2019 RT188 with Mercury Pro XS 4 with CT Option - RPM vs Prop size

[Surferboy120]

I have been enjoying my boat a lot since getting it used about 3 months ago now. I recently got the Vessel View module so I could keep tabs on my 115 hp Mercury Pro XS 4s with the Command Thrust option and keep up with maintenance times and monitor the engines activity. I am running a Tempest - 48-825862 21P with the motor mounted at 2nd hole from the top and the prop DOES NOT have solid plugs. I am not sure what size the pass-thru is on the plugs but they are not solid.

With vessel View I was able to see my top RPM was 5430 at 43 mph. As I understand it this Pro XS has a lot more RPM in it as its redline is 6300.

I write this looking for a bit of advice from other with this motor, an RT188 and have confirmed speeds (GPS). The analogs gauges are crap. Mine say 6000 rpm when I am running 5400 and the speedo shows 48 when I am really going 43. This is fairly common as I understand it so I hope to get some data from GPS verified sources.

I have been doing some searches but it seems the prop I have should be the correct one 21P?.... from what others have but this low rpm tells me I do not have the right prop unless I am missing another variable.

Thanks for the thoughts in advance. I am not trying to get every drop out of it but 5400 rpm seems to be a bit low considering this is the Pro XS model.

[Tim Kelly]

That's 18% prop slip according to the calculator, which seems high. You're fully trimmed to the point where it's nearly porpoising and the steering is fairly neutral?

[Surferboy120]

I am fully trimmed at max throttle but I have zero issue with porpoising and the steering seems neutral.

[Tim Kelly]

Assuming you're an average size, the boat has a 24v system, not massively loaded with stuff etc I think you sound on the slow side. When Ranger started with the CT motors they supplied 23p props, which were too tall, the 21 was supposed to be a better match and get closer to the 6000ish RPM range. Be interesting to see what others say who have the same rig. I don't know whether the 3rd hole is a more usual setting for the outboard, but doubt that would make a significant difference. Be interesting to see what a 19p prop would do for you, but usually that would not be enough prop.

[Surferboy120]

Assuming you're an average size, the boat has a 24v system, not massively loaded with stuff etc I think you sound on the slow side. When Ranger started with the CT motors they supplied 23p props, which were too tall, the 21 was supposed to be a better match and get closer to the 6000ish RPM range. Be interesting to see what others say who have the same rig. I don't know whether the 3rd hole is a more usual setting for the outboard, but doubt that would make a significant difference. Be interesting to see what a 19p prop would do for you, but usually that would not be enough prop.

I completely agree with your thoughts and I am eager to see what is said by others playing with this setup. I think what misleads some of this is the fact that the analog gauges are not good and people report data from them rather than via GPS and tools such as vessel view. I was very surprised at my RPMs using vessel view. I am using a 24v setup for my Ultrex and I am a little rascal. I have the dual console but thats light and so is my wife he he he.... I was also curious what people said about my plugs and engine height being 2nd from top.

[Tim Kelly]

My understanding is that the plugs only affect hole shot, and were designed for 2 stroke motors and not relevant for 4 strokes, for some reason I don't understand, but that seems to be the consensus.

[Surferboy120]

The 4s has more grunt down low by nature vs the 2s is top end.

[GeauxTide]

This is why I don't like CT on light hulls. Being almost 1k below max will eventually harm the motor. I run at hole #3 with my X7. The Tempest has good cup, so I'd raise it.

[Surferboy120]

This is why I don't like CT on light hulls. Being almost 1k below max will eventually harm the motor. I run at hole #3 with my X7. The Tempest has good cup, so I'd raise it.

I am ignorant to all the benefits of raising the motor but with that said it makes sense that it would reduce drag which increases speed\rpm. Now with that logic I do not believe it would be much in the way of raising the mph\rpm? I guess what I am saying\asking is shouldnt I play with the prop pitch first before raising and get the RPMs right under the 6300 and then raise it? Thanks in advance.

