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Prop for Al Duck boat Options
Muddywater
Posted: Thursday, August 09, 2012 1:09:40 PM
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Joined: 8/9/2012
Posts: 3
Points: 9
Location: LA
On a custom 15' welded (1/4" al) flat with Merc 40 (2-stroke 4-cylinder), is it likely I would get much if any performance gain going from a 15 pitch Mercury Vengeance to some form of Powertech prop of the same pitch?  As far as usage goes, it has been everything from duck hunting in shallow marsh, fishing in marsh and some very rough bays, kneeboarding, wakeboarding, carrying heavy loads to the camp, and even search and rescue in New Orleans after Hurricane Katrina.  If I am in the marsh I run 90+% throttle pretty much 100% of the time, but if I am getting pounded in the bay, then often slower than cruising speed.  I have never really had any complaints about performance, but I usually have a pretty heavy load slowing me down and was wondering if something with more lift would help.  Also, I just got it back from the shop and was curious so I checked the anti-ventilation plate and found it was a good inch below the hull bottom which can't be good so as soon as I can find whatever marine caulk they use for mounting intend to raise it a couple holes.  It does blow out on sharp corners already, but I probably take them sharper than most boats are even capable of (it has some really serious runners on the bottom).  I do not need it to go super shallow as I also have a teflon'd smooth bottom hull with a surface drive mud motor for that.   Hopefully I can get some numbers with a clip on tach this weekend. 
Ken2
Posted: Friday, August 10, 2012 10:30:05 AM
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Location: Sarasota
The Vengeance isn't a bad prop, just depends if its hooking up well or not.

Powertech makes some props that would have stronger traction and allow you to run the motor higher and turn sharper without ventilating as easy.

I'd probably stick with a 3 blade.

depending on the rpm and speed you end up with after you raise the motor, you may end up just having some cup added to that Vengeance and be fine.
Muddywater
Posted: Saturday, August 11, 2012 5:30:09 PM
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Joined: 8/9/2012
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Location: LA
Thanks for the info!  The prop is an 18 year old Quicksilver 48 76232 A5 15P that has been on it since I bought the engine in 1994.  It seems to have a tiny bit of cup on the half of the trailing edge of the blade closest to the hub.    I wanted to test before modifications so I went out today.  The river was at flood stage so I took readings going both ways with a moderate load (2 people (183 and 200 lbs)and most of my fishing gear)
5190-5200 rpm (clip-on Oppama Pulse tach) @ 30.8 mph (Garmin gps) against the current
5220-5260 rpm @ 34 mph with the current
Average of both: 5230 rpm @ 32.4 mph
According to the owner's manual, the top speed rpm range should be 5000-5500.  I am guessing that raising the engine a couple holes will raise the rpm's a bit putting me closer to where I would prefer it to be.  I am still debating on a small setback jackplate or not.

Ken2
Posted: Monday, August 13, 2012 6:33:13 AM
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Location: Sarasota
Gear ratio is 2.0:1

5230 RPM at 32.4 MPH is 13% slip, which is pretty decent.

When I mentioned sticking with a 3 blade, I had 4 stroke on my mind.
I just looked again and saw it was a 4 cylinder 2 stroke. You could probably turn a 4 blade just fine.

I think the Vengeance you have will probably ventilate pretty easy if you raise it up much.
A little Powertech 4 blade would give more lift and bite a little better in those hard turns, even with the motor jacked up.
As for pitch, I'd see what happens when you raise the motor, but I'm thinking 14 pitch SCB4 its a 10 3/8 diameter 4 blade

Muddywater
Posted: Saturday, September 29, 2012 3:29:47 PM
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Joined: 8/9/2012
Posts: 3
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Location: LA
I decided to remount the engine 1 hole higher instead of two.  Having tested it, I could have gone higher because the anti-ventilation plate is still fully submerged while running but it is 3/4" higher than it was.
I put 3 80-lb concrete piers in the boat which is a little more than my Dad weighs (he was with me on the previous test).  The current changed on me so at one point I was getting better speeds going upstream.  Speed seemed to vary quite a bit for some reason.  Upstream I hit 5550rpm@35.9 and downstream 5460rpm@35.7 although there were some momentary better speeds like 5560rpm@36.7 at one point, which is over redline.  If I was by myself with no weight in the boat I would definitely be over redline which is fine since I do not run by myself very often.  Ventilation was not any worse than before and rpm's are right about where they should be.  It is pretty amazing that raising the engine 1 hole would give me over 200 rpm and 2-4 mph.  I probably don't even need a new prop at this point, although I need to run it in chop to see if it ventilates too much. 
Ken2
Posted: Saturday, September 29, 2012 7:50:55 PM
Rank: Administration
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Joined: 2/27/2007
Posts: 5,689
Points: 16,719
Location: Sarasota
Its amazing sometimes how much 3/4 of an inch can make on motor height.

As you mentioned, I think your fine with what you have too.
However, if it ventilates a little too much, you can have a little cup added to your existing prop and you'll be set.

No need for a new one.

Once you run it in some different conditions. Rough water, sharp turns, whatever. We can modify that prop if needed.
It may grip just fine like it is.
Just let us know once you get to play with it in different conditions.
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