Venting a prop??

[Bryan Sumrell]

I have a 1974 70hp Johnson on a 15' Thunderbird tri-hull swinging a 13 1/4 X 18 plain jain stainless prop. WOT rpm's are good and speed is good. All around great performing setup. Any truth to drilling vent holes in the prop and improving holeshot or getting on top performance? Thanks for any advice

 

[MikDee]

Re: Venting a prop??

Can you imagine drilling holes in your prop like swiss cheese, it should be much faster then because there's no resistance to water flowing through it! Then again it wouldn't be able to push against the water with all those holes in it! :lol:

 

[bruceb58]

Re: Venting a prop??

Can you imagine drilling holes in your prop like swiss cheese, it should be much faster then because there's no resistance to water flowing through it! Then again it wouldn't be able to push against the water with all those holes in it! :lol:

LOL, you don't understand venting do you?

To the OP...I actually vented an extra prop I had on a 21' boat with an I/O as an experiment. Didn't do a darn thing.

 

[Bryan Sumrell]

Re: Venting a prop??

Thanks Mr. Bruce, I'm thinking its best to leave it alone. And I'm thinking it best to leave the "Swiss cheese" comment alone as well lol.

 

[Frank Acampora]

Re: Venting a prop??

Venting is really primarily effective on boats with very high horsepower, swinging a very high pitched prop. The high pitch loads the engine at low speeds, making it difficult for the engine to get into its power band, and making the hole shot slower. Drilling vents in the hub WILL bleed a controlled amount of exhaust into the prop blades. This aerates them, causing slip, and allowing the engine to rev up easily into its torque band. Thus, the controlled slip the venting supplies DOES improve hole shot.

At high speeds, the flow of water over the prop hub and the vacuum behind the engine effectively closes the vent holes.

Venting can not be done willy-nilly. you must start with small vents and progress upward in hole size. AND, a 1/16 inch can mean the difference between a good prop and having gone too far.

On a normally powered and propped general purpose boat? I would not expect much improvement.

 

[Bryan Sumrell]

Re: Venting a prop??

Thanks Mr. Frank. Sounds like good advice.

 

[dan02gt]

Re: Venting a prop??

Bryan,

A good prop shop can drill your prop to accept Mercury PVS plugs. This way you can tune the hole shot by choosing how much exhaust to vent. This is very effective in improving hole shot on 2 stroke outboards. I would recommend Mark at Mark's High Performance Propeller for this. Mark's High Performance Propellers, Inc.

Here's some info on the Mercury PVS:
PVS - Performance Vent System | Mercury Marine

 

[rallyart]

Re: Venting a prop??

I put a vented 4 blade 21" prop behind my Merc 470 on an 18' boat. There was a noticeable difference with venting. With the vents plugged the boat was slow off the line. With the vent plugs removed the engine revved up faster but my boat did not accelerate much better, but some. I slowly drilled out the plugs to find the best vent size for the prop and got to a point where there was enough prop slip that the engine ran in a better power range but did not keep slipping as the boat speed got higher. Overall, with the right size vent hole, I could get out of the hole noticeable better and had no negative affect at planing speeds.

 

[MikDee]

Re: Venting a prop??

I was just kidding with you Bryan, I have heard of venting, & how it is supposed to help, but not the mechanics of it? Thanks for the info Frank, Dan, & Bruce. I'm sure there's a lot to it, just like trying to set up a surface drive system like I have on my Fast Electric R.C. Boats, but they are easy to figure out what prop to use. Maybe raising your engine, and finding the right surface drive prop for your rig might help get more top speed, but the only thing is you would lose some of your hole shot this way. With surface drive, 1/2 of your prop is out of the water, being ventilated, but it also means swinging a larger diameter, as well as more pitch, for thrust. I would imagine this is quite a chore trying to figure, your horsepower, total weight, and thrust of your motor. Plus for it to work, you'd need to keep all these factors/conditions the same all the time, or maybe minor variations of these wouldn't matter much? I don't know for sure?

 

[bruceb58]

Re: Venting a prop??

Surface drive? With a 70HP Johnson? With a tri hull! :roll:

 

[MikDee]

Re: Venting a prop??

Surface drive? With a 70HP Johnson? With a tri hull! :roll:

Not surface drive, But, raising your engine, to ventilate your prop if done right will give you more speed, but if your looking for more low end grunt the right ventilated prop should do the trick.

 

[H20Rat]

Re: Venting a prop??

Not surface drive, But, raising your engine, to ventilate your prop if done right will give you more speed, but if your looking for more low end grunt the right ventilated prop should do the trick.

Different topics... Vented props are actually sealed once you are up to speed, the vent makes zero difference of top end. The idea is to cause ventilation when you are starting to move from a dead stop. Basically let the engine build up some rpm's and get through the dead band areas in the torque curve.

 

[gm280]

Re: Venting a prop??

Uh OH! Does this mean I should stop drilling 1/2" holes in my 9.8 Mercury O/B prop that goes on the 16' Jon boat? ound:

Have a great day folks...

 

[MikDee]

Re: Venting a prop??

Maybe a 4 blade 17" pitch prop would give you a better holeshot? If that's what your goal is.

 

[dazk14]

Re: Venting a prop??

I have a 1974 70hp Johnson on a 15' Thunderbird tri-hull swinging a 13 1/4 X 18 plain jain stainless prop. WOT rpm's are good and speed is good. All around great performing setup. Any truth to drilling vent holes in the prop and improving holeshot or getting on top performance? Thanks for any advice

Your engine in question was one of the engines (Back in the day) that was 1st to receive the factory service bulletin about venting the props and where the holes should be drilled. footer, run 40+ mph with a 21" prop and be a DOG to plane.

