merc 115 mpg?

Helpmetomcruise

Seaman Apprentice
Joined
May 19, 2017
Messages
41
Hi long time reader only second time posting a topic, I have a two stroke oil injected mercury 115/jet80 that I've converted over to a prop drive lower unit, complete with shift kit and remote, turning a 21 deg. prop. behind a 1969 chrysler tri-hull Sport Furry. My question is this what would be my best estimated mpg and at what throttle setting would I be able to achieve this?
 
Last edited:

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,559
fuel economy in a boat is based on load and where you have the throttle.

a 2-cycle outboard will burn aproximately 0.5# of fuel per hp per hr.

so if you use all 115hp, expect to burn about 8.6 gallons per hour

your best fuel burn will be at idle, just tooling along. your next best will be just on plane

what those speeds are has too many variables for any of us to know. your hull design, weight, prop pitch tilt angle, weight distribution, wind, currents, and a few other factors come into play
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,965
I thought the rule of thumb for 2 strokes was about 1 US gal per hr for each 10 hp. At least that's what I was told for the past 40 yrs.
The only 2 strokes I've ever seen that got their best economy at idle were the FICHT and Optimax. Others like OMCs early V4s(1958 -1968) burned just about as much at Idle as they did at 3/4 throttle
 

444

Senior Chief Petty Officer
Joined
Jul 16, 2010
Messages
704
Depending on your boat weight, your 115 might not have enough jam for that 21 pitch prop but give it a try. I find my 175hp merc does best on fuel when putting around just up on plane which for me is around 2500rpm.
 

Helpmetomcruise

Seaman Apprentice
Joined
May 19, 2017
Messages
41
fuel economy in a boat is based on load and where you have the throttle.

a 2-cycle outboard will burn aproximately 0.5# of fuel per hp per hr.

so if you use all 115hp, expect to burn about 8.6 gallons per hour

your best fuel burn will be at idle, just tooling along. your next best will be just on plane

what those speeds are has too many variables for any of us to know. your hull design, weight, prop pitch tilt angle, weight distribution, wind, currents, and a few other factors come into play
sorry about the lateness of my reply but your answer to my question on estimated mpg was very helpful and much appreciated as it gave me the needed inputs to estimate fuel requirements for our trip. however the volume I wound up burning turned out to be only a fraction of the estimated volume.... so I factored for a roundtrip distance of about 38 miles with one refill while near our destination before return to be more than adequate to allow for some sport and leisure cruising and fuel consumption while at location to be well in excess of our needs as i started out with a 40gal tank full of fuel and in no hurry to make time during transit.... Alas we only made it about 2/3rds of the way there luckily we had just stopped and topped off the tank before we suffered a catastrophic failure of the motor due to a sudden failure of the oil injection system ("of which the previous owner had unhooked the warning light for and then neglected to tell me when I bought the motor") while underway and under load while in a very unfortunate stretch of our plotted course. Good thing I had all that fresh un mixed gas on board because I wound up needing to feed the beast that happened to stop an offer us a tow almost 15 miles back to our port of departure
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,559
prior owner probably converted to pre-mix and when you topped off, you didnt ad oil.
 

Helpmetomcruise

Seaman Apprentice
Joined
May 19, 2017
Messages
41
I thought the rule of thumb for 2 strokes was about 1 US gal per hr for each 10 hp. At least that's what I was told for the past 40 yrs.
The only 2 strokes I've ever seen that got their best economy at idle were the FICHT and Optimax. Others like OMCs early V4s(1958 -1968) burned just about as much at Idle as they did at 3/4 throttle
it was an oil injected optimax
 

Helpmetomcruise

Seaman Apprentice
Joined
May 19, 2017
Messages
41
prior owner probably converted to pre-mix and when you topped off, you didnt ad oil.
nope he never "converted it" at least not properly as I owned it for a season and a half before it failed on me and the oiler system worked just fine throughout that stretch. he might have been forgoing the use of the oiler and just mixing his own but he didn't warn me that I would have to do that... my own fault for taking him at his word and not doing my own due dilligence and having it properly inspected and reading up on the problems that that particular system are prone to... which as it turns out are sudden failures in the oiler system which is why there is a warning buzzer and light for when the oil stops flowing... alas it was never functional while I owned it. The first thing that the shop did was locate the isolated wiring for the warning light hook it back up and guess what.... the thing lit up like a christmas tree. After that all they had to do was pop the head cover off the motor and show me the inside of the cylinder walls to make it clear she was now nothing more than a replacment lower unit for someone elses project.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,965
If it was an Optimax, using premix, would have destroyed the engine in a few minutes, or even quicker, when the Motor's Oil tank ran dry. The Crankcase only sees Air, and injected oil, not Air, Gas, and Oil like a regular 2 Stroke
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,306
Agreed-----I am not sure if an ---OPTIMAX ---can even run on a 50:1 mix of fuel !!-----No different that an E-TECH .
 

Helpmetomcruise

Seaman Apprentice
Joined
May 19, 2017
Messages
41
as I said for the first season and a half it worked fine running on the oiler tank and mixing it's own fuel and oil then all of a sudden it locked up hard on me after making a horrid racket and the shop confirmed that the warning lamp had been unhooked for the oiler. it's a moot point now as that was a while ago and the motor has already been replaced. I just hadn't signed into iboats since then and felt compelled to respond to the guy that had originally answered my question. to let him know that his answer was spprectiated and did help me plan for the right fuel consumption after all even though it turned out not to be my boat burning it... LOL
Agreed-----I am not sure if an ---OPTIMAX ---can even run on a 50:1 mix of fuel !!-----No different that an E-TEC
 

Helpmetomcruise

Seaman Apprentice
Joined
May 19, 2017
Messages
41
It can, just not very long
while I have your attention do you know anything about the big ole carver santa cruz vdrives? and how and where to measure on the old prop to find all the needed specifics to find a replacement prop
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,965
Never heard of them, but that goes for all V Drives
As for the Prop. Diameter and Pitch are usually stamped somewhere on the Hub. A Prop Rebuilder, would have the Tools needed to measure things like Rake and Cup
 
Top