How To Tune Your Prop

[timg19]

Re: How To Tune Your Prop

Thanks everyone! QC, the reason it is odd is because my boat is a 2003. I'm the second owner, so not only the builder would have to be lazy, but the owner and the maria I bought it from would all have be to lazy. I can't imagine I'm the first one to note the high RPM... at the same time, the guy that water tested it with me didn't note it, and I wasn't smart enough at the time to know any different... who knows, as long as it works in the end

Jestor68, that calculation seems to fit. Here is my tuning data (so far):
- Gear ratio: 1.81
- Current prop: 3 blade ~14 x 19 prop (size isn't marked, only notes pitch)
- Recommended Prop: 14.25 x 20
- Recommended RPM: 4400-4800
- Current RPM: TBD
- WOT Speed: 38 MPH (via GPS)

Top speed is noted to be around 49 MPH (source)... If anyone else has a 4.3L MPI / 1.81 it would be nice to hear what your WOT speed is.

I negotiated a new prop when I purchased the boat from the used dealer, and I think he gave me a rebuilt piece of crap. It felt like there was a LOT of prop slip when I was running WOT.

I needed a spare prop to carry on the boat, so rather then wait until I can test the RPM, I just purchased a prop based on the manufacture specs. I wanted a four blade for towing, so went with an aluminum Turning Point (I didn't want to invest in a SS until I can get accurate RPM readings). Because a pitch of 20" is recommend, I decreased 1 inch (to 19) for the four blade. It is 14" even, so I"m hoping the .25" doesn't make a big difference.

This thread notes Captiva 192 owners using 14x20 four blade props after tuning, but I believe they are using the 5.0 engine (and also have trim tabs), so I wanted to try a lower pitch on my 4.3 MPI. I'll be testing this in 2 weeks... Agh, it can't come fast enough! I LOVE this tuning stuff!

 

[QC]

Re: How To Tune Your Prop

Thanks everyone! QC, the reason it is odd is because my boat is a 2003. I'm the second owner, so not only the builder would have to be lazy, but the owner and the maria I bought it from would all have be to lazy. I can't imagine I'm the first one to note the high RPM...

So common an issue I start many posts with "check the tach".

FYI, pitch is inches not degrees We'll get you fixed up, but the data after each change is everything. Are you trimming out enough? Slip will be up if you don't trim properly. Trim too high and she'll "blow out" which is a ton of slip as well, but you'll hear and feel that.

 

[timg19]

Re: How To Tune Your Prop

My trim gauge is broken, so I can't fine tune the trim, but I am starting with the trim all the way down/in. Some people say they can "feel" when they hit the sweet spot with the trim, I guess that takes practice. It gets on plane VERY fast and it seems to be trimmed correctly when on plane. I trimmed out some while on plane to see if I could notice any adjustment, but didn't. I'll play with the trim some more though, thanks for the tip.

I can't believe the tach is a common issue, the RPM reading seems like the most important gauge! Thanks for the info Supreme Mariner, and I fixed the pitch to use inches :facepalm:. Hopefully going to test everything out this weekend!

 

[QC]

Re: How To Tune Your Prop

If you can't hear the splash off of your hull starting to drift toward the stern as you trim up then your bow may be too heavy. Try shifting as much weight aft as you can. Has she ever "porpoised"? This is a rhythmic bouncing of the bow as you are under way at higher speeds. If you have never felt that then bow may be too heavy. By any chance is there a whale's tail, hydroplane thingy on the drive? That can change all of this discussion.

 

[timg19]

Re: How To Tune Your Prop

From what I can remember, the water spray was toward the stern (not up front). She has never porpoised, I just think my hull is quick to plane (I've heard that from other captiva owners). There's no whale's tail / hydroplane or tabs on the hull.

The boat does lean toward the port (passenger) side a little bit until I'm up on plane and really cruising, and you can even see the lean in the wake (there is a little more wave on the port side). I'll take some photos while I'm out this weekend. I'm thinking adding trim tabs will fix this? It may be due to weight distribution, I've heard my boat does that since the battery and additional storage is on the port side.

 

[doyall]

Re: How To Tune Your Prop

... I can't believe the tach is a common issue ...

I have yet to own a boat on which the tach didn't eventually malfunction ... some more than once ... and usually sooner rather than later. I chalk it up to severe service usage. Since none of my boats have had high-dollar instrumentation built to [hopefully] withstand the rigors of boating, I have just resigned myself to replacing them often.

 

[QC]

Re: How To Tune Your Prop

From what I can remember, the water spray was toward the stern (not up front). She has never porpoised, I just think my hull is quick to plane

What should be happening is that the sound of the spray should move aft as you trim from full down to as high as she can go. Whenever someone says they can't porpoise and up trim doesn't seem to do much I think think about weight shifting. I want to be able to make my boat porpoise, and then control it with a slight bump down on the trim. If she can't porpoise, she won't be as fast as she could be.

 

[timg19]

Re: How To Tune Your Prop

If she can't porpoise, she won't be as fast as she could be.

That is the type of advise I'm looking for, I definitely didn't know that! So you would trim all the way out or until it starts to porpoise, then trim in until it stops proposing, and that is the sweet spot? Is there any risk of blowing out the engine by trimming it up too high / out of the water or will the engine stop because of an over rev? That is the one thing I'm nervous about.

