Top Speed RPM Question

[Adrenalinejunky]

I'm only asking as I have only seen the same as you are now. An occasional 48mph. Have not seen a 50 ever. Are you using the sigle lever option?

I was seeing 50 on pretty much any water condition, with or without bimini, and with both single or double throttle levers. I got my plugs last night and will change them this evening but will not get to test it out until this weekend.

One observation is that my engine rpms stay pretty even until what would be max speed. This issue started at around 25 hours.

 

[WickedPorter]

I change my plugs yearly during winterization. Have had what I would consider a "low RPM" issue since purchase (new) ... Always blamed the CARB goodies that are bolted on to my engines. Still wondering if air filter/ribbon delete would make more of a difference on a CARB boat vs. non-CARB. Anyone with a CARB boat notice a difference after removing these things?

 

[Stpndeep]

I was seeing 50 on pretty much any water condition, with or without bimini, and with both single or double throttle levers. I got my plugs last night and will change them this evening but will not get to test it out until this weekend.

One observation is that my engine rpms stay pretty even until what would be max speed. This issue started at around 25 hours.

I do not remember the top end rpms exactly but I do feel like both engines were right at 7800 at 50mph with full tank and family and now 7600-7700 port and 7300-7400 starboard WOT at 46-47 mph with family and full to empty tank...

Same boat here same issue. 7400 port 7100 starboard at wot in single or dual... Interesting everyone I've read with this issue the port engine runs a few hundred higher.

 

[swatski]

Same boat here same issue. 7400 port 7100 starboard at wot in single or dual... Interesting everyone I've read with this issue the port engine runs a few hundred higher.

It's because they use impellers w/the same pitch now, ever since the introduction of articulating keel .
Should be a relatively easy fix. I'm still breaking-in/babying mine, so not sure exactly how she will do, but it is pretty much a given starboard impeller will need depitching by some 0.5-1mm on trailing edge to even those RPM out.

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[Ronnie]

I don't understand the correlation between the articulating keel and using the same impeller on both pumps. I've always read that since they spin in the same direction one pump / side is more efficient at taking in and putting out water such that one impeller needs to be detuned / re pitched to keep the rpms in synch and the boat tracking straight.

I assume that yamaha decided that since the boats with keels track straighter than those without they no longer needed to put impellers with different pitches into the boats with keels, which saves them a few dollars as well. That sucks IMHO.

My service manual states that the starboard impeller has a 15.6 degree pitch and the port has a 16.8 degree pitch.

At $158 for a new stock impeller (part 24 port in th pic), it may be easier to just replace one than have one repitched.

Since the new boats are running what used to be the starboard impeller on both pumps isn't it the port impeller that needs to be repitched?

 

[swatski]

I don't understand the correlation between the articulating keel and using the same impeller on both pumps. I've always read that since they spin in the same direction one pump / side is more efficient at taking in and putting out water such that one impeller needs to be detuned / re pitched to keep the rpms in synch and the boat tracking straight.

I assume that yamaha decided that since the boats with keels track straighter than those without they no longer needed to put impellers with different pitches into the boats with keels, which saves them a few dollars as well. That sucks IMHO.

My service manual states that the starboard impeller has a 15.6 degree pitch and the port has a 16.8 degree pitch.
View attachment 43976
At $158 for a new stock impeller (part 24 port in th pic), it may be easier to just replace one than have one repitched.
View attachment 43975
Since the new boats are running what used to be the starboard impeller on both pumps isn't it the port impeller that needs to be repitched?

View attachment 43977

Here we go again...

Most people pitch to get the RPM they want. Most people like to get higher RPM /go faster at WOT. Hence, one would pitch the slower impeller to reach higher RPM, ideally approaching the max/ RPM rev limit that hits at approximately 7800+.

That slower impeller is the starboard in new boats with keel.

It is generally assumed that those engines generate maximum torque at around 7700 RPM, albeit Yamaha does not officially publish torque /power curves.

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[Ronnie]

Still not following . My boat came with two different impellers yet both engines reached max Rpms between 7600 and 7800. Now on the new 240s with keels both impellers are the same, it's the starboard implller. If the starboard impeller was and is reaching max Rpms on the starboard side why mess with it at all? It seems like the port impeller would need to be repitched to match what Yamaha used on the early 240s.

 

[swatski]

Still not following . My boat came with two different impellers yet both engines reached max Rpms between 7600 and 7800. Now on the new 240s with keels both impellers are the same, it's the starboard implller. If the starboard impeller was and is reaching max Rpms on the starboard side why mess with it at all? It seems like the port impeller would need to be repitched to match what Yamaha used on the early 240s.

Well, in new 240/242 boats (with keel) starboard impeller is the SLOWER one (at WOT).

There are exceptions (of unknown etiology), but as a rule the new boats rev the starboard impeller about 200RPM SLOWER than the port (at WOT). Therefore, depitching the starboard impeller trailing edge (to a less aggressive, lower angle) would bring the RPM UP, to be equal with the port. (Assuming nothing else is limiting of course.)

Hope this helps.

To put this in context, older boats WITHOUT keel used to come with starboard impeller pitched a bit lower (less aggressively, as compared to port side) on the trailing edge. There are reasons Yamaha decided not to vary the pitch between the two sides in boats with keel, but that is a different conversation.

As far as specific pitch, the numbers posted above are referring to the trailing edge only. It is important to keep in mind that these are "progressive pitch" type impellers and the leading edges are also different between port and starboard (in older non-keeled boats).

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