RMPS of 115 Mariner

[stalker170]

I have just bought a ProCraft bass boat with a 115hp Mariner. It has a manual jack plate. In a earlier post I asked the question about the jack plate, And what will happen if I adjust it.<br />My concern is that the motor will run aroung 48 to 50 mph at WOT. She is turning 5600rpm on the tach. Is she turning to high, or is that were the 1991 115hp Mariner makes it full horsepower.<br />The guy I bought it from must have been looking for top speed out of the boat. I say this because it came with four props. The one he bought this spring is a HIGH FIVE five blade stainless prop. He only tried the boat twice since his investment of the new prop. He did say maybe the jack plate could go down a little. What is anyone suggestions? Should I try to get the rpms down closer to 5300, And do you think by lowering the jackplate would help. And maybe 5600rpm is fine. Open for all suggestions.<br /><br />Stalker

 

[sho305]

Re: RMPS of 115 Mariner

The jack plate will give more speed the higher it is, until the prop blows out; as there is less motor in the water. Typically, more blades on a prop will make handling more stable, less likely to blow out, but a bit slower top end. Might be faster if the plate can be raised though, no doubt what he was trying. If he said lower it then it is getting blow out and cavitation, and that gets old real fast. Since over-trimming the prop can lead to a big prop angle(pointing up) the jack plate makes more efficient use of thrust by aiming it back instead of up. You only trim enough to get the front of the hull out of the water. Make sure you are getting good water intake when you get max trim!<br />Find out what the max rpm is for your motor and prop it to run that way empty & with just the driver. Unless you want to pull skiers or whatever. Should be in a manual or on the flywheel sometimes, or call a nice dealer. 9 time out of 10(unless a guru here knows better) that max factory rpm is fastest top end, but maybe not the quickest holeshot of course.

 

[stalker170]

Re: RMPS of 115 Mariner

Sho305<br /><br />Thanks for your reply, I have a couple more questions. What she does is run at around 45mph at 5200 to 5300 rmp. As I keep triming up she will run around 49 to 50 mph. But than she is turning 5600 to 5700. What are you calling trimed all the up? When she is turning the proper rmp? Or where she runs the fastest without be able to take in water?

 

[sho305]

Re: RMPS of 115 Mariner

Well, of course you need the trim down to get on plane and then it needs to go up to pull the front of the hull out of the water for less drag and more speed. This is normal operation and you put it where it works best. To high and the boat is unstable or prop comes out of the water, and to low is too slow. You use what works best for your water/loading/use/boat combo/prop/etc. As high as you can trim that goes fastest, but not more trim, would be your normal top speed and you would not want to exceed the factory max rpm much normally at that point. Most of the time the motor will make less power above that rpm so you will go slower. The possibility of damage exists too from over-rev, but usually is not a problem(at your risk). Maybe you normally have a loaded boat and rpms/speed are good, then only take and empty speed run once in a while, so you make better use with a little high rpm empty and don't stay on it for too long(for example).<br /><br />The jack plate is like trim; you play with it until it works for you. Normally higher is better providing you can get enough trim to put the hull at the right lift/angle for the best speed, as every boat is different-and the prop will change all of this too. Too high and you will get cavitation on turns and holeshots and such.<br /><br />If you trim way up, you can get the water intake out of the water at speed depending where it is. Very BAD! Check it while running to make sure you are in the safe zone here. Some boats have water pressure gauges for this or maybe fast acting temp gauge. Newer motors have lower intakes for this reason.

 

[stalker170]

Re: RMPS of 115 Mariner

Sho305,<br /><br />I see what you are saying, I guess the best thing is to play with the jack plate a little and also watch what the water is doing. The boat does have a temp guage. I am going to look at the motor to see if I can find the rpms stamped on it. As far as loaded or empty, when both livewells are full of water, she will be allot heavier. If I can't see it stamped on the motor, than maybe I can call a dealer for that info. Oh, she handles excellent when trimed for 50 mph I just noticed the tach being up in the 5700 range. I will look at the water coming from the motor when I try it next time. I also have a three blade stainless that came with her. Like I said before, he was doing some tweaking for speed,<br />The five blade HIGH FIVE looks like he spent a dime or two.<br /><br />stalker

 

[sho305]

Re: RMPS of 115 Mariner

Stalker17,<br /><br />Yes, it is very much a "tune your own combo" sort of thing. The factory will give a rpm range on the motor or in the manual for max rpm. This might be 5400-5800rpm for example. The idea would be to run 5800rpm empty maxed out, and 5400 with a loaded boat. Make it closer to what you use it most for. Whatever you are doing, it should run fastest near the higer rpm for most motors. You can sharpen the leading edge of the prop blades for a couple mph unless you have kids playing with it or something. I never let anybody around the motor running or not.<br /><br />Try different setups, but be careful of trimming way up in rough water as I have seen many people get hurt and break bones. It's like going 100mph down an ice filled road; you might make it, or not. (If you run in big water areas anyway.) Here in MI they use the same rules as for driving a car, particularly the drunk driving one and take your license(and boat!). Never was that way until people got hurt from being stupid at the wheel. Not a problem on smaller lakes here though.<br /><br />I looked at my motor to see the water intake, then had someone run it fully trimmed up and went back there. The intake was still about 3" under the water flow, so it was good on my boat. It was easy to see from the back seat. It is neat to see how the water flows off the hull and around the motor; you never see that otherwise. You might also get a nosecone for more speed.

 

[MIKE F]

Re: RMPS of 115 Mariner

stalker 170<br />I have a 1996 Merc 115HP - same engine? and the max RPM is 5,250.

 

[what200]

Re: RMPS of 115 Mariner

I do believe that the merc inline 4 is supposed to be turning in the low 5's, but I have friend with the same procraft 115 combo that spins a laserII 22P 5500-5800 for 10 years without a problem. If I had to guess the recommend rpm range is a little lower for that motor because it doesn't make any extra power up there, but I wouldn't feel comfortable spinning it past 6. From people I've talked to about the inline 4, merc designed that motor to have good low rpm torque but not really be a twister.