Prop venting

Prop venting DEFAULT

By Thomas P.

The Performance Vent System, designed and patented by Mercury marine, is a management system of the exhaust gases that exit from the propeller's tube allowing us to control the venting of the propeller blades in order to optimize the acceleration for quick planing.

Propeller Vent System (PVS)

It is nothing more than one or two holes located behind each blade, on the outer propeller’s hub, through which the exhaust gases are driven between the propeller blades.

Each hole has the same diameter of about two centimeters, but we have the ability to completely close it or reduce its diameter by installing the specially shaped plastic plugs provided by Mercury. Four types of plugs with different size are available: a Small one with 7mm of diameter, a Medium one with 9mm diameter, a Large one with 12mm diameter and a Solid one that allows us to close completely the venting hole.

In this way we have the control and we can ensure the exact amount of venting needed.

Propeller Vent System (PVS)

How PVS works:

As the boat accelerates an amount of the exhaust gases exiting the propeller’s tube «escapes» through the vent holes and is driven between its blades. The direct consequence is the aeration of the water between the propeller blades, which simply means that the water becomes «lighter» and therefore the load applied to the propeller is reduced. Thus, the propeller is able to slip notably allowing the engine rpm to increase rapidly, until the propeller bites, leading to a quicker acceleration and shorter time to plane.

So the role of PVS is only to allow the engine to raise faster its rpm. In fact, it reduces the time it takes for the engine to reach that level of rpm, where the torque is as high as required so as to overcome the big resistances making the boat «get out of the hole», and so therefore the total time we need to get on plane is reduced.

At this point it should be made clear that the venting system is effective only during the start , and once the boat gets on plane and running, it makes no difference at all on the rpm, performance and, of course, on top speed.

The explanation is very simple:>

When the boat is running, the water that externally is flowing over the propeller vent holes prevents the exhaust gasses from coming out the holes. In a way, it blocks the holes, as the flowing water pressure becomes clearly much higher than the one of the exhaust gases and so the prop blades operate in solid, homogeneous and non-aerated water.

Vented or non-Vented, and if Vented how much?

Propeller Vent System (PVS)

When is the venting of the propeller’s blades useful?

When our propeller is not lagging during the planing, then obviously there is no reason of propeller’s venting. On the other hand, when it is difficult to get the boat on plane, for example in the case of heavy loads, or of using longer pitch props - often choosing so as to achieve higher top speed - or even when our engine doesn’t make plenty of power at lowest rpm, then will we see a significant improvement in the planing time by leading the exhaust gases between the propeller's blades, allowing it to operate in a «lighter» environment, thereby reducing its load and causing an instantaneous engine overrevving.

But how can we know the percentage of ventilation needed for our propeller so that it is not excessive and won’t cause over-ventilation leading to a complete loss of its bite?

There is a simple way to determine the certain amount of venting that is most proper, which of course requires some time of sea trial.
Place the Mercury’s solid plugs in the holes. Drill a 7 mm hole in the plug’s center and go out for testing. If you don’t have a good holeshot or your start looks like as having 3rd gear in your car, you need additional venting. Widen the holes further at 9mm and retry to see the effect you get. If you find a slow holeshot again, re-widen the holes with a 12 mm drill. On the other hand, if you see that the engine is overrevving and the prop doesn’t «hook-up», which means the water is getting too much aerated you need to choose the plugs of smaller diameter.

In this way you can find the «sweet spot» in which you have the best results for your use and you are ready order the proper diameter of plugs from Mercury.

...keep Ribbing!

 

boat center

 

Sours: https://e-ribbing.com/en/propellers/491-propeller-s-performance-vent-system-pvs-en.html

Venting a prop??

Re: Venting a prop??

I was just kidding with you Bryan, I have heard of venting, & how it is supposed to help, but not the mechanics of it? Thanks for the info Frank, Dan, & Bruce. I'm sure there's a lot to it, just like trying to set up a surface drive system like I have on my Fast Electric R.C. Boats, but they are easy to figure out what prop to use. Maybe raising your engine, and finding the right surface drive prop for your rig might help get more top speed, but the only thing is you would lose some of your hole shot this way. With surface drive, 1/2 of your prop is out of the water, being ventilated, but it also means swinging a larger diameter, as well as more pitch, for thrust. I would imagine this is quite a chore trying to figure, your horsepower, total weight, and thrust of your motor. Plus for it to work, you'd need to keep all these factors/conditions the same all the time, or maybe minor variations of these wouldn't matter much? I don't know for sure?

