Props for the ride..

[MackDad]

Looking for prop advice out here..

I own a 1987 Formula 272, running twin 350 Mags with Alpha I's both RH. Props are 23" Mirage Plus (which are double cupped) for Alpha I outdrives. WOT yields only 4000rpm with outdrives and planes trimmed out. I have yet to GPS, but would guess low 60's. Formula spec sheet says the boat left with 23" Mirages with approx weight of 5750lbs, specced at 62 mph.

My recommendation would be to drop to either a single cup 23" SST or possibly down to a 21" double cupped SST to yield the WOT range of 4400 - 4800 rpm. Any thoughts? Have a good friend with similar sized boat, same engines, same outdrives but running larger diameter 23" Mirage single cupped blades...and he's ~2-3 mph faster, but he won't lend the blades...

Recommendations?

[Griff]

A Mirage Plus is generally 200rpm than a regular Mirage which the boat is originally spec's with.

I've never heard of "double cupping" but adding even more..and it sounds like a lot more.... cupping to the Mirage Plus's could be costing you 200-400rpm.

I would try some stock 21 Mirage Plus's and go from there. You want to run as close to 4800rpms as possible to make max power.

[MackDad]

Griff,

Appreciate the advice. From my understanding, to go from a Mirage to a Mirage Plus, I was under the impression the prop was cupped to a greater degree (or am I mistaken, or is there any difference)??. I have read info that stated there was a diff of 200 rpm from a Mirage to a Mirage Plus. Does Merc even make Mirage props anymore (not Mirage Plus), or are they under a different name today?

Now, with going to a 21" Mirage, my question is: can you run a larger diameter prop, and if so, is there any gain to this? The Alpha I's are at 1.50 gear ratio.

Apologize for all the questions, just trying to get my bearings straight.

[Griff]

Mirage Plus's are a reworked version, with more cupping, of the older Mirage. The regular Mirage is no longer made.

I'm not sure exactly how much diameter you can run with an Alpha or if the diameter would change much more than the rpm. I've read a 1/4" diameter is about 100rpms.

[MackDad]

Sounds great bud. I'll see what I can find out here locally and give it a whirl. The next steps are to GPS to see what I have going now. The Raytheon and the Speedo are not accurate by any means and I'd definitely (besides the Tach) see what I've got from a speed aspect to see whether I've improved.

Any advantage of 3 blade vs 4 blade?

[bbladesprops]

We have test Mirage Plus props if you are interested.

The M+ propellers have a different geometry than the Mirage propellers. When we went to the + series in the late 90's(Iwas still at Mercury), we incorporated the PVS vent system and the FloTorq 2 hub system. They do have a much more aggresive cup and tend to average a 150 rpm decrease over the straight Mirage.

Diameters vary throughout the M+ series with pitch just as the Mirage props did. The ability to react more to trim and the reduction of ventalation are some of the advantages to the + series.

Your boat is a 3 blade boat but a 4 blade done correctly will help low and cruise speed numbers and can maintain the same top speed numbers.

[MackDad]

Bbaldes,

Thanks for the advice. Took the 'ol 272 (dry wt 5750 lbs) for a run over the weekend. With 1/2 tank of fuel (70 gal) and two adults with 2 children, we maxed out (at a measely) 54.8 mph WOT, trimmed and planed for max mph, but yielded 4000/4200 rpm on the 350 mags.

Knowing everything said, if I drop in pitch from a 23" M+ to a 21" M+, I will improve to ~4400 rpm WOT, but will gain (most likely) a fraction of the speed (~1 mph). Speaking with the local marina, I first need to make certain the throttle linkages are properly set as one engine runs slightly higher rpm WOT than the other and the lower rpm engine has considerable play in the throttle. Once these adjustments are made, I can rerun the boat, review the WOT rpm and mph data and come back to the drawing board.

Totally understanding the relationship between pitch and rpm, what speed differences would occur (for a similar pitched 3 bladed prop) running from a smaller diameter Alpha M+ prop to a larger diameter Bravo I M+ prop? Or, is it even worth it?

Still trying to put all the math and performance together so I can understand which way to go. :oops: Thanks Bblades!

[Revd Up]

MacDad,
The best thing to do is try and borrow several different props 21 to 23 pitch. (I know that is harder to do with twins) They all will run a little different. Record GPS speeds and RPM's and write them down. With that info Brett can dial in the right set up for your boat.

When I dialed mine in I took about 7 different props to the ramp, on an off day, so it wasn't busy . Had my brother wait with the truck. I would make a run and back , about a mile. Pull it back on the trailer, change the prop, record the speed and RPM. And do it again and again. Then sent that info and my dyno sheet to Brett. He took a new prop and labbed it to run the best off of those specs. That's about as good as it's gonna get.:) Like the others have said you want to run at the top of your rpm range.
The good thing about doing it this way is you can get the borrowed props right back to your friends. Of coarse if you hit with one of there props its yours or you fix it.[smilie=106.gif]

[MackDad]

Rev,

Great to hear from you.. That does sound like a plan. I know my speed and rpm's at WOT however with so much that has been done to this boat over the past 6 years, I am not confident the throttle cables nor the throttles are set properly. I really need to get these in order first, then begin the prop testing. I heard from Brett several times and I agree with you to do a trial by error analysis with substantial data to optimally determine the best prop combo. But, I need to start at the engines first.

By the way, I know you all received a lot of rain up north. Our Boat Club is flooded (actually an island right now). We had to pull boats and campers from the low lying property as we have reached today the 5th highest flood in history at Peoria.

Here's a pic of the Lock and Dam at Starved Rock...

Take care bud!

[Griff]

At 4000 rpms, you are not making the max HP of the engine. You will more than likely pick up 400-500 rpms by going down to 21 pitch prop. Every inch of pitch is good for 200rpms and being that you are that low on rpm, you should pick up more. If you pick up more than 400rpms, you will gain mph.

Right now you are making the engine work harder than it has too at all rpms. Think of it like going up a hill in your car. With props that are too big, its like going up a steeper hill all the time.

[MackDad]

At 4000 rpms, you are not making the max HP of the engine. You will more than likely pick up 400-500 rpms by going down to 21 pitch prop. Every inch of pitch is good for 200rpms and being that you are that low on rpm, you should pick up more. If you pick up more than 400rpms, you will gain mph.

Right now you are making the engine work harder than it has too at all rpms. Think of it like going up a hill in your car. With props that are too big, its like going up a steeper hill all the time.

Totally agree. When I bought the boat, I had the shift cables replaced in the outdrives. Well, the outdrives were wedged in the engines 'cuz the couplers were shot. So, we had the engines pulled, replaced the couplers, had the quadrajets rebuilt, new plugs, wires, distributor, etc. Everything was all replaced back into the boat, but when I run wide open, the port engine throttle lever is maxed out (~3900 rpm) but the stbd throttle lever is not quite at 3/4 (turning 3900 rpm). I can trim in/out, k-plane in/out, regardless WOT still 54.8 mph (gps). The throttling on this boat is not right...Unfortunately I am nearing the end of our season and look to ride this one out.

I agree that dropping the props (once the throttles are corrected) to a 21" M+ would be a huge help (efficiency and slightly on top speed)...

Now, let's just get it done..