How does the three 7/32'' drill holes modification on the prop help

[nitro277]

I was reading a seloc repair manual and it stated that by drilling three holes (7/32" each)one behind the base of each blade of the propeller at 5/8" back from the inner lip would help exhaust gasses to bleed onto the propeller blades causing controlled ventilation during the acceleration period, this action will allow the motor to turn at a higher rpm under acceleration providing more power to plane the boat faster. I Asked Has Anyone Try this ? Does it Work ? I'm almost going for it but not 100% sure YET!!

 

[Dhadley]

Re: How does the three 7/32'' drill holes modification on the prop help

I can't think of a decent V4 or V6 prop manufactured today without them. Unless it's an OT hub. It works very well and does not affect top end.

 

[nitro277]

Re: How does the three 7/32'' drill holes modification on the prop help

I have 1992 evinrude 70hp 3cyl motor with 14x17 alluminum props' would it work on this type of engine that I have?

 

[larky]

Re: How does the three 7/32'' drill holes modification on the prop help

i would like to see some pics, if anyone has any. very interesting.

 

[Snowdrggn]

Re: How does the three 7/32'' drill holes modification on the prop help

The first I saw of this mod was on an OMC service bulletin in regards to the 70's triples having trouble getting a boat onto plane. IT WORKS!

I personally have done it on two different different outboards, an 89 15hp Evinrude, and a 25hp Yamaha, both on smaller inflatables (10 ft range)

In both cases it turned the holeshot from quick to borderline violent! Usually being on a full plane in their own length.

http://img.photobucket.com/albums/v48/Snowdrggn/Zodiac/Dscf0051.jpg

Above is a picture of one of my boats lauching itself out of the water, you can see the "hole" in the water where I hit the throttle from a dead standstill.

You can fine tune the reaction by altering the size of the holes and the postion of them. Be warned though it is a fine line between a wild launch and never getting the prop to "grab". As the boats speed increases, the exhaust area of the prop becomes a negative pressure, stopping the bubbles exiting in the blade area, thus reacting like a normal prop.

Sorry about getting long winded.

Cheers,
Snow.

 

[Dhadley]

Re: How does the three 7/32'' drill holes modification on the prop help

Yes, it will work on a 3 cylinder as well. And a 2 cylinder......any prop with a thru hub exhaust.

It does work on a OT (over / thru) hub prop as well, just most are manufactured without it since the smaller hub itself creates venting.

 

[jbjennings]

Re: How does the three 7/32'' drill holes modification on the prop help

this is very interesting. I would like to see a couple of close-ups of a prop that has the holes in it!
Thanks,
JBJ

 

[wildmaninal]

Re: How does the three 7/32'' drill holes modification on the prop help

I'm curious on this subject as well, I would love to see a pic or 2. Jbjennings if you find the pics please let me know about them. I'm all ears or eyes in this case.

 

[Dhadley]

Re: How does the three 7/32'' drill holes modification on the prop help

Here's a typical prop with vent holes --

 

[Dhadley]

Re: How does the three 7/32'' drill holes modification on the prop help

Now if you really want to vent one big time, try a slot. If you run a slot on anything but a V6 Merc it'll probably blow out so much it won't get on plane.

 

[Silvertip]

Re: How does the three 7/32'' drill holes modification on the prop help

Here's one that's drilled (vented).

http://forums.iboats.com/showthread.php?t=245547

 

[kynolan2183]

Re: How does the three 7/32'' drill holes modification on the prop help

I run an 84 evinrude 70hp on a 17 foot bass boat. It turns an 18 pitch 2x cupped prop with Vent Holes. Yes it works great !! I was able to move up from the 17 pitch 1x cupped I had.

My motor has these stats
49 cubic inches. 3 cylinders
2.42 gear ratio
wot 4500 to 5500

I turn 5400 mild load at 36 mph
Heavy load about 5250 at 33 mph

I am running about 8% slip.

Vent it

Kyle

 

[fishrdan]

Re: How does the three 7/32'' drill holes modification on the prop help

Is this mod specific to 2 strokes (or 4 stroke) outboards, or would it help an I/O engine?

