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Hello - New member - my first post. I have a 1979 18' Mckee Craft. It's a Whaler style semi-v hull that weighs about 1400 pounds. I am replacing my current 100hp Johnson with a mid/late 90's Johnson or Evinrude two stroke. I am debating between a 115hp and a 150hp. My fuel tank is only 25 gallons. In addition to fishing and lobstering in South Florida and the Keys, I also want to be able to take the family tubing and skiing. My question is: Will the 150hp drain the small fuel tank significantly faster than the 115hp? Any input is greatly appreciated. Thanks.
You will not notice a significant difference in fuel consumption at any given speed. The 150 will turn lower rpms at any speed and this will make up for the extra displacement. As an example, at 30 mph, you'll might be burning about 7 or 8 gallons per hour on either motor. At 30 mph, the 115 will be about 4000 rpm and the 150 will be about 3500 rpm if they are propped typical for a boat that size. If you run WOT a lot, obviously the 150 will burn more fuel, but you'll be outrunning thunderstorms better too.
You will not notice a significant difference in fuel consumption at any given speed. The 150 will turn lower rpms at any speed and this will make up for the extra displacement. As an example, at 30 mph, you'll might be burning about 7 or 8 gallons per hour on either motor. At 30 mph, the 115 will be about 4000 rpm and the 150 will be about 3500 rpm if they are propped typical for a boat that size. If you run WOT a lot, obviously the 150 will burn more fuel, but you'll be outrunning thunderstorms better too.
Whatever size motor you run, you might want to remember that new motors today are significantly more fuel efficient than older motors... A new fuel injected motor (E-Tec or Merc Opti) will use a lot less fuel than one of the older carbureted models no matter what size it is...
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Capt Bob LeMay
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All else being equal a 150 will burn about 30% more fuel.
A 115 should be enough for your needs. A newer engine should provide a little better economy. For water sports you may want a four blade prop for a little better hole shot and a little less top speed.
An Evinrude E-Tec would probably be perfect but many others will be fine, too.
Predictions of the economy you will get are worthless. There are just too many variables. You will need to experiment with engine mounting height and different propellers for optimum efficiency. You will also find surprising differences in performance with differences in loading.
IMO, after tuning your boat/engine/prop setup the single most important factor in economy is how fast you want to go.
I can say that with my Verado 150 that running at cruise 4200 rpms I am burning 6 gph and running 34mph that equals 5.6 miles per gallon. This is figured watching my Smartcraft gauges and with a full load (3 big guys full gear ice chest and full live wells both of them) in my boat. I find it hard to believe that you guy's are getting less than 3 miles per gallon as stated
Now at WOT on my Verado I am burning 18 gph and running 50 and 6200 rpms same load and that makes 2.77 miles per gallon. This is a 2007 Gen I Verado. I would think the newer outboards would be more efficient.
I would also think that a 150 would be more miserly on fuel than a 115 because it wouldn't work as hard to acheive the speeds or performance that you wish to run. Just my 2 cents.
__________________ 2007 Lowe FM 185
2007 Mercury Verado 150
Motorguide Wireless
Lowrance LMS 111 HD
Mercury Temptest Plus prop
Location: Los Angeles, FL.or Georgia, depends. In Fl now.
Posts: 5,979
Level ride and consumption
Quote:
Originally Posted by sparkybell627
There's something wrong with your setup. I have a carb'ed 200 HP on a boat 2 to 3 times your weight and I get 2.6mpg at 3750 and 2.5 at 3500 and 4000.
I agree.
I had a 20' boat with a Classic Rude 150 and averaged 3.5mpg @ 25 to 30 MPH 3500 to 42oo RPM. If I held steady speed at 3900/4000 RPM on the run to Biscayne it jumped to 4.25 mpg. I measured consumption with a 3 gallon tank and a gps just out o curiosity. Running on full plane with boat level gave the best results.
Whatever size motor you run, you might want to remember that new motors today are significantly more fuel efficient than older motors... A new fuel injected motor (E-Tec or Merc Opti) will use a lot less fuel than one of the older carbureted models no matter what size it is...
I am thinking a 150 might be too heavy for that boat. Would affect your performance. Check the weight of your current motor and compare to the motors you are contemplating. A big weight difference, and now you are stern heavy and posting for solutions to that problem!
Location: Los Angeles, FL.or Georgia, depends. In Fl now.
Posts: 5,979
Savings bs..
Quote:
Originally Posted by bone-a-fide
I am thinking a 150 might be too heavy for that boat. Would affect your performance. Check the weight of your current motor and compare to the motors you are contemplating. A big weight difference, and now you are stern heavy and posting for solutions to that problem!
I agree with above.
Its a boat designed for classic supe light weight 2 strokes. 4 strokes weight a lot more and put out less torque.
If you put a 4 stroke on it you will sink the stern too much and boat may not run economically or correctly. I will probably porpoise. If you do make sure you do so to keep up with the crowd You will not recover your investment while you are aliv
I have 4 strokes because that is all that was available with boat brand
Guy's the OP and I have almost the same type hull and I have no problems at all with the Verado not too heavy nothing. I don't know where that comes from however my boat is 96" wide don't know what the 18 McKee Craft is but we are the same length and my boat is aluminum. There is no way I would want a 115 on my boat and no I don't run WOT all the time mostly cruise 4200. WOT when I can because I can and it is fun
__________________ 2007 Lowe FM 185
2007 Mercury Verado 150
Motorguide Wireless
Lowrance LMS 111 HD
Mercury Temptest Plus prop
The 150 Johnson is a V6, the 115 is a V4. If you want more power for watersports look for a V4 140, shouldn't be much heavier than the 115. What is the max HP for the boat?
Fuel Consumption - 115hp vs 150hp? 
