Prop venting with the 3.0 merc.

[Suprathepeg]

Here is my prop venting thread. I will try and be as "scientific" as I can.

So here is where I started:
-89 maxum 1700 deep V hull
-new floor and interior this year
-3/4+ tank of fuel
-New 3.0 Merc with about 25hrs
-14.5x19p aluminum prop
-Factory setting of 6* timing
-always run premium fuel 91 octane with no ethenol

Everything I've done or will do is with a very experienced mechanic on board who has about 20+ years experience working on boat motors.

WOT
Easily revved over 5k top recorded and held speed 33mph with a little bit of chop (~2-3") with a pretty strong current on the river. Hole shot was good with two people. With 4 pepple and gear (~850lbs) and a full tank of gas hole shot was OK.

 

[littlerayray]

Operating range for the 3.0 is 4400-4800 you may be under proped

 

[Suprathepeg]

So here is today:
I picked up a 14.5x21p aluminum. It's used like the original 19p unit. Conditions on the water were calm, water was glass. Current on the river was minimal had maybe a .5mph effect. Two people in the boat at around 425lbs

-Advanced timing to around 8/9deg
WOT
-Hole shot was OK
-max RPM 4700/4800
-max held speed 35MPH
It wasn't bad, I could have lived with it, but no skiing etc with this setup


We drilled 3 vent holes in the prop 5/32"

WOT
Hole shot was much improved
Max RPM 4900
Max held speed 36.5mph
Probably no skiing but it was very usable.
Also prop bite in the corners was equal to the 19p

We upped the hole size to 7/32

WOT
Hole shot equal to the 19p for sure
Max RPM 5k
Max held speed 38mph
Any water sports I could have done with the 19p were a no brainer with the vented 21p.
Also it carves corners like a boss, better than the 19p, like so hard it almost threw me off the controls. The boat is a beast in the corners.

We couldn't drill bigger holes as we were hitting the hub spokes so I can't say how much better things can get. We will have the current holes filled and move them about 1/8" over so that we can mess with bigger holes.

So far I conclude that prop venting is certainly not just for outboards or two strokes. I would recommend it for I/O setups like mine all day long. We were hoping for better performance on the bottom end but we were shocked that it not only helped hole shot with a steeper prop but it also picked up a few MPH on the top end as well. With a little more movement on the water we might pick up another 1-2mph yet. I am so happy.

 

[bruceb58]

If you are increasing your WOT, your holes are too big. Prop venting should not increase WOT.

 

[steelespike]

Pretty sure your tach is not accurate un vented prop; 4800 @ 35 calculates to 25% slip.
The tach is always suspect when slip is high. more normal slip would be about 12%
If we calculate for rpm 2.00 ratio,21" prop 12% slip 35 we get about 4000 rpm. actual.
Gotta get the tach verified before any other testing.

 

[Suprathepeg]

I agree the tach may be out, I have another on the way.

Even if the tach is reading too high, venting improved perforance all over the powerband.

Keep in mind that this is an older used prop that physically measures 14" in diameter rather than the stamped 14.5". The goal was to see if venting had any effect at all.

 

[steelespike]

The tach error I used was with the unvented prop.
They say that venting isn't as effective with an I/O.
While your holes may be a little too big it appears venting does work.

 

[Suprathepeg]

The biggest hole performed the best overall especially out of the hole and while turning.

My guess is that with a V8 that has a lot of grunt all over the powerband the performance increase might be negligible. However my goal is to chase a top speed without losing hole shot so I'm upping the pitch to chase speed. If your boat has the standard prop and your overall performance is good then venting probably isn't going to make a noticeable difference.

 

[Suprathepeg]

Well yesterday was going to be a big day.

We drilled the Accelerator pump jets on the carb to try and see how the hole shot could be improved. Then we adjusted the float so more fuel can sit in the bowl. This was to address two issues.

1. The jets allow more fuel out of the hole to compensate for the reduction in fuel quality that has come on during the last 30 years.

2. The added fuel in the bowl was to offset some fuel starvation coming out of the hole when the engine is warm and idling.

The above did the trick for sure, no more bogging at all.

