Drilling holes in Prop

[Matysiak]

My boat shop has recommended drilling some holes in the prop on my 1980 115hp Johnson. He wants to drill them just in front of the blades close to the gears housing. The prop is a 15.<br /><br />Please can you tell me what the benefits are and if they are as good as what the mechanic said, why didn't it come out as standard issue?<br /><br />Stephen

 

[ddaigle]

Re: Drilling holes in Prop

My understanding of holes in prop is to let exhaust gases escape and ventilate your prop blades. this allows your prop to spin up faster and should give you a better holeshot or takeoff without affecting top speed. most newer more expensive props have them. you must be careful on size and placement of holes so you don't overventilate and cause cavitation(loss of grip) of your prop. it is good if done right.

 

[MarkAB]

Re: Drilling holes in Prop

Do a search in this forum for "holes in prop". Look for a post I made a while back - there is a link in one of the replies that spells out the process.

 

[WillyBWright]

Re: Drilling holes in Prop

Are you sure he wants to drill the holes in Front of the blades? All of the ones I've seen have been drilled just Behind.<br /><br />You're running a 15 pitch on a 115? How much Hp is your boat rated for? A 15 pitch is more of a stump puller than a speed prop. Maybe you just don't have enough motor for the boat.

 

[Matysiak]

Re: Drilling holes in Prop

The boat is a 16 ft run around.<br /><br />Yes, he definately wants to drill them in the front of the blades. <br /><br />Stephen.

 

[ob15]

Re: Drilling holes in Prop

Weather it the front or back. The holes should be drilled between the blades and the gear case. Just wanted to clarify that in case meant different things (i.e front means toward the gear case)

 

[walleyehed]

Re: Drilling holes in Prop

In relation to the direction of rotation, the holes must be drilled BEHIND the leading edge of the blade approximatly 5/8" from where blade joins hub, and 5/8" from front of hub.

 

[walleyehed]

Re: Drilling holes in Prop

Matysiac, I've got to wonder about that 15" pitch prop.....That would be an OK prop for a 70HP on a 16ft runabout...not a 115. And hole-shot shouldn't be an issue for you with 115 on a 16ft'r, so the question comes to my mind that the holes are a band-aid fix for another problem...

 

[ob15]

Re: Drilling holes in Prop

walleyehed <br /><br />Good point

 

[Joe Reeves]

Re: Drilling holes in Prop

Maty.... If you like, contact me at my e-mail address below and I'll send you the OMC bulletin pertaining to the prop holes. Just remind me what the e-mail is about in the subject matter.

 

[reeldutch]

Re: Drilling holes in Prop

I would first check the compression on your cylinders.<br />i have a 115 with a stainless 13 3/8 x 17 on a 20 ft mako and i run about 5500 rpm at wot i have the hydrofins installed and probbebly when the water is flat il make a topspeed of about 35mph give ore take a little.<br />I had this engine when i bought it first on a 17' winner bud with a 13 1/4 x 15 alluminum. and i run it at 5400 rpm. probbebly 30 mph.<br />than i took an outboard repair class and start to learn more. I did a compressin test and found out that 2 pistons were cooked and they put a smaller prop on to make the right amount of rpm's.<br />so the engine was rebuild and i puttet the 13 3/6 x 17 and i run it on the 17' winner may be close to 40 mph.<br />so check the compression before you rig your engine.<br />I'm not a mechanic bud it sounds familiar to me..

 

[Matysiak]

Re: Drilling holes in Prop

The engine has been checked oyt and has good compression. 115-118 psi on all cylinders. <br /><br />The mechanic says it will just improve hole shot with no downsides.<br /><br />Joe, i have sent you an e mail asking for the OMC bulletin on this, thanks.<br /><br />Stephen

 

[Dhadley]

Re: Drilling holes in Prop

The reason for holes in front of the blades are for cooling the prop hub. That should not be an issue on your motor unless you are not running a trim tab. Still highly unlikely.<br /><br />It would be nice to know your top rpms and more about your set up. Generally speaking, the vent holes cannot hurt unless they are too big. <br /><br />Good luck!

 

[reeldutch]

Re: Drilling holes in Prop

I still dont understand what you are trying to accomplish with a 12 1/4 x 15 while you could according to Michigan wheel corporation spin a <br />12 3/4 x 21 ore 14 x 21 prop.<br />your rpm's must be very high with your 15 ??<br />If somebody could explain this to me because i am lost here?

 

[rodbolt]

Re: Drilling holes in Prop

hello<br />the purpose of venting the prop is to allow exhaust gasses to "ventilate" the blades causing them to slip in the water. kinda like side stepping the cluth and allowing the rear wheels to spin tobring the engine rpm up on your daily drver/street racer. a little is good to much is bad. its mostly a plus on large props when trying tomove a heavy v-hull from sitting 2 ft deep in the water toplaning speed. a 16 ft runabout does not fall into this catagory. vent holes should not be nessasary on a 16ft runabout with a 115 unless your loaded with fuel drums and seriuos supplies. what is the WOT with the 15 pitch? sounds like its time to check the tach for accuracy then test the motor with a dyno or test wheel to insure the power head is actually capeable of making power. then check the gear ratio to make sure its correct . this is not a bitg issue on the v4 jonnyrudes but a serious one on the inline 4 and 6 mercs. happened a lot. if the motor can make power and the tach is accurate and the prop allows for 5500 rpm or so with your normal load then a vented prop may help. if the above conditions are not met no amount of prop playing will compensate for a motor that does not run correctly. I did a rebuild on a 17 mckee craft a few years back that had a smoked piston and the guy had ran it that way for almost a year. when I fixed it we had to go from a 15 to a 19 on the pitch. the guy could not believe it had been so slow and he never really noticed. but it was like that when he bought it and thought it was normal to be so slow out of the hole.<br /> good luck and check the motor over carefully before ya start chunking props and drill bits at it.

 

[reeldutch]

Re: Drilling holes in Prop

totaly agree with you rodbold it makes sence.

 

[Matysiak]

Re: Drilling holes in Prop

The prop is marked 5153. These are the only markings i can see. Is it a 15 pitch??<br /><br />Stephen