How To Tune Your Prop

[timg19]

I just purchased a 2003 Rinker Captiva 192 - Mercury 4.3 MPI 220 HP.

I'm finding the recommend RPM for my engine is 4800. We had it out this past weekend and if my memory serves me correctly, my RPM at WOT was around 6000 - 6500! Speed was 38 mph via GPS (speedometer gage was not working).

Obviously I need to get my RPM down and I feel like my speed can go up? It felt like I had prop slip. I made sure my trim was all the way down too.

My question is, how do people go about tuning their props? Mine is 14, 19 pitch, 3 blade and I can't afford to go out and buy multiple props to test how they effect RPM. I'd like to have a 4 blade prop for our lake since I primarily pull tubes and wakeboards. When I use it on the river I focus on speed, so I'm guessing I should switch to a 3 blade on the river?

What's the roll of thumb for doing this type of tuning? I'm coming from a 1981 70HP Johnson outboard (on a 15' Thundercraft), so tuning wasn't a big deal to me prior

 

[BobGinCO]

Re: How To Tune Your Prop

What's the roll of thumb for doing this type of tuning? I'm coming from a 1981 70HP Johnson outboard (on a 15' Thundercraft), so tuning wasn't a big deal to me prior

The rule of thumb is for each inch of change in prop, you will have a 200 RPM change in WOT.
That rule has so many exceptions, it should hardly be considered a rule at all.

Obviously, you need MORE pitch. But how much is a guessing game. At least a 21"
A 21" four-blade might be a good choice - but I know squat about your situation.
Yes your speed will go up.

*that's it for the serious stuff I have to say*

Now some people swear by the "tuning fork" method of tuning, where they put a 440 Hz tuning fork on the hub, at WOT, and listen for the "third harmonic distortion". Still others will swear by the digital guitar tuner method, clipping it onto the lower unit, and adjusting for "High E". Personally, I was taught by an old-school master to use the "hamster/gerbil" method. But that is a trade secret, and I can't reveal it at this time.

 

[Bondo]

Re: How To Tune Your Prop

I just purchased a 2003 Rinker Captiva 192 - Mercury 4.3 MPI 220 HP.

I'm finding the recommend RPM for my engine is 4800. We had it out this past weekend and if my memory serves me correctly, my RPM at WOT was around 6000 - 6500! Speed was 38 mph via GPS (speedometer gage was not working).

Obviously I need to get my RPM down and I feel like my speed can go up? It felt like I had prop slip. I made sure my trim was all the way down too.

My question is, how do people go about tuning their props? Mine is 14, 19 pitch, 3 blade and I can't afford to go out and buy multiple props to test how they effect RPM. I'd like to have a 4 blade prop for our lake since I primarily pull tubes and wakeboards. When I use it on the river I focus on speed, so I'm guessing I should switch to a 3 blade on the river?

What's the roll of thumb for doing this type of tuning? I'm coming from a 1981 70HP Johnson outboard (on a 15' Thundercraft), so tuning wasn't a big deal to me prior

Ayuh,.... Fix yer Tach, it's Wrong....

No way yer 4.3l was screamin' along at 6500 rpms.... It'd turn itself into an Anchor long before that....

 

[timg19]

Re: How To Tune Your Prop

I just went down and looked at the boat again and the interesting thing is, gauge sits at 1000 RPM...with no power. So that's a good indication that it is messed up.

So assuming the RPM is 1000K off, that means I was around 5000 - 5500. Has anyone had such a thing where the tack sits at 1000K when it should be 0? Do I need a new gauge?

 

[Wind dog]

Re: How To Tune Your Prop

The rule of thumb is for each inch of change in prop, you will have a 200 RPM change in WOT.
That rule has so many exceptions, it should hardly be considered a rule at all.

Obviously, you need MORE pitch. But how much is a guessing game. At least a 21"
A 21" four-blade might be a good choice - but I know squat about your situation.
Yes your speed will go up.

*that's it for the serious stuff I have to say*

Now some people swear by the "tuning fork" method of tuning, where they put a 440 Hz tuning fork on the hub, at WOT, and listen for the "third harmonic distortion". Still others will swear by the digital guitar tuner method, clipping it onto the lower unit, and adjusting for "High E". Personally, I was taught by an old-school master to use the "hamster/gerbil" method. But that is a trade secret, and I can't reveal it at this time.

Do you need to use a strobe tunner or will any clip on guitar tuner work? ;-) ;-)

 

[timg19]

Re: How To Tune Your Prop

Thanks, BobGinCO. So I guess what you are saying is people take an educated guess and buy the prop? The guess depends on RPM (and if you're good, the sound)? But since all of this depends on an accurate RPM reading, I need to figure that out first before I get the prop.