3. We also increased the size of the main jets on the carb 1 size. I'm not sure on this one but I will update soon.

4. Advanced the timing to around 10/11 degrees.

While the mechanic was working on the carb I drilled new holes a little over from the originals and put the prop on the back of the truck while I waited. Then I forgot to install the prop and we drove away with it on the toneau cover. Then it fell on the road at about 35mph and got bent to $#*(. I'm an idiot. I straightened most of it out at the launch and we tried it out.

Bottom line is the prop is pooched so I dropped it off at a shop and should have it back later today so more hole shot and speed testing to come. The upside is the motor really likes the added fuel and timing. It runs smoother and pulls good all over the power band. one bad blade on the prop kept us to a top speed of 36mph and we could feel the vibration at lower RPMs. I'll update ASAP.

 

[steelespike]

Your actual rpm may be as low as 4000 based on the calculations.

 

[bruceb58]

My suggestion is to Shop iboats.com for the best Boat Propellers in the industry.

 

[Suprathepeg]

Funny you say that. I took the prop to a shop that reprofiles blades and can actually rebuild a prop. Anyhow he straightened out all the dents but the prop is missing about 1/4 of material on each blade for a previous repair. So the prop isn't pulling its weight.

I've seen some good reviews of turning point props, the prop shop also recemended them. They also happen to be the the most affordable I can find locally. Any reason I shouldn't pick one up?

 

[Suprathepeg]

Also today we installed some spark plugs that are 1 temp cooler than stock. With the increased timing the stock plugs are running hot.

Don't worry if the temp seems like a persistent problem I'll reduce the timing and go back to stock plugs.

 

[bruceb58]

A good prop shop can replace the missing material. You did not go to a good prop shop.

I would be really careful about advancing timing like you are. You will burn a hole in your piston. A cooler spark plug will not prevent this. I would not be setting the timing above the stock setting unless you are only using high octane fuel.

 

[Tail_Gunner]

Funny you say that. I took the prop to a shop that reprofiles blades and can actually rebuild a prop. Anyhow he straightened out all the dents but the prop is missing about 1/4 of material on each blade for a previous repair. So the prop isn't pulling its weight.

I've seen some good reviews of turning point props, the prop shop also recemended them. They also happen to be the the most affordable I can find locally. Any reason I shouldn't pick one up?

Those props very widely in results, the 3 blade solas here is a 100 dollar bill can't beat that on a whim or a prayer with very uniform results. Verify the tach and as to fuel and timing the 3.0 cam is the only thing holding you back unless the factory carb is slopped out and can no longer meter gas.

 

[steelespike]

Often it is suggested to reduce pitch 1" when going to the Hustler. This would depend on getting your actual rpm before buying a prop.

 

[littlerayray]

Wonder why merc doesn't recommends keeping the same pitch on their spitfire then

 

[Suprathepeg]

The prop shop couldnt add material because someone in t past has a material using something that isn't aluminum. Cell prop I fu of some sort o material that I heavily Laden with zinc an I won't take an arc properly. Otherwise Yea , could h brought I back.

We are only using premium fuel with no ethenol.

 

[Suprathepeg]

Reference timing. I'm a little nervous about timing too. But so far we've added fuel, using the best quality premium gas I can find locally and have reduced the temp range of the plugs to reduce the chance of pee ignition at high RPM. If we don't like how the plugs look we will reduce timing.

Reference the cam I don't want to push it past 5k but is there a cam that would yield better results? Especially at the low end?

 

[Chris1956]

First of all, a TP Hustler prop is a good choice. I had a SS Rapture and a Hustler in the same pitch. Both were vented and had the same aggressive rake. I used them on both my Mercuriser 4.3LX and my Merc 135HPV6 outboard. I did not vent the props on the MerCruiser, but allowed venting on the OB, to get to the powerband sooner. I therefore think the Hustler is a good value, and much more advanced than other aluminum props. For performance you will want the 3 blade version.

If you are handy with fiberglass and mechanical issues, you could swap a 4.3LV6 motor for your 4 banger. You could run it stock, on regular fuel, and have a lot more performance than the 3.0L motor you have.