Drilling vent holes in a prop

[jason8887]

Hi everyone. I apologize if this question has been covered before, but does anyone know of any guidelines to follow when drilling vent holes in a prop? I'm looking to get a little bit more acceleration when coming out of the hole (18' searay, 3.0L mercruiser, alpha 1 stern drive).
Recommended prop to achieve desired engine rpm is a 14.25", 21P, which I have on the boat now. It allows me to hit the target rpm at WOT, but I don't get as much of a hole shot as I'd like
Thanks in advance for any help

 

[bajaman123]

Aluminum or stainless steel? Unless you have a good drill press or milling machine like a Bridgeport, it is going to be difficult to achieve what you want without mucking it up. Instead of using a drill bit you should use an end mill, preferably carbide. Why not just buy a prop with the holes already there and the plugs with it to help 'tune' it to how you want?

 

[airshot]

Hi everyone. I apologize if this question has been covered before, but does anyone know of any guidelines to follow when drilling vent holes in a prop? I'm looking to get a little bit more acceleration when coming out of the hole (18' searay, 3.0L mercruiser, alpha 1 stern drive).
Recommended prop to achieve desired engine rpm is a 14.25", 21P, which I have on the boat now. It allows me to hit the target rpm at WOT, but I don't get as much of a hole shot as I'd like
Thanks in advance for any help

21" pitch sounds high, what rpm are you trying to achieve ? Two neighbors have 3.0 engines and out drives on their aluminum boats ( probably lighter than yours). And they only run 17 & 19" props.....curious here.....

 

[Scott Danforth]

Your 3.0 will be running between a 15p to 19p

21p will be too much prop

 

[jason8887]

Thanks for the tips guys. I started with an aluminum 3 blade, 17P. The boat definately wanted to over rev. I looked up the recommended prop and it said a 21P would be right for the RPM, so I ordered an aluminum 4 blade. It seems to be right for the rpm, but it of course hurt the hole shot.
I've got an aluminum 19P on the way, but failing success, I thought I would play around with one of them instead of spending more $$ on another prop (the vented ones seem to be a bit pricey). Being as they are aluminum I don't anticipate much problem drilling them.

 

[StewartL]

Drilling vent holes can help hole shot start with three small holes 1/4" or 5/16" near the hub. Too big or too many will cause excessive venting and RPM flare. Test and enlarge slightly if needed. A 19P prop would also improve acceleration but drop top speed. Try holes first since you already have the right WOT RPM.

 

[airshot]

Thanks for the tips guys. I started with an aluminum 3 blade, 17P. The boat definately wanted to over rev. I looked up the recommended prop and it said a 21P would be right for the RPM, so I ordered an aluminum 4 blade. It seems to be right for the rpm, but it of course hurt the hole shot.
I've got an aluminum 19P on the way, but failing success, I thought I would play around with one of them instead of spending more $$ on another prop (the vented ones seem to be a bit pricey). Being as they are aluminum I don't anticipate much problem drilling them.

What is the rpm your trying to achieve ? Drilling them will be easy....drilling them in the right place and the right size is the bigger problem !

 

[Chris1956]

There was a template that showed exactly where to drill the holes. I would think that a drill press and a jig to hold the prop in the right position would work fine.

However, the holes are to let the exhaust ventilate a bit and help the prop slip more when first accelerating. This helps the motor to spin up to higher RPM sooner, getting you into the powerband faster.

I can see this being a plus on a 2 cycle motor, where low end power is lacking, in some cases. I am not sure your sterndrive would benefit.

Does your boat drag it's butt? If so, trim tabs may be a better alternative to improving hole shot.

 

[cyclops222]

It took me lots of WOT take offs at different drive angles to find the best angles for loaded boat of people. At first they laughed. Then they did not.
Drag racing against yourself? Great way to put STRESS CRACKS all around the sterndrive mounting plate.

