I am switching my props from 21's to 23 mirage plus.. The plus's have no vent holes. Would I be better off drilling holes ? they are going on my 29 scarab with twin 350 mags.. Thanks

Do not drill holes in them. Really, you should not need the vent holes.

What Griff said +100

If your exhaust is routed thru the transom, they won't make any diff. anyway. It's an outboard thing.

Well, actually it's for I/O's too - if you don't have thru hull exhaust. I know, picky, picky, picky.

Thanks.. It is thru hull.. I thought that it had to do with exhaust but the ones I took off have holes so I figured I would make sure .. Thanks

Ya, but if you don't have thru hull exhaust, you are a sissy and you need to get it, and then you are not a sissy and you don't need vent holes, see eeh?

ARE YOU A SISSY TEST http://www.nerdtests.com/mq/uttake.php?id=55676

It said I was normal, hmm. It might need some tweaking

Mercury makes three different size vent plugs. They make a difference whether you have hub exhaust or transom. You want to run the biggest size you can without blowout. The boat will come out of the water MUCH faster once you have the optimum size. I have tried no plugs and the boat still came out of the water but there was blowout. They make no difference on cruise or top speed. Hope this helps.

One more thing, do not drill holes.

How do you make the holes?

So if you have thru hull exhaust, the holes do absolutely nothing.

As a bit of an explanation, here's what the holes are designed to do - if you have thru hub exhaust. They allow a small amount of exhaust to ventilate the prop - effectively allowing it to slip a bit when coming up on plane. Think of it as being sort of like slipping a clutch. Most older outboards have very little power in the lower RPM range, so they need some help with a stiff prop that works well at top end.