Diesel used per hour

[Stu Jackson]

mw,

I offer that this is simply incorrect.

If the engine is running at 2600 rpm, it doesn't care what your bottom condition may be, nor what the currents for against you may be.

That's all.

Here's another comparison for you:

I have a friend who bought the same engine I have (uses 0.5 GPH), and before he installed it in his boat, he had it in his GARAGE on a dolly. He ran it at 2600 rpm for 20 hours and used 10 gallons of fuel. He put it in his boat and ran it at 2600 rpm for 20 hours.

Guess what the fuel consumption was.

Sorry, I disagree. I think a dirty bottom ABSOLUTELY affects your gallons per hour. Why?

Most of us have a certain sweet spot that we run our engine. In my case, I like 2600 RPM. So, the only thing that is static is RPM. If you have a dirty bottom, you *WILL* have extra drag, which your engine WILL feel. For instance:

1) Clean bottom, 2600 RPM, 6 knots, throttle POSITION is at 55%, GPH=.50
2) Dirty bottom, 2600 RPM, 5.0 knots, throttle POSITION is now at 65%, GPH=.65

[The above is just an exaggerated example]

Why? Because we always set RPM to be 2600 RPM, or whatever you use.

Your THROTTLE POSITION will vary based on how much DRAG your boat is creating on the engine and WILL change your GPH calculations (and speed).

 

[Allan12210]

Sorry, I disagree. I think a dirty bottom ABSOLUTELY affects your gallons per hour. Why?

Most of us have a certain sweet spot that we run our engine. In my case, I like 2600 RPM. So, the only thing that is static is RPM. If you have a dirty bottom, you *WILL* have extra drag, which your engine WILL feel. For instance:

1) Clean bottom, 2600 RPM, 6 knots, throttle POSITION is at 55%, GPH=.50
2) Dirty bottom, 2600 RPM, 5.0 knots, throttle POSITION is now at 65%, GPH=.65

[The above is just an exaggerated example]

Why? Because we always set RPM to be 2600 RPM, or whatever you use.

Your THROTTLE POSITION will vary based on how much DRAG your boat is creating on the engine and WILL change your GPH calculations (and speed).

I need some help on this one. Shouldn't throttle position determine rpm regardless of bottom condition? I see with drag that speed will be less at a given rpm (and throttle position). So you can still burn the same gph, but over a fixed distance it will take longer to get there therefore more fuel will be consumed to complete the trip. Or, to overcome the drag, throttle position can be advanced to restore the speed, but rpm will then be higher resulting in additional fuel consumed by completing the trip in the same time as the no drag condition. The physics of rpm and throttle position as a function of drag escape me unless speed is kept constant.

Quick edit: Man that Stu is fast!

 

[woodster]

mw,

I offer that this is simply incorrect.

If the engine is running at 2600 rpm, it doesn't care what your bottom condition may be, nor what the currents for a against you may be.

That's all.

re read his post Stu it is more about throttle position than bottom condition if you have a hard load to pull ...it will take more fuel to pull it at the same rpms hence the 55% to 65% throttle position at the same 2600 rpms

 

[mz4wheeler]

mw,

I offer that this is simply incorrect.

If the engine is running at 2600 rpm, it doesn't care what your bottom condition may be, nor what the currents for against you may be.

That's all.

Here is a test. Tied at the dock, put it in gear and run the RPMs up to 2600 RPM and NOTE the POSITION of the throttle. Now take it out and note the position of the throttle. If what you are saying is true, the position in both situations will be identical. Doubtful.

 

[Stu Jackson]

re read his post Stu it is more about throttle position than bottom condition if you have a hard load to pull ...it will take more fuel to pull it at the same rpms hence the 55% to 65% throttle position at the same 2600 rpms

woody,

That is not the issue. I read it a few times.

"...it will take more fuel to pull it at the same rpms.." is simply wrong.

What some folks fail to recognize is that fuel consumption is an AVERAGE of the throttle position BASED ON OVERALL USE.

In one of my earlier posts I provided a link to my fuel consumption spreadsheet that I've been doing for 17 years.

Yeah, sometimes I'm lighter on the throttle than other times.

But consistently, FOR FIGURING OUT FUEL CONSUMPTION, the AVERAGE is 0.5 gph.

If I ran my boat at 1200 rpm ALL THE TIME, my consumption would be reduced. If I ran at 3000 all the time, my consumption would be increased. My cruising rpm is between 2400 and 2650 for 3000 WOT engine. I normally run my engine at that rpm, except when leaving and return to my berth or any other stopping point.

So, while I may run my engine at a lower rpm for a short amount of time each time I am motoring, MOST of the time we all run at cruising rpm.

BUT, the AVERAGE is still the average of how each of us uses their boat.

I do not care if sometimes I run at a higher or lower rpm sometimes, because over a long period of time, it all averages out, and that's all one needs to know for the purpose of AVERAGE fuel consumption.

No one is arguing that lower rpms uses less fuel, or that higher rpms uses more fuel.

Distance and speed and bottom growth have nothing whatsoever to do with AVERAGE engine fuel consumption in gph.

 

[Stu Jackson]

Here is a test. Tied at the dock, put it in gear and run the RPMs up to 2600 RPM and NOTE the POSITION of the throttle. Now take it out and note the position of the throttle. If what you are saying is true, the position in both situations will be identical. Doubtful.

If it is 2600 rpm in both cases, it would be identical.

