Gas mileage on 2008 280 with 8.1 litre

minnesota

Hi guys, I just finished about 60 gallons of gas on the 280 we just bought,  its a 2008 with the 8.1 litre with only 75 hours.  I mostly cruise at 3500 rpm at 30mph.  The Merc monitor indicates this is optimized for fuel consumption but I am getting only 1.4 mpg with 3 on board, half gas and water.  Plus the boat takes about 14 seconds to get on plane.  The engine feels struggling to get on plane.  Is this typical for this engine?  Any input is appreciated. 

Ezgoin

Not sure what your boat weighs, but my 2005 262 BR with the 8.1 and Bravo 3 loaded with 2 people, a bunch of stuff, water, and 50 gallons of gas will jump up on plane in just a few seconds.  I think my boat is 5600 lbs empty, so loaded it's probably an easy 1,000 - 1,500 lbs over that.  The engine should not feel sluggish!

Stodge

I wish I got got 1.4MPG LOL  I'm happy when I squeeze out 1.2MPG.

My old 280 had twin 4.3L V6s and we got 1.4 to 1.5 MPG most of the time.  I think you are doing ok.

Handymans342

Is your bottom clean? Props clean? 

minnesota

My boat is 7640 lbs empty, bottom not clean for sure. Props look clean, not stainless shiny but clean.

Cableguy Greg

Are your tabs down when trying to get on plane? Once on plane are you bringing them back to the up position? Are you trimming the drive up after the tabs are retracted and on plane?

I don't have exact times, but my 280 with a 6.2 gets on plane faster than what you are stating. When starting out, I have the tabs down, and drive down. Once starting to plane out, I retract the tabs then trim the drive up until I feel the boat lift higher out of the water.

LaRea

Bottom fouling can KILL performance.  A couple years ago, my 342 (a 42-mph boat) had moss so bad, I struggled to make 27 mph at WOT.  Thought it was bad fuel -- then I power-washed the hull and it was like a new boat.

Light slime might not hurt you, but moss or barnacles will definitely slow you down.

MarkB

On the weekend with 3 adults and 2 kids on board, 3/4 tank gas, full tank of fresh water, on my 260EC with 350 MAG got onto plane in about 7 or so seconds. At 30 mph, 4000 rpm, my fuel economy is 2 mpg. 

The 280 EC is a bit bigger, maybe 500 lbs. However, is 7640 lbs with the 8.1 litre on board, or another engine? With the 8.1 litre, I would think you are more than compensating for the extra load.  Besides my boat had around 60 lbs more fuel, and probably 250 lbs more people, so that would mostly close the weight gap. I'd say the majority of the difference in weight would be the engines 5.7 litre vs. 8.1 litre.

So based on that comparison I'd say something doesn't sound right, especially that time to plane. I would expect your setup to plane in about 8 seconds (similar to my boat) especially with only 2 people on board.

Make sure your trim tabs are fully down before opening up your throttle. Throttle to at least 80% of WOT.  Once up on plane, back off on your trim tabs enough to keep the boat stable and level.  After that lift the drive until you experience porpoising and then lower a little to eliminate that.

Other concern is bottom fouling - could easily be costing you 5 mph or more. I had this happen once on a Tahoe I owned, could not believe the difference after cleaning the boat.

Lastly, at 3500 rpm, it seems your fuel burn is around 21.5 gph (30 mph /1.4 mpg).  At 4000 rpm, my fuel burn is around 14 gph.  Your displacement is 8.1 / 5.7 = 1.42 of mine, or 42% bigger. It's not an exact science but I would expect your fuel rate at 4000 to be 42% or so bigger than mine, which is 14 gph x 1.42 = 19.9 gph. If I adjust the consumption down to 3500 rpm (1 complete displacement per rpm), so x 3500/4000) then the fuel burn is estimated to be 17.4 gph at 3500 rpm vs. 21.5 gph you are getting. This means you are burning 24% more fuel than estimated.

What could be the difference which would effect both fuel burn rate (gph) and plane time? Propeller specifications. What pitch is your props?

Stodge

Check out http://www.boat-fuel-economy.com/mercury-mercruiser-496-mag-8.1-fuel-consumption-us-gallons

It says the 8.1 should burn about 20GPH at 4000RPM.  Mind you that's an estimate.

MarkB

Stodge and that is exactly what my calculation determined. At 4000 rpm, based on engine displacement in comparison with my boat, it should burn 19.9 gph (close enough). 

The site you posted also says that the burn rate should be 15 gph at 3500 rpm. That's a lot less than the 21 gph Minnesota is posting.  I'm thinking prop size is the issue. 30 mph at 3500 rpm seems to be too fast.

Unless ..... your tach is reading wrong. Oh wait, maybe that could be it.  You should be at 20 gph and 30 mph at 4000 rpm. That would all make sense.

pepmyster

Great topic. I also have a 2008 280ec with the 8.1 Merc. Antifouling, all clean...I know that with a full tank of gas, 3\4 of fresh water and holding tank empty, I would take a long time to get on plane with 2 people on board. As Marc stated, you must have the 2 trim tabs all the way in the down position at the start. I never start with the throttle at WOT. Never. I don't believe in the straining it must give the coupler at that point. Once at planning speed, do all my trimming till perfect.  I swear, I would love to see my boat with a bigger engine, sometimes water current, weight in the boat etc affects all.

Stodge

I had an older (1997) 280 with twin 4.3s.  I think the newer ones would benefit from having twins.

minnesota

Great inputs guys, thanks. My props are 24" pitch, the bottom has some slime but I don't consider it real dirty.  I have the Merc Monitor so I don't think my RPM is off. Cable Guy, Mark B, thanks I will start with the tabs down. I came from a smaller boat and  didn't have trim tabs. So I am hearing the consensus is that I should get about 15gph or 2m/gallon at 3500rpm.  Pepmyster, what is your prop pitch?

MarkB

I get 2 mpg on my 260 EC, at 4000 rpm, 30 mph.  With that engine, you should be able to achieve 30 mph at a lower rpm, 3500 rpm could be right. I would think you would consume around 17 gph at 3500 rpm - so a little less than 2 mpg fuel economy, but not much less, may 1.8 or 1.9 mpg.

Also, you probably would see a noticeable difference in performance if you went to a 22P prop.  But that's for another day.

Absolutely, while running at idle, get your tabs ALL the way down. Only then start accelerating. Bring throttle to around 30%, 50% than 80% and hold (not good to jump straight to 80%, better to ease the drive into the force). Time yourself to plane. It should be definitely less than 10 seconds. 

pepmyster

My pitch? **** of a question. Not sure, E flat minor? Sorry, music joke.

Cableguy Greg

FWIW, my props are 22's.