Fuel economy

[DavidD]

I have an 86 Ebbtide 17' with a 175 hp mercruiser.According to the manufacturer
boat and motor new weighed around 3100lbs full of fuel.Dragging the kids around on tubes and kneeboards what kind of fuel usage should i expect? I know it would be a ballpark figure but just kinda curious.

 

[Boatist]

Re: Fuel economy

If runing at planing speed in the 18 22 range about 3 MPG. If subplaning speed it would be less than 3MPG.
Ballpark

 

[marine4003]

Re: Fuel economy

David, Thats not easy to answer,there are so many factors involved...prop/current/type of usage/ ect..and a motors usage of fuel is calibrated by GPH, or this method : Miles Per Hour ? Gallons Per Hour = Miles Per Gallon, which is hard to determine unless you have a GPS & a flowmeter. So to answer the original question,with heavy usage : ALOT. I f you want some more info go to - http://www.floscan.com/html/MostImportant.asp

 

[Silvertip]

Re: Fuel economy

The highest MPG on nearly any boat is at displacement speed. However, you would need to run for a much longer period to get to your destination than you would on plane. In the end, the actual cost for the trip will be very nearly the same. You either burn fuel at a higher rate over a short period of time, or you burn fuel at a lesser rate for a longer period of time. It all depends on how much of a hurry you are in.

 

[DavidD]

Re: Fuel economy

Thanks,I would imagine an I/O 175 hp would still use less fuel than the 96 175hp
evinrude we had.

 

[Silvertip]

Re: Fuel economy

Not necessarily. It depends on the boats the engines are attached to An I/o weighs considerably more than an outboard so there is a significant weight penalty.

 

[External Combustion]

Re: Fuel economy

Actually I dissagree with Silvertip a bit here. Displacement speed will not cost you the same in fuel usage as planing speed IF and only IF you use an engine that is appropriate to displacement speeds. If you are idleing a five thousand horsepower engine,then displacement speeds on a twenty foot boat will not give you the economy of a six horsepower engine straining at the stops for the same hull.

Your 175 horsepower engine will give marginally better fuel economy at displacement speeds as compared to just above planing speeds. Personally if I had to feed the ponies in the stable I would not be caught dead below planing speeds in your rig.

Have I mentioned the economy of steam???

 

[tomh59]

Re: Fuel economy

No such thing as "fuel economy". Used to be. Not anymore.

 

[DavidD]

Re: Fuel economy

When you load that boat up with wood I bet you dont have much room to move around. LOL

Actually I dissagree with Silvertip a bit here. Displacement speed will not cost you the same in fuel usage as planing speed IF and only IF you use an engine that is appropriate to displacement speeds. If you are idleing a five thousand horsepower engine,then displacement speeds on a twenty foot boat will not give you the economy of a six horsepower engine straining at the stops for the same hull.

Your 175 horsepower engine will give marginally better fuel economy at displacement speeds as compared to just above planing speeds. Personally if I had to feed the ponies in the stable I would not be caught dead below planing speeds in your rig.

Have I mentioned the economy of steam???

 

[marine4003]

Re: Fuel economy

Have I mentioned the economy of steam???[/QUOTE]

Is that the stuff that comes out of my thru hull exhaust after running my twin 825Hp Full Tilt Boogie offshore ?????

Economy,what economy....i dont need no stinkin economy..

 

[External Combustion]

Re: Fuel economy

^^^^^^^^^

Good ones! When the bunkers under the seats are full we have a 240 mile range. The cockpit is 7' X 18' with plenty of room to walk around.

Now if I could only get a long enough pipe I could use marine4004's exhaust and get better milage!

 

[Silvertip]

Re: Fuel economy

I don't have a ready reference for I/Os but here are a few figures from Yamaha's performance bulletins regarding outboard fuel economy. I also agree that few boaters choose to run at displacement speed for any distance. But -- here are some numbers to chew on.

90 HP 4-stroke (best MGP at displacement speed)
http://www.yamaha-motor.com/assets/...e_midthrustjetport_al_g3b-eagle170-f90tlr.pdf

Here's one that agrees with you: 50HP 4-stroke. This is an engine mis-match (I know that because I had one)
http://www.yamaha-motor.com/assets/...e_midthrustjetport_al_g3b-eagle170-f90tlr.pdf

And another: 40 HP
http://www.yamaha-motor.com/assets/...trokePerf_MidThrustJetPort_40p_0184-G3B-Z.pdf

And one more -- 150 HP:

http://www.yamaha-motor.com/assets/...letin_4stroke_hpv6_bss_wlc-215bay-f150txr.pdf

Three of four examples indicate displacement speeds provide the best MPG. However, in the last example, the fuel cost for the trip would be significantly less at displacement speed. At displacement speed (4 mph) the engine is burning .7 g/hr and at wot its 15 (at 47 mph). So a 47 mile trip would consume 15 gallons of fuel at wot but at displacement speed the engine would use only 7 gallons. Yes -- the trip would take 10 hours. In the third example, the same wot/displacement speed factor is still about 10 but the difference in displacement/wot fuel consumption is much closer. Still in favor of displacement speed.

The above comparisons were displacement speed vs wot. If you study displacement speed vs sweet spot (or best cruise MPG) then those numbers come even closer together so I stand by my statement that you either burn less fuel over a long period of time or lots of fuel over a short period of time. Cost for the trip (ignoring time) gets very close in most cases.

