High RPM's

[edjunior]

Okay, so I finally got my boat out after getting my tach installed. I have an '86 Merc 75hp 4ycl on a 16 ft fiberglass runabout. My initial concern in a previous post was "do I have the right prop", since I felt that a top speed of 33mph might be a little low for that boat/motor. I needed the RPM's to see what was going on. Well, I took it out yesterday, and the RPM's shot up to near 6500! I mean, wow! I wasn't expecting that. Could it be the tach is not set properly, os something else? It idles at between 800 - 1000, and at about 5200 RPM (around the supposed rated WOT), I'm only pushing about 26 mph! I couldn't hold top speed (WOT) too long because the lake was pretty rough, and it was beating us up. But those are my numbers. If the RPM is indeed that high, what is wrong? I have a 21p SS prop.

 

[Texasmark]

Re: High RPM's

I ran a 4 cyl, approx year model of '84, 85 hp Merc on a 16' glass bass boat at 39 max with a 19P prop as I recall (but it's been a very long time and I could have had a 21P.....but I doubt it, but I remember the 39 mph very well as I wanted 40 and could never get it) with the normal load which was me and tackle, batteries, fuel, etc. Didn't have a tach, but she was purring right along, singing nicely, which I would relate to 5500 give or take rpm's.

That says that 35, to me would be realistic for a 75 on that size/type boat and anything above that would be because of hull design, weight, etc.

If you got a new tach did you set the selector on the rear for your engine? I assume you set your tach to the correct pole position for that engine.....6P as I recall for most Mercs with 12P alternators. If you didn't get a chart with your tach, Teleflex has one on line. My current engine (3 cyl 90 hp) has 12 pole magnets which supports the 6P position on the back of the tach........I just verified that from the service manual for it. 6500 really sounds high for that prop on that boat with that engine. Besides, your ear would tell you that the engine is screaming, very loud and noisy, not purring or humming.

The gear boxes on our engines were probably identical as they are very close in the series so the numbers you gave us don't make sense. I'll buy the 33 (give or take) mph part after thinking about is as it is close to the number I'd expect given my experience.

Mark

 

[edjunior]

Re: High RPM's

Okay, so I got out and looked at the settings on the tach. It is (and was for the run) set on 8C/4P. I'm pretty sure it should be set on 6P, for a 12 pole setting? Now just a question, what is meant by 12 pole, or 8 pole, or even 4 and 6 pole on other engines. It appears that all Mercury's except the 8 and 9.9hp 4 stroke are either 8 pole or 12 pole. So, time for my education to continue!

 

[Jimmy70]

Re: High RPM's

The poles are located on the stator of the engine. Pull up a picture of a stator and you can count the poles on it for your engine.
Jimmy70

 

[j_martin]

Re: High RPM's

So in reality you were running about 4200 RPM. You're a little overproped. Marks recollection of running a 19 was probably accurate.

John

 

[edjunior]

Re: High RPM's

So in reality you were running about 4200 RPM. You're a little overproped. Marks recollection of running a 19 was probably accurate.

John

Okay, John, now you went and threw some math in the mix. Just for my edification, and since I really suck at math, how did you come up with 4200? Now it's going to be real interesting to see how close you were!

 

[Texasmark]

Re: High RPM's

Probably by applying the ratio of 4P-6P. 4/6 x your stated 6500 WOT rpm's = 4300. Must have done it in his head and got close.

Mark

 

[edjunior]

Re: High RPM's

Wow, you guys are just about spot on. 4200 - 4300 RPM, sure enough. Top speed right at 32 mph. Okay, so now that that is settled, let's work on the proper prop. As I said, I'm currently running a 21P SS. I know I should be at least down to a 19P SS, but what about going down to a 17? Thanks, and you guys have been a wealth of information and education.

 

[MercFan]

Re: High RPM's

I believe it's 400-500rpm for every 2 inches of prop pitch