Boat MPG ? 120HP Evinrude

I'm every so looking to summer in my new boat, but i have no idea how much gas my 120hp VDR or VRD or something 1989 evinrude will drink on the water... Obviously like a car, it will all depend on how i drive it...

So Questions...

At what Water speed, is a boat comprible to a car at getting max MPH (You know, cars at 55 get the best gas mileage.

Ive got a 25 gal tank, if i was going at 30mph lets say, how long/far could i get...

Or, if i was going full out 45-50 mph, how long would it last ?

Clearly these numbers vary in a large number of circumstances, but just looking for general info, so i can get a better idea how much i'll need for what lakes/rivers i'm going to go on this summer...

PORTLAND TO ASTORIA BABY !

Re: Boat MPG ? 120HP Evinrude

Two of the manufacturers that I know of have performance sites that will give you that information for the boats their motors are mounted on. Since you have an older motor and you gave no boat information, it will be hard for any of us to tell you what the most efficient motor rpm will be.
Lets just say that for most of those I have been looking at, any where from 3000 to 4000 rpm seems to be most boat/motor combo's sweet spot.
Be prepared for many other opinions!
You will need to track some of that data for yourself. Just don't trust your fuel gauge. They are notorious for being wrong. Go by fill ups. And having a gps with tracking data will make all of this a lot simpler. Good luck!

Re: Boat MPG ? 120HP Evinrude

Sorry, its a 1989 17 Ft SeaSwirl Tempo

Re: Boat MPG ? 120HP Evinrude

OK, now I remember your boat and your story. Quite interesting......

Here's the best I can do for now. This data is from Yamaha's site. I used a 115 2 cycle with a 17' boat and it gave these numbers. This should be a ballpark. Remember, you will need to do something that you can rely upon so you don't get too far in the wrong direction and get stranded!

http://www.yamaha-motor.com/assets/...in_2stroke_hpmidport_pro_str-476sf-115tlr.pdf

Re: Boat MPG ? 120HP Evinrude

You have a V-4 Rude with what is called the VRO system for blending TCW-3 oil in the proper mix with the fuel. This is vital. As to fuel consumption you will get answers all over the place here. If you want to nail it buy or borrow a 6 gal tank and take it with you next trip and use it to measure whatever you like.

Re: Boat MPG ? 120HP Evinrude

Fuel consumption can vary greatly each time you go out depending on how the boat is loaded, current or tidal conditions, and wind.

On a boat fuel consumption is usually measured in gph or gallons per hour not mpg. Getting just a little more speed past your "sweet spot" of fuel economy can be very significant for example my current boat burns approx. 11gph @ 3200rpm going 28-30mph if I increase my rpm to 3400 I only gain 2-3mph and my fuel burn goes up to 19gph in my opinion its not worth it to go a little faster. you will need to experiment with your boat or you can install a fuel flow meter to monitor it more consistently.

Re: Boat MPG ? 120HP Evinrude

triumphrick said:

OK, now I remember your boat and your story. Quite interesting......

Here's the best I can do for now. This data is from Yamaha's site. I used a 115 2 cycle with a 17' boat and it gave these numbers. This should be a ballpark. Remember, you will need to do something that you can rely upon so you don't get too far in the wrong direction and get stranded!

http://www.yamaha-motor.com/assets/...in_2stroke_hpmidport_pro_str-476sf-115tlr.pdf

Click to expand...

Holy crap, 4.67 MPG at best ! Jeeeeze, need to get a second job, to pay for gas, no wonder people put 50hp on 16ft boats lol. I can't remember if my boat has an RMP or just Speed gauge.

Re: Boat MPG ? 120HP Evinrude

Take it easy, don't blow a hoolie...

Solittle gave some good info. My stats were just ballpark, using a 2cycle carb newer Yammy. We don't know what your boat is going to get yet, so you will need to run some tests yourself. As said previously, stats for your boat will be very hard to find....

