How fast does your Mercruiser 120 push you?

[egleas84]

Just wondering; I've heard some wild variations from 25-50MPH from people. Thought it would be cool to collect it into one spot

Please share your speed and RPM's @ WOT and your boat w/ prop pitch (if you know it)!

 

[superbenk]

Re: How fast does your Mercruiser 120 push you?

I have a 31 year old OMC 140 which is the same block as the Merc 120 I believe with a slightly larger bore in a 18' Century 190. I get about 32-35 MPH tops at around 4800-5000 RPM.

 

[dozerII]

Re: How fast does your Mercruiser 120 push you?

I have a 1972 Starcraft 18'SS 120 mercruiser. WOT 3800 weight approx.1800lb, prop, Solas aluminum 15,3/10 x 21, speed 36mph gps, I know I should be spinning higher rpm at wot but I cruise at 2400 at 25mph and it has a heck of a hole shot.
Glen

 

[HT32BSX115]

Re: How fast does your Mercruiser 120 push you?

I have a 1972 Starcraft 18'SS 120 mercruiser. WOT 3800 weight approx.1800lb, prop, Solas aluminum 15,3/10 x 21, speed 36mph gps, I know I should be spinning higher rpm at wot but I cruise at 2400 at 25mph and it has a heck of a hole shot.
Glen

You're lugging the engine.

It would have a better hole shot and better top speed if it was turning at the top of the recommended RPM range at WOT.

 

[dozerII]

Re: How fast does your Mercruiser 120 push you?

You're lugging the engine.

It would have a better hole shot and better top speed if it was turning at the top of the recommended RPM range at WOT.

Thanks HT, what would you recomend for pitch? Maybe drop down to 17 or 19 , my problem is I can't borrow so each trial is a prop purchase.
Thanks again Glen

 

[HT32BSX115]

Re: How fast does your Mercruiser 120 push you?

Thanks HT, what would you recomend for pitch? Maybe drop down to 17 or 19 , my problem is I can't borrow so each trial is a prop purchase.
Thanks again Glen

Hi Glen,


Yeah it's a hassle if you don't have your own personal supply of props!

The best way to select what you should use first, is to determine what your drive ratio and recommended WOT RPM is. (is it something like 4400-5000 for a 120 and maybe 1.98:1?)

Then use a prop-slip calculator to get in the "ball-park" with what you have now. (be sure to use GPS for speed and verify your tach......some tachs are WAY off at the higher RPM's.........maybe find another tach and just see if it indicates the same as what you have now......) inaccurate RPM will throw EVERYTHING off!!

Go to a prop shop and buy that prop and ask them if they'll let you try it before final sale (most will as long as you don't take it out and plow it into the ground!!)


Using the following calculator, http://www.go-fast.com/Prop_Slip_Calculator.htm

1.98:1 ratio, 3800 RPM, 21p prop and 12% slip (common for single prop runabouts planing) ------------I get about 33 MPH.

(remember that pitot speedo's are FREQUENTLY inaccurate which is why you should use an el-cheapo GPS......don't do it on a fast moving river though)

I would try a 17p 3 blade prop on yours first.

Just about ALL engine manufacturers suggest "propping" engines (I/O, Inboards & Outboards) to *just* make MAX RPM (towards the higher RPM in the range) when lightly loaded at the pressure altitude (elevation) you will be running the boat.

This way, if you load the boat up or take it to a higher elevation, it will still operate close (although lower) to the recommended RPM range.


Regards,


Rick

 

[egleas84]

Re: How fast does your Mercruiser 120 push you?

I'm moving to a 17p this weekend. People on the forum and several different marina's recommended it. With 14.5 x 19 I'm getting 32MPH @ 4000 RPM's. I probably wont be going any faster with the 15x17, but the engine will be at the recommended RPM's. I still have some outstanding carb issues... on top of the wrong pitch. One of the reasons I was wondering how many MPH others are getting with the 2.5L.

My 2.5L says 3900-4300 on the plate on the engine.

 

[fishrdan]

Re: How fast does your Mercruiser 120 push you?

I'm moving to a 17p this weekend.

With 14.5 x 19 I'm getting 32MPH @ 4000 RPM's.

14.5 x 19 stainless steel prop? (I noticed in another of your posts you have a SS prop on the boat.)

What material is the new prop going to be, aluminum, stainless steel? Just changing from SS to an aluminum prop might get your engine at the correct WOT RPM or darn close to it, while swapping from a 19P SS to a 17p aluminum prop will most likely overshoot your WOT RPM, since you are only 300 RPM off.

BTW, I have a 140HP Mercruiser in my 18' Crestliner (about 2500# loaded) and running 33-34MPH off the speedo while turning 4400RPM with a 15.5x19P aluminum prop. Not the same engine, but kind of close.

For your boat's speed, I think you are in the ball park from what I've seen others post in regards to their 120/140 I/O's. Most I've seen say their speed is from the low to high 30's, a lot of it depends on hull design and loaded weight.

 

[dozerII]

Re: How fast does your Mercruiser 120 push you?

