Fuel burn question.
 

How much fuel would you realistically expect to burn cruising at 23 knots for 32 miles and just puttering around while fishing and drifting? 1990 merc 115 on a light 20 SF. calm seas. 50 gal tank started at 3/4. Thanks

Might be of no help whatsoever but, my 18SF with a 1998 Merc 115 has so far burned 20 gallons (half tank), which includes:
A roundtrip run from Meetinghouse pond in Orleans to Chatham Light in Pleasant Bay.
A roundtrip run from Sesuit harbor in Dennis to Billingsgate Shoal and trolling for 3 hrs
And tubing on Long Pond for about an hour.
Had to fill up the oil tank once so far, but she's still burning up that first tank of gas.

Sorry for the fairly vague parameters...

thanks Zak. Heres why i ask. i installed a brand new 49 gal fuel tank and sender this winter. left barnstable harbor 2 weeks ago with just over 3/4 tank and steamed to ptown to fish which is roughly 16 miles. once there we just idled and drifted for the most part. 16 miles home to barnstable. gauge read, about an 1/8th at the pier. yesterday, i wanted to fill it up again and was only able to put 17 gallons in before it came out the vent and burped out the fill twice.

got u/w last night and guage read between 1/2 and 3/4. 15 miles at cruising speed and again idiling around looking for fish. than back at the pier last night read 1/2 tank.

assuming that when i left the first time i had about 36 gallons. when i returned by gauge told me that i had about 6 gallons left. should i have burned 30 gallons during that ptown trip? or is it more likely i only burned 17 gallons which is all the tank allowed me to put in yesterday before my night trip? thanks for any help.

My guage is way off at anything but displacement speed. Just under full on plane can read just under half. Trolling against the stream here, I can see negative mpg on some days. Crazy, I know.

So is it safe to assume I have either a bad gauge or more likely a sender, despite the sender being brand new this year? Thanks

No. At displacement speed, the gauge is most accurate. On plane the go go juice splashes around and reads much higher as the sender is at[normally] the rear of the tank
or close to it.

The boat was filled on the trailer which sits pretty level. I was expecting to put in 40 gallons according to my gauge reading 1/8. I was then only able to put in 17 gallons before it came out the vent and burped out the fill twice. The gauge then read 1/2 but all my senses were saying the tank is full. The sender is at the very rear of the tank and the fill at the front. There must be something wrong. Right??? As for my first post, is it more likely I burned 30 gallons over 30 miles or 17 gallons over 30 miles?

I always like to run with a full tank. You just never know. I do your runs almost to a T.
I can`t slow down and get 10+ mpg with a strong wind and stream. Not even close.

Cheers,
GFS
Pigs on the wing.

Chance,

I made 6 trips to Abaco in the Bahamas back in the late 70's/early 80's with a 1975 115 Evinrude, back when they were rated at the powerhead instead of the prop, so it probably wasn't much over 100 hp at the prop. Running with a very heavy load (dive gear, 2 wks worth of food, etc.), and 65-80 gal of gas, (35 of which was even in the main tank!), I averaged 2.8 mpg pretty consistently, burning about 8 gal/hr and cruising about 20 kts. With a lighter load and more modern motor, you should do at least that well. Denny

I have an older Merc I-6 on a 20 Seafari. At the cruise speed you mention I am consistently getting 2.5 mpg w an all day fishing load and a full 22 gal bait tank in front of the splash-well- barely 1.5 mph at idle speeds. I would expect you would do 10 to 15% better with a 4 cyl.

Your meter readings are similar to something mine was doing. Is it possible that your fuel meter and sending unit are operating at different ohms? I found someone had put an automotive or euro fuel meter on a marine sender in mine and units were behaving like unfriendly neighbors. One operated at 0-90 Ohms, the other at 33-240 Ohms. My old sending unit had even been tweaked (ineffectively) to accommodate to ohm variance. I swapped it all out for a WEMA reed sending unit and meter and am very happy with the results.

Heres a simple explanation from Autometer:
http://www.autometer.com/tech_faq_an...x?sid=1&qid=36

GFS, I'm not trying to get 10 mpg. I'm trying to determine what the problem is. Thanks bushwhacker and McGillicuddy, excellent info of your experiences. That is what I was looking for. Also thanks for the link. I will do those tests and check the ohms on the gauge.