Re: Boat MPG ? 120HP Evinrude

4.76 MPG seems pretty good to me. Not sure what my 21' with Merc 260 HP gets but I'm sure it's not that much. The whole boarding thing probably doesn't help MPG though. It's very true that a couple MPH past the sweet spot can significantly increase fuel consumption. Once you get into the enrichment circuit of the carb you're dumping fuel into it for small gains in speed.

Re: Boat MPG ? 120HP Evinrude

slasmith1 said:

On a boat fuel consumption is usually measured in gph or gallons per hour not mpg.

Click to expand...

slasmith, this is not an attack on you, more of a pet peave of mine . . . but I dispute the above statement as ultimately MPG is what you are trying to understand so that you can calculate range . . .

slasmith1 said:

my current boat burns approx. 11gph @ 3200rpm going 28-30mph if I increase my rpm to 3400 I only gain 2-3mph and my fuel burn goes up to 19gph

Click to expand...

And, I believe my point is made here. When you add in the speed (which we always do) you have just calculated MPG. Without speed, GPH is useless, with speed you have MPG . . .

Re: Boat MPG ? 120HP Evinrude

You should do better than 5, but I really don't know. I get 1.6 mpg at optimum cruise.

Re: Boat MPG ? 120HP Evinrude

I don't think he'll ever do significantly better than 5. An 18 Stingray with a 140 I/O (4 cycle) eeks out 6. One of the best there is and although both are carbed, carbed two cycles usually suffer in this comparison . . .

Re: Boat MPG ? 120HP Evinrude

Solittle said:

You have a V-4 Rude with what is called the VRO system for blending TCW-3 oil in the proper mix with the fuel. This is vital. As to fuel consumption you will get answers all over the place here. If you want to nail it buy or borrow a 6 gal tank and take it with you next trip and use it to measure whatever you like.

Click to expand...

I've got plenty of 6gal tanks, but the motor is connect direct to the inboard fuel tank, not sure if i wanna try and take that off, to plug on a 6gal, not sure if the line is even big enough to power the motor, then again, i have no idea for sure.

Re: Boat MPG ? 120HP Evinrude

A 20 year old carbed 2-stroke? Probably be lucky to see 4 MPG at best. If you like running wide open or pulling skiers, tubes, etc, expect around 2-3 MPG. Your most efficient cruise speed will probably be in the mid-to-high 20's.

Very nice looking boat!

Re: Boat MPG ? 120HP Evinrude

Well, here you go now. It's time to learn a little and time to get familiar with your boat. The more you can do mechanically the better prepared you will be when things break. And, yes...things will break.
Is there a water/fuel separator that you can break the line from your tank and tap into it? They usually use black tie wraps. ( You will need a bundle of them)
If there is not one, buy one and install it. It will have 1/4 or 3/8 fittings. Your motor will probably use 3/8. Now you have a disconnect point to tie in the temp tank to help you track your mileage. It would be good to have a handheld or portable gps to track your miles. That gas used and miles navigated will give you the mpg you are looking for.
Hopefully we can get a fellow iboater from your neck of the woods to help you out a bit. If you were here it would be me...
If you buy your separator at a marina, you may get some help from them as well.
Anyway, not too daunting of a task and a good way to get your feet wet...
By the way, get a manual for that motor!!

Re: Boat MPG ? 120HP Evinrude

I dunno about anyone else, but I've always been happy to get 2mpg from an old carbuereted 2 stroke engine. I do my planning for 1mpg.

One key thing is to use no more than 1/3 tank on the trip out, not 1/2 the tank's capacity. That way, you have margin to get you back home in case the tides, wind, 500 pounds of caught fish now on board, or other issues crop up on the return trip.

