340 DA mpg problems

[BahamasBlair]

On a recent trip of 86 nautical miles, on very calm water, no wind, 4 people on board, she burned 143 gallons of gas. That works out to almost 1.7 gallons per mile! We ran at 3250 RPM that gave us 19.5 Knots. 8.1S Horizons.
Hull recently painted, Propspeed on the shafts and props, new spark plugs, new engine oil, new exhaust manifolds, boat in great condition.
Any help in solving this problem is appreciated. Based on the numbers in boattest, she should be getting around .80 miles per gallon, not .60 miles per gallon.

 

[Rum Runner]

Someone with better knowledge on this issue will probably give you more definitive information. But, from my experience, what you described sounds about right. I have a 340 with twin 454's, granted it is a "87", but my fuel use is about the same. One thing however, if your running at 3250 rpms for an extended period of time, you'll burn up a lot of fuel. If someone feels differently, let me know, but I think most will run under 3000rpms after the boat is planed out. Hope a little bit of this helps. R

 

[magster65]

On a recent trip of 86 nautical miles, on very calm water, no wind, 4 people on board, she burned 143 gallons of gas. That works out to almost 1.7 gallons per mile! We ran at 3250 RPM that gave us 19.5 Knots. 8.1S Horizons.
Hull recently painted, Propspeed on the shafts and props, new spark plugs, new engine oil, new exhaust manifolds, boat in great condition.
Any help in solving this problem is appreciated. Based on the numbers in boattest, she should be getting around .80 miles per gallon, not .60 miles per gallon.

What were the tides doing?

 

[fc3]

2005 340 is a different hull from the 1999-2002 340 and is a very different hull from the older boats in the 80s. I'm thinking, though, that your speed was a bit low and the hull was operating inefficiently. But I don't have hard data to support that.

Best regards,
Frank

 

[osd9]

Just for conversation, you averaged ~.7sMPG.

The numbers.....143 gallons....is that from Smartcraft or the fuel pumps? The 86 nautical miles....is that from GPS or ST60 wheel and where you diligent in reseting at any stops? Do you feel confident in the accuracy of the numbers?

 

[osd9]

I just re-read your post. I see you are only making 19.5 Kts at 3250 RPMs. That in itself seems low. In Hampton's recent trip, he was posting a higher cruise speed at similar RPMs in the same boat...and he was running heavy. Do you know what WOT RPMs are? You may be underpropped and need to run at a higher RPM to get the boat fully on plane and efficient. Is the speed from GPS or the ST60 Wheel?

 

[barryg]

I also think you were actually running too low speed and were not as efficient as you needed to be. 19.5 is probably not even close to optimum cruising speed.

On a recent trip to Bimini, which is only 55 miles, because of tides, winds and sea conditions (6-8footers) I was not able to get on plane and was going up and down the waves as much as moving forward, (so 55 miles became around 90 miles) but running at almost -3000 rpms and only doing 15-17mph and burned twice as much gas as I usually do on this trip. on the way back when seas were following, and calmer and I was able to get up on plane, go 25-27mph at 3800 rpm the burn was about 85 gallons vs 132 gal going over.. So many variables involved, but you should be able to look up the sweet spot (mine is usually about 3600-3700 rpm and 24mph) and get much better gas milage at that speed.

There are quite a few 340 owners here that can point you in the right direction.

 

[Vince_nj1]

Did your fuel contain Ethanol?

 

[EricW]

I have some experience running a friends 05 320 with 6.2 V-drives.

That boats most efficient cruising RPMs seem to be between 3600 and 3850 rpm. Speeds are between 22 and 26 knots. In that range MPG doesn't change much -- about .9 mpg.

Slower than 3600 rpm/22 kts, the bow comes up a bit more and efficiency seems to drop.

Certainly your 340 w/ 8.1s is a bit different, but 19.5 kts does seem a bit slow for your most efficient cruise. I would think your 8.1 powered 340 would achieve a slightly higher speed for a given rpm than a 6.2 powered 320. Maybe 3400 rpm would give you at least 22kts, your hull might be more efficient, and your speed might increase more than gph.

 

[Hampton]

Yep, they nailed it - too slow. You may have been able to push it up to 3400 - 3500 RPM, gotten up on a better plane, then pulled the power back to about where you had it and still maintained over 25 mph. Even if you left it up at 3400, you would most likely have gotten better mileage.