Thanks for the great reply Rick,
"Go to a prop shop and buy that prop and ask them if they'll let you try it before final sale (most will as long as you don't take it out and plow it into the ground!!)"

I have no place withing four hours of driving to get a prop to try, so I pretty much just buy them here on Iboats.

"1.98:1 ratio, 3800 RPM, 21p prop and 12% slip (common for single prop runabouts planing) ------------I get about 33 MPH."

Those numbers are almost exact, ratio is right, but 36mph on gps.

"(remember that pitot speedo's are FREQUENTLY inaccurate which is why you should use an el-cheapo GPS......don't do it on a fast moving river though)""

My speedo says 34mph

If I drop from 21 to 17 P what would my rpm go up, I'm thinkin 4 to 6 hundred rpm? 42 to 4400 rpm?

Thanks again Glen

 

[HT32BSX115]

Re: How fast does your Mercruiser 120 push you?

200-300 RPM per 2 "change in prop pitch I think.....

I went back to the calculator and using your numbers I got 5% slip. Which is probably impossible. (single prop drives on planing boats is usually 12-15%)

This indicates that your tach might be off by more than 200 RPM.

Since you KNOW the ratio, prop pitch and speed, you can use the calc to figure your theoretical speed (zero slip)

Once you know your theoretical speed, you can leave the RPM field blank and calculate what it should be with that theoretical speed.


I got about 41 mph (theoretical speed ) and about 4000 RPM. To get 41 MPH with a 17p Prop, the engine would need to turn 5000 RPM.

to get it with a 19p prop, you'd need to turn about 4500.


Sounds like a 19 might be a better choice.


But before you buy any props, you need to verify what your tach is actually showing for WOT RPM. 200 RPM can make a difference at WOT when you don't have a LOT of HP to play with!!

 

[egleas84]

Re: How fast does your Mercruiser 120 push you?

My tach is probably off. Says I idle at 1,000.... So if it's reading 300 RPM high the. I'm actually doing 32MPH at 3700 WOT... Def need a different pitch

 

[HT32BSX115]

Re: How fast does your Mercruiser 120 push you?

My tach is probably off. Says I idle at 1,000.... So if it's reading 300 RPM high the. I'm actually doing 32MPH at 3700 WOT... Def need a different pitch

Well, you shouldn't make any assumptions until you KNOW for sure what your tach is REALLY indicating.

The error might not be linear. I.E. 200 at low RPM and 400 at high (WOT) RPM or vice-versa........

The best thing to do might be to get another tach and compare them before changing props...... It's unlikely that BOTH would be the same amount off!

 

[HT32BSX115]

Re: How fast does your Mercruiser 120 push you?

Oh, and by the way,

If your tach IS accurate at idle, 1000 RPM is WAY too high. That can damage the clutch dogs in an Alpha.

 

[egleas84]

Re: How fast does your Mercruiser 120 push you?

It probably isn't that high. I just had the carb rebuilt, they did an idle set too. Tac should be easy to wire another in for a test. Just have to find the right two wires, right?

 

[HT32BSX115]

Re: How fast does your Mercruiser 120 push you?

It probably isn't that high. I just had the carb rebuilt, they did an idle set too. Tac should be easy to wire another in for a test. Just have to find the right two wires, right?

Yeah, a Tach is pretty easy to hook up.

points side of the coil, 12v, ground etc.

 

[dozerII]

Re: How fast does your Mercruiser 120 push you?

Thanks again Rick. I just bought a tach/dwell meter so I can check the onboard one. It will be while before we can get on the lakes up here anyway.
Glen

 

[not enuf toys]

Re: How fast does your Mercruiser 120 push you?

I don't remember the facts but my 81 16' arrowglass had a 120 in it. I seem to remember around 35 36

 

[HT32BSX115]

Re: How fast does your Mercruiser 120 push you?

I don't remember the facts but my 81 16' arrowglass had a 120 in it. I seem to remember around 35 36

Yeah. My previous boat, a 1966 18' Mark Twain 150 Merc (230 cu-in I-6) , did about 40 when it was new. Dad could have ordered it with a with the smaller 4 cyl engine (a 120 hp 4-banger) but the performance was somewhat less.....

A couple of years ago, I installed a reman 3.0L longblock in a friends 2050 Bayliner.

It ran great. But did about 40 mph flat out.

35mph for a (not so new) 120hp boat might be about right.

 

[jimbo_jwc]

Re: How fast does your Mercruiser 120 push you?

Yea its wild fastest , funnist boat I have owned , read on net it has unique hull it's a three speed . 1987 Thompson 196 Sea Rage 196 CC 130 hp 3.0 liter Mercruiser Alpha One I/O 14" X 17P have seen 5000 rpm . Also now jet skis apear to jump wake and can open it up and leave some of them couldn't do with old boat

48-78118-a40-17p prop so IT's 14.5" X 17Pitch still don't know if its cupped or not but was told its orignal

 

[85 CobiaMint]

I have a 1965 Cobia Sterndrive with a 2.5 GM ( Iron Duke ) So it's 120 hp and at full throttle at 3,500 rpm I get 32 mph out of it. Has a 13.5 / 17 prop Alum. I am still playing around with it as I feel it should go a bit faster