Best fuel economy

My best economy is with the keys out of the ignition.......anything after that hurts
 
For what it's worth, I had to go back to my log books from the late 90s to see what the performance was like on my last gasser. It was a 370DA with fuel injected 7.4s. The best cruise was at 3300 giving 1.5nmpg. at 3450 the efficiency dropped to 1.25nmpg. That's a 17% difference (about 60 nm range on a tank of gas) which I consider to be pretty significant.
 
1.5nmpg is pretty damn good for a gas boat.
 
Slight topic change here...

Is it safe to say that you should run any engine at an RPM that is as low as possible to produce comfort and efficiency? On my 340 it just seemed that anything over 3300 RPM was a "strain." Does higher RPM produce more wear and tear and degradation over the life of the engine?

Take an outboard motor. I'm making these numbers up... If you run it at 4400 RPM you can get, let's say, 2.5MPG b/c most of the boat is out of the water and you're flying along at 52 MPH. If you run it at 3600RPM and you drop to 1.7MPG, would it be true that the lower RPM, lower speed and lower economy numbers are healthier for that engine?
 
3500rpm in my boat is the best economy, but the boat feels sluggish. 3800rpm is where I cruise for the best performance/economy mix, and the boat just feels like it is cruising right. 4100rpm is even better, but to avoid excessive wear I don't run it that fast. Last boat I'll buy with the smaller of the engines offered.

It's not underpowered, but just can't cruise where it wants to without really spinning the engines.
 
Slight topic change here...
Is it safe to say that you should run any engine at an RPM that is as low as possible to produce comfort and efficiency? On my 340 it just seemed that anything over 3300 RPM was a "strain." Does higher RPM produce more wear and tear and degradation over the life of the engine?

Take an outboard motor. I'm making these numbers up... If you run it at 4400 RPM you can get, let's say, 2.5MPG b/c most of the boat is out of the water and you're flying along at 52 MPH. If you run it at 3600RPM and you drop to 1.7MPG, would it be true that the lower RPM, lower speed and lower economy numbers are healthier for that engine?

Good question. The lower RPM and slower speed increases your engine hours to get to you destination. To expand the question: Are less hours at a higher rpm healthier than more hours a bit slower?

Different set-up but I find it more efficient to be running a bit faster, on top of the water, 3800 rpm with v6's. You can feel the engines don't need to push as hard at a certain point, RPMS increase only about 200 but mph jumps almost 5. That sweet spot - water conditions permitting.

It's all a big balancing act. I found you just need to get out there and keep track of how your running and try different scenarios to see what ends up working best.
 
Who's sparring? I'm still trying to wrap my fingers around what the big freakin' deal is in the first place. We're talking tenth's of a mile per gallon here, what am I missing?

But since I am an administrator, I am forbidden from defending my position? Really? Should I just refrain from answering any questions on this forum, of which I have participated in, going way back to the SROC?

There have been no personal attacks on either side, thus no need of moderation there....so where are we going to go now?

I respectfully disagree.

As of late you seem to have taken to verbal sparring with several of the members who dare to disagree with the thoughts you advocate. If you wish to do that, I am of the opinion that you should hang up your administrator/moderator hat and move back to the general membership and let someone more level-headed act as the administrator. I do not believe that you are not representing Jim's interests very well when you get into name-calling of the members here or act in a disparaging manner, close posts after you put in the "last word", or delete posts entirely because of personal reasons (what happened to the ECM thread), just to name a few.

On the other hand, as a regular member not endowed with super-powers, feel free to engage in all the name calling the new administrator will allow.
 
As another data point, my 250DA with the 7.4L and Bravo 1 was most efficient around 3200 RPM's according to my Navman fuel flow meters. My new boat, does not have any type of fuel measuring devices, but it seems that if my sweet spot is 3000-3200 RPM's with my 30' Chaparral running twin 5.0 Volvo's with duoprops. I haven't run a full tank through to confirm numbers though.
 
OK. Time to throw my hat in the ring... After cruising for some 85 nautical miles yesterday here is what I found. At 3200 rpm my 340 Sundancer would barely hold plane and felt sluggish and like it was mushing. Fuel fuel and 4 people, no water in the tank. It would barely plane with full tabs. At 3400 rpm it would maintain cruise. At 3500 it cruised nicely after I burned off some fuel but at 3600 rpm it sounded like I hit a sweet spot, and it didn't need that much on the tabs. Round trip I burned slightly over 1/2 tank. I filled up prior to departure and now the needle is barely off the the 1/2 tank mark. (I know a scientific estimate.) Let's call it 110 gallons. That is .77 NMPG. So right in the ballpark I expected. Even if I burned 120 gallons... that is .71 NMPG. When I departed the wind was S-SW at 6 or less. Waves less than 1' with a smooth ride. Halfway home the wind turned out of the NE and got stronger. Waves 1-2' with little patches of "whitecaps" Choppy ride the last few miles into the wind..

