115hp 2 stroke on a Stracraft Mariner 21 ft?

jonnyfish

jonnyfish

Petty Officer 3rd Class
Joined
Nov 28, 2016
Messages
79
I'm shopping for a new motor to put on my boat, the maximum it's rated for is 140hp. There are very few motors in my market used in the 135 - 140hp range.

I wanted to get some feedback from folks on if a 115hp 2 stroke would be too underpowered for this boat?

With my old 135hp blackmax i got into the mid to upper 40 mph range. That's not a speed i need normally but having the upper end is kind of nice.

Mainly I'm wondering:
- is 115hp too underpowered for this boat size?
- for those of you who may run this setup, what is your top speed approximately?
 
C

cyclops222

Lieutenant Commander
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Mar 21, 2024
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1,962
I would need about 30 data points on the boat & current motor condition. And weights that would be carried on different trips.
 
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Chris1956

Supreme Mariner
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Mar 25, 2004
Messages
28,073
A good rule of thumb is to have 75% of max HP on the boat. If the max HP for that boat is 140HP, then a 115HP is within 75% of max. I assume the hull is aluminum?
 
MNhunter1

MNhunter1

Lieutenant Junior Grade
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May 12, 2014
Messages
1,077
I think you'll be just fine with a 115hp on a 21' Mariner. They are not heavy hulls and the old 115hp 2-strokes are much lighter than the current 4-strokes. Prop it correctly for your needs and you'll be just fine.

I have a 115hp sitting here waiting to go on my 18' MR and its the max rating for the hull. The boat had a 70hp when I bought it and pushed it along for 30 years just fine.

Not sure of the year of your Starcraft, but in '82, Mercury started measuring HP at the prop rather than the powerhead. Not sure if the dates were similar for other outboards, but your 70's and pre-82 140hp is essentially the same HP as the '82 and beyond 115hp.
 
jonnyfish

jonnyfish

Petty Officer 3rd Class
Joined
Nov 28, 2016
Messages
79
Thanks for the feedback guys. It's a '76 starcraft. Sounds like 115hp isn't a bad bet especially if I can find one at a reasonable price. Just trying to get up and running again this year.
 
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racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
38,411
A 115 HP today is rated at the prop.----In 76 motors were rated at powerhead,.-----A 115 HP built from the mid 1980's to today will be plenty.
 
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cyclops222

Lieutenant Commander
Joined
Mar 21, 2024
Messages
1,962
Take the motor out for a rie ...BEFORE... buying it. Cold start Run it until heated up. Come back to the dock. Tie up. THEN stop it. try to do a instant restart with no choke or advanced throttle lever. Ask to check plugs after engine cools down a lot.
Haggle on price. Enjoy the motor.
 
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