How many miles per gallon???

[Asainz320]

I took my boat out today for the first time, i still need to fix the gas tank so i had to use those small red fuel tank the 6 gallon ones. I have a 200hp mercury. I knew it was going to drink lots of gas but not that much. In less than 10 miles it took more than 12 gallons!!!

How many miles per gallon should it be more or less? Maybe this doesn't exists with boats but im just curious.
Thanks

Alain.

 

[Asainz320]

Re: How many miles per gallon???

Or maybe is gallons per mile!!!8)

 

[LubeDude]

Re: How many miles per gallon???

If you were running flat out durring your run that`s probably about right, My 150 will do that too. If you back off to around 2/3 throttle you will improve significantly.

 

[Mike722]

Re: How many miles per gallon???

Rule of thumb is 10% of horsepower rating per hour. 200HP = approx 20 gals/hour.

 

[OldMercsRule]

Re: How many miles per gallon???

Hi Alain, Fuel consumption in boating is generally discussed in terms of gallons per hour, (GPH) @ wide open throttle, (WOT). The pros here state that 10% of your horse power should be the GPH @ WOT. Everything depends on a lot of factors. Boat weight, and efficiency, engine propeller configuration and engine tuning. Does the engine turn the proper RPMs for that particular engine @ WOT? This rough formula would state that the ten miles you covered took about 36 minutes @ WOT, (which would indicate a very heavy boat going 16.7 MPH which may not even be on step). It would be hard to know the actual distance anyway, unless you were using global positioning. Hope this helps. JR

 

[Silvertip]

Re: How many miles per gallon???

I have a fuel flow monitor on my 75 HP 2-stroke and it burns exactly 7.5 gph at WOT. 200 HP is 2.66 times the HP of my motor so 7.5 x 2.66 = 19.95 gallons/hour. The easiest way to estimate fuel consumption is to put a decimal point one number to the left in the HP rating (or use the 10% figure as was explained earlier). 200 HP = 20 gph. Since we don't know what your speed was, we don't have anything concrete to go on. But it does appear you have a heavy boat or one that may have some weight issues (such as trapped water).

 

[gss036]

Re: How many miles per gallon???

I run a 1989 carbed 200 hp Black Max on a SeaCraft 23 WA and I figure I get about 2 miles per gallon at about 4100-4200 rpm. So if I go 20 miles out, I can figure on using at least 20 gallons of gas by the time I get back to the dock.

 

[Asainz320]

Re: How many miles per gallon???

Hey guys thanks the answers.

My boat does 43 mph at WOT or 5500 RPM. That was using a GPS. I ran the engine most of the time at like 3500 RPM which was traveling at 21-23 mph.

 

[Silvertip]

Re: How many miles per gallon???

OK -- now we have something to work with. You went 10 miles at an average speed of 21 MPH which is roughly half of your WOT speed. So, at WOT theoretically the engine burns 20 GPH. But you only went 10 miles or 1/2 hour of travel so that equates to 10 GPH. Since the actual amount burned was 12+ gallons in that period, that's about right since 3500 RPM is not half of the 5500 WOT rpm. Again, at 4200 RPM my 75 HP engine burns 5 gph. 5 x 2.66 = 13.3 GPH for your 200 HP engine at roughly the same RPM. Amazing how that works eh?? You should have gathered by now that although the fuel burn rate is less at lesser RPM, it takes longer to get to your destination so you might as well run wide open, The amount of fuel burned to cover a given distance will not vary that much.

 

[OldMercsRule]

Re: How many miles per gallon???

Hi Guys, Silvertip's analysis is excellent, and prompts another question from me. It would seem that Alian's boat has extra weight or maybe a mossy bottom that is causing some inefficiency, or maybe he has not found the "sweet spot" where the boat runs most efficiently, (as far as distance per gallon of fuel). I run a number of fairly lightweight boats in mostly fresh water, with less power and likely less weight then Alian's boat. I lack a flow meter and usually do not bother with GPS so my impressions are not strictly varifiable, and maybe false, (although I doubt it). I have found that speed and distance on my boats are not sprictly linear functions of how much throttle I give them. If I have a lot of weight in a boat, or I intentionally keep the boat just off step, (planing speed), or just barely on step, the various engines on various boats use much more fuel per distance traveled, then if I find the "sweet spot" at 2/3 to 3/4 throttle where the hull speed is still decent but the fuel consumption is far less. Silvertip's analysis would indicate that I would just use the same fuel for the same distance traveled which seems very different then my experience where running 10% slower at 25% less throttle saves roughly 25% less fuel used for only 10% less distance traveled. Am I dreaming? Thanks JR

 

[OldMercsRule]

Re: How many miles per gallon???

Hi Guys, I just spoke with my brother in Alaska, who is also convinced that fuel constumption over various distances is much less then the consumption @ WOT if one significantly cuts back the throttle, (once on step: as Alian stated he did). He is a commercial fisherman and has run a lot of different kickers in various configurations and he frequently travels significant distances over salt water. He thinks (as do I) that either Alian's boat is either significantly underpowered, has the wrong prop, or most likely his boat has a lot of extra weight onboard. At 21-23 MPH the kicker would have run less then 30 minutes to cover 10 miles, and consumed 20% more fuel then the model indicates at far less then WOT, seems like a problem to me. I would weigh the boat and look for some water in the hull somewhere, or check for a fuel leak. JR

 

[LubeDude]

Re: How many miles per gallon???

