Correct WOT for 86 115

[redjmp]

I am trying to determine the correct prop for my I6 merc.
It spins 5800 with what I think is a 17 pitch @ 43 mph.
When I look at the prop selector here at iBoats, It says that the correct rpm range for all years of I6 115's is 4750 to 5250.
The 135 - 150's are listed here as 5000 to 5600.
Now I thought that my 115 is the same motor as an older 140 or 150, the difference being the low dome pistons and the ignition system.
So I don't know if I should go up to a 19 which should bring down the rpms to around 5400-5500 and speed up to 45mph, or to a 21 which should bring it down around 5000-5100 and speed up to 47mph. Then again it will be a steel prop so the rpm drop will be a little less and I do want the most speed and fuel efficiency. I will swap to my existing prop for pulling/towing.
With the aluminum prop I have now, I tend to run motor around 3500 which is the most fuel effecient at around 5.9 mpg @ 27 mph on my cvx/cobra style 15' skiboat weighing about 1600 fully loaded.
Before you go saying this belongs in the prop forum, my only questions are:
What is indeed the correct WOT range for this motor, and am I safe running at 3500 rpm for extended periods or is that considered "lugging"?

 

[JB]

Re: Correct WOT for 86 115

Let's try this in the Prop Forum

 

[Texasmark]

Re: Correct WOT for 86 115

I ran the '89 I6 at a constant 6000 rpm at WOT with me in the boat....which was my normal load.

I ran a Merc SS prop, forget what it was then, but it had some fancy name with either 19 or 21P. Probably was 19. It had port holes for the hole shot which were super for getting a skier up when we went skiing. The boat was a Ranger 680C and had a deep V padded hull so when you get her hooked up the hull came up outta-da-watta and she took off. I don't remember the top speed but it was in the high 40's.

I could have gone up in pitch to pull the rpm's down, but the occasional times we went skiing, I wanted the extra hole power the lower pitch gave me to get my 250# arse up on slalom in a deep water start.

That baby just sang like a meadow lark at 6000. I'd trim out till speed started dropping off and that's where I ran her.

Got tired of fishing and after 7 years of ownership, sold it to the daughter/son-in-law who still have and enjoy it.

Mark

 

[redjmp]

Re: Correct WOT for 86 115

JB This not a prop question. Why did you move it or did you not bother to read it?

Before you go saying this belongs in the prop forum, my only questions are:
What is indeed the correct WOT range for this motor, and am I safe running at 3500 rpm for extended periods or is that considered "lugging"?

 

[Bob_VT]

Re: Correct WOT for 86 115

You are running the best of all worlds. I have the same results with my 115 I-6. I really never paid too much attention to the cruising rpms but as long as you are up on plain you are not lugging that inline. If you go to a 21 or 23 pitch then that would lug it.

I try to keep my wot below 5800 with 5500 being optimum.

 

[redjmp]

Re: Correct WOT for 86 115

Thanks for the info guys but I still would like to know which is correct.
5000-5600, or as the selector says here on iBoats 4750-5250?
I'm going to repost back where it belongs.
According to what you all have said, the selector is wrong.
Perhaps it needs to be changed

 

[Texasmark]

Re: Correct WOT for 86 115

I think you are splitting hairs my man. You need something to keep your mind occupied.

Mark

 

[redjmp]

Re: Correct WOT for 86 115

I don't think the difference between 5600 and 4750 is exactly splitting hairs.
That's about 3 prop sizes difference.
As for keeping my mind occupied, I actually try to run my boats and fish when ever the weather permits.
From a guy that has spent just about more time on here than anybody else, with thousands of replies posted, and pointless nonsense like your last, obviously to occupy your idle time, don't you think that's a lot like the well used pot calling the shiny new kettle black?

 

[Texasmark]

Re: Correct WOT for 86 115

Guess I got tangled up in about the third paragraph "So I don't know if I should.......". Sorry if I didn't understand what you were really wanting to know.

And your question about 3500 rpm and lugging is a great place to run and get good mileage and low engine wear and tear.

I chat on here to keep my typing skills honed, have something to do, and reminisce. Sometimes I even have something to say of use to someone. Then there are times that I learn things that I have wondered about for a long time. Then there are times when I just blab. Then there are times that I just read. Then there are times that I feel that I should control myself.

Mark

 

[walleyehed]

Re: Correct WOT for 86 115

What the book says and what it is now (current time) is different due to the lower quality (not octane) fuel, and we've found the higher RPM to cause less coking and carbon build-up with best economy and all-around use, is right where you are...5800 RPM.
Remember, your carbed 2-stroke is just an air pump really...that I6 will handle alot more.