LAGOON 450 FUEL BURN

[chilihead]

Any idea of how I can determine a general fuel burn rate for various rpm''s etc? Not trusting gauges (of course) so general idea is fine.
Thanks
Fred

[StuM]

Which engines do you have?

[StuM]

You should be able to get power curve and fuel burn per HP data fror your engine on the WWW.
If not, you can calculate approximate values if you know the WOT RPM and max HP of your engine:

1. Calculate actual HP for a given RPM for your engine.

Horsepower at RPM = (RPM/MaxRPM) ^3 * MaxHP

Example: Max RPM: 3600, Max HP: 40
Horpsepower at 2000RPM
= (2000/3600) ^3 * 40
= 0.556 ^3 * 40
= 0.172 * 40
= 6.88 HP.

2. Calculate fuel burn for calculated HP:
The rule of thumb for fuel burn is:
.255 litres per hour per horsepower.

So for the above engine at 2000 RPM, burn would be
6.88 * 0.255 = 1.75 LPH.

[donradcliffe]

Using the power curve assumes that the props were sized for max hp at max rpm-- a big assumption.

[StuM]

Originally Posted by StuM

You should be able to get power curve and fuel burn per HP data fror your engine on the WWW.
If not, you can calculate approximate values if you know the WOT RPM and max HP of your engine:

1. Calculate actual HP for a given RPM for your engine.

Horsepower at RPM = (RPM/MaxRPM) ^3 * MaxHP

Example: Max RPM: 3600, Max HP: 40
Horpsepower at 2000RPM
= (2000/3600) ^3 * 40
= 0.556 ^3 * 40
= 0.172 * 40
= 6.88 HP.

2. Calculate fuel burn for calculated HP:
The rule of thumb for fuel burn is:
255 .225 litres per hour per horsepower.

So for the above engine at 2000 RPM, burn would be
6.88 * 0.255 .225 = 1.75 1.55 LPH.

Oops!

Rule of thumb is 0.225 litres/hr/HP (0.4 lbs, 0.06 US gallons)

[a64pilot]

Originally Posted by donradcliffe

Using the power curve assumes that the props were sized for max hp at max rpm-- a big assumption.

Yes, but without flow meters or something similar, you have to assume.
Stu’s assumption is I believe a pretty accurate one, I know of no better way other than taking actual measurements, and even then there will be variables such as wind and waves.

[chilihead]

Originally Posted by StuM

Which engines do you have?

4jh57

[StuM]

Originally Posted by chilihead

4jh57

OK, the datasheet here: http://magelia.demo.epitomy.com/fren...monRail-HR.pdf
gives the power curve and fuel consumption curve.

Specifically, the fuel consumption curve is attached.

As you can see, that shows a fraction under 5 lph at 2200 RPM.

The specs say max = 57HP @ 3000 RPM

Using the formulae above would give (2200/3000) ^3 * 57 * 0.225 = 5.06 lph which is close enough

[a64pilot]

Now what gets interesting is when you graph fuel flow and speed.
I did this for a couple of Sportfishermen and it’s amazing at how steep that graph is.
Look at the above graph, fuel flow at 2000 is less than 4 liters, but just 50% more RPM more than triples fuel consumption.

[StuM]

Originally Posted by a64pilot

Now what gets interesting is when you graph fuel flow and speed.
I did this for a couple of Sportfishermen and it’s amazing at how steep that graph is.
Look at the above graph, fuel flow at 2000 is less than 4 liters, but just 50% more RPM more than triples fuel consumption.

But 50% more RPM also more than triples the output HP. ( approx 15HP v 57HP)

Of course the problem is that speed v HP required is not a straight line either. As you increase speed, the drag increases dramatically and the additional HP requirement goes up (exponentially? ). It takes a lot more additional HP to go from 7 to 8 knots than it does to go from 6 to 7 knots.

[chilihead]

Thanks very much.

[a64pilot]

Originally Posted by StuM

But 50% more RPM also more than triples the output HP. ( approx 15HP v 57HP)
.

Yes, but at possibly a 25% increase in speed. That 25% was just pulled out of the air of course, it’s going to differ. It’s a steep curve of course and with us being displacement hulls, it almost gets to where an infinite amount of power is required for no increase in speed, eventually.
We had flowscans so it wasn’t hard to set an RPM, let speed settle down and write down the fuel flow, increase 250 RPM, repeat.
I was certain that I would find a magic speed where you go the most speed for just a little more fuel, I supposed that would be just where she broke onto plane, being a planing hull of course.
However what I found was surprising, it was a steep curve, the faster you went, the steeper the fuel burned curve was, there was no magic speed where you got a good compromise of speed over fuel burn. It did lessen its steepness just after plane, but did not decrease like I had supposed it would.
The best fuel mileage by far was idle, the more fuel range became critical, the slower you had better go.

[a64pilot]

The other thing, now just going from memory, but the wear on a cylinder and bearings increases with the square of speed? Not sure I said that correctly, but doubling RPM increases the energy in the moving parts by way more than double and the wear follows the energy on those moving parts pretty closely.

[StuM]

Originally Posted by a64pilot

The best fuel mileage by far was idle, the more fuel range became critical, the slower you had better go.

Yep, at 4 knots on one engine, I can get 4 times the range I do at 8 knots, which requires both engines. About 800 v 200NM.

[DavefromNZ]

My Lagoon 450, Yanmar 54hp, I set max continuous RPM 2800 and fuel flow per engine is about 5.5 to 6 liters per hour. 1 eng gives 7kts, 2 eng gives 8 kts. However with 1000ltrs fuel and I reduce RPM and speed and on 1 eng, range is 1500nm.