To vent or not to vent

In my experience, removing the vents increases prop ventilation when a boat brakes over onto plane. Try lowering the engine a little. Scott

Heres the basic idea. If the prop bites solidly, the engine cant wrap up into the fat part of the horsepower/torque band, making launches sluggish. So you want to decrease the bite at take-off, kind of like slipping the clutch in a manual transmission vehicle so that you can get the motor up into its power band. So the first step is to get enough ventilation that the prop will slip at takeoff and let the motor wind up a bit. Once you get to moving, the bow is usually up and the boat has to be pushed up and over the water being displaced. As the bow starts to drop, the prop (which is ventilating) will slip even more for a moment or two, until all the turbulence caused by the roll-over onto plane dissipates and the water slipstream smooths out. It is not uncommon for the prop to slip significantly for the second or two that the hull is “rolling over” onto plane. Once on plane, the plugs have no significant effect on performance as the water slipstream is moving too quickly, the exhaust that leaks through the PVS holes slides along the prop hub and doesnt get out into the blade area where it would cause slippage. Heres the best way to set the PVS plugs. 1. Get yourself some ear plugs. 2. Tape a cover on the tach so you cant see engine RPM. 3. Find a location where you can make repeated runs in the same direction without worrying about other boat traffic. Point the boat in the direction you want to use for all the tests and let it idle in gear until speed is stable. Punch it to WOT (not 1/2 throttle or anything in-between). Once the bow rolls over, watch the speedo and note the next major number it passes (say 30mph, or 35, but not odd numbers like 32 unless it is digital). Now you are ready to test. 4. Back to the starting location at idle in gear. Punch the throttle and start the stop watch. When the “magic number” shows up on the speedo, stop the stopwatch and record the time. 5. Now for the experimentation. Remove a PVS plug. Repeat the test. If the time is shorter, you are going in the right direction. Remove another plug. Continue testing until the stopwatch shows you are taking longer to reach the magic speed. Now back up and replace the last plug you removed and try a smaller hole there. Keep testing until the stopwatch says you are getting to speed X in the shortest amount of time. The reason to cover the tach, and to plug your ears, is to eliminate extraneous data. All that counts is “time to speed”. If the prop breaks free at rollover, who cares if that produces the fastest 0-30mph time? Ever heard a chopper? Normally you punch it, it will wrap up to 3-4K, then as the motor rolls over it will break free and wrap up to 5500-6000 for a second or so, then start to bite and settle back down to normal speeds. That wrap-up in RPM will make you think it is taking longer to get to speed when it is not. If you cant hear it, and cant see it, it wont influence you. There are other ways to measure this if the above doesnt feel comfortable. You can always start from a standing start at some point you can return to accurately for each run. And you can let someone on shore time you to the same point such as going between him and a tree on the opposite bank. But shortest time wins, regardless of how things sound.2008 Pantera Classic2014 Mercury Pro XS 200

I agree completely. The good thing is, if you set it up to do well when heavy, it will do well when light although perhaps not optimal. But if you set it up light, and then load it down, it might not even get up on plane at all which would be a pain at blast-off time… Also you need to use a little common sense about going for absolute optimal value, because weather conditions change. Optimal in the winter is not optimal for the summer, if you are a “fix it once and leave it alone” type, you probably should pick the hottest Summer day with the highest possible humidity to set it up. Cooler air will only help and make it get even better in the winter. BTW, this really is a good place for “scientific method”. Take a small notebook and pencil with you so that you can record every test and the results. As the testing goes on, you can even backtrack every now and then and make sure the result matches what you got the first time you tried it, to prevent unexpected error from creeping in. When I play with prop or engine height, I try to record the date, time, temp and water conditions (slick, slight chop, etc)…Last edited by oldtimer57 on July 22nd, 2008, 8:53 pm, edited 1 time in total.2008 Pantera Classic2014 Mercury Pro XS 200

I agree. The stopwatch is the key in how long it takes to go from zero to X. We (my son and I) drag race (autos) regularly, and I have had great difficulty with him at times in choosing the proper “pill” to put in the shift light we use. Humans are very subjective creatures and often think too much. For example, if you over-rev a motor, you go past the motors ability to breathe and horsepower drops precipitously. But sometimes it is the right thing to do as if you shift at peak horsepower before that drop, the next gear might drop you down into the weak part of the torque band, actually hurting the ET. I finally convinced him to experiment with the shift light pills and rely on the time slip to tell him which shift point is better, rather than going by feel/sound. Same goes for boats. The stopwatch wont lie…2008 Pantera Classic2014 Mercury Pro XS 200