending on throttle setting you should get approx. 13 to 18 hours at approx 5.5 to 6 knots for a distance of roughly 70 miles erring on the safe side. (again, depending on wind and seas). BTW, the difference in speed between 2/3 throttle and full throttle is barely 1/2 knot.
Run Hard
Tom is right. A diesel will only squirt as much fuel as is set by the trottle. When the trottle setting it too high and the engine is overloaded it will blow black soot. This is unburned fuel. When the trottle setting is too low it squirts less fuel but will not completely burn it. The difference between the too high setting and the too low setting is at the low setting the black soot from the unburned fuel is not forced out of the engine. Instead it coats everything (especially the valves)and dramatically shortens the life of the engine. I did not suggest overloading the engine. They are designed to be run at 80-85%. This is not considered overload for a diesel engine. If the engine cannot run at 80-85% of rated RPM without overloading and blowing black soot there is likely a problem with the prop size or pitch.
Tom is right. A diesel will only squirt as much fuel as is set by the trottle. When the trottle setting it too high and the engine is overloaded it will blow black soot. This is unburned fuel. When the trottle setting is too low it squirts less fuel but will not completely burn it. The difference between the too high setting and the too low setting is at the low setting the black soot from the unburned fuel is not forced out of the engine. Instead it coats everything (especially the valves)and dramatically shortens the life of the engine. I did not suggest overloading the engine. They are designed to be run at 80-85%. This is not considered overload for a diesel engine. If the engine cannot run at 80-85% of rated RPM without overloading and blowing black soot there is likely a problem with the prop size or pitch.
Pretty wide estimates of range. Sorry but
but my calculations do not have you faring much better. I was thinking that with all the range estimates that you are getting it may encourage you to calculate the burn rate for your boat. A burn rate of .5 to GHP sounds about right for your engine. You should have a capacity of 12 usable gallons in your tank. The gas in the filter, water separator and fuel line will protect against shut down from air due to any fuel splashing in the tank. So 24 hours would be a fair estimate for your total motoring range. If you calculate a safety reserve of 5 Hours or 2.5 gallon you are left with 19 hours of motoring. With a strong head wind and current at 2 knots of travel you could safely cover 38 nautical miles or with a following wind and current at 7 knots of travel you could cover 133 nautical miles. That is while maintaining a 20% reserve. These approximate figures with a safety factor built in by the reserve may serve you well until you record and develop your actual figures. Your ultimate range will be dictated by your knotmeter and is to be calculated underway, with the knowledge that you will have a safe 19 hours of motoring before refueling. Obviously sailing time will not reduce your hours but you wil have to deduct any time for motor sailing. Remember the speed of the boat and distance travelled has nothing to do with the burn rate and available motoring hours. An automobile engine can be considered to be mechanically attached to the road by its tires so fuel consumption can be reliably calculated by distance travelled but a boat or an airplane moving in a fluid enviroment can only calculate fuel consumption against time. We are not looking for absolute figures but for average, useful figures. Good luck on your trip and let us know what is your actual burn for that engine after a couple of hundred hours.
but my calculations do not have you faring much better. I was thinking that with all the range estimates that you are getting it may encourage you to calculate the burn rate for your boat. A burn rate of .5 to GHP sounds about right for your engine. You should have a capacity of 12 usable gallons in your tank. The gas in the filter, water separator and fuel line will protect against shut down from air due to any fuel splashing in the tank. So 24 hours would be a fair estimate for your total motoring range. If you calculate a safety reserve of 5 Hours or 2.5 gallon you are left with 19 hours of motoring. With a strong head wind and current at 2 knots of travel you could safely cover 38 nautical miles or with a following wind and current at 7 knots of travel you could cover 133 nautical miles. That is while maintaining a 20% reserve. These approximate figures with a safety factor built in by the reserve may serve you well until you record and develop your actual figures. Your ultimate range will be dictated by your knotmeter and is to be calculated underway, with the knowledge that you will have a safe 19 hours of motoring before refueling. Obviously sailing time will not reduce your hours but you wil have to deduct any time for motor sailing. Remember the speed of the boat and distance travelled has nothing to do with the burn rate and available motoring hours. An automobile engine can be considered to be mechanically attached to the road by its tires so fuel consumption can be reliably calculated by distance travelled but a boat or an airplane moving in a fluid enviroment can only calculate fuel consumption against time. We are not looking for absolute figures but for average, useful figures. Good luck on your trip and let us know what is your actual burn for that engine after a couple of hundred hours.