So I pulled the sender and tested while hooked up to the gauge still and it works fine. The gauge reads about half right now. With the sender out I manually sounded the tank with a stick and sure enough it is only half full. The pump I was using is by no means fast, so what would cause fuel coming out the vent at half tank and burping out the fill. I guess I will try to fill it super slow next time but any thoughts out there or experience with this? Thanks

Are you absolutely 100% positive that your vent line runs uphill from the top of the tank to the vent fitting?
Any sags, kinks, pinches or loops can fill with gas and give you a premature burp.
And I mean any. Just a tiny low spot can collect liquid, and if you've had to squeeze the hose at all (i.e. make it not round) to fit through anywhere or have pinched it with something, it's pretty much game over.
Same goes for the fill.

I have the same issue. I have to fill very, very slowly or the pump will kick and stop. The vent line may be too small in diameter for the flow. I recently went to a different station where the boat was tilted forward a good bit. I could pump a bit faster and didn`t get the burp til full.

Now your thread got me thinking. Dangerous, I know. If this e 10 is swelling the inside of delivery lines, would it not do the same to vent lines? Alcohol is a strong solvent and I avoid e10 like the plague. The station on Blue Heron and Old Dixie also has rec 90 now.

Is the vent line also foamed in? What I cant figure out is why the vent burps at all before full.

Cheers,
GFS

thanks for the reply guys. the fill is dead vertical. i fill through the top of the seat on the console and it goes straight into the tank. there are no sags in the vent line, i fixed that prior to sealing the deck up. i have filled it to 3/4 before with no burping. i'll just try to fill it slow next time and see how that goes. i have emailed the builder to see if he can offer any intel. i'll let you know what i find at fillup at a different station.

I think Zach is right about gas in the vent line restricting the vent. If there isn't a positive slope from the tank all the way to the vent outlet, you could have fuel trapped in there. Another potential issue is where the vent is located on the tank relative to how the boat is sitting in the water or on the trailer. The fuel fill on my Seafari is on the starboard side a couple feet from the transom. The tank vent is near the front of the tank a couple inches in from the port side. It's designed so that if I'm refueling at a marina the boat the boat will be heeled to starboard and down at the stern with my weight in the back, the vent is at the high spot on the tank.

However when boat is on trailer at a gas station, the front of the tank is lower than the back, so the vent will spit gas before the tank is full. When I get home and raise the front of trailer, it will typically take about another 5 gallons to get the tank completely full.

Regarding your fuel gage, those wiper type sending units can be notoriously inaccurate because if you don't use the boat frequently, oxidation can build up on the resistance coil and wiper and change the resistance and gage reading! I've noticed mine often does not show full until I've burned off enough gas for the float to start bouncing around to clean off the wiper surfaces, and of course by then it's not full anymore, but it usually reads higher than it did when I first started out with a full tank! A Weema sending unit like Gillie has should eliminate that problem.

One thing I did when I first got the boat was to calibrate the fuel gage. I was running premix then, so it was a big help to know about how much it would take so I could add the proper amount of oil if refueling at a marina. The calibrated gage was accurate enough that I could usually predict within a couple of gallons how much it was gonna take! Here's how to calibrate your gage, starting with an empty tank:

1. Put a piece of white vinyl tape across gage. If it's just a simple 90 degree sweep gage, maybe use two pieces of tape, above and below needle, so you can put reference marks at each end of needle. A fine Sharpie marker works good and the ink doesn't fade.
2. Pull the sender, measure the inside height/depth of the tank, and then make sure the float is adjusted so it just touches bottom of tank at one end of it's travel, and that it's also at the other end of it's travel when it touches the top of the tank.
3. Hook up battery charger so battery is at same voltage it'll be at when motor is running.
4. With the sender still out of tank but connected to the gage and ground, turn on power to the gage and mark Empty and Full positions of needle when sender is at the extremes of its travel. Then install sender and recheck your E mark.
5. Put boat on a level surface and use trailer jack to get it level fore & aft.
6. I then filled the tank in 6 gallon increments, since I was running the 50:1, 1 pint/6 gallons oil mix, and I had 3 portable tanks. Wait for gas to quit sloshing around and needle to stabilize and then mark gage after each increment. If not running premix, maybe 5 gal. jugs would be more convenient.

Although the E-TEC's I-Command gage system tracks fuel usage and is usually accurate within a gallon or two, I did notice on one occasion on a long trip that the gallons used number had stopped increasing so I had to reset the gage to get it working again. Since running out of gas can ruin your day, I like to have an independent measurement to keep the electronics honest!

