Removing vents on a Merc Prop

[Lake Wisconsin]

Here is my situation:

I have a 250 horse merc that was running a 4 blad 21 pitch rev 4. I could hit a average top speed of 57 mph and 5900 RPM with a full load. Excellent hole shot.

I would like to get my RPM up a little higher, so went to a Tempest 3 blade 21 pitch. On my first run, I hit 57 mph with 5900 RPM. The prop seemed to get too much air and would not stick to the water. Also notice a lot more exaust noise. I believe this is called slip, but not sure. Excellent hole shot.

I noticed my 4 blade has the vent plugs closed. My 3 blade, it appears they are the mid level vent holes. I would like to try my solid vent plugs but tried to push them out, and wouldn't go. What is the trick to pushing these out? I don't want to force them and break them, as I still want the solid plugs in my 4 blade.

[PCDOC]

Leave the plugs in and fill the holes with black rtv silicone. If you don't like the way it works after that, just remove the silicone and you will be back to normal. Ran my last stainless prop with silicone in the plug holes for two years. No problems.

[Thalweg]

There are two tabs on the back side of the plug that lock the plug in place. These catch on the inside of the hole in the prop. I have gotten mine out by reaching into the inside of the prop with a screwdriver and prying the tabs over a bit.

From your post it is not clear if you feel are getting air to the prop at full speed or nearer planing speed. If near full speed, the plug size will have little or no effect. If near planning, it could make a big difference. I have a 1800 Pro-V, 175EFI and a 19 inch Tempest. With the medium plugs, I would drop off plane at about 30 mph. I ended up plugging two and putting a plug with a small hole in one of them. I could then plane down to a little over 20 mph. You may or may not see a degradation in hole shot with the hole plugged or partially plugged.

I've played around with changing vents many times. Vents have no effect on top end speed. It changes the low end, and maybe mid range with larger plugs. Depending on the size of the plug, once the water pressure overcomes the exhaust pressure coming out of the plug, it is sealed by pressure and no more air comes out causing slip. I'm assuming that with a 250, you're pushing somewhere around 60mph top end. Even with no plugs in there, the vent is sealed off somewhere in the 40mph range.

They are an excellent way to tweak your hole shot and getting the boat to plane out at lower speeds.

[1860Angler]

If your Holeshot is good with the PVS plugs that are in it, leave it alone. The PVS plugs are only used for Holeshot and will have no affect on prop slippage once on plane. Here is the Mercury website that explains how PVS works.

http://www.mercurymarine.com/propell...hnologies/pvs/

[Lake Wisconsin]

Top end speed I don't notice the slippage. It is at low and med speeds. I would say once I hit 40, I feel a full bite.

[1860Angler]

Originally Posted by Lake Wisconsin

Top end speed I don't notice the slippage. It is at low and med speeds. I would say once I hit 40, I feel a full bite.

Changing the plugs won't change your low and mid-range venting, it only affects your holeshot. How is your motor height? It might be to high causing it to blow out? What boat is the 250 on?

Originally Posted by 1860Angler

Changing the plugs won't change your low and mid-range venting, it only affects your holeshot. How is your motor height? It might be to high causing it to blow out? What boat is the 250 on?

Sure it does. I got my boat to plane out at 20mph vs 30 at moderate throttle by going from the large plugs that were in it, to the small ones. With the big plugs, my boat was definitely venting still and riding the bow high at 25mph. Allot has to depend on operation, thus pressure. If you "hole shot" it at high rpm's/higher exhaust pressure, the effect is going to be much different that if you are much easier on the throttle. Best is to buy them all for how cheao they are, play around with them and choose the plugs that best suit YOUR type of driving habits. If you run and gun, go bigger, if you are more laid back on the throttle, go smaller.