Hey guys thanks the answers.

My boat does 43 mph at WOT or 5500 RPM. That was using a GPS. I ran the engine most of the time at like 3500 RPM which was traveling at 21-23 mph.

This seems to me to be a very inefficient speed. The boat would be pushing a lot of water even if it is on plane. Most boats without smat tabs are just barely on plane at those speeds. I think like others, he hasn`t found the sweet spot.What kind of a boat is this anyway???????

 

[Silvertip]

Re: How many miles per gallon???

My statement about running wide open was made "tongue in cheek" because in my case there is relatively little difference in fuel consumption. Here's why. At 39 - 40 MPH the burn rate is 7.5 g/h. At 30 MPH its down to 5 - 5.5 and that is a nice comfortable cruise speed (boat does have smart tabs). That says the burn rate is 27% less at 30 than it is at 40 MPH. That may seem great but consider that we are discussing burn rate, which has nothing to do with distance. Using those same figures if I had to travel 40 miles I could do it in an hour and I'd use 7.5 gallons of fuel. Or I could travel at 30 MPH and only burn 5.5 g/h. But since in an hour I've only gone 30 miles, I need to run longer -- 25% longer. .25 x 5.5 = 1.38 additional gallons of fuel I need to make the last 10 miles. 1.38+5.5 = 6.88 gallons or roughly 9% total fuel savings in a 40 mile run over running wide open. I certainly agree these numbers are not the same on all boats, all engines, all speeds, all loads, and all water conditions. Every combination does indeed have a sweet spot which will equate to a certain amount of fuel savings. What anyone considers "significant" is in the wallet of the beholder. I again remind everyone, I use a fuel flow monitor. If one is using the gas gauge as a fuel economy tool it is terribly inaccurate and only carefully measured fuel vs distance covered or speed can provide meaningful fuel economy numbers. I have three measures of speed (GPS, locator speed, and pitot speedometer). I would also steer those who have fuel economy concerns to visit the Yamaha web site. They have more boat/engine test reports than anyone, complete with fuel economy (M/G and G/H). Pick your boat, pick your engine (HP and 2/4 stroke) and read the reports. Just be certain you understand the net effect of speed, "burn rate" and miles per gallon numbers. And by the way, pay attention to the WOT numbers for comparable 2 and 4 stroke engines. You'll see they are very close on the same boat. I could rattle on here but this topic gets beat to death so I'm outa here.

 

[OldMercsRule]

Re: How many miles per gallon???

Thanks a bunch Silvertip, That is a real eye opener! I would have guessed an approximate 15% savings but that is not very far (in absolute magnitude) from the 9% in your actual measured example, (not just a raw guess based on uncalibrated experience: as mine admittedly was). I love these forums, and the knowledge one can pick-up here. I think I will check out how much head ache a fuel flow moniter to duplicate your findings on one, (or if I could make it portable: more then one), of my old boat/motor combinations would be. Do you know where I would start, (all of my large kickers are fairly old inline 6s and the fuel is mixed, which may rule out a fuel flow monitor like you have). My power to weight ratios seem much more favorable on my high powered combinations then Alian's is, as my boats are faster then 43 MPH, and much faster then 21-23 MPH @ 3500 RPMs. As to Alian's set up; he still looks like he is burning 20%+ too much fuel and that could be costly on a 200HP kicker, so I still recommend he looks into the weight of the vessel. Thanks again JR

 

[Silvertip]

Re: How many miles per gallon???

I have a Navman 2100. They have a couple of models depending on HP. The turbine needs to be sized for the flow rate of the particular engine. The standard version handles outboards to about 200 HP as I remember. They are reasonably priced ($150 or so depending on where you buy), easy to install, and could be made portable by just installing a separate turbine on each boat. Put the head unit in a plastic box with a connector arrangement. These units are nice because they work just like the fuel computers in your car. They track total fuel burned, total since fill, and fuel flow rate and fuel remaining. I installed it because my tank is long and shallow so boat attitude really messes with fuel gauge accuracy. The monitor eliminates any guessing since I routinely make 50 mile trips in a remote area where there are no fuel pumps. The only setup is an initial calibration which is close out of the box but can be verrrry closely when dialed in over time. You need to tell it how big the tank is. It makes no difference if you premix fuel or have oil injection. As for fuel savings, 10% in my case is not a big deal but when flow rates get above 20 g/h 10% or more improvment in economy can amount to some serious cash.

 

[Asainz320]

Re: How many miles per gallon???

Ok guys BIG MISTAKE in my part. Today i was checking with google earth all my route that day and messured it with the mesuring tool on google and for my surprise i traveled 27.6 miles give or take a few. i didn't think i had traveled so much since it was just in the bay from the dock to my friends house and some test runs from marker to marker.

Im really sorry for the mistake. Also i need to clean the bottom of my boat since it has some "barnicles"(spelling) living under there.

 

[JasonJ]

Re: How many miles per gallon???

Other things that affects your fuel economy is wind (obviously) and water condition. You will achieve your best economy with a light chop on the water (about 6 inches or so). Perfectly smooth water can actually hold the hull back whereas the light chop breaks the surface tension of the water. heavier chop creates resistance, you go slower and invariably give it more throttle to even keep it on plane let alone go at a desired speed.

 

[timmathis]

Re: How many miles per gallon???

Face It. We have to pay to play at fuel cost today for sure. Only thing we can do to help is back off a little, And to keep them well tuned. Tim