Boat Performance

[O C Diver]

Great discussion that I am struggling to understand.

Load has been mentioned so I’ll ask a silly question. For wear and tear on an electronically controlled common rail engine, what load % typically be efficient and “best” for the engine?

It's been my understanding that best efficiency usually occurs within a few hundred RPM of maximum torque. My goal when setting up my drivetrain, was to have my cruise RPM occur at peak torque. Didn't quite get there (about 250 RPM below). As far as percentage of load, the manufacturers want you to stay away from the extremes (low and high). With no further guidance, 50% seems to be a good safe target. Interestingly, if my boat wasn't overpropped, the percentage of load for my 7 knot cruise speed would drop from 50% to around 25%.

Ted

 

[dhays]

In our application (well, mine anyway) I have an oversized engine that I am not going to run any significant percent of the time at or near full power. So a question might be asked, in my 380 hp Cummins that I am going to run at 7 knots requiring about 80 HP @ 1400 rpm, would it prefer I do that at 200 more rpm and 40 ft-lbs less torque*? A little less load on bearings and rings, but the same proportion more surface travel per mile run. I suspect with those kind of minor variations, it makes very little difference. These marine diesels in yachts get killed by corrosion and neglect far quicker than most of them wear out.

You and I have the same engine. I generally run it between 1400 and 1500 rpm. Now, I could run it slower and faster and if there was an engine wear consideration, I might. I will try and remember to check tomorrow what my % load is at various RPMs. Of course, right now I have a dirty bottom and dirty running gear so if I understand things correction, the % load will be higher than normal.

Max torque for our engine is at 2000 rpm. That also burns 6.6 gph and on my boat just digs a big hole in the water. Max rpm is supposed to be 3065. At a typical 1400 rpm, I’m at 62% of max torque.

Not sure what that means...

 

[DDW]

It is possible that maximum efficiency occurs near peak torque, but that would be full throttle efficiency. Due to the unique character of the prop load on a marine diesel, if you operate the engine at it's peak torque rpm with anything close to a sane prop, you will also be operating at a much lower than peak available torque. This is why the fuel map is important - it tells you the fuel efficiency over the range of loads and rpms.

Again it is very different than the same engine in a truck. With a transmission, you can shift to overdrive or go down 3 gears and run the engine at several different operating points with the same load. On a boat, you have to change props (or transmissions) to do that.

Dhays - As you get close to hull speed wave making drag dominates, so a slightly dirty bottom might not make much difference. A dirty prop (barnacles) can though. I'd be curious as to what load you are seeing - your boat is quite a bit larger and I'm sure heavier than mine. Here is the chart for our engine:

 

[O C Diver]

I probably wasn't clear when I said peak torque. I was referring to peak torque at 100% load not propeller absorption. This would be the graph published by the manufacturer as the torque curve. On my John Deere 4045TFM75 135 HP @ 2,600 RPM, the peak torque occurs at 1,800 RPM.

Ted

 

[FF]

"This is why the fuel map is important - it tells you the fuel efficiency over the range of loads and rpms."


Truer words were never written , but obtaining the necessary fuel map for most engines is really hard!!!

 

[Art]

"This is why the fuel map is important - it tells you the fuel efficiency over the range of loads and rpms."


Truer words were never written , but obtaining the necessary fuel map for most engines is really hard!!!

Why do mfg's make it hard to obtain fuel map? Or... does fuel map accuracy for an engine specifically rely on a boat it powers as well as prop it turns??

 

[Nomad Willy]

Art wrote;
“Why do mfg's make it hard to obtain fuel map?”

Probably supply and demand. Few want to know.
Very useful stuff here on TF but in the big world?

 

[Art]

Art wrote;
“Why do mfg's make it hard to obtain fuel map?”

Probably supply and demand. Few want to know.
Very useful stuff here on TF but in the big world?

Should be a page on operating inst.

 

[dhays]

'd be curious as to what load you are seeing - your boat is quite a bit larger and I'm sure heavier than mine.

Well, here are the numbers for the range of RPM that I normally cruise at. I think they are interesting only in there non-interestingness.

Rpm. % Load
1350 26
1400. 27
1450. 28
1500. 29
1550. 30
1600. 31

I’m surprised it is so linear, even if it is through a limited power range.

 

[DDW]

The load probably means torque. When multiplied by rpm to get power, it's a bit more non-linear (approx 2nd power which is what you'd expect of the resistance curve). It would be hard to call that engine overloaded.

What were your speeds out of curiosity?

 

[markpierce]

On my full-displacement boat, engine load versus speed and RPM:

100%: 7.3 knots, 2400 RPM
72%: 7.1 knots, 2200 RPM
43%: 6.3 knots, 1800 RPM
38%: 6.0 knots, 1600 RPM

(Load as defined as actual fuel consumption versus maximum fuel consumption.)

