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.
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.
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?
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.