Also, do you do this at WOT?

 

[QC]

Re: How To Tune Your Prop

I would try all positions of trim. When they start to lose bite (blowout) you can hear it. You have time to reel her back in. Does your trim limit work? Yes I would try at WOT. That's the idea. And actually, I like trimming to the point of porpoise and slightly past and using trim tabs to control, but without tabs, then you have to be able to control with trim or they can be unusable. Many guys fight porpoise forever. From what you are telling me, she still seems bow heavy, and you have the opposite problem. Can't trim up and make a difference. At WOT, you should be able to pick up at least 5 MPH from full down (in) trim, and full up (out).

 

[timg19]

Re: How To Tune Your Prop

Thanks for all the info QC! My trim limit does work, but I'm not 100% sure if it is properly adjusted (I'm going to double check that before this weekend).

It's an open bow (with nothing in it) so I can't image it is bow heavy. I'm sure it is me, I just wasn't trimming up enough (I was just tapping the trim up). It's my first time having adjustable trim, so the idea is weird to me. I'm excited to test it out now!

That is great to hear I could pickup some strong MPH to! This sounds like it is my main problem with speed and the prop slip I was feeling.

 

[QC]

Re: How To Tune Your Prop

It's an open bow (with nothing in it) so I can't image it is bow heavy.

You may be right, but a Cuddy can be heavy too. With that said, it does sound like you are being too cautious. Go for it!!

 

[callen369]

Re: How To Tune Your Prop

DO not be afraid to trim it out! My Turning Point prop likes it trimmed almost at the max. Almost all trim units have a built in limit/stop these days using the thumb switches, ans won't allow you to trim dangerously high without using your trailer button. Like others said, when you are running wide open, and trim it up, the MPH will creep up to a point, then start to drop off again, or you will hear/fell the prop slip/cavitate, then just bump it back down again.

Think you will like the TP prop. I switched from a 3 blade 14.5/21 that ran great but had lots of vibration, to a 14/21 4 blade. Performance all about the same.

 

[QC]

Re: How To Tune Your Prop

when you are running wide open, and trim it up, the MPH will creep up to a point, then start to drop off again, or you will hear/fell the prop slip/cavitate, then just bump it back down again.

Agree with this description with the clarifier that it is ventilation, not cavitation, that occurs at blowout. Common mistake, but cavitation is not felt like ventilation is. They can vent at almost any speed if trimmed too high, or tight turn, or bottom flow disturbance etc. Cavitation is extreme low pressure on the blades that causes pitting.

 

[timg19]

Re: How To Tune Your Prop

Thanks guys! I tested the boat out this past weekend and verified the RPM gauge is reading correctly after adjusting the setting on the back. Here are my results so far:

- 3 blade aluminum prop 14x19
- @WOT:
- RPM 4800-4900
- Speed: 46 MPH

Trimming out did the trick, I could feel the speed increase as I trimmed up!

I definitely need to get my trim gauge fixed though, it will make all of this so much easier. I believe I blew out before I hit the trim limit, because my engine alarm went off. I throttled back right away. I'm not sure if it was related to blowing out... but not sure what else the alarm would have went off for. I also was able to get it to porpoise at about 80% throttle. At WOT I couldn't tell if I was proposing because the boat was really flying and started to skip around on the water a little bit at that speed.

I put my new 4 blade prop on for this weekend, and looking forward to testing it out. I'm guessing I'll see a lower RPM since the pitch is the same, but I'm OK with that since with 5 people on the boat it took me a wile to plane with the 3 blade (I'm assuming the 4 blade will help with that).

 

[timg19]

Re: How To Tune Your Prop

Had the boat out this weekend with the new 4 blade. Here are the results:

- 4 blade aluminum prop 14x19
- Getting up on plain was not improved. This was disappointing.
- @ WOT:
- RPM 4500
- Speed: ~47 MPH

It felt like it was working too hard, I'm thinking it might be over propped. So the next step would be to go to a 4 blade 14x18? Hopefully that will get me on plain quicker? One other thing to note that might have affected these numbers. With the 3 blade I was running with about 15 gallons of fuel. I filled her up (42 gallons) so that might be effecting the numbers?

What do you guys think? Again my objective is towing, so the best hole shot I can get.

 

[QC]

Re: How To Tune Your Prop

We never got there, but we typically recommend a 1 inch drop in pitch when going from 3 blade to 4 blade. That assumes the 3 blade was right, and your numbers looked good . . .

 

[HopinImFloatin]

Re: How To Tune Your Prop

smart tabs work awesome, best 100$ i've spent on the boat yet. helps it get on plane real quick and keeps it there at lower rpm...if thats what your lookin for

 

[timg19]

Re: How To Tune Your Prop

Thanks HopinImFloatin. I've been looking at SX Smart Tabs since I my boat is naturally heavy on the port side, so that will be an added benefit.

 

[timg19]

Re: How To Tune Your Prop

So I installed the SX Smart Tabs on my boat, and they were definitely worth it! I'm going to do some more testing in a few weeks and will post the results when I'm done.