New outboard, from 2 stroke tot 4 stroke

xagneovox

Seaman Apprentice
Joined
Feb 3, 2017
Messages
36
Hello,

Currently I own a 15 ft speedboat with a v shaped hull equiped with a 90 hp johnson made in '94 (2 stroke).

I'm thinking in buying a 4 stroke, I'm currently looking to a yamaha 115 made in '00.

The main reason of changing is fuel consumption, smell and sound.

Will there be a big difference between the two engines and how will this act in topspeed and acceleration?

Regards jasper
 

xagneovox

Seaman Apprentice
Joined
Feb 3, 2017
Messages
36
Topspeed of the boat is 40, 42 mph at 5500, 5800 rpm and goes to plane very smoothly
 

robert graham

Admiral
Joined
Apr 16, 2009
Messages
6,908
I'd just keep what you have....new 115 4 stroke will cost big $$$, 100 plus lbs. heavier......some quieter, smoother, better mileage.....but is it worth it??....
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Add more maintenence parts too. It's a car engine sittting vertically inside a hood..

How loaded was combo when running 5500-5800 wot revs and with which diam/pitch prop ? Is 5800 rpm max wot revs factory stated or it's 6 K ?

Happy Boating
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
You say this engine will be on a "speed boat" so that implies you will use a lot of 3/4 - full throttle operation. A 90 HP Johnson will burn about 9 gallons of fuel/hr at full throttle. If you look at the Yamaha web site you can go to their "boat test" page and select a boat similar to yours, select the engine (115 4stroke) and you will be presented with speed and fuel consumption data. You will likely find at full throttle the 115 will burn the same or only slightly less fuel than the 90 2 stroke. The Johnson will likely plane the boat faster than the four stroke and speed across the board will be faster at any given rpm than the 4 -stroke. Read that last statement very carefully so you understand what I said. You will need to run a lot of fuel through the engine to make up the difference in cost of the switch. I agree with others to stick with 2 stroke until it dies or until a super deal that meets your definition of "deal" is found.
 

xagneovox

Seaman Apprentice
Joined
Feb 3, 2017
Messages
36
Thank you guys for the responses and I will stick to my johnson for the moment and sell it with my boat if I want another boat.

Happy boating :)
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,030
Gee, I would think that a 90HP motor on a 15 footer would do better than the 42MPH you cited. Do you have power trim/tilt?

Normally a 15 footer would be rated for no more than 75 HP, usually less.
 
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