Thru-hub vent hole

[Celeb92]

Hi. I searched the forum but could not locate the threads. I read here some time back about drilling one or more vent holes in the prop hub to vent some of the exhaust in from of the prop for better hole shot without hurting top end. Do I remember this correctly? Anyone care to educate us newcomers? Thanks.

 

[Barnacle_Bill]

Re: Thru-hub vent hole

Is this what you are looking for marinemechanic.com/site/page76.html

 

[Celeb92]

Re: Thru-hub vent hole

Hi bhile. Thanks for the response but the address doesn't work. Is it correct? Thanks.

 

[Barnacle_Bill]

Re: Thru-hub vent hole

Just copy and paste into the address at the top of your browser and it will go right to it.

 

[stevens]

Re: Thru-hub vent hole

Here's the full link: link

and here is Mercury's take on the same topic: link

Some think vent holes help their engines rev up quicker to get better accelleration on take-off, others (incl. me) think it only gives a lot of bubbles... I guess it depends on the boat/engine/load combination.

 

[guy74]

Re: Thru-hub vent hole

I vented the prop on my 60hp Johnson, since it was a small displacement, high reving motor, venting the prop really made a big difference. The slippage from venting let the motor spin-up and get into it's power producing rpm range. The same ported props didn't help on a V4 Evinrude 90hp, it had plenty of low end torque to take off without help.

 

[ddennis]

Re: Thru-hub vent hole

Hmm I have a 2002 nitro 700lx bass boat (just over 17 feet) with a 75 hp merc. prop is stoc 13x23p. rpms are dead on with me and my son in it, full fishing load at WOT. However..it is a bit sluggish getting on plane.. hull is rated for a 125...not an option to upgrade at this point. Do you think venting would help in my situation? Does venting apply to the newer torque-flo hub props?

 

[walleyehed]

Re: Thru-hub vent hole

Venting has nothing to do with the inner hub...will it help on the outboard applications???.....99% yes sterndrive??...very little if any help-only half the power strokes, so half the exhaust gases in a given period of revolutions.
If you understand controlled ventilation, you'll understand why this works as well as it does...if you don't understand the ventilation issue, just try a Vented prop and impress yourself with the difference it makes.

 

[ddennis]

Re: Thru-hub vent hole

Hmmm....sounds like time to break out the drill....

 

[walleyehed]

Re: Thru-hub vent hole

Start with 1/8" holes BEHIND the leading edge of each blade at a distance of about 5/8" then measure down about 1/2" from where the step is cut to go inside the gear case, mark that point to drill.
somewhere in the area of 19/64-5/16 would be as big as you would want to go.....much bigger may cause you to lose enough bite that it goes back the other direction.
I set-up a 17ft Tidecraft with an 85hp and 3/16" holes was the magic number for that particular rig.

 

[Frank Acampora]

Re: Thru-hub vent hole

I vented the prop on my 60hp Johnson, since it was a small displacement, high reving motor, venting the prop really made a big difference. The slippage from venting let the motor spin-up and get into it's power producing rpm range. The same ported props didn't help on a V4 Evinrude 90hp, it had plenty of low end torque to take off without help.

Venting also helps the idle on powerful engines or even low power engines swinging very high pitched props for whatever reason. The high pitch just tries to move too much water and the engine will either idle rough or stall. The vented exhaust fouls (ventilates) the blades so the prop doesn't grab as much water. At high water speeds, the pressure of the water effectively seals the holes. But DO YOUR HOMEWORK BEFORE YOU DRILL! You can go too far, so start with small holes and work up. You can always drill larger but welding them smaller is a pain.

 

[Celeb92]

Re: Thru-hub vent hole

Hi guys. Thanks for all of the replies -- particularly the links posted by Stevens. It is an interesting topic and I suppose there are a lot of opinions and a great deal of experience with regard to how effective a mod the venting can be. I'd like to hear more of the "I did it and it worked/didn't work" experiences as I sure would others. Again thanks -- great forum.

 

[walleyehed]

Re: Thru-hub vent hole

celeb92, go to the top of the prop forum and look at the thread; "Walleyehed's prop test results"....these are just a few of what I have run.
I can assure you, if you drill the holes in the correct location and of the correct size(start small and increase in small amounts) you will see improvement.
I've played with, drilled, flattened, cupped, shaped, in fact flat ruined more props testing than most will ever own in 5 lifetimes. I'm not the perfect expert here, my Mentor is on this forum as well, but I do have a pretty good idea as to what to tell you to expect.
You might punch in "vent hole jig, or diagram" in the search function....
One of the guys had a great copy from an OMC SB that showed exactly what and where they placed the initial holes.

 

[Celeb92]

Re: Thru-hub vent hole

Thanks Walleyehed. That was an incredible amount of test work that you completed and posted. Contributions like that from knowledgeable people really make this board a great place for newbies to learn. BTW -- I tried a number of key word searches but could not locate the post that you had mentioned. I'll try again using other terms.

 

[walleyehed]

Re: Thru-hub vent hole

I'll see if I can find it as well...if I do, I'll send the link to that thread to ya.

 

[walleyehed]

Re: Thru-hub vent hole

Here ya go....

Here is a good copy of the OMC Outboard Service Bulletin:<br /><br />

<br /><br />Save/print a copy for yourself!

 

[Celeb92]

Re: Thru-hub vent hole

Hey great! Thanks Walleyehed. Sorry for the delayed response but I've been on business travel and this is the first time I've logged on in a while.