Correct me if I am wrong, but on props with vent holes, the vent holes....

[Philster]

Correct me if I am wrong, but on props with vent holes, the vent holes (when opened) allow for a slight to modest low end performance increase, but might sacrifice a little top end -- is this correct? This holds true only for setups with thru-prop exhaust, because the vent holes allow the water to be less solid and more exhaust air-filled, which lets the prop/engine spin up faster, which can help time-to-plane results.

I've had a number of people explain the vent holes improve everything, so just take the plugs out. The last time I heard this was from a Merc certified dealer, and it made me cringe that he categorized it as an overall improvement (boat was an outboard). Prior to that, another Mercruiser tech said to take them out for better low-end punch and this was on my boat with through-hull exhaust.

My novice understanding makes me think that it's akin to letting the tires on a car spin slightly to get engine RPM up for a better launch, but as long as the the tires hook up at the right RPM, it's worth the slightest tire spin (ultimately an acceleration delay) to get the return on the investment in that delay (engine RPM/power in a sweeter zone) so that overall acceleration is improved. Problem is that on a boat, the prop never 100% 'hooks up' because slip is always present.

This leads me to conclude that prop vent holes, and how they are used, are all about changing/improving low-end accel characteristics and are best left plugged if one feels low-end bite and acceleration are fine, because open vent holes means bite is lost even at upper RPMs/speed possibly hurting top end.

 

[Frank Acampora]

Re: Correct me if I am wrong, but on props with vent holes, the vent holes....

Yes: Vent holes are for improving the hole shot by venting exhaust gases into the water behind the prop blades. This allows the engine to get into its torque range easier. If correctly sized, they do this well and do not affect top end performance.

At higher speeds, (boat speeds, not RPM) the water flow over the holes and so called negative pressure behind the prop effectively seal them. Thus, they have no effect on higher speed performance or ultimate top end speed. ----you just get there a little quicker.

 

[John_S]

Re: Correct me if I am wrong, but on props with vent holes, the vent holes....

I have only used one prop with vent holes, a Hustler on an I/O. It did provide some slip at launch. I didn't need that slip, so plugged the holes. There was not any change in wot rpm or speed. It is my understanding that the vents don't bleed exhaust at higher speeds. On boat/motor/prop/load combinations that are struggling to get into the engines powerband and on plane, that the slip allows it to reach the powerband.

 

[Philster]

Re: Correct me if I am wrong, but on props with vent holes, the vent holes....

Okay, so:

The venting is sealed at higher speed by pressure/flow
The exhaust gases are used, so no need to concern one's self with vent holes in a boat with through hull exhaust
If you want to spin the prop up faster to get an engine into the RPM power band faster, then take them out, if this happens to work with one's engine(s).

 

[Texasmark]

Re: Correct me if I am wrong, but on props with vent holes, the vent holes....

I had a Quicksilver Laser I years ago with ports and loved it. I had a fairly deep v padded hull and the boat was heavy. Once on plane and "on the pad" I could go fast. The prop would spin up fast in the hole, but wouldn't loose grip like happens when one "ventilates". Once up and running the rpm's would drop about 200-300 when the prop "locked up"......quit ventilating, and it was just like a car with overdrive.....rpm's would drop and boat would continue to accelerate. A really neat experience.

Sooooo some 25 years later, I have a boat that I like to run fast for what it is but didn't want to burden the engine with any more time in the hole than I had to with the high pitch prop I like to run. Additionally, I wanted to feel the "lock up" again. So I bored my Ballistic SS 1/4" (per blade).

Yesterday I had it out and I had plenty of time to play with it. It worked really well, hole was about 2-3 seconds with 450# of passengers and gear and top speed was right where it was before I ported it. Rpm's actually ran a couple hundred higher at WOT than before at max trim but speed didn't change....that was kind of confusing. I think the holes were porting (slightly relieving some stress on the engine HP) which allowed it to spin the prop faster and even though there was some slip the extra rpm's made up for it.

In short I love it and would do it again in a heartbeat. Buttttt don't kid yourself, SS is tough to drill.

HTH,

Mark