 

[FF]

"Why do mfg's make it hard to obtain fuel map? Or... does fuel map accuracy for an engine specifically rely on a boat it powers as well as prop it turns??"

With a fuel map the efficiency of an engine can be compared with other similar engines , for pumps , noisemakers and other devices a mfg can see if most of his gear can be operated in an efficient range.

A selling point for customers .

Since it could hammer sales of inefficient engine makers the useless prop graph is usually provided, by all.


A fuel map looks like a bullseye, HP ot torqye up one side , RPM on bottom and fuel burn per HP labeled in the concentric circles .


Here are a bunch,


https://www.google.com/search?q=die...1MKHd50A5MQ9C96BAgBEBg&biw=1366&bih=654&dpr=1

 

[dhays]

The load probably means torque. When multiplied by rpm to get power, it's a bit more non-linear (approx 2nd power which is what you'd expect of the resistance curve). It would be hard to call that engine overloaded.

What were your speeds out of curiosity?

All over the map. Lots of current today so really no way to gauge. At 1450 rpm, typical cruise for me, I saw a low of 3.5 knots and a high of 10 knots.

In general, when I have a clean bottom and running gear (which I don’t now) I get about 7 knots at 1400.

 

[DDW]

On my full-displacement boat, engine load versus speed and RPM:

100%: 7.3 knots, 2400 RPM
72%: 7.1 knots, 2200 RPM
43%: 6.3 knots, 1800 RPM
38%: 6.0 knots, 1600 RPM

(Load as defined as actual fuel consumption versus maximum fuel consumption.)

So a 1 knot reduction in speed yields a 57% reduction in load.

By the way, we are neighbors now - my sailboat is out further on the dock. The sky blue hulled one with the funny looking rig.

 

[markpierce]

So a 1 knot reduction in speed yields a 57% reduction in load.

By the way, we are neighbors now - my sailboat is out further on the dock. The sky blue hulled one with the funny looking rig.

Yeah, neighbor. One knot below hull speed works well for this fat, heavy boat.

 

[Nomad Willy]

What I usually do to determine speed is to observe my speed on the gps in a fairly stable (current wise) area. By stable I mean mostly no swirls or eddies. Then make a 180 turn, wait for speed to resume and stabilize. Then make another observation. If the first was 6.5 knots and the second 7.5 then My fwd speed is extremly close to 7 knots. I average the indicated speed readings also.

 

[Art]

What I usually do to determine speed is to observe my speed on the gps in a fairly stable (current wise) area. By stable I mean mostly no swirls or eddies. Then make a 180 turn, wait for speed to resume and stabilize. Then make another observation. If the first was 6.5 knots and the second 7.5 then My fwd speed is extremly close to 7 knots. I average the indicated speed readings also.

That makes for considerable accurate calcs! Running performance tests, back in the 60's, we did same thing... except we used a measured mile.

 

[BruceK]

Lehman Manual says max torque of L120 is achieved at 1600rpm with 3 blade prop.
Interesting to see comments above. Without having checked this before,I`ve long found 1500-1600 a good comfortable engine cruise speed.

 

[sunchaser]

Here is a two part question-

Do any of us have an engine on our boat that has a 5X or more fuel variation at constant RPM? I can think of at least two logical "yes" answers.

In both these cases how is the variable fuel flow managed with zero input from the skipper?

 

[O C Diver]

Here is a two part question-

Do any of us have an engine on our boat that has a 5X or more fuel variation at constant RPM? I can think of at least two logical "yes" answers.

In both these cases how is the variable fuel flow managed with zero input from the skipper?

I would guess most generators fall in that category.

Ted

 

[Art]

Here is a two part question-

Do any of us have an engine on our boat that has a 5X or more fuel variation at constant RPM? I can think of at least two logical "yes" answers.

In both these cases how is the variable fuel flow managed with zero input from the skipper?

"...5X or more fuel variation... is a huge #; at constant RPM!

- 1 gal to 5 gal"
- 5 gal to 25 gal
- 25 gal to 125 gal

"... or more fuel variation at constant RPM?"

 

[DDW]

5x is possible. If you set the "throttle" at 1800 rpm with the transmission out of gear, the only loads on it are friction and pumping losses. It will not use much fuel. The same engine loaded to max torque will use a lot of fuel.

Remember, diesels aren't like gas engines, there is no throttle. There is a lever we call the throttle which chooses the set point of the rpm governor. The engine's injection system then tries to add fuel until the set point is reached. You don't control the fuel, only the rpm - the injection system controls the fuel. Once the lever is set, the injection system will attempt to maintain rpm regardless of load.

It would be harder to reach 5x on a gas engine. It has a real throttle and consequently large pumping losses at high rpm and low throttle settings, requiring more minimum fuel to keep it going. On a gas engine as the load is varied the throttle must be adjusted to maintain the desired rpm, as you are controlling the fuel delivery with the throttle.