Here's another tip that makes it easy to drain the tank. Most Racor type filter heads have more than one outlet, so just install a male outboard fuel line fitting like those in portable tanks in one of the outlets. I made up a 3/8" drain hose with a female fuel connector in one end, a primer bulb about 6" away, and then about 6' of fuel line at the primer bulb outlet. I use my boat as my emergency fuel supply for my vehicles during hurricane season, so if we have a storm that knocks out power to the gas stations, it's easy to just run that hose out the bilge drain hole into a 5 gal jug to get gas to keep the cars or a generator running without the hassle of trying to find a gas station with power when everyone else is trying to do the same!

bushwacker, what a great post. Thank you. after over thinking this i have come to the hypothesis that when i lower the trailer onto the truck that the fwd end of the tank is lower than the back end. this would cause the vent to collect fuel before the sender sees the same amount. my vent is on the fwd 1/4 of the tank the fill is aft of that by about a foot or so and the sender is 11 inches fwd of the aft end of the tank. i may be able to solve this by just getting a 4 inch raised hitch vice the 1 1/2 inch i have now. i have the wema brand sender and it works fine when i pulled it yesterday. i'll fill it today and try to monitor cruising RPMs and distance traveled when i head out tomorrow. i still think i'm buring to much fuel. i am terible with mechanical stuff so what are some quick easy things to imporove fuel burn, ie, plugs, timing, etc... i will definetly do that gauge cal you talked about later as well. thanks guys.

Hi Elijah,
I replied to your email yesterday. Just to go over the points I mentioned. If it gurgles from the fill, then it's not venting properly. If it spurts and gurgles from the vent, then the vent is under or near the fuel level in the tank when filling. Next time you go to fill up, fill up as usual and stop when it spurts fuel from the vent. Go ahead and disconnect from the truck and jack it up a bit. Continue filling. If it doesn't spurt any more, then you have isolated the problem. You may have to jack up again if it's really sitting low in the front. You can use a raised hitch or you may be able to tweak your trailer a bit so the bow sits a little higher. Let me know what the outcome is.

Tony

Thanks Tony. I did jut that at the pump today and it filled up perfectly. I guess I'm a little paranoid. Thanks for all the info guys

1. Make sure engine height is correct. Top of the anti-ventilation plate should be out of the water when you're up on plane, so next time you're out have a buddy drive and take a look over the transom to see what's going on. With a motor hanging on the transom, this typically requires the it to be mounted with the AV plate at least an inch above the bottom. Some props with lots of cup like a Stiletto can run even higher, so one option would be to keep raising motor until it ventilates on sharp turns or has to be trimmed down all the way and then drop it down one hole. Less lower unit in the water = less drag = better mpg.

2. Make sure your tach is accurate. Some analog tachs can be off by several hundred rpm, and if it is, you'll end up picking the wrong prop! On the E-TEC's the digital I-Command gages get rpm from the engine computer, so no question on their accuracy. Don't know about carb'd Merc's - maybe have a dealer or mechanic with a known good tach or a strobe type tach check it out.

3. Find out the OPTIMUM WOT rpm for your motor and pick a prop that turns that with your max load, or at least the load you normally carry. This is a much narrower band than the "Recommended Operating Range", which is 4850-5850 on my E-TEC, but it's OPTIMUM range is 5300-5500, so I'm propped to turn 5400-5450 with max load. A Recommended Operating Range is published for all engines, although I've only seen Optimum rpm recommendations published for BRP motors. If you can't find a published optimum, you'll probably be close if you shoot for the middle of the range. (Or PM Big Shrimpin - he seems to know Merc's pretty well!) In general a motor that revs easily and ISN'T lugged down to a low WOT rpm by too big a prop will have better hole shot and will have better durability because the pistons will run cooler and cleaner with less carbon build up.

4. I'm not familiar with Merc's, but on the carb'd OMC/BRP motors, with the motor off, if you have someone advance the throttle at the helm while you watch the linkage, you'll see that the carb throttle plates barely move for the first 2/3's or so of the throttle travel . . . all it does is advance the spark! Then the throttle plates open almost 90 degrees during that last 1/3 of throttle travel! I used to cruise at 4500 rpm at 20 kts, and at about 4700 the motor got much louder (visualize the sound of a toilet flushing with all the gas pouring thru the carbs!) and seemed to put out a lot more power, so I think that's where the carbs were really starting to open up. You'd obviously like to be able to cruise at a throttle setting below that rapid-opening threshold. The service manual described a "Link & Sync" adjustment on the linkage where a roller on the carb throttle lever is supposed to line up with a mark on a cam that's connected to the spark advance lever. If this spark advance and throttle linkage isn't adjusted right, you'll lose performance, so this is worth checking, as is the basic timing at idle. The Merc service manual probably shows a similar adjustment.

