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Old 06-29-2017, 07:16 AM   #1
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GPH and MPG cruising speeds???

Last year I ran at 1800 rpm, 8.5 its, 2.36 GPH. There is a sweet RPM at 2100, 10.5 it's. The 2003 34 Mk3. Perkins turbo 200hp. On a nice plane at 2100rpm. Will the MPG be higher????
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Old 06-29-2017, 07:35 AM   #2
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Sometimes a boat will get better fuel economy (MPG) on plane with a higher fuel consumption (GPH). There is a point above hull speed where the boat becomes terribly inefficient because it's plowing a big bow wave. Only trial and error will give you the answer to optimal speed relative to fuel consumption.

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Old 06-29-2017, 07:45 AM   #3
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Most boats on plane will burn about 2-3 times more fuel per mile than at hull speed. I have lots of data on my 38 with 450 Cummins in both modes. 7.5kts 950rpm 1.9gph 3.9nmpg. 20kts 1900rpm 11gph 1.8nmpg.

2.2 times as much fuel per mile to go fast.
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Old 06-29-2017, 08:58 AM   #4
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Most boats all get lower MPG the faster they go.

I subscribe to BoatTest.com. They perform presumably rigorous testing on a wide variety of boats and publish speed, rpm, mpg and gph data. I have never seen one that didn't do this except near the hump where the engine has to put out a lot of power (which burns fuel) to get up on plane which then drops off as it planes. But the mpg on plane will always be worse than near displacement speed before the hump.

Semi-displacement hulls which comprise 90% of what most of us call trawlers don't even do this.

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Old 06-29-2017, 09:14 AM   #5
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Howdy - Rockland eh! You know realtor Chris Cokinis?? He's with Camden RE. Chris is an icon in the general area!

Here's a few close estimate stats on twin screw 350 cid. / 255 hp. gasoline powered, 34' Tollycraft tri cabin... regarding speed and nmpg and gph:

- 4.5 to 5 knots running on one engine = nearly 3 nmpg. Which equals around 1.5 gph

- 6.5 to 7 knots [7.58 is calced hull speed] running both engines = 2 +/- nmpg. Which equals some 3.5 gph.

- 16 to 17 knots on full plane = 1 nmpg. Which equals in the vicinity of 16 to 17 gph.

- WOT at 22 +/- knots = OMG nmpg. Which equals a hole in my wallet gph!

Usually, but not always with every boat [and, I bet so with yours], the slower you go the more $$ you save while the more time you take to get where you're going. That said... taint always the place arrived at anymore than the fun of relaxing travel to get there that makes time aboard a boat so enjoyable.

Happy "Boat-Speed" Daze! - Art
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Old 06-29-2017, 09:22 AM   #6
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Before I repowered my trawler, with the 450 HP Cummins, it would do 10 knots at 18 GPH (.56 MPG). The boat was plowing a big bow wave and the swim platform was under water. It would do 14 knots (sort of on plane) at 21 GPH (.67 MPG).

Ted
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Old 06-29-2017, 10:01 AM   #7
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"On a nice plane at 2100rpm. Will the MPG be higher????"


Yes - the fuel used to cover speed over water will increase by twofold or more on your boat if you move up from 6-7 knots. Here are a few past sources for you to read about your early model 34 (I had a 78 myself - great boat).




BoatUS - Boat Reviews - Mainship 34 Motor Cruiser


Mainship 34 models
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Old 07-01-2017, 01:57 PM   #8
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Foggy day. I applied some numbers. Given that the fuel injector squirts a determined amount of fuel unto a cylinder when it fires. Double the RPM will double fuel consumption. Therefore: 1800rpm = 8.5 kts= 2.36gph = .277 gallons per knot mile. recorded over 3 years. Increase rpm to 2100rpm= 10.5kts. 17% increase in rpm should increase fuel consumption to 2.76gph = .308 gal per knot mile. This is an 11% increase in mileage ( I think ) I am really bored. We will see . After 100 gallons
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Old 07-01-2017, 02:03 PM   #9
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Not exactly, load along with rpm determines how much fuel is being burned the way I understand diesels.
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Old 07-01-2017, 02:40 PM   #10
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Not exactly, load along with rpm determines how much fuel is being burned the way I understand diesels.
So.... what's the way to formulate fuel use for marine gas engines... similar as diesel I'd imagine?? And, I'd also guess that load amount, corresponding with fuel amount required for said load at any rpm, will create a fuel burn number??

I know that if I travel 60 mph down road with or without trailer on back that my vehicles [gas or diesel] doing same rpm while traveling... but due to trailer load factor my foot is deeper into accelerator. I'm confident there is more fuel burned at same rpm and same road speed pulling trailer. With less fuel burn at same speed/rpm without trailer.
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Old 07-01-2017, 02:41 PM   #11
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Given that the fuel injector squirts a determined amount of fuel unto a cylinder when it fires. Double the RPM will double fuel consumption.
No, no, no!!!

A diesel engine injector is controlled by the injection rack which in turn is controlled by the governor to squirt just enough fuel through the injector tip to make just enough power to hold the rpm at its set point.

At idle where there is very little load the rack is set to divert 95% of the injection pump stroke to the return line. Even at wot rpm with no load on the engine this is nearly true. Only with load does the rack let more of the stroke go to the injector tip rather than being diverted to the return line.

Only under full load will the injector deliver the full injection pump stroke to the injection tip and therefore into the combustion chamber.

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Old 07-01-2017, 02:44 PM   #12
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No, no, no!!!

A diesel engine injector is controlled by the injection rack which in turn is controlled by the governor to make just enough power to hold the rpm at its set point.

At idle where there is very little load the rack is set to divert 95% of the injection pump stroke to the return line. Even at wot rpm with no load on the engine this is nearly true. Only with load does the rack let more of the stroke go to the injector tip rather than being diverted to the return line.

Only under full load will the injector deliver the full injection pump stroke to the injection tip and therefore into the combustion chamber.

David
Yea - What David said!
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Old 07-01-2017, 03:41 PM   #13
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GPH and MPG cruising speeds???

Yup, you are correct. I am old . Did not remember the rack control in there. So much for my numbers 🤣. There is no throttle plate. Rpm is controlled by amount of fuel. ( possibly why getting diesel to burn clean for EPA is so hard
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Old 07-01-2017, 04:08 PM   #14
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Actually diesels inherently burn very clean for some things and not so clean for others. Diesels produce almost no CO due to the high excess air in the combustion. When running right, they also produce little hydrocarbons.

But they do produce higher NOx than emissions controlled gasoline engines by virtue of their high compression (heat) and high excess air (the O in NOx). They also produce much more particulates than gasoline engines.

The solution to NOx is urea injection and catalytic conversion of the urea/NOx to just N2. Particulate filters were used on some engines, but the use of high pressure, tightly controlled common rail injection has pretty well taken care of particulates. You used to see belches of black smoke out the tailpipe when a diesel car accelerated. No more with common rail.

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Old 07-07-2017, 07:41 PM   #15
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LOL.🤣. After 95 gallons of fuel Ski was correct. At 2100 rpm and 10.5 kts I burned more then twice the fuel per hour and almost exactly 2x the kph. 5.19 GPH and 0.5 kpg. Vs 1800 rpm , 8.5 kts, 2.36 GPH, 0.277 kpg. Back to 1800 rpm, happy old motor. But she ran strong for that tank of fuel, good to know my 35 year old Perk is capable of prolong hard runs
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