21' boat with 260hp - how much fuel?

dirtyoldman

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I'm trying to guess how much fuel my boat will use on a cruise and what kind of range to expect. I just bought a 1987 21' sea ray sorrento open bow boat with the 260hp v8 (350 cu in). It weighs 2800 lbs dry and will likely be around 4k lbs to 4500 lbs displacement with my family and gear on board.

I have a 44 gallon tank and am guessing I'll be using 8-10 gph at a cruising speed of 3000-3500 rpm which is probably about 25 mph. This equates to a range of about 100 miles.

I'll never be able to check this in the small lakes around here, it's all sitting in a cove or skiing. Can anyone tell me if I'm close in my expectations or not?
 

Fl_Richard

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Re: 21' boat with 260hp - how much fuel?

Where is the boat going? Coastal and lake use can change fuel consumption dramatically. Dont forget 10-20% of your equation for reserve in fuel.
 

dirtyoldman

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Re: 21' boat with 260hp - how much fuel?

Where is the boat going? Coastal and lake use can change fuel consumption dramatically. Dont forget 10-20% of your equation for reserve in fuel.

It's all lakes around here, and little chop. We stay away from the lake on windy days.
 

JoLin

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Re: 21' boat with 260hp - how much fuel?

Your consumption estimate seems about right. Best rule of thumb is to use the 1/3 rule- 1/3 out, 1/3 back and 1/3 in reserve. You may not start out on a windy day, but that can easily change once you're out there.

Fuel consumptiion increaes dramatically when you're headed into the wind.
 

magster65

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Re: 21' boat with 260hp - how much fuel?

I'm trying to guess how much fuel my boat will use on a cruise and what kind of range to expect. I just bought a 1987 21' sea ray sorrento open bow boat with the 260hp v8 (350 cu in). It weighs 2800 lbs dry and will likely be around 4k lbs to 4500 lbs displacement with my family and gear on board.

I have a 44 gallon tank and am guessing I'll be using 8-10 gph at a cruising speed of 3000-3500 rpm which is probably about 25 mph. This equates to a range of about 100 miles.

I'll never be able to check this in the small lakes around here, it's all sitting in a cove or skiing. Can anyone tell me if I'm close in my expectations or not?

You'll probably see a little higher fuel consumption than your estimate. My 5.0's are 10 gph (each) @ 3500 rpm.
 

dirtyoldman

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Re: 21' boat with 260hp - how much fuel?

You'll probably see a little higher fuel consumption than your estimate. My 5.0's are 10 gph (each) @ 3500 rpm.

Thanks for the input. I checked mercurymarine.com and found a lot of test results. The 350 mag MPI engines (more powerful than mine and probably mroe efficient too) use about 8 gpm at 3000 RPM and 12 GPM at 4000 RPM. THe boats they checked seemed heavier than my boat and the speed at those RPM's were significantly higher than what I figured (25MPH at 3000, 40 MPH at 4000).

I'm guessing that since these engiens are more powerful than mine in a heavier boat, but probably more efficient too - that it's a pretty accurate comparison? That shows my figures as pretty conservative - but still good news. My old V-Sonic 18' used every bit of 6-8 GPH to go 20 MPH.
 

TilliamWe

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Re: 21' boat with 260hp - how much fuel?

You can't compare your boat with a 4 barrel carburetor to ANY boat with MPI.
The best way to tell... install a fuel flow meter on it. Then you'll know for sure. Guys on here that have them, swear by them.
 

dirtyoldman

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Re: 21' boat with 260hp - how much fuel?

You can't compare your boat with a 4 barrel carburetor to ANY boat with MPI.
The best way to tell... install a fuel flow meter on it. Then you'll know for sure. Guys on here that have them, swear by them.

Why? Are you saying the fuel consumption will be better or worse?
 

smokeonthewater

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Re: 21' boat with 260hp - how much fuel?

figure on about 3 mpg and hope for as many more tenths as you can get
 

dirtyoldman

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Re: 21' boat with 260hp - how much fuel?

Yours will be worse.

See the above post for the best guess.


That's why I was confused. That's better than what I was figuring - but it was implied that it would be worse.
 

QC

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Re: 21' boat with 260hp - how much fuel?

figure on about 3 mpg and hope for as many more tenths as you can get
Exactly. For carb'd 18 - 22 footers almost all test data you can find will confirm this ;)
 

TilliamWe

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Re: 21' boat with 260hp - how much fuel?

That's why I was confused. That's better than what I was figuring - but it was implied that it would be worse.

You are confused because you are comparing apples to oranges. My 1997 Chaparral 2135 cuddy had an EFI (GM TBI) 5.7 and it got nearly/over 4mpg when cruising. My dad's 2001 Chaparral Signature 300 with twin 5.0 EFIs (GM TBIs) got 1 mpg when cruising. No comparision, due to type of boats and weight.
 

smokeonthewater

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Re: 21' boat with 260hp - how much fuel?

if your carb is spot on they are about the same..... the advantage of the fuel inj is that it self tunes..... changes in air temp altitude fuel quality are automatically adjusted for.... generally less than 1/4 to 1/2 mpg difference but that is a VERY broad statement.... on a big boat that gets 1 mpg you could pull teeth to try and save 0.1...
 

dirtyoldman

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Re: 21' boat with 260hp - how much fuel?

if your carb is spot on they are about the same..... the advantage of the fuel inj is that it self tunes..... changes in air temp altitude fuel quality are automatically adjusted for.... generally less than 1/4 to 1/2 mpg difference but that is a VERY broad statement.... on a big boat that gets 1 mpg you could pull teeth to try and save 0.1...


This is why I am surprised - on other EFI applications that I am familiar with, they may be at most about 5% better - if that. I would expect marine applications to be closer to the same as EFI because of the constant high loading, no variable loads.

But what do I know?
 

TilliamWe

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Re: 21' boat with 260hp - how much fuel?

Marine EFI systems are not as good as they could be of course, because until 2010 and catalytic converters, they have had no Oxygen sensor in the exhaust. So they always ran in "open loop" mode. And Mercruiser, for sure, sets them up to run rich. But they are still better than a carburetor, and they start without having to touch the throttle.
 
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