Like Tree0Likes

Single v/s Twins : Fuel consumption ?

Reply
Old 11-07-2007, 09:23 PM
  #1
Registered
Thread Starter
Trade Score: (1)
 
littledcsrodshop's Avatar
 
Join Date: Apr 2005
Location: TN
Posts: 2,194
Default Single v/s Twins : Fuel consumption ?

This similar situation has probably been covered here before but here it is :

I have heard a few differant ideas on this but lets see what you guys have seen from experiance :

I am woundering how these 2 setups would compare in fuel consumtion under normal cruising condions say ( 3,000-3,500 rpm's ) & yes I understand that there are several variables ect...

Twin 325-350 hp Small block's ( 650 cfm Holley's )

V/s : A single 750 hp (+ or - a few hp depending on boost #'s ) supercharged BBC ( Centrifical with single blowthrough 850 cfm carb )

Boat : True 28' conventional V
4800-5000 lbs. Single BBC
5800 lbs. orso Twin SBC's

Like I said I know that there are variables ect.. But I am just tossing around Ideas in my head B/C I am getting close to rigging my 28 Saber & I have both setups availible. This is not my sole deciding factor on how I will rig the boat but just trying to weigh it out from all directions... So lets hear it !! & Please only real world experiance here...

Thanks in advance , Scott

Last edited by littledcsrodshop; 11-07-2007 at 09:33 PM.
littledcsrodshop is offline  
Reply With Quote
Old 11-07-2007, 09:50 PM
  #2
Registered
 
OkieTunnel's Avatar
 
Join Date: Mar 2001
Location: Oklahoma City, OK
My Boats: 35 Motion "Bring It" is "For Sale", 31 Warlock
Posts: 872
Default

I have had both but currently have twins and triples. Never really measured but figure the twins to be less fuel efficient. Your real question is which will you be more happy with. Spend the money on twins. Two mild engines should be less temperamental than one boosted motor. It will handle rough water alot better and be more maneuverable around the docks. When you break a shaft, prop, gimbal, etc. you can still limp home without a tow. You will pay more know but your resale will reflect this also.
OkieTunnel is offline  
Reply With Quote
Old 11-07-2007, 09:51 PM
  #3
Registered
 
Join Date: Jun 2006
Location: Sunland, CA
My Boats: 2000 Fountain 32 Sportfish Cruiser
Posts: 377
Default

Fuel consumption is directly related to work.

Moving more weight is more work. So if its heavier, it will burn more fuel. For planing boats, the function is related to the

Moving against more friction is more work. So if it has more wetted surface, it will burn more fuel: two lower units are more drag. More cylinders also means more friction, so two V8s are more drag. More drag burns more fuel.

Blower motors, however, take a lot of power to make power: just turning the blower might consume 250 HP, so to get 750HP to the drive shaft, you burn enough fuel to make 1000 HP. That is such a huge hit, that the twins might be more fuel efficient than the blower motor.

It looks like the BBS will be faster (85 mpg .vs. 76 mph) flat out,
but will burn more than a gallon per mile flat out, while the twin SBCs will get about 25 percent better mileage (1.2 mpg).

At 60, and at 20, both burn almost identical fuel, because the
weight at wetted surface advantage gets lost by the blower.

Go normally asperated but still very hot 700 HP BBC -- go the same speed because of the weight savings (blower, intercooler, plumbing, coolant, 1 mph slower), and the single BBC beats the twins
at all speeds by a bit more than 10 percent.
carcrash is offline  
Reply With Quote
Old 11-07-2007, 09:54 PM
  #4
Registered
 
Join Date: May 2004
Location: Okanagan Falls BC. Canada
My Boats: 1990 Mirage 257 , twin 6.2 MX
Posts: 595
Arrow

I have twin small blocks [ 6.2 or 377 cu ] with ecm 555 and SmartCraft system . Between 3000-3200 rpm's I'm burning 7-8 gal/hr each . Total SAE HP is 700 . I assume a 700 HP
carbureted BB would burn similar or a little more with same rpm's .I like the twin setup for safety reasons going further from shore in the ocean .
spectras only is offline  
Reply With Quote
Old 11-07-2007, 10:42 PM
  #5
Registered
 
OkieTunnel's Avatar
 
Join Date: Mar 2001
Location: Oklahoma City, OK
My Boats: 35 Motion "Bring It" is "For Sale", 31 Warlock
Posts: 872
Default

Quote:
Originally Posted by carcrash View Post
Fuel consumption is directly related to work.

Moving more weight is more work. So if its heavier, it will burn more fuel. For planing boats, the function is related to the

Moving against more friction is more work. So if it has more wetted surface, it will burn more fuel: two lower units are more drag. More cylinders also means more friction, so two V8s are more drag. More drag burns more fuel.