I could put the engine on a dolly in my garage (or yours) and it would be exactly the same.

2600 rpm is 2600 rpm, tied to the dock, out in a nasty current, or on a test bed in the shop.

Could you please help me understand why they would be any different.

Sure, I have never said that if you're fighting a current you wouldn't go as far.

But your engine doesn't know what's happening outside the boat. All it knows is what the rpms are, therefore the fuel consumption in VOLUME per unit of TIME.

 

[mz4wheeler]

If it is 2600 rpm in both cases, it would be identical.

2600 rpm is 2600 rpm, tied to the dock, out in a nasty current, or on a test bed in the shop.

Could you please help me understand why they would be any different.

Sure, I have never said that if you're fighting a current you wouldn't go as far.

But your engine doesn't know what's happening outside the boat. All it knows is what the rpms are, therefore the fuel consumption in VOLUME per unit of TIME.

You are at a dead stop, punch it up to exactly 2600 RPM and let the boat come up to speed. After you reach speed, the RPM will read about 2800 RPM, because there is now less drag on the propeller. The throttle position has not changed.

I am not talking about AVERAGE fuel consumption, but rather MOMENTARY fuel consumption. However, I would argue that a real dirty hull will increase even the average over time.

 

[Allan12210]

By this example, if the boat were tied to the dock and the engine was at 2600 rpm then cutting the line should result in an increase in rpm as the boat gains way, requiring the throttle to be backed down. I've never heard of that happening. I also never see as conditions change (current, wind direction and speed) that I ever need to adjust throttle position to maintain a constant rpm. It just always stays put. What is the role of the engine governor at this point? Maybe that is the true throttle position. I know on my rototiller I set the throttle and then as go from compacted soil to loose stuff the engine sounds like its running the same speed all the time and I don't have to touch the throttle.

 

[Stu Jackson]

1. You are at a dead stop, punch it up to exactly 2600 RPM and let the boat come up to speed. After you reach speed, the RPM will read about 2800 RPM, because there is now less drag on the propeller. The throttle position has not changed.

2. I am not talking about AVERAGE fuel consumption, but rather MOMENTARY fuel consumption.

3. However, I would argue that a real dirty hull will increase even the average over time.

1. If the throttle position has not changed, the rpms won't either. This makes no sense.

2. We agree.

3. Nope. Never. Has nothing to do with the ENGINE'S fuel consumption. It DOES affect the distance you can travel. You WILL use more fuel to go X miles, but you will NOT change the GPH. This is simple mathematics.

 

[Cool_Change]

My thinking is that the only thing that will affect the relationship between throttle position, RPMs and fuel consumption is the load on the engine (aka on the prop) itself.

As the friction of water on the prop as it spins in water is basically constant regardless of condition of the hull, the load doesn't change and therefore neither does consumption nor RPM at a set throttle position.

Now however envision the prop trying to spin in lets say mud. Much more load on the prop so for the engine and to try and turn at the same RPMs as above it will need more throttle consuming additional fuel so the engine can work harder.

The point here is that hull condition will effect speed not fuel consumption (at the same RPM). Fuel consumption is governed by the load on the engine.

 

[Jerry Clark H356 SV Persi]

I have kept fuel consumption figures on my H356 for 13 years and every hour of operation on my Yanmar 3GM30F and my Northern Lights 5KW generator. I average .25 gallons per hour for the generator and .4 on the Yanmar and cruise my boat At 2650 RPM which is around 75 to 80% of Max RPM of 3400. We made a long cruise in 2013-14 and my fuel consumption was very variable, using up to 1 gallon per hour average to as low as .75 ( for both combined since I run my generator full time when not docked). At first, I attributed it to a very loaded boat and went from Kentucky Lake to Punta Gorda FL and then returned to Mobile. I had soot on my stern which I had never had before, and my oil consumption on the Yanmar was noticeable where in the past, it had hardly used a drop between changes. Crossing Mobile Bay enroute to Turner Marine, I decided to run it up to full RPM and had only 3100 max. Got a mechanic at Turner, changed injectors on both engines and then pulled the boat after I had my mast lowered. The reason my consumption had been so variable was that I had been running a much higher Percent of Max (2650/3100 or 85%) or greater horsepower to get my normal 6.2 knots because my MaxProp gears had worn and the prop pitch had changed as a result of the wear. On my return north To Kentucky Lake, up the Tenn-Tom , I ran at 2500 RPM (2500/3100 - 80%) and my consumption reduced, but not to the levels before the cruise. The point I learned is that the prop pitch change made it a different situation. I also only made 5.5 knots where before it was 6.2 at the same percentage power. We had a lot of extra stuff on the boat and I think that was a contributing factor too. If your consumption changes, run your throttle up to see what your max rpm is. You may be as surprised as I was to see the max reduced.

 

[Johann]

By this example, if the boat were tied to the dock and the engine was at 2600 rpm then cutting the line should result in an increase in rpm as the boat gains way, requiring the throttle to be backed down. I've never heard of that happening. I also never see as conditions change (current, wind direction and speed) that I ever need to adjust throttle position to maintain a constant rpm. It just always stays put. What is the role of the engine governor at this point? Maybe that is the true throttle position.

Once you set the RPM with the speed control lever, the governor will then increase or decrease fuel flow to maintain the set RPM. That is why you don't have to adjust the "throttle", and the RPMs remain the same under varying engine loads.