Prop Vent Results
06-11-2007, 05:58 PM
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Prop Vent Results
Thought I would post my results from playing with the PVS vents on my Rev-4's. After I installed the Rev 4's I noticed the boat would shudder a little when first advancing the throttle to get to plane. Since they were inexpensive (althought a PITA to install) I thought I would try opening the PVS vents a little. I installed the mid-size vent, and as expected the props slip some getting up to RPM and allow the motors (250 Yamaha) to rev up to about 2500 RPM before starting to pull the boat on plane (it is a 32 CC and weighs about 8000#).
What I hadn't expected was the impact on being able to run at mid-RPM. The boat has never been able to be held between 3000 and 4000 RPM without one prop or the other blowing out and letting the motor rev up. With the vent plugs opened it will now hold RPM at 3300, 3600, 3900 RPM with no problem. It still varies RPM as the props raise and lower with trim or in turns, but they no longer blow out.
I really didn't see any change at top end, which is 5200-5300 RPM and 53 mph on GPS.
I expected the impact getting on plane, but was surprised at the change to being able to hold a low RPM cruise.
-Greg
What I hadn't expected was the impact on being able to run at mid-RPM. The boat has never been able to be held between 3000 and 4000 RPM without one prop or the other blowing out and letting the motor rev up. With the vent plugs opened it will now hold RPM at 3300, 3600, 3900 RPM with no problem. It still varies RPM as the props raise and lower with trim or in turns, but they no longer blow out.
I really didn't see any change at top end, which is 5200-5300 RPM and 53 mph on GPS.
I expected the impact getting on plane, but was surprised at the change to being able to hold a low RPM cruise.
-Greg
06-12-2007, 02:15 PM
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The mid-range effect surprised me too, which is why I mentioned it. I guess the vent lets them slip just enough that the motors don't want to keep kicking in/out of the high speed system on he carbs, as right at 4000 RPM is where they start to pull harder.
I've run the 3B Yamaha props that came on the boat, a pair of 22 Bravo 1's that I borrowed from Scott C., and the 21 Rev-4's that I have on now. I don't have the HP or top speed to pull as much pitch as yours.
The B-1's may have held in the 3000-4000 RPM range a little better than the Rev 4's with the vents fully closed (the B-1's are pre-vent models), but not as well as the Rev's with the vents half open.
The B-1 and Rev-4's pull exactly the same top RPM and speed, but the trim response seems nicer with the Rev-4's. Actually, it's pretty easy to lift the bow too much, it runs better if I trim it a little flatter.
-Greg
I've run the 3B Yamaha props that came on the boat, a pair of 22 Bravo 1's that I borrowed from Scott C., and the 21 Rev-4's that I have on now. I don't have the HP or top speed to pull as much pitch as yours.
The B-1's may have held in the 3000-4000 RPM range a little better than the Rev 4's with the vents fully closed (the B-1's are pre-vent models), but not as well as the Rev's with the vents half open.
The B-1 and Rev-4's pull exactly the same top RPM and speed, but the trim response seems nicer with the Rev-4's. Actually, it's pretty easy to lift the bow too much, it runs better if I trim it a little flatter.
-Greg
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