Ross's suggestion Makes Sense
Ross's suggestion makes the most sense. With so many variables a reserve of 5 gallons will give you roughly a 10 hour fudge factor. Has the boat been used much recently??? If there is dirty old fuel in the tank it might clog up your filters. I'd check that out before getting underway. Be sure to have a few extra filters just in case stuff gets stirred up while you are underway. It is always nice to know that with a sailboat that if you run low on fuel that you can sail!!!! Be sure not to run out!!! I'd start motoring when I had about 1/4 of a tank and keep that for reserve if you must motor. That way you always have an option.
Ross's suggestion makes the most sense. With so many variables a reserve of 5 gallons will give you roughly a 10 hour fudge factor. Has the boat been used much recently??? If there is dirty old fuel in the tank it might clog up your filters. I'd check that out before getting underway. Be sure to have a few extra filters just in case stuff gets stirred up while you are underway. It is always nice to know that with a sailboat that if you run low on fuel that you can sail!!!! Be sure not to run out!!! I'd start motoring when I had about 1/4 of a tank and keep that for reserve if you must motor. That way you always have an option.
Fuel consumption
When I first read all of these comments on fuel consumption they made sense to me. The logic of Benny's comment on fuel burn being fairly constant in GPH was reasonable. But I happened to be looking at the Yanmar online manual for my engine and I noticed there was a graph preented for fuel consumption and it showed a significant change depending on the RPM. E.g. at 1800 RPM it was below .3 GPH; at 2400 RPM it was almost .5; and 3400 RPM it was 1.2. That suggests to me that it consumes like the gas engine in my car, more fuel at higher RPM. What am I missing?
When I first read all of these comments on fuel consumption they made sense to me. The logic of Benny's comment on fuel burn being fairly constant in GPH was reasonable. But I happened to be looking at the Yanmar online manual for my engine and I noticed there was a graph preented for fuel consumption and it showed a significant change depending on the RPM. E.g. at 1800 RPM it was below .3 GPH; at 2400 RPM it was almost .5; and 3400 RPM it was 1.2. That suggests to me that it consumes like the gas engine in my car, more fuel at higher RPM. What am I missing?
govenor
A diesel's RPMs is set by a govenor that injects just enough fuel to maintain RPM.s if the load increases the engine slows a little and then injects more fuel to get back to the set RPM. Drag increases (ie load) the faster that you go so that your engine will require more fuel. Load is also increased when you are going into the wind so at the same RPM's you will burn more fuel into the wind than with the wind. Loading was what I was talking about when a boat starts to approach hull speed. It might take 10hp to approach hull speed but to go a little over hull speed migh take 50 hp and it increases exponentially so that it might take 150hp to go twice hull speed and this assumes that things don't break first. One of the few times that you go farther on a gallon of fuel by going faster is when you are going against a current. Obviously if you are going 1 kt against a 1 knot current you are standing still over the bottom. Under these conditions there would be a mathematically optimal speed to cover the most distance with a given amount of fuel. Then a given engine will also have an optimal RPM where it operates most efficently. Ideally the engine and prop and boat match up so that they all have about the same optimal speed. You can attach a fuel meter to your fuel line and with a GPS and calculator figure the best MPG under any given circumstance. They also sell computers that will calculate it for you. But most of us don't worry that much about MPG but at $2.50/gallon some of the big stinkpotts probably worry about it more than us sailboaters. There is a certin amount of fuel recycling in some diesels and I don't know how you would account for the bypass fuel.
