My 120hp Boat Is Slow,,,WHY?

[bradly2072000]

I just bought a 1985 Sunrunner 165, Mercruiser 120. Boat runs great, fires right up, never fails. The only problem is, it only does 24-25 or so. Fishing boats were passing me, pissing me off. For being a 120hp, I would think that it would cruise at least 40 or so. Whats going on? Do I have a power prop on? Or should I get a 25" pitch prop, whould that solve my problem?

 

[180shabah]

Re: My 120hp Boat Is Slow,,,WHY?

Agreed - that boat should be in the mid-upper 30's

However, a 120 will NEVER push a 25" prop.

If you want suggestions you need to provide the information outlined in the stickies at the top of this forum. Primarily we need WOT numbers, and prop information.

 

[nstato00]

Re: My 120hp Boat Is Slow,,,WHY?

I agree that it should be goin a little faster. However, a 120 hp is not a very big motor and probably doesn't have as much power as you might expect.

 

[steelespike]

Re: My 120hp Boat Is Slow,,,WHY?

Your prop numbers are probably under the prop nut or on the barrel.
Get us your prop numbers,max rpm and speed.Problem could be as simple as a plug wire off or not firing or the throttle plates in the carb not opening all the way.
Does the boat respond when you trim up?How is the hole shot?

 

[45Auto]

Re: My 120hp Boat Is Slow,,,WHY?

No way it's EVER going to cruise at 40 MPH, you'll be lucky to see 40 MPH at WOT (Wide Open Throttle). I wouldn't consider that cruising speed, how long would you be willing to run your 25 year old car at full throttle?

Is the 25 MPH speedometer or GPS?

You need to use a GPS, speedos are very inaccurate.

My 1982 16' boat with a 120 would top out at 34 MPH, I would expect yours to also be in the mid-30's if it's running right. Cruising speed will be in the mid to high 20's.

 

[steelespike]

Re: My 120hp Boat Is Slow,,,WHY?

He's saying at wot he is being passed by fishing boats I assume he means like a 14 footer with a 15 or 20 hp.40 would be afar reach and certainly not cruising speed.

 

[bradly2072000]

Re: My 120hp Boat Is Slow,,,WHY?

I agree that it should be goin a little faster. However, a 120 hp is not a very big motor and probably doesn't have as much power as you might expect.

Well I was able to contact the original owner. He said that he was getting to old to ski, so he put a 15" on. He said that it used to have a 17" so, I went ahead and bought a 17, we'll see how that goes. He also said that it usually ran about 35-38mph with the 17 on.

 

[bradly2072000]

Re: My 120hp Boat Is Slow,,,WHY?

Well, it does ok getting out of the hole. The previous owner used to run a 17" prop when he would ski, said it usually ran about 35-38mph

 

[indy440]

Re: My 120hp Boat Is Slow,,,WHY?

2 inches of pitch will not make 10mph...

 

[bradly2072000]

Re: My 120hp Boat Is Slow,,,WHY?

No way it's EVER going to cruise at 40 MPH, you'll be lucky to see 40 MPH at WOT (Wide Open Throttle). I wouldn't consider that cruising speed, how long would you be willing to run your 25 year old car at full throttle?

Is the 25 MPH speedometer or GPS?

You need to use a GPS, speedos are very inaccurate.

My 1982 16' boat with a 120 would top out at 34 MPH, I would expect yours to also be in the mid-30's if it's running right. Cruising speed will be in the mid to high 20's.

Well the boat runs great, I got all the service records from the previous owner, He pretty much rebuilt the entire motor. Im not even joking, it looks new. I went ahead and ordered a 17" prop, to replace the 15", that should make a difference. He told me that thats what used to be on it to begin with.

 

[45Auto]

Re: My 120hp Boat Is Slow,,,WHY?

I went ahead and ordered a 17" prop, to replace the 15", that should make a difference.

It should make a difference, about a 200 RPM difference. In other words, if your motor was running at 4400 RPM wit the 15" pitch prop it'll be running at about 4200 RPM with the 17" pitch prop. Don't expect it to make any difference in top speed, only way it could do that is if you were already running WAY past the peak of the power curve. It will give you a more comfortable, lower RPM cruise though.

The power curve for the 120 is pretty flat, I'd be surprised if you see any difference in speed with it running WOT (Wide Open Throttle) anywhere between 4000 and 4500 RPM.

 

[180shabah]

Re: My 120hp Boat Is Slow,,,WHY?

WOT rpm is the piece of the puzzle we are still missing.

Was 28mph as fast as it would go, or a fast as you let it go? If it WOULD not go faster, then a 17" prop will only make it worse.

 

[robjen1019]

Re: My 120hp Boat Is Slow,,,WHY?

Sunrunners are pretty heavy hulls. I would guess that 30mph is top speed for that motor and boat combination. I had a 19ft sunrunner with a 151 and it would top out at 32 mph.
Make sure when you are comparing mph your not using the speedo on the boat. The only way to be sure is to use gps.