 

[QC]

Re: Fuel economy

Funny, and Silvertip is gonna flip out, . . . but . . . since he specifically asked how much fuel he'd use "Dragging the kids around on tubes and kneeboards" shouldn't this maybe a GPH number? Or at least a kind of avergae for a day? I agree, tough to answer. But if you are simply hanging out in coves etc. dragging them around in circles, I'd guess you'll use around 15 gallons a day. That depends if you stop, eat lunch, throw a hook, drink a beer or two, stare at some bikinis etc . . . It really is tough as we would need to know how far you go and how long you are out.

 

[External Combustion]

Re: Fuel economy

Good info Silvertip! I would have expected the smaller engines to get better milage at displacement speeds but I would have thought that around 150 hp that would change because of propellor design.

Are inboards less efficient at idle? That is where I have recieved my information.

 

[QC]

Re: Fuel economy

All throttled engines are least efficient at idle . . . it is hull efficiency that makes them more efficient at idle speeds (usually under or very close to hull speed).

 

[Silvertip]

Re: Fuel economy

Nope -- I won't flip out QC. Just so everyone knows, QC sees no value in a flow monitor and prefers MPG as his measurement. I prefer a flow monitor as it presents real-time numbers and can instantly tell you whether you have a fuel consumption issue. Without a flow monitor, MPG can only be calculated from a very accurate fuel gauge, and few of them are accurate enough to provide anything resembling "real-time" flow rates, or to measure the amount of fuel used after a trip is completed. Speed (instant and average), and distance are easily obtained with any type of GPS. It doesn't matter to me whether one is interested in MPG or real time flow rate. If one has a flow monitor, MPG can be very accurately determined and some of the fuel monitor systems provide the capability of doing both calculations. The long and short of this is that I find a very basic fuel flow monitor very useful. QC and I just differ on the numbers we want to see. I would love to see the U.S. make a full conversion the metric system as well but it's apparent the vast majority of citizens don't feel that way.

 

[QC]

Re: Fuel economy

Nope -- I won't flip out QC. Just so everyone knows, QC sees no value in a flow monitor and prefers MPG as his measurement.

This isn't accurate . . . I see no value in a flow meter for a moving piece of equipment UNLESS you calculate instantaneous MPG which I understand and agree with you on. But hang on as we may finally be getting to the end of this disagreement/misunderstanding . . .

I prefer a flow monitor as it presents real-time numbers and can instantly tell you whether you have a fuel consumption issue. Without a flow monitor, MPG can only be calculated from a very accurate fuel gauge, and few of them are accurate enough to provide anything resembling "real-time" flow rates, or to measure the amount of fuel used after a trip is completed. Speed (instant and average), and distance are easily obtained with any type of GPS. It doesn't matter to me whether one is interested in MPG or real time flow rate. If one has a flow monitor, MPG can be very accurately determined and some of the fuel monitor systems provide the capability of doing both calculations. The long and short of this is that I find a very basic fuel flow monitor very useful. QC and I just differ on the numbers we want to see. I would love to see the U.S. make a full conversion the metric system as well but it's apparent the vast majority of citizens don't feel that way.

We do not differ. With this complete explanation we are in sync. My difference was, (and you can look it up) that you said multiple times that MPG was meaningless . . . Flow rate alone is IMHO meaningless unless I am trying to calculate when my GenSet is going to run out of fuel. With speed and flow though, like you have said, you absolutely have the most accurate data with which to base conclusions. I always prefer more and ACCURATE data for any decision I have to make.

With the absence of instantaneous fuel numbers I use my previous fill and GPS trip info to keep a running average fuel economy in my on board trip computer (my head) . . .

 

[Silvertip]

Re: Fuel economy

I agree that average over time can be a fair predictor of what MPG for a trip in the future will be. How accurate is that number? Only as accurate as your records. So, we almost agree and that's a good thing. I also confirm that I said MPG is a rather useless measure for a boat and I still feel that way. For the viewing audience -- the only difference we had through all of this discourse boils down to instant vs average fuel consumption data. There are some benefits to knowing fuel flow rate however. Finding the sweet spot, having very accurate fuel remaining and fuel use data instantly available, and immediate indication of a fuel consumption problem are some of them. If that's not important to you then you will likely be happy with MPG. That number provides for lively discussion as you've noticed. But that number may not be what you get on the next trip. You can say the same thing for flow rate. But again, it's instant -- so you know immediately if there is an issue.

 

[marine4003]

Re: Fuel economy

Both Silver & QC are right its a matter of personal preference, for a while there I was delivering boats up & down the east coast,as the trick is to deliver as fast as possible,fuel consumption was watched, i much prefer a boat w/ a fuelflow meter,a chartplot coupled with GPS was the only way to get somewhat accurate fuel consumption. to determine speed over ground/GPH between waypoints,as it was stated there's alot of variables in this, current,being a big factor,a sweet spot on a 42' Grand Banks traveling with the current consumes less diesel than the same fighting the current,the same applys to smaller boats,more so considering the total lack of "economy" pulling skiiers and start stop conditions.

IC : 240 Mile Range...NICE!!!!!!!I think,wide open i get 24 MR