[Tim Kelly]

I think you need to wait until someone with the same setup as you reports their figures, yours sound sub optimal, so if others are getting genuinely closer to 6000 it might be a problem with the motor/fuel/hull

[Surferboy120]

Yes I do agree.

[tcesni]

I don't have the same motor so I will only add that you are running your set up in the "slowest" time of year during heat and humidity so you might gain some performance when cooler weather arrives. But there has to be more to be found in your rig as I run 43 mph with a smaller motor on the same boat. Something is definitely not right.

[Surferboy120]

I don't have the same motor so I will only add that you are running your set up in the "slowest" time of year during heat and humidity so you might gain some performance when cooler weather arrives. But there has to be more to be found in your rig as I run 43 mph with a smaller motor on the same boat. Something is definitely not right.

I agree and one simple thing to verify is that the throttle linkage is also actually making it to max throttle on the motor. Sounds simple but also another item to just check off the list. I have seen stranger things in my life. If I have read some of the legacy posts for the 4s XS with CT option then I 'believe" the optimum prop was this Tempest 21p. The only two differences would be the motor height being on #2 from top vs #3 and the prop plugs not being fully plugged.

[Rangerjeff]

I run the same motor on a rt188 with a 21p tempestplus. I get 6250 rpm on a vesselview and 48 mph Gps. I am not sure what to check next if I were you but it sounds like we are running the same stuff. I do agree the analog gauges are junk. My motor is in the number 2 hole as well.

[Surferboy120]

I run the same motor on a rt188 with a 21p tempestplus. I get 6250 rpm on a vesselview and 48 mph Gps. I am not sure what to check next if I were you but it sounds like we are running the same stuff. I do agree the analog gauges are junk. My motor is in the number 2 hole as well.

Are you running plugs on the prop? I dont think it plays into the top speed just curious. Sounds like I seriously need to check the throttle linkage to make sure its seeing full range of throttle at the motor. Thank you again for your input it baffles me but confirms I should be seeing more with the current setup.

[Rangerjeff]

Are you running plugs on the prop? I dont think it plays into the top speed just curious. Sounds like I seriously need to check the throttle linkage to make sure its seeing full range of throttle at the motor. Thank you again for your input it baffles me but confirms I should be seeing more with the current setup.

Yes...not solid...it sounds like we have the same setup...I have no idea what is causing the differene

[Surferboy120]

Yes...not solid...it sounds like we have the same setup...I have no idea what is causing the difference

Yes we are identical motor wise. I am going to check the cable travel asap and report back. There isnt a lot of things that should be limiting rpm like it seems to be. I sure do appreciate you responding as it at least gives me some direction.

Thanks again

[NewJay]

I have the same boat, motor, and prop with trim tabs cut off, 36v 50ah lithiums, 112lb ultrex, half tank of gas, full gears tourney loaded with dual 8’ talons and was able to get 50.2 mph GPS. I have vessel view but did not check what rpm I was at when running full throttle. I would assume it’s probably around 5700-5800 since a 24p can only go 5500 the last time I checked.

[NCSURodney]

Have a 2016 RT 188 115 Pro XS Command Thrust. Purchased used from local dealer with 10 hours on it. Boat came with 24P Tempest and I was worried it was to large (Swindle has 23P on his tin rig). Although I rarely run WOT I've had boat to 49 GPS and RPMs I'm guessing were in the 5800 range.

[Surferboy120]

I have the same boat, motor, and prop with trim tabs cut off, 36v 50ah lithiums, 112lb ultrex, half tank of gas, full gears tourney loaded with dual 8’ talons and was able to get 50.2 mph GPS. I have vessel view but did not check what rpm I was at when running full throttle. I would assume it’s probably around 5700-5800 since a 24p can only go 5500 the last time I checked.

So your motor is a 4 stroke not the two stroke and you are running a 24P?