[slight hijack]

If the 340 is running the 9x22 tabs, try getting them enlarged to 18x22. I did it and it made all the difference in the world in ride and fuel consumption. I average 1nmpg @ 3450 RPM @ 22kts.


Now back to our regularly scheduled program.

[/slight hijack]
 
That does seem high. Maybe that was only the fuel burn of one engine, meaning 0.75 nymph was the actual burn with both engines. My memory is failing me.
1.5nmpg is pretty damn good for a gas boat.
 
I'm still impressed you have logs that far back. I can't find what I wrote down yesterday.
 
That does seem high. Maybe that was only the fuel burn of one engine, meaning 0.75 nymph was the actual burn with both engines. My memory is failing me.

That was what I was thinking when I read your numbers (the .75 part not your memory is failing). My 3 former 7.4L gassers ranged from .6 to around .8...one was smaller and two larger than your 370. The two 400's never met a fuel dock they didn't like.
 
1.5 is good for a diesel boat!

I get 1.4mpg...at about 1400 rpm! Still only gives me a usable range of 360 st miles.
 
yep, it is. for comparison if anyone's interested...

fuel.jpg
 
[slight hijack]

If the 340 is running the 9x22 tabs, try getting them enlarged to 18x22. I did it and it made all the difference in the world in ride and fuel consumption. I average 1nmpg @ 3450 RPM @ 22kts.

Now back to our regularly scheduled program.

[/slight hijack]

David,

I’d love to talk with you about that! I’m seriously thinking of doing the same thing, I saw that another member who has 340 did it and I have had that on my research and maybe do it list!

Matt
 
welcome to the forum, Forever Mates:grin:
 
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David,

I’d love to talk with you about that! I’m seriously thinking of doing the same thing, I saw that another member who has 340 did it and I have had that on my research and maybe do it list!

Matt

you won't regret it. I brought Saint Max from Wilmington, NC to RIC and with full original tabs at 3800 RPM I could barely get the bow over. Now granted, the boat was mis propped plus a number of other things Marine Max did not disclose, however, with the new tabs and properly fitted and tuned PropScan props, my numbers went from a bow high 3800 RPM 38 GPH 20 knot dog to a bow proper 3450 RPM 22GPH 22knot cruiser.

YMMV.

Let me know if I can assist in any way.
 
If I ran my previous boat (1989 twin 7.4 MIE Carb'd) at 3500 RPM, I'd burn 36gph and make 21mph. 8 open barrels is an arterial bleed of money. No way is that the speed I would be able to run the boat. My sweet spot was around 3250, 19mph, 31gph (as per GPS and FloScans). Give me a tail wind and I'm up to around 20mph at that RPM. 10-15kt headwind, down to less than 18mph. 1/4 tanks of fuel, up to darn near 21mph. Depending on conditions, I'd make 0.59 - 0.70 mpg. Thus my former nickname ("Pokey").

Just checking the math here. 3500rpm, 36gph, 21mph. So,in 10 hours of running you burned 360 gallons and covered 210 miles. The same trip, running at

This boat is a different animal. 2k lbs heavier, no flybridge, 8.1s... and at 3400RPM I'm making 23mph at less than 29gph. Much less dependent on wind. Weight is always a factor. Thrilled to be averaging 0.80mpg at cruise. On the trip back from AC, which involved a 36 mile run on the mighty Atlantic climbing up and down 4' rollers, I still averaged 0.80mpg (these numbers pulled off the engine computer, displayed on the Garmin).

I would say that each and every gasoline engine is going to have it's own most efficient RPM, when it's installed on a boat. GPH is not the only measure. Sure, on a dyno, you may have a tolerable fuel burn at 3500RPM, but running an a$$-heavy 10 ton gasser at that RPM will make your MPG drop like a rock. Run that clip in a pocket cruiser and it may turn out to be optimal.

Just respectfully checking the math here. 3500rpm, 36gph, 21mph. So,in 10 hours of running you burned 360 gallons and covered 210 miles. The same trip, running at 3250 rpm you're burning 31gph at 19mph. To cover the same 210 miles, you need to run 11.5 hours @ 31gph,burning a total of 356.5 gallons. So, over 11.5 hours of running you only saved 3.5 gallons total! That's what is deceiving about fuel economy on the water. Burning more fuel per hour can be MORE beneficial if the added speed cuts down on the total running time.

Edit - 11.05 hours of running time (at 19mph) = 342.5 gallons, not 356.5! At 342.5 you saved $78.75 over 10 hours, which s a decent savings for sure.
 
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you won't regret it. I brought Saint Max from Wilmington, NC to RIC and with full original tabs at 3800 RPM I could barely get the bow over. Now granted, the boat was mis propped plus a number of other things Marine Max did not disclose, however, with the new tabs and properly fitted and tuned PropScan props, my numbers went from a bow high 3800 RPM 38 GPH 20 knot dog to a bow proper 3450 RPM 22GPH 22knot cruiser.

YMMV.

Let me know if I can assist in any way.

Hi David, PM sent!

Matt
 

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