why is my gas mileage so bad

[itshamortime]

I have been reading what others are getting and mine seems to be way off. I have a 1986 sea ray 340 exoress cruiser with twin 454 mercruisers. At 25 mph I am turning almost 4000k rpms and blowing through a litlle over 50 gph (26 gph per motor). I just bought this boat a few months back and just assumed they were all this bad but i ready people on here burning half that. What do you think my problem is? And this is on Clear lake in Houston Texas not rough seas.

Any thoughts suggestions? My only guess is props. Not sure whats on there or of their condition.

 

[chuck1]

IMHO - you will find most are not running at 4000K rpms for long.

 

[jason78]

I have been reading what others are getting and mine seems to be way off. I have a 1986 sea ray 340 exoress cruiser with twin 454 mercruisers. At 25 mph I am turning almost 4000k rpms and blowing through a litlle over 50 gph (26 gph per motor). I just bought this boat a few months back and just assumed they were all this bad but i ready people on here burning half that. What do you think my problem is? And this is on Clear lake in Houston Texas not rough seas.

Any thoughts suggestions? My only guess is props. Not sure whats on there or of their condition.

I don't know about speed for that particular boat but that GPH sounds about right for Big Blocks running at 4K RPM- especially if they are carb engines.

 

[larrytbull]

Sorry I can't help my self, but
you have a boat with 2 big 454's , there is no such thing as good gph

 

[Quint4]

My 454 burns about 12-13 gph at 3500 rpm. How are you measuring fuel burn? When was your last tune up? Hull cleaning?

 

[relentless1996]

I have a 370 sundancer with 454's We cruise to Catalina island off LA Cal 1.5 hours each way at 3400 rpm I burn 18 GPH your RPM is whats killing you.
pump the brakes !!!

 

[itshamortime]

I guess the other question is why am I turning 4k rpm's to go 25 mph. Everthing I have been reading seems to say I should be under 3k rpm's with fuel burn around 15 gph per motor.

Hull is clean. plugs, caps and rotors this weekend. same gph before and after.

Could someone in the past have put a horribly wrong prop or maybe prop damage cause this problem?

 

[itshamortime]

relentless1996

Join Date: Aug 2010
Location: seal beach ca
Boat: sea ray
Details: 370 sundancer
Engine(s): 454 merc gas
Posts: 5


Re: why is my gas mileage so bad
I have a 370 sundancer with 454's We cruise to Catalina island off LA Cal 1.5 hours each way at 3400 rpm I burn 18 GPH your RPM is whats killing you.
pump the brakes !!!

​

What speed does this put you at?

 

[itshamortime]

I have a 370 sundancer with 454's We cruise to Catalina island off LA Cal 1.5 hours each way at 3400 rpm I burn 18 GPH your RPM is whats killing you.
pump the brakes !!!

what speed does that put you at. Really wondering why I have to run such a high rpm to go 25 mph.

 

[Whitenights]

I basically have your boat, only a few years newer. I have a flow scan on mine along with gps so I know my numbers are pretty good.

At 3300-3400 rpm I am running about 22 mph and burning 32-33 gph which is .66-.68 mpg.

At wide open I am running 4300-4400 rpm, about 31 mph and burning about 46 gph for about the same mpg. But I seldom run this speed for more than a minute or two.

Mostly I >run< at 1100 rpm and get about 1.25 mpg

I would say to check the props for starters, haul it out, clean the bottom, check for bilsters, and see how much moisture is in the hull. If that boat has been in all its life, and it has blisters, it could be holding a lot of water increasing the weight a great deal.

My boat spent the first 10 years of its life near Dallas and had some blisters which had to be ground out, and left open for hull to dry out. Hull tested dry after a month or so of drying. Then was epoxied and bottom coated before bring back here to Kentucky. This lasted the last 10 years with no more blisters. I think that the warmer water may increase the risk of osmosis.

Good luck.

 

[relentless1996]

around 20 mph

 

[quality time]

Nobody's asked the obvious question - what's your WOT RPM?

 

[SCORPIO]

+1 on WOT, also, you may want to go to a travel lift and have the boat weighed. That would tell you about how much extra gear/water you are carrying around.

 

[johngus]

I would say slowwwwww down.you should be only running 3200.these boats max was around 4200RPM.if you are running 4000 all the time wait til you get the repower bill.the only good thing will be then you can put in those nice 8.1's and be able to cruise at that spped all day.stop beating the crap out of the boat and enjoy the ride....

 

[sbw1]

I had an 86 week ender and spent a fair amount of time on 34s. The engines should turn 4200 to 4400 rpms at wot. Cruise is 75% of that. Typical mpg was .5 mpg. Don't recall the speed but it was probably around 23-24 mph at cruise. This is a nice boat with a fairly ineffecient hull insofar as fuel economy is concerned. The range is limited.

 

[Away Point]

For what it's worth...

Our `98 330DA, 7.4's. v-drives, depending on load and conditions, makes 25mph at 3,000 or a little above. Also consistently get 1 mpg at anything from 18-19 (2,800 rpm) to 28-29 mph (3,400 rpm). Don't do it often but, WOT is 4,400 at about 37 mph.

 

[Whitenights]

Our older boats have a deeper prop pocket than most newer hull designs. Great for shallow water, bad for fuel economy. Of course back then they never worried about fuel being $4-$5/gal.

 

[Nehalennia]

I have a 370 sundancer with 454's We cruise to Catalina island off LA Cal 1.5 hours each way at 3400 rpm I burn 18 GPH your RPM is whats killing you.
pump the brakes !!!

I need a ride to Catalina in Nov? That's pretty good fuel burn if it's accurate. I assume that's per engine?

 

[Redhook98]

My '83 runs at 21mph at 3000-3100 RPMs with a clean hull and running gear. It should be at 29mph at that RPM according tot he original build sheet. With slightly fouled bottom and running gear, I am at 3300 RPM at 19-20mph.

Changing the carbs out with newly rebuilt units right now. have experienced surging and other issues (including leak-down on both carbs). Hopefully this will bring back some performance and fuel economy.

 

[Four Suns]

For those posting fuel burn numbers, how are you calculating this? Do you have fuel flow meters?... or are you using the BS approach of dividing the hours on the meter into the gallons you took in at a fill up?