 

Sours: https://forums.iboats.com/threads/venting-a-prop.617160/
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  1. 05-22-2012, 12:21 AM#1
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    Venting a prop

    I've done some searching and found nothing.. Is it possible *and advisable* to drill vent holes in a SS prop? Can it be done at home without extensive shop tools?

  2. Just did the Bravo 1 four blades for my twin O/B. Tried at home w/a hand drill and some good, high speed steel drill bits. Started small too and dulled two bits trying. The metal in those props anyhow, is way too hard. Took it to a friends machine shop and he let me use his Bridgeport to bore 3/8" holes by all the blades.
    HUGE improvement in hole shot and to plane time.

    I'd rather be competitive w/junk I built in my garage than win w/stuff I bought.


    I refuse to allow common sense to interfere w/my boat buying decisions.


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  3. 05-22-2012, 12:57 PM#3
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    Where should the vent hole be in relation to the "start" of the blade? In the above picture it appears to be equal distance from the hub and start of the blade.

  4. 05-22-2012, 02:05 PM#4
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    I did a Mirage prop by hand....it was tough, but I got it done...
    JOHN MASON
    '83/VECTOR/MERC 200-- SOLD
    '98 Quartershot T-3/ MERC 200




    "If your not living on the edge, your taking up to much space"

  5. I plugged the holes in my large hub Trophy with Marine Tex and drilled the holes to 23/64", was the best for giving good hole shot and least slip when at a slow speed plane. The centre of the holes were 9/16" from the lip where the recess is cut to fit inside gear case and 1" from the edge of the back side of the blade at ft of the prop. Looks like about where Instigator drilled his.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

  6. 05-22-2012, 10:55 PM#6
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    I did a Mirage prop by hand....it was tough, but I got it done..
    hey how did the prop performe on a outboard. i have a 25 i could do that to cause its to hard to plane now. might make a good cruising around prop with 4 up

  7. 05-23-2012, 07:15 AM#7
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    Is it possible *and advisable* to drill vent holes in a SS prop?
    i dont think i would do that to a sst i dont think it has the rake and cup . well maybe the cup, but i dont think they bite in air good enough

  8. I had it done at a prop shop it does take off out of the hole big improvement. Rick

  9. 05-23-2012, 07:49 AM#9
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  10. It was a rampage made by michigan looked just like a omc raker. Took to rj props and he said it was good so he put the vent holes in and it went onto plane quicker. They weren't real big holes but it seems to grab better. Rick

  11. 05-23-2012, 04:07 PM#11
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    what style props benefit most from venting? High rake, big cup?

  12. Quote Originally Posted by BajaSvrView Post
    what style props benefit most from venting? High rake, big cup?
    Had friend with a 16' Sea Ray with a 115hp 4cyl Merc, drilled some vent holes in a stock aluminum Merc prop that made it so he could deep water start on a slalom ski and still have good cruise speed whithout dropping down one size. Think the holes were 1/4".


    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

  13. If you do it on a bridge port you can actually buy the PVS plugs from a prop shop. Then you drill a pretty large diameter thru hole (dont remember the size) then counter bore for the top of the PVS plug and tune hole size with plugs. The bigger the better untill it cavitates with a heavy load (water ski) and will not go any where. I had a prop that I drilled too large before PVS and had to use silicone in the holes for a day of sking with freinds.
    On the Bravo 1 a 1" hole is common. There is so much blade area they are just about impossible to cavitate completely out of the hole. Some shops put TWO 1" holes by each blade.

    http://marksprops.com/services.html Im guessing these prices are for the entire prop? two PVS holes per blade for $70. If its too much just plug them up.
    L.T.


  14. 05-24-2012, 11:13 AM#14
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    Do vent holes hurt midrange speed...or top end?
    JOHN MASON
    '83/VECTOR/MERC 200-- SOLD
    '98 Quartershot T-3/ MERC 200




    "If your not living on the edge, your taking up to much space"

  15. Quote Originally Posted by WATERWINGSView Post
    Do vent holes hurt midrange speed...or top end?
    I have never seen any loss that I am aware of. Good question.
    L.T.


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Propeller Vent System plugs

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