21' Mariner top speed

[mjames76]

I have a 115 Johnson on mine and was a little disappointed in the top speed I am getting. I had it out this weekend after a carb job, new plugs and fuel lines and it ran great, but according to my speedometer I am only getting about 27mph. I need to take my gps with me next time cause I dont trust the speedometer 100%. I had a 16' SS with a 70 evinrude and it would run 36mph. I also dont have a tach so I am not sure what rpm I am running at wot. I checked to make sure the carbs are fully open at wot, compression is good at 123-126 on all 4. Maybe I am expecting too much out of my 115 I was hoping for high 30's to low 40's.

I was wondering what kind of speed yall are getting.

 

[ammo]

Re: 21' Mariner top speed

mine is a 16' with a 80hp on it getting around 40mph, as big as that boat is i would say the speedo isnt to far off....also the prop on mine is a stanless 23pitch 3 blade ....

 

[North Beach]

Re: 21' Mariner top speed

M, that's the semi V so it's sposed to be faster than the Deep V. It's rated up to what, a 140hp?

 

[mjames76]

Re: 21' Mariner top speed

M, that's the semi V so it's sposed to be faster than the Deep V. It's rated up to what, a 140hp?

yep 140 is the max so I am giving up 25hp right of the bat.

 

[fshngho]

Re: 21' Mariner top speed

M, I would suggest two things. First get a tach, you must know the rpm's to adjust anything. Optimum rpm for you should be 5500. If you aren't getting that you may look into a different prop. Second is to check the motor height, the cavitation plate should be about even with the bottom of the hull. I will assume you have t&t and the weight distribution in the boat is correct. Good Luck

 

[ezmobee]

Re: 21' Mariner top speed

With everything set up correctly, I would think you'd be able to hit 35. You're definitely slow now. We hit 27-28 with my buddy's 21' Chieftain with only a 90 on it and that thing's got a big cabin to push around!

Definitely get a tach. When we took the Chieftain out the first time the tach was showing we were about 400 RPM low on the WOT RPM. My buddy bought a new lower pitch prop and while our top speed hasn't really increased, the hole shot improved and the motor is how reaching very close to the optimal WOT RPM.

 

[mjames76]

Re: 21' Mariner top speed

I have a tach I just need to figure out how to wire it. It is an old aqua meter and it has 4 terminals on the back in a straight line across the top, but none of them are marked. One of them has a jumper to the one next to it but other than that I would just be guessing. Looks like I am back to searching.

 

[ezmobee]

Re: 21' Mariner top speed

The tach should have a positive, a ground, and a pulse from the gray wire in your control box.

 

[mjames76]

Re: 21' Mariner top speed

The tach should have a positive, a ground, and a pulse from the gray wire in your control box.

well I wired it and one way when I turn it over it jumps over to 6K so I switched the power wire and pulse wire and when I turn the key to acc it just pegs out to 6K so I dont know if maybe I have a bad tach or whats going on.

 

[mjames76]

Re: 21' Mariner top speed

this is what I am working with

 

[ezmobee]

Re: 21' Mariner top speed

Well you could try a tach newer than 1932......

 

[mjames76]

Re: 21' Mariner top speed

Well you could try a tach newer than 1932......

But what fun would that be???