 

[BobGinCO]

Re: How To Tune Your Prop

Do you need to use a strobe tunner or will any clip on guitar tuner work? ;-) ;-)

I tried a strobe tuner once. Turns out, that I'm susceptible to seizures from the strobe. I shoulda remembered that from the 70's, but thought it was maybe the recreational substances we were using.

Now I don't have a guitar - I have a banjo, so is the tuner the same? Every time I try that method, I get this great sense of foreboding, like I'm taking a one-way trip in a canoe. If you use a clip-on guitar tuner, do you also need a capo?

 

[BobGinCO]

Re: How To Tune Your Prop

Thanks, BobGinCO. So I guess what you are saying is people take an educated guess and buy the prop? The guess depends on RPM (and if you're good, the sound)? But since all of this depends on an accurate RPM reading, I need to figure that out first before I get the prop.

Hey, some people here (NOT ME) are very experienced, and can make better guesses than others. I just know how it's done - sort of an educated guess. But you REALLY need to get an accurate WOT RPM reading before you try anything. If you can work with your marina, or a prop shop, they can usually let you "try" different props, without too much difficulty (as long as you don't ding them, and return them "like new".

 

[Don S]

Re: How To Tune Your Prop

You have an MPI engine, so the first thing that is going to happen is the ECM on the engine will not allow the engine to go past 5000 rpm, it will drop injectors to prevent it from going further up.
A shop can hook up directly to the ECM and read the rpm.
Until you get an accurate rpm reading, switching props is a waste of time.

 

[timg19]

Re: How To Tune Your Prop

Thanks for the information everyone! I will work on getting an accurate RPM reading first and go from there. I already emailed Rinker to try to get my hands on a replacement tach.

 

[timg19]

Re: How To Tune Your Prop

You have an MPI engine, so the first thing that is going to happen is the ECM on the engine will not allow the engine to go past 5000 rpm, it will drop injectors to prevent it from going further up.
A shop can hook up directly to the ECM and read the rpm.
Until you get an accurate rpm reading, switching props is a waste of time.

That's great to know about the MPI Don S! Always nice to have that type of a fail safe in there.

 

[Frank Acampora]

Re: How To Tune Your Prop

many tachs will stop wherever they are when the key is turned off. They may not necessarily return to zero. An off reading of 1000 RPM does not necessarily mean the tachometer is malfunctioning.

 

[timg19]

Re: How To Tune Your Prop

Thanks for the observation Frank. I took the gauges out and checked all the connections. Put them all back together turned on the accessories and the tach went to zero. So it may or may not have been a wiring problem... Looks like I need to get it back out on the water and test the RPMs at WOT again. I won't get to do that for a few weeks, but when I do I'll check back in with the results.

 

[timg19]

Re: How To Tune Your Prop

I looked at the back of my tach and found the following. Any chance this could be set wrong? I'm not really even sure what this is...

 

[BobGinCO]

Re: How To Tune Your Prop

Ic he led the back of my tach and found the following. Any chance this could be set wrong? I'm not really even sure what this is...

Looks like it's set for a 4 cylinder engine.

 

[timg19]

Re: How To Tune Your Prop

That's what I found from my research last night. Mine is a 6, which means it requires more "pluses" (3) based on the label on the switch . I'm assuming that means having it set at a 4 cylinder the RPMs would read high because they are reading a rep as 2 pluses rather then 3. I'm hoping that is the problem!

 

[lakegeorge]

Re: How To Tune Your Prop

That's what I found from my research last night. Mine is a 6, which means it requires more "pluses" (3) based on the label on the switch . I'm assuming that means having it set at a 4 cylinder the RPMs would read high because they are reading a rep as 2 pluses rather then 3. I'm hoping that is the problem!

I think Rinker is still in business, why not contact them and find out what it should be. If the tach is original I don't know why anyone would set it up wrong.

 

[timg19]

Re: How To Tune Your Prop

Quick update: I contacted Rinker and they said the RPM gauge should be set to 3P 6C. It is weird that someone would set it to a 4 cylinder setting though... Right now I'm waiting to water test this to see if this actually fixed the gauge output.

 

[jestor68]

Re: How To Tune Your Prop

With a 1.62 gear ratio with a 19 pitch prop; your theoretical rpm at 38 mph (with 10% slip)is about 3782 rpm according to the prop slip calculator. With a 1.81 gear ratio, it works out to about 4225 rpm.

Your tach is way off. And as Bondo said, your motor is equipped with a rpm limiter that kicks in at 5100(+/- 100)rpm. The motor will not run at 6000 rpm; when the limiter kicks in, it will sit at that rpm and "stutter", and not go any higher.

Since it appears that somebody messed with the settings on the back of the tach, you probably found the problem.

 

[QC]

Re: How To Tune Your Prop

It is weird that someone would set it to a 4 cylinder setting though...

Very common. They use the same tach in multiple boat/engine combos. Simple laziness on the builders part.