-BWR

Re: Boat MPG ? 120HP Evinrude

Boats get terrible fuel economy. It's a lot harder to push water out of the way than air. As a general rule, a typical powerboat with a big engine and a planing hull gets its best fuel economy near the minimum speed at which it will reliably stay on plane. If you speed up, the hydrodynamic resistance increases faster than your speed. If you slow down, hydrodynamic resistance increases dramatically as you come off plane. Resistance drops dramatically if you slow down even more (to the point where the bow drops back down and you are no longer generating much of a wake), but using a 120 hp engine to produce the minimal power you need to stay at that speed is very inefficient, so your fuel economy won't improve that much.

I can get about 2 km/L (5 MPG) out of my old 2-stroke outboard, but it's a 70 hp on a 14.5 foot boat rated for 75 hp maximum.

Re: Boat MPG ? 120HP Evinrude

And I disagree (to a point) with QC on fuel flow vs MPG. MPG cannot be calculated until the trip is over UNLESS you have a flow meter and an accurate speedometer (as in GPS). Without GPS you have no idea how far you've gone so MPG cannot be computed accurately since your watch/clock and an inaccurate speedometer is your measure. The purpose of a flow meter is to find the speed or rpm that provides the lowest flow rate yet acceptable (to you) cruise speed. That number will vary greatly depending on water conditions, load, wind etc. So if you made a very calculated run and actually computed MPG, lets say you came up with 4.0 MPG to keep things simple. On a 25 gallon tank that would get you 100 miles. But the question is -- "at what speed and what load". So my point is, if you used that number and the next trip out planned to go 75 miles but with a heavier load, higher wind and bad water you might not make it. Since you very likely have a rough idea how far it is to your destination, you can run up to whatever cruise speed you want, check the flow meter and tell in an instant if you can get there on the available fuel. Not only that, but if the engine typically flows somewhere around 10 gallons per hour and you suddenly show 15 at the same speed it used to flow 10, you know you have a problem and can head back to the dock. Here is where I agree with QC -- when the trip is done those numbers can translate to MPG but only after the trip is done (or you are out of fuel). The more instrumentation you have the better but MPG is still a trip average or an instantaneous measure like on your car. But even on your car -- coasting downhill yields an instantaneous 60 MPG while the actual trip average will be whatever the vehicle normally gets -- and it won't be anywhere 60 MPG.

Re: Boat MPG ? 120HP Evinrude

Play it safe the first 2-3 times out. If you think you'll get around 4 mpg, calculate your range for 3 and use that. Once you have 2-3 tanks worth of data you'll have real numbers to live by.
Also, when calculating range use 80-90% of full fuel load (20-22 gallons in your case) so that you have a reserve calculated in.

FWIW-My old 15ft Trihull with an 1984 50hp 2 stroke Merc averaged 3.1mpg.
My 21ft Wellcraft with a 300hp carbed 454 V8 averages 2.4mpg.

Small motor =/= dramaticly better mileage.

Re: Boat MPG ? 120HP Evinrude

QC said:

slasmith, this is not an attack on you, more of a pet peave of mine . . . but I dispute the above statement as ultimately MPG is what you are trying to understand so that you can calculate range . . .

And, I believe my point is made here. When you add in the speed (which we always do) you have just calculated MPG. Without speed, GPH is useless, with speed you have MPG . . .

Click to expand...

Not exactly. Range is usually measured between points on a map, or distance over ground. A gps gives you distance over ground traveled and speed over ground.

However, a boat is different from a land vehicle in that the "ground" can move under it if it's stationary. Using a paddle wheel to calculate mpg works a bit better since it measures water flow under the hull, but it's still possible for the boat to hold stationary relative to the ground while expending fuel that might move it at full speed otherwise (like a fast river) or vice versa, expending less fuel to move a given distance if the tide is with it.

Generally I like to make my calculations of range based on gph because that averages in the slow running, fast running, maneuvering, and water flow directions and rates over that amount of time.

But since I really wanted to know where it was most efficient to run the boat I put in one of the lowrance fuel flow meters... cost was about $200 and it displays gps position and over ground speed too.

Erik