Vent hole locations

[Mjohnpilot]

As stated in item 1, the boat has an extremely poor hole shot. I've been boating since 1969, owned 2 other boats, 18' Glastron Tri-Hull and a 24' Pontoon boat. The only time I've seen as bad a hole shot as the Gambler has is when I was trying to pull a bunch of skiiers with the Glastron. Once the Gambler finally starts to plane out and the speed picks up it goes like a stripped ***** ape! With about 450 lbs of driver and passenger we have seen close to 70 MPH GPS at WOT. I am considering drilling, or having someone drill, vent holes in the hub but not sure how to locate the holes or how big to drill them.

ESSENTIAL DATA REQUIRED
When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
1) Performance issue you are trying to correct. Very poor hole shot
2) Current prop manufacturer, model, aluminum or stainless as a minimum. Suzuki Stainless Steel
3) Current prop diameter and pitch (required). 14.25 x 24
4) Wide open throttle RPM and speed with an average load (very helpful) 5800 - 6100
5) Engine/drive make, model, year, and HP Suzuki DT175 1989 175HP
6) Boat make model, year, length and weight Gambler 1989 18' 3" not sure of weight

 

[ttankmoran]

Your top RPM is good, I assume since you have been boating for a long time, you have the motor trimmed all the way down and under for the hole shot, the motor is at the right height, etc. weight that can be shifted has been shifted to the front..

Vent holes in the prop are generally placed right at the base of the prop Behind the leading edge of the blades when you are going forward. Mercury and some aftermarket props have holes already, and have plugs available to lessen the amount of exhaust thats allowed to escape. You might have to do some experimenting, start by drilling small, maybe quarter inch holes, testing, and deciding if you want to go bigger. You don't want to go too big though unless you can find some plugs to put in.

 

[Texasmark]

Go to the archives and look around. Somewhere around in there is the picture of an OMC SST that has been drilled. Position is approximately 3/4" behind and below the leading edge of each blade and about 1/2" between the blades and the center of the holes. The idea is to decrease the density of the water flowing across the blade coming up from below the hole. Start with a 5/16" hole and go up from there. 1/2" is about as big as they get as I know it. I went up to 3/8" and that was all I needed for a stellar shot. My engine was 90 hp and used the 13" props while if you have a 150 or such you will be in the 14" prop range and the holes probably will be 1/16 to 1/8" larger in diameter to get the same effect.

I had a 13x24 SS prop that I drilled using my shop drill press. SS is tough to drill but I got her done without any problems. Go up in size slowly, like 1/16 to 1/8" at a time. Remember the "area" of the hole is the square of the radius meaning (if you don't know) the area goes up exponentially fast as compared to the width, aka diameter of the hole and it's much easier to go on up to the next notch than to......oh !@#$% I went too far.

On how much hole to bore, here's my rule of thumb and it works: What you want to have happen is for your engine to jump up around 800 -1000 rpm over where it currently revs when firewalling the throttle in the hole. Less of an rpm increase, due to the hole not being large enough, is less of an effect and more will not only delay your shot but can be a problem in getting on plane plus the engine overreving, and in rough water at a high trim position, could cause some unnecessary ventilating. Once the boat is on the water, it shouldn't take more than 15-30 seconds for the prop to lock in and the speed increase and rpms drop (while pushing the trim on out if you tucked it in for the shot)....like putting an old stick shift car in overdrive, letting off the gas and feeling the car lock in at lower rpms....same sensation...for me anyway.

The boat was not a real heavy stepped hull, like popular glass bass boats with a pad, so this trim position I used may not apply to you if you have one of those boats. You'll just have to experiment and see what works for you.

I usually did not tuck my engine in for the shot. I'd leave it trimmed vertically or sometimes out farther. The idea is to get your rpms up fast which ups your hp fast and hp (torque x rpm) is what twists the prop shaft and produces thrust. Coming out of the hole in that configuration the boat would porpoise 3 or 4 times and every time it did, the engine would rev (prop would ventilate and grab) and each time it did this, you'd wind up going faster and the rpms would build. Was a lot of fun and a very fast hole shot.

If you overdo it, you can JB weld the holes shut, but be careful and don't increase the size too fast, giving yourself time to see if what you have is the right size.

Good Luck,
Mark