Mercury PVS Plug Help Needed

[rpr]

I have a 22p Bravo XS that I just received and I’m trying to develop a PVS plug strategy so I am at least close to what I need with my first shot. I only got 5,650 rpms with a 24p Bravo XS, and it was boggy in the holeshot with Large plugs in all 8 holes. So I’m about to swap it out with a 22p to try to get my rpms up to ~5,950 and hopefully also improve my holeshot.

Here’s my objective, I want to minimize the PVS venting to the maximum extent practicable to improve low speed low rpm rough water handling. I really don’t understand the autonomy of plugging some and leaving others wide open, I would think using the same amount of venting on all 8 plugs would generally be beneficial? Again, I don’t need a perfect holeshot, the priority is to minimize or eliminate cavitation in low speed low rpm rough water conditions where I’m just barely on plane. So I’m thinking of initially trying either all Small or all Medium plugs.

Any other recommendations?

[fishnfireman]

It's possible you may reach your objective with all holes plugged in the 22..
Dropping down 2" in pitch should raise your RPM to where you are wanting and help your holeshot.

[JR19]

PVS holes only effect hole shot. They allow air bubbles to escape from the barrel thru the holes. This allows the prop to spin up faster because it actually needs to slip some on hole-shot. Too much slip and it will blow out with no bite. Not enough slip and it will not spin up fast enough to break the bow over. Opening and closing PVS vents allow you to fine tune how much venting/slip your prop has on hole-shot.

The initial sudden change in rpm pushes exhaust air bubbles out the PVS holes. Once the prop is spun up to operating RPM the exhaust is escaping out the exit of the barrel so it does not matter what your PVS configuration is once your up and running. PVS configuration only effects hole-shot...this is how it was explained to me. If I am incorrect then maybe someone can clarify because I don't want to steer you in the wrong direction.

On another note...If that 22P bravo I XS does not work for your set up and your interested in selling it let me know. A used 22 P Bravo I is hard to find. I "think" my set-up (150 HO with a heavy load/two shallow water anchors) could turn it so I would be interested if you want to sell it.

[rpr]

You’re spot on with the conventional wisdom, the issue is that conventional wisdom is rarely 100% accurate. In actuality, as I understand it, there is insufficient water pressure to completely “seal” the PVS holes at low speeds/rpms, so the exhaust pressure overcomes the water pressure flowing past the outside of the barrel and the PVS holes pass some degree of air. This can cause slippage/cavitation at low speeds and rpms. I view the balance here as kind of a bell curve, the PVS holes pass a small amount of air at low speeds/rpms, almost no air beyond that point up to the extreme case of holeshot where the engine is almost pinned and then it passes a lot of air.

Less than perfect explanation I’m sure, but it seems to correlate with way my rig performs. For my rough water handling objectives it would be great if I could just plug them all like Fishnfireman suggests, but I think I would really struggle to get her up on plane. I may start there and see how it goes.

Thanks to both of you for the feedback.

[mdtritn21]

Since you said it has eight holes try the 24" bravo again and open up two holes all the way. You may need to raise your motor higher to get your rpm's up to 5800+ at WOT. If your going to try the 22" prop plug the four holes further from the motor first and try it. Your trying to hit 3500-4000 rpm's at take off.

[rpr]

No jackplate (rig is too stern heavy) and I already sold the 24p.

Is there any specific rationale behind plugging 4 holes and leaving 4 open as opposed to using (for example) Mediums in all 8 holes? All other things considered equal, I would think more smaller bubbles would provide a more forgiving uniform “zone” for the prop than fewer larger bubbles. Not a great technical analysis, but hopefully you get my point. Just curious.

[fishnfireman]

Keep in mind that different motors vary in how the exhaust is relieved. Thus changing how much exhaust is forced thru the prop at take off.
Your 250XS vs my 300XS is a prime example .. On boats such as mine the 250 actually has a better hole shot even with 50 less hp when all other things are equal. The experts say it's because of the exhaust.
I was told by one prop guy to leave the outer plugs solid at first and start your relief testing on the front holes.. However when working with so many different variables that is just a starting point because they are all different.

I have had props do just as you say in rough water.. If there is to much exhaust relief they will lose bite in rough water at the most inopportune time.
Good luck and let us know how it works out.. I still think you should be real close to what you want just by dropping down 2" in pitch with all holes plugged.

[Rip Bass]

RPR. like FnF said,with the 22 I would start with all holes plugged solid. Then if you need some slip,put one small hole plug in one hole,then two holes and so on.I know you said your rig is stern heavy but a 6"or 8" manual plate will only add a few pounds and it will likely help you a lot. It's a lot easier to dial in performance if you can alter engine height also.

[mdtritn21]

The 24 was bogging with large plugs in all holes right then what you do is open one,two or four holes wide open so the prop can vent and spool up to 3500-4000 rpm's. All the holes don't have to have the same size plugs in them per Mercury who makes them. All your trying to do is let the motor spin the prop to 3500-4000 rpm's so it will push your boat up out of the water from a dead rest and any combination of plugs that allows you to do that is what you use. PVS plugs only effect take off not low speed running or high speed running. 2" less pitch will gain you 300-400 rpm's but nothing else will change and if your blowing out at low speed something is wrong with your setup not the PVS plugs.

[rpr]

Well, looks like the new prop is going to have to sit in the garage a while. Snow in GA...if I was a few years (decades) younger I’d get out there, but I’m not built for the cold.

[LOSTTEXAN]

You’re spot on with the conventional wisdom, the issue is that conventional wisdom is rarely 100% accurate. In actuality, as I understand it, there is insufficient water pressure to completely “seal” the PVS holes at low speeds/rpms, so the exhaust pressure overcomes the water pressure flowing past the outside of the barrel and the PVS holes pass some degree of air. This can cause slippage/cavitation at low speeds and rpms. I view the balance here as kind of a bell curve, the PVS holes pass a small amount of air at low speeds/rpms, almost no air beyond that point up to the extreme case of holeshot where the engine is almost pinned and then it passes a lot of air.

Less than perfect explanation I’m sure, but it seems to correlate with way my rig performs. For my rough water handling objectives it would be great if I could just plug them all like Fishnfireman suggests, but I think I would really struggle to get her up on plane. I may start there and see how it goes.

This is consistent with my experience.
Venting greatly increases minimum planing speed and increases slip in sloppy conditions.

My priority is lowest possible planing speed and maximum grip and response in rough water.
I run the highest pitch Bravo 1 style prop that I can run unvented and still achieve a decent hole shot on my rigs, and can stay on plane at around 2500rpm and 16-17 mph.

[redxdu]

Well, looks like the new prop is going to have to sit in the garage a while. Snow in GA...if I was a few years (decades) younger I'd get out there, but I'm not built for the cold.

Snow? What's that got to do with anything?? As long as there's no ice I don't understand what the problem is! Ya know there's some guys (me) who would love snow on the ground to be the worst thing they have to deal with in a day of prop testing!

Get your long johns on and get your kiester out on that lake! I'm interested to hear how it goes for you