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Author Topic:   Vented Props- How do they work???

DeepSouthWhaler posted 03-14-2005 10:57 AM ET (US)
I have a vented prop on a 150 Mercury. The vents are open with no plugs in them. What are these vents for? How does this effect the boat ride? What happens if the vents are plugged?
blkmtrfan posted 03-14-2005 01:56 PM ET (US)
The vents in your prop allow a certian amount of slippage at low RPM. This can be very helpfull with an outboard that dosen't really get into it's powerband until the RPM's increase. Kind of like a torque converter in your car. Let the motor spool up where is starts making power, then it can pop you up on plane faster. Plugging the holes could make you boat much more sluggish at the bottom as the prop now has much more bite but the motor dosen't have the power to overcome this bite.
Please note , it is different for every application and yours make work better with the plugs in or some plugs with smaller holes (which are also available). Experement a little and find out what works best.
jimh posted 03-15-2005 08:58 AM ET (US)
The Mercury Performance Vent System (PVS) provides controlled venting of the propeller. The vents can be adjusted in size with plastic insert plugs to control the amount of exhaust gas ventilating into the propeller. By aerating the water the propeller loses its grip. The venting is naturally reduced as speed increases due to the increase in the venturi effect of the exhaust gases rushing out the propeller hub. Thus at low speeds the vents have an effect, and as speed increases they become less effective. In theory, this provides the propeller with less grip at low speeds, which has the effect of reducing its effective pitch.
Driving a boat with a vented propeller takes some getting used to. As the boat accelerates, the propeller becomes more efficient and the boat speed often increases significantly. There is a bit of elasticity in the propeller, and you will find that the boat speed will keep climbing until all the venting has subsided. Then it is possible to reduce engine speed but maintain the non-vented propeller operation.
I think this works best on boats which have a very high speed potential and are running propellers with high pitch.
Backlash posted 03-16-2005 03:37 PM ET (US)
DSW,
I agree with blkmtrfan, experiment with different vent plugs. I use the vent plugs with medium sized holes and it works well with my setup. Any Mercury dealer will carry various sizes and they're only about 95 cents each.
If you're not running any plugs, your engine probably has to really rev up before the prop bites. A medium plug will allow the prop to hook up sooner.
Steve
DeepSouthWhaler posted 03-17-2005 11:59 AM ET (US)
Thanks for the replies. The RPMs really run up when getting the boat up. I will try to get some plugs.

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Author	Topic: Vented Props- How do they work???
DeepSouthWhaler	posted 03-14-2005 10:57 AM ET (US) I have a vented prop on a 150 Mercury. The vents are open with no plugs in them. What are these vents for? How does this effect the boat ride? What happens if the vents are plugged?
blkmtrfan	posted 03-14-2005 01:56 PM ET (US) The vents in your prop allow a certian amount of slippage at low RPM. This can be very helpfull with an outboard that dosen't really get into it's powerband until the RPM's increase. Kind of like a torque converter in your car. Let the motor spool up where is starts making power, then it can pop you up on plane faster. Plugging the holes could make you boat much more sluggish at the bottom as the prop now has much more bite but the motor dosen't have the power to overcome this bite. Please note , it is different for every application and yours make work better with the plugs in or some plugs with smaller holes (which are also available). Experement a little and find out what works best.
jimh	posted 03-15-2005 08:58 AM ET (US) The Mercury Performance Vent System (PVS) provides controlled venting of the propeller. The vents can be adjusted in size with plastic insert plugs to control the amount of exhaust gas ventilating into the propeller. By aerating the water the propeller loses its grip. The venting is naturally reduced as speed increases due to the increase in the venturi effect of the exhaust gases rushing out the propeller hub. Thus at low speeds the vents have an effect, and as speed increases they become less effective. In theory, this provides the propeller with less grip at low speeds, which has the effect of reducing its effective pitch. Driving a boat with a vented propeller takes some getting used to. As the boat accelerates, the propeller becomes more efficient and the boat speed often increases significantly. There is a bit of elasticity in the propeller, and you will find that the boat speed will keep climbing until all the venting has subsided. Then it is possible to reduce engine speed but maintain the non-vented propeller operation. I think this works best on boats which have a very high speed potential and are running propellers with high pitch.
Backlash	posted 03-16-2005 03:37 PM ET (US) DSW, I agree with blkmtrfan, experiment with different vent plugs. I use the vent plugs with medium sized holes and it works well with my setup. Any Mercury dealer will carry various sizes and they're only about 95 cents each. If you're not running any plugs, your engine probably has to really rev up before the prop bites. A medium plug will allow the prop to hook up sooner. Steve
DeepSouthWhaler	posted 03-17-2005 11:59 AM ET (US) Thanks for the replies. The RPMs really run up when getting the boat up. I will try to get some plugs.