Low RPM cruise

[freshalaska]

Thank you Dockhead you are a breath of fresh air.

[daletournier]

Originally Posted by Dockhead

Here you are:



https://www.maretron.com/products/ffm100.php


Attachment 178440


Attachment 178441


It's part of our NMEA2000 network, which has many different functions.


It will read out liters/mile directly and so will give you instantaneous directly measured information on the subject of this discussion.

Hi Dh, how accurate is this in the real world? I need to change my getting old raymarine displays, ive been thinking of going to mareton.

[Colin A]

Originally Posted by a64pilot

I have graphed fuel consumption over speed on three different boats, all with flow meters, although the last was picked from the can bus so it was calculated fuel flow, not actual measured, it was also the only gas motor and an outboard.
They were all three planing boats and I was certain that If I looked hard enough that I could find a sweet spot, a spot just when they broke onto plane that I actually got better fuel mileage than a speed greater of less than the sweet spot.
Well what I found surprised me, that was regardless of anything else, trim tab setting etc, the faster you go, the worse the fuel mileage, yes the graph had steep spots where you burned a lot more fuel for very little increase of speed, like right before you broke onto plane, but it never dropped, there was no single speed that got better fuel mileage than a slower speed. This was on two Sportfishemen, a 36 twin engine and a 45’ twin, and my single engine outboard center console Dive boat.
They are not sailboats of course, but with all three the faster you went, the more fuel you burned over a distance, best fuel consumption was single engine, idle.
Now this was in a no current situation, current may change that of course.

I haven't looked at fuel graphs lately on planning boats. But back in the carb'd 2 stroke outboard days getting a slight bump in MPG was common once on plane.



Looked it up looks like it can happen even on modern engines.

https://mercurymarine.widencollectiv...rformanceTests

[Colin A]

To add a little from my experience. I haven't seen a diesel primary actually die a premature death from under loading but I have seen plenty of Generators that have. Remember load is a function of available HP. There is less HP at lower RPM. Which is why constant speed gens have more trouble they have to run at one speed to meet the hz requirement. This means they are closer to their peak HP and easier to under load. It's a fairly common problem on some larger boats. I first came across it with a Sport fish with a rather large gen (20KW maybe) that was sized to power 4 AC units ice maker home style fridge etc. He would fire it up when ever he left dock for the ice maker and fridge. Yeah thing had to be overhauled with I think 500 hours on it maybe less.

[Simi 60]

Originally Posted by Kenomac

Nice. But like I wrote, I’d rather put $1500 worth of diesel in the tank than buy such a thing.

So would I but I have been considering 2 of these at $40 each.
One on fuel in one on return

At start of run set both to zero
Run for an hour at 1000rpm
Subtract one reading from the other and that's usage at that speed

Repeat for 1050, 1100, 1150 etc





https://www.ebay.com/itm/Liquid-Fuel...MAAOSwJ4hY-hIu

[Kenomac]

Originally Posted by Simi 60

So would I but I have been considering 2 of these at $40 each.
One on fuel in one on return

At start of run set both to zero
Run for an hour at 1000rpm
Subtract one reading from the other and that's usage at that speed

Repeat for 1050, 1100, 1150 etc





https://www.ebay.com/itm/Liquid-Fuel...MAAOSwJ4hY-hIu

Great idea! I’ll add it to the list.

[Colin A]

Originally Posted by daletournier

Hi Dh, how accurate is this in the real world? I need to change my getting old raymarine displays, ive been thinking of going to mareton.

I think they are fairly accurate. I have seen them installed on big sport fish that also had a older floscan setup. They gave similar results. If you have a display that can get the NMEA 2000 data you don't need the Maretron display.

[Simi 60]

Originally Posted by a64pilot

However a very good point has been brought up, if we sailors want to learn how to motor most efficiently, maybe we ought to listen to our Trawler brothers, cause I bet they have that down to a science as it’s their only means of propulsion.

Exactly.

Its a simple fact that our engines are more efficient at lower rpm giving far higher range at slightly reduced speed.

As an example our 60 ft 65 tonne ex working trawler has a 14 litre Cummins that produces 350hp @ 1800 rpm

At full noise she'll guzzle 65 litres per hour and do 12 knots
She needs about 270hp @ 1700 rpm burning 55 lph to get to her 11 knot hull speed and pushes a sizable wave.
She needs about 80hp @ 1150 rpm burning 15 lph to travel at 7.5 knots and almost zero wash.
And about 55hp @ 1000 rpm burning 10 lph to to travel at 6.5 knots

Basically works out we can do 1000nm at 11 knots
Or 3500 nm at 7.5 knots.

