Vent Holes

[TomP]

My boat is an 18.5 foot Cobalt with a 205 hp 4.3L Mercruiser I/O. The dry weight is about 2,850 pounds. I currently use a 5-blade Ron Hill prop that is better for water sports than my 3-blade. RPMs at WOT for the 5-blade are around 4,700, which is near the top of the 4,400-4,800 range for my engine. While this seems to be the right prop, I wish that the hole shot was better, particularly with more than 2 people in the boat. I have read on this forum that vent holes help the hole shot, but more so for an outboard motor than an I/O. If that is true, would it still make sense to try the vent holes on my 5-blade prop. If so, should the holes be smaller because there would be five of them instead of three? If so, what would be the correct diameter of the holes? Thanks for you help.

 

[guy74]

Re: Vent Holes

I've always been told that venting a propeller doesn't help much a an I/O. Venting the prop is a trick used mostly on 2 cycle outboard motors.

 

[Dhadley]

Re: Vent Holes

If your exhaust still goes thru the drive it should help. Obviously if you have thru-transom exhaust the vent holes won't do anything.

Many 5 blade props have what's called an OT (over / thru) hub. The barrel of the prop is slightly smaller than the ID of the lower unit housing. At slow speed some exhaust goes over the hub, as you get on plane the exhaust goes thru the hub which vents the prop at slow speed.

 

[rndn]

Re: Vent Holes

Smart tabs or trim tabs will help with the hole shot.

 

[TomP]

Re: Vent Holes

Thanks Dhadley. The barrel of the 5-blade prop is not smaller, so maybe the vent holes will help. Can anyone guess how much? Will it be noticable? I am interested if any of the technical experts know why vent holes work so well for outboard engines, but not so much for I/Os.

 

[steelespike]

Re: Vent Holes

Smart tabs or trim tabs will help with the hole shot.

What he said,
Tabs of some sort would be a big help with hole shot,ride and handling.
Are said to iliminate no wake speed wander as well.
I'm a little outside the prop venting technology but I assume any reference to venting not working on an I/O refers to those with through transom exhaust.Venting allows a little exhaust to flow over the blades during hole shot letting the motor rev quicker.

 

[guy74]

Re: Vent Holes

Thanks Dhadley. The barrel of the 5-blade prop is not smaller, so maybe the vent holes will help. Can anyone guess how much? Will it be noticable? I am interested if any of the technical experts know why vent holes work so well for outboard engines, but not so much for I/Os.

The main reason that it is not supposed to do as much for an I/O is that I/Os with larger displacement engines are devloping their torque at a much lower rpm. Hence, venting the prop to let the motor spin up to develope the power to hole shot isn't necessary. It is easy to try, it might do what you want it to, if not you can fill the holes back in with JB weld. Goodluck at whatever you try.

Brian