As I understand it, the mod helps vent exhaust and wouldn't specific to the type of engine (2 stroke, 4 stroke, outboard, I/O).

Any suggestions on sizing of holes of for different HP engines, or what size of hole is recommended for a specific HP of engine?

This mod sounds interesting as I have had problems getting on plane at high altitude, every little bit would help.

 

[kynolan2183]

Re: How does the three 7/32'' drill holes modification on the prop help

The Mod does work better on two strokes than on four strokes due to the torque curve differences between the two types of engines.

The idea is to get the engine into its power ban quicker and since a four stroke makes most of its torque under 3500 rpm and a two stroke does not poor it on until about 2500 rpm it becomes a dance between venting and power. Never start with to much of a drill bit or you will end up with scrap.

I would start with very small holes on a four stroke because you are only wanting to allow the prop to slip off the start to drop any hestitation ect.

On a two stroke I have 3/8 holes for my 70hp as I want it to slip more and get into the power ban.

If you ever had a 2 stroke dirt bike then you know the power ban. Two Strokes like RPM.

 

[larky]

Re: How does the three 7/32'' drill holes modification on the prop help

I bit the bullet yesterday after reading this thread and drilled some holes in my prop. I figured I have nothing to lose because I can just tap the holes and fill with j-b weld if i don't like the results. I started with a 5/32 bit. its on a 71' 50hp. excited to see how it works

 

[guy74]

Re: How does the three 7/32'' drill holes modification on the prop help

Here is a picture of the vented prop on my 1988 Johnson 60hp. My advice is to start small when it comes to hole size, 1 prop I vented only needed 1/8 holes to give the same slip as another I did with 5/16 holes. It's cerainly easier to go bigger if needed than to fill them in and start over, been there done that. http://i273.photobucket.com/albums/jj201/g3farms74/IMG_3096.jpg

 

[audiobill5]

Re: How does the three 7/32'' drill holes modification on the prop help

Hello,
Has anyone vented a 1988...110 HP Johnson's prop, 13.25 x 15p rh 3 blade alunimum?
What size hole worked on ?
The boat is a 16 ft bowrider about 2k pounds.
thanks, Bill

 

[Scott Danforth]

Re: How does the three 7/32'' drill holes modification on the prop help

Dude. its a 4 and a half year old thread. read the rules, no replies to dead threads (older than 3 months).

Welcome to iboats.

 

[fishrdan]

Re: How does the three 7/32'' drill holes modification on the prop help

I thought we were supposed to welcome them aboard first, then rip them a new one,,, instead of ripping them a new one and then welcoming them aboard



Oh yeah, welcome aboard.

SD is right though, start a new thread with your question

 

[phillnjack2]

Re: How does the three 7/32'' drill holes modification on the prop help

i cant see why older threads should not be allowed.
this is my first time ive seen this topic and im sure its many peoples first as well.
its a very interesting thread and very upto date type of thing to be posting on.......
no point in having a forum if your not allowing people to answer it or post about it is there ????
this is a great forum, dont make rules that will send people away from it....
The 3 month rule is stupid rule, it proves its wrong by people posting, including you.
No way can anyone read everything on the forum in 3 months, even if they read each post has its typed.

anyway, i had a 1992 60hp merc triple on 15ft dell quay dory sportsman.
the 12 pitch prop was good, 13 inch would only plane with 1 person and wot 38-39 mph.

i tried a 14 and it was a disaster, then i drilled three holes that were 7/16th in the hub about 1/2 way between the blades.

this prop was the business, great holeshot and 42mph top end with 2 people onboard, 36 with 4 people and a dog.

i would always drill a slightly larger prop for sure,it gives a better all round performance.

This weekend comming im gonna try my 13 x 19 stainless on my 60hp triple evinrude,if the top end is good then
i will drill the prop to give me very fast holeshot with a heavy load.
you can always put plastic gromets in to make them like normal if you like as well !!!!!!!!!

the props dont have to be stainless, my merc prop was standard merc alloy prop, and ive still got it in my garage.