 

[Chris1956]

Gee, the best angle for all "take-offs" is motor full down. If you have trim tabs, the heavier the boat's butt (people included), the more down tabs necessary.

i.e. on my 18 foot SeaRay bowrider, with a 205HPV6 sterndrive, 6 people and a skier, it was tabs full down, spread the people out, hit it, and as she planes, lessen the tabs until full up, and trim the OD a bit. She handles like a barge, until on plane and tabs up.

 

[Texasmark]

Years ago I found a diagram for where to put the holes and how large a diameter. As I recall I found it on here and it was OMC oriented but the process is the same. Using a HF drill press and coated drill bit with a premium cutting oil, I bored out one of my SS props. Took awhile but I got-r-done.

The hole diameter determines the hole shot engine RPMS. The larger the holes the more "dig" and longer it takes for it to clear out once on plane. Some props, like the Merc. Laser II have plugs of different diameters (available) and one that closes off the hole so that you can tune the prop for your desired outcome.

No doubt porting makes 2 props out of one. Most of my dozen or so props were ported.

 

[sangerwaker]

This may be the diagram you seek.....

http://i20.photobucket.com/albums/b225/bassworm1/ventingprops.gif

 

[airshot]

Gee, the best angle for all "take-offs" is motor full down. If you have trim tabs, the heavier the boat's butt (people included), the more down tabs necessary.

i.e. on my 18 foot SeaRay bowrider, with a 205HPV6 sterndrive, 6 people and a skier, it was tabs full down, spread the people out, hit it, and as she planes, lessen the tabs until full up, and trim the OD a bit. She handles like a barge, until on plane and tabs up.

My side console might be an exception to the full down for best hole shot idea. While it worked well on my I/O, my outboard likes to be out a bit or it just plows along. Move it out a bit and it jumps up on plane much more quickly. Experiment to see what works best. On smaller boats with full loads of people....where they sit can mean even more then motor trim...

 

[cyclops222]

Some people hold the DOWN TRIM button until they feel / hear the trim motor stop.
CAUTION !! All the way down can also be,,,,,,,, prop now angled UPWARD. Also causes the bow to be pushed down more and more as speed increases. Boat becomes erratic.

 

[Chris1956]

Airshot, I am not sure I ever saw a true hull get adverse performance from prop thrust that is parallel to the hull. It begs some questions about your boat/motor.

Some motors have a negative angle tilt stop, which tucks the gearcase under the hull a bit. This will lift the stern and could cause the bow to plow. If you have this setting, you might move the tilt pin out one hole.

Alternatively, you could have a hook in your hull.

 

[airshot]

Airshot, I am not sure I ever saw a true hull get adverse performance from prop thrust that is parallel to the hull. It begs some questions about your boat/motor.

Some motors have a negative angle tilt stop, which tucks the gearcase under the hull a bit. This will lift the stern and could cause the bow to plow. If you have this setting, you might move the tilt pin out one hole.

Alternatively, you could have a hook in your hull.

No hook and yes the motor does tuck way in, haven't bothered to change the tilt pin due to the power trim.

 

[cyclops222]

Do it soon. I value your logic on boating problems.

 

[Texasmark]

Thanks for the tips guys. I started with an aluminum 3 blade, 17P. The boat definately wanted to over rev. I looked up the recommended prop and it said a 21P would be right for the RPM, so I ordered an aluminum 4 blade. It seems to be right for the rpm, but it of course hurt the hole shot.
I've got an aluminum 19P on the way, but failing success, I thought I would play around with one of them instead of spending more $$ on another prop (the vented ones seem to be a bit pricey). Being as they are aluminum I don't anticipate much problem drilling them.

Most of not all of my Turning Point Hustler aluminum props for my 90 and 115 engines come ported from the factory....they are designed to mimic SS prop amenities except for the metal and it is a special, high density, aluminum. I get mine right here.

 

[tpenfield]

Mercury already has vented propellers and various sized hole 'plugs' to fine tune performance. Might be a better approach vs. the guess-work of a drill press.