Prop recommendations please: 20' Seafari w/ 150 Johnson OB...

[JUST JOHN]

I'm getting sucked into researching my tach issue. The tach is getting correct voltage, and zeros out when ignition is on. I don't believe I'm getting voltage from the sender wire. The gauge pulse setting is set to 5, which I think is odd (shouldn't it be 6?)... Next I'll try to test the regulator/rectifiers/stater/confirm # of magnets in the flywheel. I have no idea what I'm doing...

[Bushwacker]

I think you will be surprised at the mid-range performance improvement with the 4B Vortex prop. That extra blade will provide extra stern lift and blade area which should help you hang on plane at a lower speed, which is a significant benefit when you get into rough conditions.

I think I’d forget that existing tach and look for a good digital tach! You need an accurate WOT rpm reading with an existing prop so you have a good baseline before looking a stainless prop. In general, the SS prop blades are thinner than aluminum props, so they’ll spin a little faster than an aluminum prop with the same pitch and diameter. For example, the Michigan Apollo is the 4B SS version of the Vortex; the comparable Apollo to the 14 x 20P Vortex was 14 1/8” diameter x 20” pitch, so the thinner SS blades allowed them to increase the diameter by 1/8” for the same input horsepower.

[JUST JOHN]

Hey Thanks Denny, I just read back through all the previous input from you and Frank...

The Michigan wheel I was looking at locally was a 3B (992003), not 4B. Same goes for the PowerTech prop I mentioned, 3B. Sorry for missing that earlier:
https://www.deepblueyachtsupply.com/...-vortex-992003
I don't think the 17p pitch will work out with a 3B aluminum, due to the thicker blades, flex, and lighter weight...

I see you already commented on the Rebel props before I mentioned them Hard to turn and intended for higher HP, noted and thank you!

I've tinkered with the tach and motor for two evenings now with many of the troubleshooting techniques I've found online and am frustrated. I can't track the grey sending wire to my engine...

I should be back out on the boat tomorrow

[Bushwacker]

Didn’t realize that the Michigan Vortex series included 3B props, as the one I bought was a 4B.

Perhaps it would be useful to share my experience with 4B props. I had never experimented with 4B props before I replaced my transom-mounted 31 year old 300 lb V-4 with a ~130 lb heavier V-6, set back 30” on a bracket. I knew the extra setback and heavier motor would move the CG back a lot, which is why I stayed with a 2-stroke to minimize the weight increase when I repowered. However I was NOT prepared for the dramatic change it created in the boats planing performance! A boat I had run for 31 years that would plane easily at 12-13 mph and run comfortably thru the 2’ square waves on the Bahama Bank at 15 mph while sitting down, would no longer hang on plane below 22-23 mph! Initial tests with a dealer-provided aluminum 3B 17” pitch OMC prop (I think about 14.5” diameter) gave exactly the right WOT rpm (and would probably be about right for your boat), but it’s low speed planing performance was unacceptable and it was so sensitive to throttle setting that it was very difficult to even maintain a steady 22 mph! I tried another 3B aluminum 19” pitch OMC prop but WOT rpm was too low, although it did seem to have less throttle sensitivity.

I concluded that more blade area would better absorb the E-Tec’s awesome mid-range torque. (The torque curve is nearly vertical in the 2000-3000 rpm range, after the injectors switch from a very lean stratified charge mode at about 1800 rpm, to a much richer mixture to keep from overheating the pistons!)

Not only did the 4B prop completely eliminate the throttle sensitivity issue, the extra stern lift also dramatically reduced min planing speed, down to about 15 mph! The addition of a fin on the AV plate got min planing speed down further to the 12-13 mph that I had previously found to be so useful.

So I would suggest that you try a 4B prop if possible . . . You may be pleasantly surprised at the results! But before you do that, I’d bite the bullet and install an accurate digital tach! If you don’t know what your WOT rpm is, any prop testing you do is a complete waste of time!

[Fr. Frank]

Originally Posted by Bushwacker

... If you don’t know what your WOT rpm is, any prop testing you do is a complete waste of time!

That's almost Gospel, Denny.

In 2002, I re-powered a previous 20' Seafari from a 120hp Chrysler motor to a re-manufactured XR4 2.4L making about 230 hp on the dyno, after balancing and blueprinting the motor. I tried a lot of different props. I was running a lower unit off a 225 hp with a 1.65 gear ratio, so pitch was an issue.
I got a Solas 3B stainless 17P to get me 6000 rpms, but it was only giving me 38-39 kts trimmed out at WOT.
I next tried a Mercury Offshore 4B at 15P, but I only got 5600 rpms at WOT, producing about 37 kts.
I tried a Laser II 17P 3B, and got 5900 rpms, but it was a dog out of the hole and only produced 35 kts. (Lots of slip with that one).
I next tried several props in succession, finally settling on the Quicksilver Mirage 17P 3B, which got me 5850 rpms, and 40 kts, and still had decent handling and acceleration. I ran that for a couple of months, but still didn't think I was getting all the performance I could from the motor.
After talking with Jerry Schiaccitano of AAA Outboard Machine Shop in Crystal River, who raced classic Chrysler Outboards, I got in touch with an engineer at PowerTech. After sending him the specs of the Seafari and on my motor, including photos of the hull, they designed and made for me a custom 15.25" X 16.5" three blade propeller based op their OFS design.
Max RPM was 6150 rpms trimmed out at 43 kts. Hole shot was just under 4 seconds, and while maneuverability was slightly down from the Offshore 4B, I could maintain plane down to 13 kts with the trim tabs. The prop did, however, cost me $735.


The point of this story is show that performance can be tweaked, and sometimes come from unexpected experimentation.

[JUST JOHN]

Originally Posted by Bushwacker View Post
... If you don’t know what your WOT rpm is, any prop testing you do is a complete waste of time!

I hear ya. After tinkering with the tachometer/wiring for a few evenings after work last week, I gave up with that and prop shopping for now, without doing further research on figuring out the tach situation. This past weekend the boat ran great...

Frank, The XR4 2.4L was a transom-mounted outboard, correct? The Quicksilver Mirage 17P 3B sounds promising.

[jtharmo]

Originally Posted by Bushwacker

I selected a 15x20 version of the Vortex that I later had repitched to 15x18....

John, this Vortex 4B prop is now in my garage collecting dust. The Cyclone is the primary with the superior performance Denny found. The Powertech is in the cuddy as backup. I don’t know that I would ever have a need to use this. If you want to try it, it’s yours. I would rather see it used. I checked shipping rates to Long Island. It’s maybe $30 if you’re a little patient.

[JUST JOHN]

Originally Posted by SSPBill

John, this Vortex 4B prop is now in my garage collecting dust. The Cyclone is the primary with the superior performance Denny found. The Powertech is in the cuddy as backup. I don’t know that I would ever have a need to use this. If you want to try it, it’s yours. I would rather see it used. I checked shipping rates to Long Island. It’s maybe $30 if you’re a little patient.

Wow Bill, thank you!! PM'd.

[Bushwacker]

You’ll need to get a splined adapter from any Michigan dealer to fit that Vortex prop. Bill needs his for the PowerTec prop. They used to be about $20.