A diesel's RPMs is set by a govenor that injects just enough fuel to maintain RPM.s if the load increases the engine slows a little and then injects more fuel to get back to the set RPM. Drag increases (ie load) the faster that you go so that your engine will require more fuel. Load is also increased when you are going into the wind so at the same RPM's you will burn more fuel into the wind than with the wind. Loading was what I was talking about when a boat starts to approach hull speed. It might take 10hp to approach hull speed but to go a little over hull speed migh take 50 hp and it increases exponentially so that it might take 150hp to go twice hull speed and this assumes that things don't break first. One of the few times that you go farther on a gallon of fuel by going faster is when you are going against a current. Obviously if you are going 1 kt against a 1 knot current you are standing still over the bottom. Under these conditions there would be a mathematically optimal speed to cover the most distance with a given amount of fuel. Then a given engine will also have an optimal RPM where it operates most efficently. Ideally the engine and prop and boat match up so that they all have about the same optimal speed. You can attach a fuel meter to your fuel line and with a GPS and calculator figure the best MPG under any given circumstance. They also sell computers that will calculate it for you. But most of us don't worry that much about MPG but at $2.50/gallon some of the big stinkpotts probably worry about it more than us sailboaters. There is a certin amount of fuel recycling in some diesels and I don't know how you would account for the bypass fuel.
So TOM........
......."what kind of range we might expect from the engine - fuel capacity is listed at 12.5 gallons"?
......."what kind of range we might expect from the engine - fuel capacity is listed at 12.5 gallons"?
Increase in fuel use at higher rpms
Ted You're right, higher rpms will use more fuel. The thing to remember here, though, is that boats are NOT like cars. Usually, once out of the dock area, the boat is set on course and the engine pegged to cruising speed which is usually 2/3 to 3/4 of the max rpm. That's why, after almost 8 years, I can verify that my fuel use is 0.439 gallons per hour. Every time we fill up, I record the engine hours and fuel quantity, and include it on a spreadsheet that calculates the fuel per hour and hours per gallon. Sure it's varied on individual fill ups, but not by much. Cars, OTOH, keep changing speed (unless you do an incredible amount of freeway driving). Yes, dirty hulls and bad or incorrectly matched props can increase fuel consumption, but so can running at WOT forever. Current will NOT affect gallons per hour IF the engine is run at cruising speed, all it will do is decrease miles per hour over ground. Stu
Ted You're right, higher rpms will use more fuel. The thing to remember here, though, is that boats are NOT like cars. Usually, once out of the dock area, the boat is set on course and the engine pegged to cruising speed which is usually 2/3 to 3/4 of the max rpm. That's why, after almost 8 years, I can verify that my fuel use is 0.439 gallons per hour. Every time we fill up, I record the engine hours and fuel quantity, and include it on a spreadsheet that calculates the fuel per hour and hours per gallon. Sure it's varied on individual fill ups, but not by much. Cars, OTOH, keep changing speed (unless you do an incredible amount of freeway driving). Yes, dirty hulls and bad or incorrectly matched props can increase fuel consumption, but so can running at WOT forever. Current will NOT affect gallons per hour IF the engine is run at cruising speed, all it will do is decrease miles per hour over ground. Stu
All diesel engines meter the fuel
according to the horsepower demand. Most diesel engines are rated at about .285 liters per horsepower per hour. So if your fuel consumption rate is .439 gallons per hour you can work a precise conversion to know the exact horsepower demand is for the speed that you push the boat. Just because you have a 25 horsepower engine does not mean that you are using 25 horsepower. By my calculations in the example above the engine is putting out 5.8 horsepower. The conversion factor is3.785 liters per gallon approximately.
according to the horsepower demand. Most diesel engines are rated at about .285 liters per horsepower per hour. So if your fuel consumption rate is .439 gallons per hour you can work a precise conversion to know the exact horsepower demand is for the speed that you push the boat. Just because you have a 25 horsepower engine does not mean that you are using 25 horsepower. By my calculations in the example above the engine is putting out 5.8 horsepower. The conversion factor is3.785 liters per gallon approximately.
Usable Fuel ?
