fuel "mileage"

[geekbyday]

I put a 76 model 60HP mariner on a smaller boat than the one it was on. I now have the motor trim pin set properly and it is running great in no small part to questions answered on this forum, y'all are great! I am just getting an understanding of prop pitch and this is the first time owning a motor as large as this so I have a 2 part question to verify what I'm already thinking. When buying a prop for this motor, I was told that going with a lighter boat, I could go with a longer pitch, so I went up 1 from a 12 to a 13. Saturday I was able to take her out and really stretch it's legs for the first time and man does this thing run great, right on top of the water like it should be. I ran out for about 30 minutes flat out and then back and I'm glad I turned around when I did because I was bingo fuel when I got back. Questions; Is it common for a 60 horse Mariner 2 stroke to get about 15 minutes per gallon at top speed? & wouldn't I be able to go just as fast and plain off just as well at less throttle if I went to a 15" pitch prop? and no, power out of the hole is no problem what so ever which leads me to think maybe I'm over taching & that is why I'm burning so much fuel although the motor runs and sounds great. 15.5' x 52" flat bottom john boat
Thanks in advance for any and all input!

 

[guyjg]

Get a tach to see what the WOT RPM are so you don't over/under rev the engine. If the engine has a WOT RPM LIMIT of 5500 and you are churning 4700 then you could add 4 degs. of pitch; 1 degree for each 200 RPM. The same is true if the WOT has a 5500 RPM LIMIT and it is churning 6000 then you are over taxing the engine and need to drop the pitch 3 degrees.

 

[H. I. McDunnough]

Get a tach to see what the WOT RPM are so you don't over/under rev the engine. If the engine has a WOT RPM LIMIT of 5500 and you are churning 4700 then you could add 4 degs. of pitch; 1 degree for each 200 RPM. The same is true if the WOT has a 5500 RPM LIMIT and it is churning 6000 then you are over taxing the engine and need to drop the pitch 3 degrees.

Is your statement backwards Guyjg?

My guess is, you could move up, but you need a tach to know for sure. I'm not sure if a 15 1/2 ft. john boat is rated for a 60hp motor.

 

[guyjg]

Is your statement backwards Guyjg?

Dang! It sure is arsebackwards. Thanks for the notice. Down in pitch to raise RPMS and vice-versa. I need to go back to bed because I'm not thinking straight...had the flu w/bronchitis and headaches for 10 days now and on my 2nd dose of antibiotics. Where are you warm weather?

 

[Curmudgeon]

Assuming the propeller is optimum for the boat (it's that tach thing, as above) and all else being equal, two-strokes will be within spitting distance of horsepower x 10% at wide-open (more or less). Unless you're very over-loaded, your best mileage (miles per gallon) should come in the 3500-4000 rpm range ...

 

[papyson]

John boats in that length and width range come in various weights and builds....it could be 100 gauge and have more ribs and runners under the bottom...even at 80 gauge i dont think it would be over powered...there should be a plate inside the boat with posted max hp...there is a weird formula to compute max hp and i do not have access to it at the moment but transom width figures into it and thats a pretty wide john...the gas usage is well within expectations running at WOT with a 60 hp...you must have run it with a pretty light load..

 

[papyson]

reread your original post....you need to run with a tach because i think you may be turning too high on the rpm...i ran two 80 gauge 15.5 john boats over the course of 20 years with 50 mercs and a 15 inch prop was perfect with a good load..and my spare was a 17 which gave me better mileage but was a tad slow coming out of the hole with a load...sometimes i wish i had stayed with a rig like that instead of spending a fortune and worrying about taking care of all that carpet and fancy seats on what i run now.....just hose that baby out and park it was better... the things we do to make a wife happy....

 

[papyson]

i will explain something referred to earlier about gas mileage...the basic rule of thumb is that you will use 6 gallons of gas per hour running wide open on a 60 hp...4 gallons running a 40...i gallon running a 10 etc..thats the 10% rule referred to...i will tell you that running a flat bottom john you will do better and the 4 gallons sound about right..add a 5 degree v like they are making now and mileage will drop somewhat...

 

[geekbyday]

Hey guys, thanks a ton for the responses and thanks for clearing up that first reply guyjg, I have to say I was a little confused when I first read it. I will buy a tach and figure out how to use it. I'm not sure about the guage of the boat but it is a heavy duty WeldBilt but I'm sure it is lighter than the old fiberglass fish/ski it came off of. Everyone has confirmed my suspitions that I can go with a 15 pitch and I will be purchasing one after a tach test confirms what we are thinking. papyson, you are right, there is nothing like a flat bottom john boat for functionality, I will be fishing mostly for catfish and this year I am going to concentrate on flathead so there is nothing like just taking a water hose to it. Never owned anything larger than a 25hp, I didn't realize how much fuel the bigger motors used.

 

[papyson]

if its all welded and no rivets then its probably 100 gauge....and if you back off the throttle to 4k or less there wont be a hell of a lot of difference between it and the 25 running WOT...i think everyone is aware of gas usage with todays prices...