86 Merc 75 top speed

[edjunior]

Hullo all. Okay, finally got the old '86 Merc 75hp out on a successful run. Really did great, but I really though I'd get a litte more top speed out of her. After she got good and warmed up, I punched it, and GPS indicated speed was 33.1mph max, averaged around 32mph. I was hoping for more around 35 - 40 mph. The boat is a 69 MFG fiberglass runabout, about 980lbs (according to the tag). Is this all I'm gonna get from this motor, or might a different prop help. I played with the trim, and when I trimmed up, the boat started bouncing enough to make the wife a little queasy, so I trimmed back down and she smoothed right out. Any advice, suggestions, anything?

 

[jmburock]

Re: 86 Merc 75 top speed

I have an 89 Mariner 75hp 3 cyl w/ 13.25x 17pitch propeller on a 17 ft Boston Whaler. Top speed is 42mph at 5200 rpm. I don't know much about your boat/ bottom conditon/ or propeller, especially. All of these will play a major role in your top speed. What rpm were you running at this speed? What prop are you running?

 

[edjunior]

Re: 86 Merc 75 top speed

I have an 89 Mariner 75hp 3 cyl w/ 13.25x 17pitch propeller on a 17 ft Boston Whaler. Top speed is 42mph at 5200 rpm. I don't know much about your boat/ bottom conditon/ or propeller, especially. All of these will play a major role in your top speed. What rpm were you running at this speed? What prop are you running?

Mine is a 4 cyl, but I don't know the specifics of the prop. Is there an easy way to determine this? I don't have much experience with props, as everything I've had in the past has performed right from the git go. I don't have a tachometer yet either. I am working on that though because I've read elsewhere that it is an important number to have to help determine maximum performance.

The current prop (it is stainless steel) is in very good condition, but if it's not the right one for the motor, then it doesn't really matter what the condition is. The boat bottom is fair condition. I did patch a little leak, and should probably get under there and sand down that area, and it has some old ugly bottom paint (I presume for saltwater use, since I've not seen this paint on freshwater lake boats), but otherwise it is as I said, fair. Just old.

Thanks for the reply.

Ed

 

[jmburock]

Re: 86 Merc 75 top speed

The prop specifics should be written on the propeller/ near hub, sometimes they are covered up by the prop nut/washer. Would take it off and find the size. However, this will do nothing until you put in a tachometer. Ones from Teleflex or Mercury/Quicksilver will do the job (rpm range of 0-6000). Set it at 12p. This should hook up easily to your existing outputs on your controller (I'm assuming its Quicksilver controller). Can't say if you have the right prop w/o these numbers. Do you have water in the inner hull? - said you had a leak, they have a way of coming back especially if it has osmotic blistering on the bottom.

 

[edjunior]

Re: 86 Merc 75 top speed

Thanks again JM. Okay, I'll look tonight or tomorrow and find the numbers on the current prop. And I'm making a run to West Marine (everyone's favorite weekend thing when not in the boat...right??) and will look at a RPM guage. Found bunches on E-bay, missed a couple already, so we'll see. My run after I patched the bottom (it was a hairline crack...I only noticed it because water was dripping out of it after I rescued the boat from my brother) was great. When I got the boat home and pulled the bottom plug, I only got a little dribble for a couple of minutes. So it's definitely not taking on water, at least in any amount worth worrying about. Keep in mind also that the boat is 38 years old. Heck, for it to not leak anymore than it does is pretty good as far as I'm concerned. So, I will get the numbers (the easy part), and get the tach (easy, but not as).

I also talked to the mech that got the motor running right for me, and he said pretty much the same thing, but even without a tach, the prop could be just all wrong, depending on what I find for the numbers. I have also found an old board (www.outboard-motors.us) with a bunch of really good Mercury (and other motors) info on it. Found a sheet for determing the proper prop based on some generic numbers (boat length, weight, motor size, use (number of people carried), etc.). Also lots of good info on the posts that has helped increase my knowlege base tremendously. Only problem is most of the posts are at least 4 - 5 years old. Looks like the board is not used anymore, but still there.

So armed with all this good stuff, I feel ready to tackle all this with a great deal more confidence, and feel I will end up getting my speed and performance up on this motor. I'll certainly keep in touch here and let you know what I find.

Thanks for the responses, and see ya on the water!

 

[ninja500r]

Re: 86 Merc 75 top speed

I have an 87 Bomber bass boat with the same 86 Merc 75 motor. I also had to do some work on my motor and now that it's running well I got to 43 on my fish finder speedo. Who knows how accurate it is but I'll take it. I have a 13.25x17 pitch stainless prop on it and the WOT rpms seem just about perfect. The motor is mounted pretty high on the transom as well.

 

[jmburock]

Re: 86 Merc 75 top speed

So now you have 2 of us w/ basically the same motor w/ the same top speed using a 13.25" x 17 pitch propeller. I have an aluminum 3 blade but I will soon be moving to a 13.25" x 17p stainless steel 4 blade propeller to get better performance in the chop (Stiletto Bay Pro I propeller). However, I might lose 2-3 mph on top speed.

 

[edjunior]

Re: 86 Merc 75 top speed

Okay, mine's a 21 pitch, stainless steel. Funny thing is, I talked to my mechanic, and he said that prop seemed to be fine. I still don't know, especially after seeing y'alls responses. I'll have to talk to him more. I also ran this through a calculator I found and it took things down to at least a 19 pitch. So either way, I really think 21 is too much. So, I'll take what y'all have given me, and what the calculator said, and talk with the prop guy. Thanks for the responses and help.

 

[ddennis]

Re: 86 Merc 75 top speed

also take your boat to a set of scales and weigh it..if you had a leak you may have some foam that is saturated and adding weight.