I know little to nothing about diesel engines except that if you suck in any air, you will have to prime the system. Is there a way to know how much you can run a tank down before air will get into the lines from bouncing around? This question is based on average to rough conditions and not on any really bad storm systems such as a Tropical depresion and up. Hopefully, I will never be caught in conditions that are THAT bad AND low on fuel at the same time, and NOOOOOOOOOOOO I cant go back to the dock if I'm 150 miles offshore.
I know little to nothing about diesel engines except that if you suck in any air, you will have to prime the system. Is there a way to know how much you can run a tank down before air will get into the lines from bouncing around? This question is based on average to rough conditions and not on any really bad storm systems such as a Tropical depresion and up. Hopefully, I will never be caught in conditions that are THAT bad AND low on fuel at the same time, and NOOOOOOOOOOOO I cant go back to the dock if I'm 150 miles offshore.
number of hours not number of miles
Hi Bob; If you read all of the replies the bottom line is that you can predict the burn rate at a given rpm with a given boat under given conditions. But you can't predict a given number of miles per gallon under all conditions. With my Volvo MD11C at about 1700 rpms I burn about a quart an hour. Into a headwindor dirty hull a little more. tailwind clean hull and little less. I figure on about 200 miles for my 20 gallon tank. But I'd never put myself at risk by planning to motor 200 miles without a backup plan. On a sailboat the backup plan is to sail or carry extra fuel in a jerry can. If I knew that I'd have to motor up a canal etc I'd keep reserve fuel forthat purpose. And remember that slower usually means more MPG. I learned that flying my airplane. At WOT the burn rate in an airplane is about 50% more than at minimal throttle to maintain altitude. Cruising at 80 mph in a cessna 172 you can fly a lot longer than at 120 mph. Will you go farther??? Well that depends if you have a tailwind you travel a lot farther if you have a headwind you might not go as far. Of course a 50Kt wind is not uncommon at altitude. Once according to my math and GPS and speedometer I had a 70 knot tailwind as the gps said 170 knots and the speedo said 100 knots. The same thing happens but a lot slower in a boat. Trying to return into St Andrews bay one day the speedo said about 2 knots. Well sometimes I was moving toward a buoy in front of me and sometimes I was moving backward. The wind finally picked up and I got into the bay without starting the engine.
Hi Bob; If you read all of the replies the bottom line is that you can predict the burn rate at a given rpm with a given boat under given conditions. But you can't predict a given number of miles per gallon under all conditions. With my Volvo MD11C at about 1700 rpms I burn about a quart an hour. Into a headwindor dirty hull a little more. tailwind clean hull and little less. I figure on about 200 miles for my 20 gallon tank. But I'd never put myself at risk by planning to motor 200 miles without a backup plan. On a sailboat the backup plan is to sail or carry extra fuel in a jerry can. If I knew that I'd have to motor up a canal etc I'd keep reserve fuel forthat purpose. And remember that slower usually means more MPG. I learned that flying my airplane. At WOT the burn rate in an airplane is about 50% more than at minimal throttle to maintain altitude. Cruising at 80 mph in a cessna 172 you can fly a lot longer than at 120 mph. Will you go farther??? Well that depends if you have a tailwind you travel a lot farther if you have a headwind you might not go as far. Of course a 50Kt wind is not uncommon at altitude. Once according to my math and GPS and speedometer I had a 70 knot tailwind as the gps said 170 knots and the speedo said 100 knots. The same thing happens but a lot slower in a boat. Trying to return into St Andrews bay one day the speedo said about 2 knots. Well sometimes I was moving toward a buoy in front of me and sometimes I was moving backward. The wind finally picked up and I got into the bay without starting the engine.
Fuel burn
A burn of approx. 1/2 gallon per hour is about normal for this engine, or almost any of this output. You will have to figure out distance between where you can get fuel, and go by that.As many have mentioned, you can't figure mpg, because of headwind, current etc. If you think you need five gallons of reserve, carry 10.
A burn of approx. 1/2 gallon per hour is about normal for this engine, or almost any of this output. You will have to figure out distance between where you can get fuel, and go by that.As many have mentioned, you can't figure mpg, because of headwind, current etc. If you think you need five gallons of reserve, carry 10.
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