Guess which end of the rev range we run at?

[Simi 60]

Originally Posted by Simi 60

Basically works out we can do 1400nm at 11 knots
Or 3500 nm at 7.5 knots.

Corrected

[a64pilot]

Originally Posted by transmitterdan

Try extending that curve all the way down to 0 MPH. You will cross over a peak at about 30-40MPH. It isn’t a simple always decreasing curve.

Yes, as I stated earlier on average a car will get its best fuel mileage at the lowest speed that the engine will run smoothly in the highest gear. Level, unaccelerated cruise that’s pretty much idle or slightly above.
Of course boats, most anyway, don’t have gears.

[sailorboy1]

Originally Posted by Dockhead

No one ever called you a liar. In this particular case, you are simply mistaken, and there is no shame in that. I am mistaken as often as you or anyone else is, and I am generally grateful to those who bring me new knowledge that helps me see that.

I'm NOT!!! Repeatedly saying my direct experience is wrong is in fact you saying I'm lying.

With that I'm just going to pick this thread on ignore so feel free to come up with and another way to call me a liar and get the last word.

[a64pilot]

Originally Posted by Colin A

I think they are fairly accurate. I have seen them installed on big sport fish that also had a older floscan setup. They gave similar results. If you have a display that can get the NMEA 2000 data you don't need the Maretron display.

Decent turbine flow meters are extremely accurate, down to .1 GPH, repeatable.
At least the ones I used on aircraft made by Electronics international were. I can assure you other aircraft name, they weren’t any better than others.
https://buy-ei.com/fuel-flow-transducers/
They are paddle wheel sensors with the wheel RPM being sensed by a Hall effect sensor, which can be phenomenally accurate. I have seen tachometers that count gear teeth and flywheel starter ring teeth.

[Kenomac]

Originally Posted by sailorboy1

I've never been call a liar so politely so many time in one thread

Bet people don't really understand that are is a lot of efficiency factors to consider. I use to design pump systems and people always assumed it it was better to pump slower because there would less head loss friction. But, LOTS of times it wasn't because you might lose 30-50% of efficiency between the pump and motor running so far below their most efficient point. An engine driving a propeller is the same thing.

Now please stop calling be a liar about my own direct years of operational knowledge about MY boat!!!!

I agree with you 100%, and have had the same experience on my boat.

[Ericson38]

Originally Posted by Kenomac

My turbo plugged up from low revs under 1800, the turbo kicks in at around 1800, so I’m not willing to chance messing up another. I personally know of three others who had the same problem. I’m doing what the manfacturer service rep told me to do in order to avoid problems.

You do whatever you like with your engine, it’s your money.

We had a turbo failure due to the engine be ran under 2000 rpm for years (yanmar 4JH2-HTE). Two things were at work here, a max prop with too much pitch, and also for the previous owner, a partially pinched fuel tank vent line that he could not diagnose (until the engine survey we paid for) that kept them under 2000 RPM for much of the time too. At 2960 hours (this year at launch) the turbo locked up completely, even though I had repitched the prop just last winter and was hoping to get the engine up into the 3000 RPM easily like it should be capable of in case the additional thrust would be needed. The turbo oil seals were good and we were not burning oil, but the HI turbo came out looking like a iron meteorite with an interior of coal. This turbo is complex, having oil lines, water lines, and inlet and exhaust air ducts of course. Taking off the air inlet cover, I could not hand rotate it, and had lots of black smoke at anything over 2000 rpm, even with the repitch. This resulted in turning the engines oil black with carbon, and raised the exhaust temp, as the turbo is not pulling energy from the flow.


Now the engine runs to 3500 rpm at WOT, and sounds smoother and stronger all around. My manual (1991) states to run the engine under load (not neutral) 1 minute at over 2500 rpm for every minute ran at a slow speed. I now run it at 2800 rpm for about a minute every hour or two, but otherwise run the engine at 2000 to 2400 rpm continuously, keeping it in the torque band. Our boat in the slings is 39K pounds.


Yanmar routes the crankcase breather line from the top of the rocker cover right into the inlet cover of the turbo. This keeps oil fumes from blow by out of the engine compartment, but does load up the inlet turbine with oil residue. Over time (and compounded if overpropped), this film will build up on the blades, which fit close to the housing, and you can get a stuck turbo if you don't perform the turbo wash procedure as outlined in the operations manual. If there was an air oil separator for the crankcase vent, then the 150 hour turbo wash procedure would most likely not be required by Yanmar.

[Kenomac]

No problems at all with our Yanmar since we began revving it six years ago. Our Maxprop had been over-pitched when we first bought the boat, so this was corrected when the new turbo was installed back in early 2013.