5. The style of prop you run can make a difference in your gas mileage. It seems like 4B props or 3B props with a lot of blade area like the Merc Mirage or BRP Rebel that have a lot of stern lift will hang on plane better at low speed and give better mpg, although they may cost you a few mph on top end. The bass boat guys that want to run fast in flat water like bow lifting props to get as much hull out of the water as possible, but that's just the opposite of what you want on an offshore boat! And of course, for max mpg you'll want to run a stainless prop with thinner blades and less drag than an aluminum prop.

Hope this helps. Denny

Denny, again an awesome reply. It will take me many reads to digest all the info you provided but I will do it. My av plate is 1 inch above the keel. I'll take a look tomorrow while running and also try to set a trip gauge on my gps and monitor cruising rpms and knots. I swing a 13 3/4 X 15 merc prop from,west marine. I'll look through the book tonight for wot and optimal.

That's the same size prop I ran on my 115; it was an OMC SST and was perfect for heavy loads - used to turn up to about 5400-5500 with a 2:1 gear ratio. Your boat is probably lighter and that more modern motor is rated at the prop instead of the powerhead, so if your gear ratio is the same I would think you could turn a little more rpm. If it overrevs, a good prop shop could put a little more cup in the blades to knock it down by 100-200 rpm which would also help efficiency, but you need to the motor height right and check the tach before fooling with the prop.

I used to figure about 8 gallons/hr at 4500 rpm and 20 kts. We didn't stop much on those trips, running for at least 3 hours at a stretch, so that may be why my gas mileage was a little better than Gillie's. Stopping often and climbing up on plane again burns lotsa gas!

If you're running the Quicksilver Black Diamond, I think that's about as good as its going to get unless you ante-up for the stainless version, the PowerTech MQF3 or 4. I may be mistaken but I believe yourl motor was known a 2+2. Designed for bass fishing circuit, it used a funky cylinder/carb deactivation where in idle it would burn on top 2 cyls, and when it hit around 1800 rpm, the lower 2 would kick in and it would take off. Kinda no mid-range slow putting - slooow or GO! Sloppy idle but nice at cruise. It's very possible that the carbs could be re-jetted for better efficiency with modern fuel. 2.07 gear ratio I think, 5200 rpm target should be about right w that prop.

Glad you got good insight on the vent burping issue -- good call, y'all.

I use a Navman fuel meter, very similar to Lowrances current version to keep me in the loop on fuel, but a 6 gallon portable tank should get you to P'town easy. Might be worth rigging just to see if your motor is burning right. I think you should see over 3 mpg overall as that idle burn with 2 cyls. should be much better than what I could ever hope to see in my I-6.

Lots of god stuff coming out of this post. Thanks, and good luck:cool:

Thanks for all the replies on this and I agree, there a lot of good info on this. Today I started with a full tank, 49 gallons. Cruised to ptown at 4200 rpms, bout 18 knots cause of some slop up to ptown which is 18 miles away. Cruised around trying different spots, trolled for a few hours drifted for a few more hours, ran up to peaked hill bar and trolled spreader bar at slack, ran 20 miles back to Barnstable at 4400 rpm at 23 knots. Trolled around the channel there for an hour and cruised back to the pier. Best I can figure is 50 miles, mostly running pretty good. Tank still above half. I put new spark plugs in yesterday and it was good I did cause they were bad. Now she starts a lot easier and prob helps my fuel a little.

My engine does go from slow to fast with not much I between, just thought that was a quirky 23 year old engine thing. Great info everyone. So I'm guessing I burned 23 gallons over at least 50 miles. I can live with that...

I forgot to mention how awesome the ride was this morning when it was sloppy out there up to ptown. I found myself saying any other boat would turn around or slow down to 10 knots. Didn't even feel em when I took them head on. It's only when I took em off the starboard bow did I notice it was rough when I noticed a lot of water hitting me in the face. Great ridding hull, a little wet, yes but what a ride! Can't wait till my buddy puts his 23 Parker in the water and runs next to me in my 20 in some cop. I'm curios how we will compare