RPM's too High?

[spudhauler]

Here is my question.

I recently bought a 1995 alumacraft competitor 170 s/c, with a 1998 115 merc. The motor has a 17" aluminum prop. Last Saturday I had it out for the first time. When wide-open the tach in the boat said 5500-5600 rpm and my handheld gps said 38mph. The owners manual for the motor states that at wide open the rpms should be between 4750 and 5250 rpms. Do I need to be worried that it is running too high of RPM's? Will a different pitch prop make a difference in regards to lower RPM's and/or perhaps higher speeds? I wonder how accurate the tach gauge in the boat is? I would appreciate any feedback.

Thank you, Spudhauler

[ChuckN]

Personally I think your motor is running in the correct RPM range. My Johnson 115 and my Merc 175 run near 6,000 RPM full trim, 5600 or so when "normal" trim.

I had a boat similar to yours once with the 115hp motor and I too had a Stainless 17 pitch prop, top speed 43mph GPS. The aluminum prop was around 40mph. A stainless prop would definitely improve performance. The 19 pitch prop would have been ideal for the boat at the time.

[kjgmh]

DO NOT RUN OVER 5250!! Above 5250 this motor can shear the crankshaft and send the flywheel out the side of the hood. Has something to do with the harmonics in the 4 cylinder motor.

[marine_man]

I agree with kjgmh... manufacturers typically have a recommended rpm range for a reason... and there typically is some factor of safety built in there, but I prefer to leave that area alone if at all possible.

Yes, a prop change will help your issue... you'll likely need a 19 pitch prop, which will pick up a little more speed (maybe 1 or 2 mph) but should get your rpm's down to where they belong. You will suffer a little hole shot, but it won't be so much that you still wont' have good hole shot.

marine_man

marine_man

[CatManLee]

I also agree, a 19 prop would probably be perfect, unless your holeshot right now is so bad you can barely get out of the water. just remember the basic rule, RPM's too high - go up in pitch. RPM's too low - go down in pitch.

Get a 19 SS and I think you'll be happy and you'll be in recommended RPM range. RPM's above the "Normal" Range is just RPM's, you're not creating any more power, just wearing on the motor more. Hope these posts help you out.

[Surface Tension]

I'd be wondering how accurate the tach is too. When you took the boat out for a ride was the load comparable to that of a day on the water? A 19 pitch prop will drop your RPMs but whats going to happen when you load the boat down, will you be under proped. It may sound funny but, you don't have to run the engine wide open all the time. After you plane out throttle back to get it in the recommended RPMs.

[Whoaru99]

Surface Tension has some good points to consider - tach accuracy, weight of the boat and load, also summer heat/humidity will affect the performance of the combination.

That said, a rule of thumb is that each "inch" of prop pitch will change the engine speed by about 200 RPM. So, going from a 17 to a 19 inch/pitch should drop the engine speed about 400 RPM. Ideally, I think you would want the combo to top out at WOT at the top of the RPM range with the average, or slightly light load. That way when you load'er up, you shouldn't fall below the minimum WOT RPM.

Depending on how you use the boat, it may be that you need an additional prop or two of different pitches or blade design if you want to optimize performance in a number of different situations - fishing, tubing, water skiing, crusing, etc.

Myself, I have several different props. I have a 21, that I sometimes put on if I really want to streak around by myself or someone else but little or no gear and only in the spring and fall when it's cooler. A 19 that is on the boat probably 75% of the time, and a 17 if I want to take a boat load of people crusing or tubing in the summer heat and humidity. Overkill? Maybe, maybe not...

[Valv]

I also want to add that it's better to back off throttle to stay in the max rpms range than have a prop too large and lug motor down at way too low rpms at WOT (wide open throttle).

[spudhauler]

Thank you all for the good advice. Since I have the interchangeable hub/prop system already in place, I think I will pick up a 19" prop and see what happens. Sounds like a 19 might be the ticket for my normal circumstances. Thanks again, Spudhauler

[cold one sd]

I have to agree with Valv. I have my boat underpropped and stay within the recomended rpm range. Then when I have a heavy load I can keep the rpms up and I don't lug the engine. You also don't want to fall below the rpm range at when wide open either. My 2 cents.

[Valv]

Now that I think better at this I might backup a little from what I said. I think less throttle will be ok if it's just slightly below WOT. The reason is many motors now are Oil Injected and the oil pump it's activated (mechanically) by the throttle lever position. Since motor is running at max rpms but oil pump is not injecting oil at the max ratio due to throttle lever backed off a little, I wonder if this could be dangerous for motors.

This is a good question for a mechanic with "balls"...anybody out there ????

Of course 4 strokes don't have this issue.