Mercury Prop PVS plugs on prop mated to a 5.0 MPI + Alpha One GenII

FliesAndFloats

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After much coaxing from others to switch to a stainless prop, I found a decent price on a used Mirage Plus on eBay and picked it up and installed it on my 24' bowrider with a single 260HP Mercruiser 5.0 MPI. The 19" aluminum prop that it had would allow me to run up to around 4800 RPM and 43 mph GPS speed. The "new" SS prop, also 19", did not include the PVS plugs.

On the hole shot the engine overrevs to 4000 RPM before coming back to around 3300-3400. I notice that at 3500 RPM I'm seeing around 35 mph instead of the 28-29 mph I was seeing with the aluminum prop, though the engine seemed to max out around 4200 RPM and 40-41 mph (as opposed to 43mph at 4800). I didn't stay on it for very long, but it felt like it may have still had another couple hundred RPM in it if I'd waited. I don't believe that the hole shot is as good as it was with the aluminum prop as I found the boat not yet on plane at the point when I would usually pull back on the throttle. With the old prop, I would start at idle and ease it all the way to full throttle and back off to keep the engine from exceeding 3600-3800 RPM as the boat would come on plane. Now it seems like it takes longer to "pop up" on plane and that peak to 4000 RPM on hole shot sounds like an airplane with a constant speed prop does if you advance the throttle too quickly and it just doesn't seem right. I figure I will go ahead and order the solid PVS plugs for the prop and call it a day, but I figure someone here could give me some competent advice/recommendations. I fear I might have purchased a prop that may be a bit oversized for my setup, though I don't think I'd lose any money on it if I had to sell it.
 

Fun Times

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I fear I might have purchased a prop that may be a bit oversized for my setup, though I don't think I'd lose any money on it if I had to sell it.
Hi there, unfortunately you're most likely going to be correct. The Mirage Plus "Diameter" is a bit bigger and typically doesn't work as well on the Alpha stern drives....The Mirage Plus normally offers a better fit/performance on the Bravo gear cases.

The PVS plugs might show a little bit of a difference from what you're seeing now on the Mirage SS prop, but again due to the increase in diameter compared to the aluminum prop, chances are you are not going to see to much of an "overall" performance increase from what you are use to with the aluminum prop at "all" RPM's.

For the typical pleasure boats that use an Alpha stern drive, the Vengeance SS from Mercruiser tends to offer comparable (< but better) overall performance over the Mercruiser aluminum prop you have now.

https://www.mercurymarine.com/en/us/...tor/#/step-one

http://boatpropellers.iboats.com/?ca...n_id=664129110

Since this is more of a prop question vs an engine repair question, we're going to move this topic on up into the Prop Questions and Topics sub-forum for you.

What model boat do you have?
 
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steelespike

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Often it is suggested to drop one inch in pitch when going to a ss prop.,
I wouldn't be surprised if the new prop requires a different hole shot technique.
As you probably know the vents are there to help the motor rev a little easier.
Will be interesting to see the results with the plugs.
Bow lift might slow the final planing some perhaps delay trimming up a little.
 

Chris1956

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I did not think the vented prop was proper for my MerCruiser. I therefore used flat head stainless steel screws and nuts to plug the vent holes in my prop, on my 4.3LX Alpha drive. Install the bolt into the vent hole and install the nut. As you tighten the nut, clip off the bolt end, until It is short enough. The plugs can be easily removed when desired..


After you maximize your performance, you could have the prop "sized down" 1" pitch at a prop shop, if that is what you need.
 

FliesAndFloats

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Hi there, unfortunately you're most likely going to be correct. The Mirage Plus "Diameter" is a bit bigger and typically doesn't work as well on the Alpha stern drives....The Mirage Plus normally offers a better fit/performance on the Bravo gear cases.

...........

What model boat do you have?

The boat is a 2005 Maxum 2400SD with a Mercruiser 5.0 MPI mated to an Alpha One GenII stern drive. I guess I should update my profile/sig line to reflect that. I've been out of town all week; the PVS solid plugs arrived while I was away and I intend to test them in the morning.

I'm guessing the hull is very similar to the Sea Ray 240 Sundeck, though those were equipped with Bravo III drives; they mated their 225WE to the Vengeance 19 SS as someone suggested earlier in the thread.
 

rallyart

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Vented props are great. With the plugs out completely or in to block the venting there is an enormous difference in behaviour though. It sounds like no plugs provides much to much slip. Get the solid plugs and you can tune it exactly right for your hull. Run it first with the plugs solid. Run it again after you drill a 1/8" hole in about the center of each plug. Try going up in size with the drill if it's better. The holes don't have to be perfect and it's much cheaper than getting a set of plugs with formed holes. You know with no plugs that it has too much vent and you'll know how it is with no vent. I suspect you'll find a great setup in the middle somewhere.
With my Vensura the boat worked so much better with some ventilation than none but it was not good with the plugs all the way out. Seat of the pants and your ears will let you know what is happening. I would lug coming out of the hole with no ventilation but got it so my slip on starts set the RPM at a much better place on the torque curve.
 

FliesAndFloats

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Update: I installed the plugs today and it made a world of difference. I also did some more sea trials. With the solid plugs the boat gets on plane nearly immediately and I'd say sooner than it did with the aluminum prop. It's almost like it starts to get on plane before it pitches all the way up on launch. It's almost much, much easier to control speed. Wide open the engine turns around 4300-4400 RPM instead of the 4800 I was seeing with the 19P aluminum prop and I seem to be able to milk as much as 42-42.5 mph GPS out of it. The water had a little chop to it today. I think I could get the old prop up to 43-45 mph, but there are all sorts of variables with the bimini up or down, weight, temperature, current and winds, all of which I think could make a difference of more than a couple mph. The boat is 3-6mph faster in the 3200-3800 RPM range which some people say is good, but I'm a bit skeptical as the engine has to be under a heavier load at those RPMs.

Also, and not to derail my own thread, I'm a little skeptical that with an engine with so many computer controls, sensors, a rev limiter and modes that will limit you to just an idle, that running at wide open throttle would be bad for the engine.
 

rallyart

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Glad the plugs worked. Running faster at 3200- 3800 is a great indicator that the prop is much more efficient. That range is where your engine has peak torque so there should be no worry about heavy load. It is the most efficient range for the engine to operate. (mechanically, not necessarily from the hull's perspective)
Running at wide open throttle just shuts off some of the engine management as they are trying to maximize power so they ignore economy. The engine running rich ensures you burn most of the air. Lower rpm and idle settings are trying to manage that you burn most of the fuel for economy. The only engine management issue would be if you ran bumping on the rev limiter for an extended period as that stops the smooth operation but protects you from the much worse damage of an over rev.
If you are getting a good hole shot, without bogging, then there is no need to add more ventilation.
 

FliesAndFloats

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Aha! So WOT == open loop mode, which increases fuel delivery and with it, unburned fuel, which can wet cylinder walls and contaminate oil and thus lead to increased oil breakdown. Maybe I'm nuts, but I think you can almost hear it in digitally with the throttle position. Going from cruise and slowly advancing you hear the engine change instantly to a different sound "mmmmmmmmmmmmmmmmmmmmWAAAAAAAAAAAA"
 
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