Dropping pitches didn't work , because the motor would over-rev at high speeds.

It wasn't uncommon to see a lighter 15

If I recall, they were small holes, maybe 1/8" or so. Boy has time changed, going much bigger. At the time, many Shop owners thought it was just a gimmick.

Reason being, the looper had zippo low end torque and made it's power at higher rpm's, so this was a helpful mod.

You can do it yourself and copy the general location from a vented prop and drill away. Your only risk is your time. I like solid cobalt bits, if yours do more smoking than cutting.

You can buy individually at Homepo and Lowes (double check).

If you don't like the Vents, plug 'em up with 5 min "Tootsie roll" epoxy. The tube type where you slice off a piece and roll it in your fingers to mix the dough together. Simple.

You can re-open the epoxy holes somewhat if you shot past the sweet spot.

The other comment about a 4 blade is spot on. They work nicely as well, but scrub off a little top speed.

It is far less common for an I/O to benefit from the vents, while your 70 is the poster child for them.

You should also be running a Progressive pitch prop, which makes a big difference. What exactly do you have?

 

[phillnjack2]

Re: Venting a prop??

VENTING prop does work, anyone who says it don't work have either done something very very wrong or
have never tried it.

now to vent a prop you only put holes down the bottom of the hub closest to the gearbox.

it will not do anything at all for mid range or top speed, but the pull away is where it works big time.

now it works on all sizes of engine ,not just the mega high horsepower v6 and v8's.

I was using a mercury 60hp triple on a 15ft dell quay sportsman and was getting 38mph with a 12 prop that gave usefull holeshot

Then I got a good 15 inch prop same diameter same make etc (mercury) that was decent pull away with just me and 42mph top end.
this was the top end I wanted but pull away could of been better, I wanted the same pull away I had when on my own with the smaller 12 pitch prop.

anyway,
I drilled 3 holes in the hub of the prop and the results were amazing, I got the pull away of the 12 prop with the top end
of the 15 pitch.
also with 4 people onboard the boat gave reasonable holeshot and good top end.
ive tried this a few times and each time the results have been the same, by venting its like going down 2 sizes for pull away but normal mid and top end speeds.

mercury 10.1/8 x 15p Black alloy prop
pat no 4873140 15 normal r/hand rotaton 3 blades.

this prop was too much for the dell quay 15 sportsman when i first got it as it would take too
long to get the boat up onto plane.
Once on plane and the revs built up it was fine and gave a top speed of 42mph.

The prop then had 3 holes drilled,one between each blade near the base of the prop hub.
The holes started off at 1/8th inch and tested, this gave a very slight improvement on hole shot.
prop was then drilled out to 1/4 inch and this saw huge improvements on pull away.
the prop was then drilled out to 3/8ths and this gave excellant results with a holeshot like a
12 pitch and the top end of the 15 pitch.
even with 4 people onboard this drilled prop gave great results on the 1991 60hp mercury triple.
The handling with the prop like this improved and didnt even show any signs of slipping or
blowing out in tight turns.
I would never hesitate to vent a prop to get a better holeshot if the top end is what you want.
the venting just lets the prop spin up a bit quicker and the engine is under less strain by doing this.
it does work, and you can always just simply plug it with plastic/rubber plugs if you want !!!

when I first heard of it about 30 years ago I thought it was just a gimic,but I tried it and got a shock to
just how good it realy works.

try it you have nothing to lose but lots to gain. you can also have it done at a prop shop and get
different size plugs for different applications.

pics of prop after venting
( I did rub it down and re-spray it before use, and got no burning on the blades neither )
.
.

 

[phillnjack2]

Re: Venting a prop??

Bruce58 said
" Surface drive? With a 70HP Johnson? With a tri hull! "

Well here is my Evinrude 60hp triple running surface on 13ft tri hull.
starts off tri-hull then merges into shallow v, similar to boston whaler classic 13.
its running a 13 3/4 x 23 omc stainless silver streak surface prop, to run good the engine has been
setback 5 inches, and jacked up by 7 inches !!!!, I think ill be going down to just 5 inches to use without
low water pick up to stay safe.
Jacked up 7 inches puts the centre of the bullet just 1/2 inch higher than bottom of the transom.


I also run full hydraulic steering with it too, anything that improves is good .

there are lots of even smaller boats running surface piercing props very successfully .
and lots of fully surface drive inboard engined boats under 15ft too.

 

[MikDee]

Re: Venting a prop??

Well phillnjack2, Thanks for the info. It's great to hear someone with the voice of experience, who had the gumsion (balls) to try all these options, tricks, or tips, and actually gotten Great results!

My hats off to you sir, for implementing all these, and posting the results :joyous:

 

[jestor68]

Re: Venting a prop??

As a point of reference, Merc's PVS vent plug sizes are; small-7mm, medium-9mm, and large-12mm.

If you're experimenting, I would suggest starting with the smaller size hole.

 

[wca_tim]

Re: Venting a prop??

Like some of the others above, I've had really good results doing this as well. if done right, it works a lot like putting a higher rpm stall converter in an automatic car. it gets the engine rpms up faster where it makes more power and can make a dramatic different in holeshot... It is best to drill the prop so you can use the inserts and tune it. you get the holes a little too big and you can start blowing out in turns easier... and then you'll want them smaller...