Blower motors, however, take a lot of power to make power: just turning the blower might consume 250 HP, so to get 750HP to the drive shaft, you burn enough fuel to make 1000 HP. That is such a huge hit, that the twins might be more fuel efficient than the blower motor.

It looks like the BBS will be faster (85 mpg .vs. 76 mph) flat out,
but will burn more than a gallon per mile flat out, while the twin SBCs will get about 25 percent better mileage (1.2 mpg).

At 60, and at 20, both burn almost identical fuel, because the
weight at wetted surface advantage gets lost by the blower.

Go normally asperated but still very hot 700 HP BBC -- go the same speed because of the weight savings (blower, intercooler, plumbing, coolant, 1 mph slower), and the single BBC beats the twins
at all speeds by a bit more than 10 percent.
Leave it to the theorists in the center consoles
OkieTunnel is offline  
Reply With Quote
Old 11-08-2007, 02:35 AM
  #6
Charter Member # 55
Charter Member
 
Griff's Avatar
 
Join Date: Oct 2000
Location: Omaha
My Boats: .......32............. ...............Active Thunder....
Posts: 15,743
Default

It will depend on the efficiency fo the engines and boat. I can tell you that My 32AT with twin 502mpi's gets about the same mileage as my 28 Pantera did with the hopped up 525SC. I think its a little better at cruise speeds and a little worse at WOT.
Griff is offline  
Reply With Quote
Old 11-08-2007, 07:16 AM
  #7
Driver-441
Racer
 
TeamSaris's Avatar
 
Join Date: Aug 2004
Location: Lake George
My Boats: 32 Cobra Raceboat, 27 Avanti Raceboat 27 Kryptonite Raceboat
Posts: 11,348
Default

We converted our boat to a Single BBC. Fuel consumption is a better but I give more credit to the DFI than the single engine
TeamSaris is offline  
Reply With Quote
Old 11-08-2007, 02:49 PM
  #8
Registered
 
CAVelocity's Avatar
 
Join Date: Nov 2003
Location: Umatilla, FL
My Boats: 2004 Air Nautique (sold the Velocity)
Posts: 287
Default

A single fuel injected non-blower motor should get the best fuel consumption. My velocity with single modifed 500 EFI gets better than 2 mpg at almost any power setting.
CAVelocity is offline  
Reply With Quote
Old 11-08-2007, 03:52 PM
  #9
Platinum Member
Charter Member
 
Join Date: Jan 2001
Location: so. WI
My Boats: Formula
Posts: 1,562
Default

Quote:
Originally Posted by CAVelocity View Post
A single fuel injected non-blower motor should get the best fuel consumption. My velocity with single modifed 500 EFI gets better than 2 mpg at almost any power setting.
I've had two twin SBC boats, one carbureted (an older Formula 242LS- modified 355's) and one with the 350 Mag MPI package (Active Thunder 28 Savage). Both weighed nearly the same, and both consistently delivered well over 2 mpg at cruise, with the MPI outperforming the carbed boat in fuel economy at nearly every power setting.
The 28 AT is a true 28' hull and is the most similar to your Saber and Pantera, and if given the option, especially with our Great Lakes and occasional ocean boating, I'd opt for the twins EVERY time. I also ran many long trips on the Mississippi River with similar size boats equipped with single BBC packages, and they stopped for fuel; I didn't (admittedly partially due to the fact that boat mfg's put smaller fuel tanks in single-engine boats). OSO member Spilman has the same package in his 28 AT, and I know he is experiencing 2.5 mpg #'s at cruise as well. I love both the boats you have, and I wouldn't hesitate to own either with 350 MagMPI power. I know there have been a myriad of documented problems with the 6.2L, so I'd stick with the venerable 350 here. BTW, our insurance was only $750/year with the AT.
Jim
jafo is offline  
Reply With Quote
Old 11-08-2007, 08:39 PM
  #10
Registered User
 
Join Date: Jan 2007
Location: Mansfield, TX
My Boats: 1990 28 Apache SOLD!, 1990 16 Wahoo DC, 1973 40 Kingscraft
Posts: 5,701
Default

Nice numbers. I wonder what the 32 fountains got with twin 6.2's
TexomaPowerboater is offline  
Reply With Quote
Reply

Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
LutzParty
General Boating Discussion
42
11-11-2007 08:00 PM
superwrench0166
Scarab
5
03-14-2007 08:47 PM
franklins
General Boating Discussion
1
08-30-2006 10:46 PM
Prater
Formula
6
05-12-2004 03:09 PM
pullmytrigger
Fountain
8
02-06-2003 10:15 PM



Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 07:10 PM.


Copyright 2011 OffShoreOnly. All rights reserved.