Enough RPM for bigger pitch?
 

I have twin, stock 5.7L Mercs, Alpha Gen Ones, with 3-blade SS Mirage Plus 19P props. Currently, I'm only getting about 4200 RPM at WOT. I would like more top end, so I'm considering 21P. I know that comes with two tradeoffs:

1) slower out of the hole - which I'm fine with as I never nail it as I feel I have to baby the Alpha Ones based on what I've read on OSO

2) lower RPMs - at 19P, 4200 sound like the bottom range of the spec'd operating range

What can I expect for RPMs if I go to 21P and is that ok?

Thanks!

You will loose aproximately 300 rpm, and the boat will probably be slower on the top end as well.

Chris

Dude that is really low RPM''s for those engines, I would not go up in pitch, if anything I would go with a 17" and see what it does, you need to be turning around 46-4800 RPM good luck...

Ditto

send what you have to someone like Brett at Bblades, he can get you an extra couple hundred rpm and mph from the props you already have.

Double Ditto.

You are not making max HP at 4200rpms. You need to get the rpms higher, not lower.

With a small block engine... many are having great results with the smaller diameter Laser II props. I think the problem here is that the Mirage plus is just too big a diameter for the horsepower you have. I would try a pair of 19 or 21 pitch Laser II's. I think you will find that your boat will come alive with these props spinning around 4800-5200 rpm.

Chris

Thanks everyone - I appreciate the advice. I'm psyched to get more out of it.

Chris - I like the diameter theory. I checked and the Mirage Plus is 15 1/4 dia vs the 14 for the Laser II. I'm going to start looking for a pair of SS 14" 19P Laser IIs. Anyone up for a swap? :-)

Thanks again.

PS - there's something funny about a username with "REDLINE" in it recommending I drop to a 17P prop! :eek:

15.25 is a very large diameter for Mirage Plus props, especially for an Alpha drive. You should try something in the 14.5 range. Mirage Plus' do come in smaller diameters as well.

I think you should look into some 21 pitch Laser II's.. see if you can demo a pair. If they are a couple of hundred rpm low.. then have then thinned and sharpened to pick up the lost rpm.

You should be shooting for around 5000 rpm.

Chris

Will do. Thanks!

told ya, these guys have it straight and the change will let the drives live longer/more rpm and less load/torque on the drives, gears etc. cya

New Props on the Way....
 

Just ordered two brand new Mercury Quicksilver Lightspeed (replaces Laser II) props, Part #: QS5072X.
Dia: 13 7/8
Pitch: 21
Blades: 3

Replacing my Mirage Plus, Part #:48-13700:
Dia: 15 1/4.
Pitch: 19.
Blades: 3

Thanks for all the advice. I HOPE THIS WORKS!!!!

:boat:

hey dude, ya know goin back to the thru transom exhaust is going to wake her back up, thru the hub is quiet, but restrictive. that is prob. why the prev.owner claimed higher top end numbers/speeds. can ya get out of work today ? know it's late timing, but Marina Bay is a callin US :eek:-:eek:-:eek:

N-I'CE idea, but no props yet. In fact, they called and said they only had 1 in stock. They need 8-10 days to get the second prop. I told them to hurry 'cause it's getting colder around here. When I get them, you, George and I can try them out in tha harba (although it will seem like going backwards for you).

dude call me tryin to set ya up on a real horse :grinser010: RIDE !

Not sure I want to. I'm sure it's like wine. Once you start tasting $30-$50 bottles, the $10 bottles that used to taste good now taste like vinegar. :drink:

your right,but remember you did not pay 30-2-50$ it was a free offer and test of your true convictions remember vinegar was once sweet tasting apple juice now lets not worry about comparing oranges to cantaloupes or any of that BS u-need devotion and a firm budget as we all can say the grass always looks greener some where else, but I like to keep some of the green in my pocket - free rides come seldomly - - :kiss:

Wow dude... is that some kind of far out psychedelic code stuff ??

To de-code he had a chance to take a lunch run ride on a 38 Cig. yesterday with twin 588's maken about 750 hp a side thru. a set of #3 speed masters and he turned it down, oh well maybe next wk. I can get him hooked up again. :D - :) ps: 242LS will keep an eye out for a nice set of exhaust manifolds for your pony.

Thanks man. You know I would have loved it! Just bad timing. Feeling the wind in my face, breathing the salt air, listening to the deep roar of the engines while jumping across the waves!

Now that is poetry !!

I love this sport !!

Chris

Results of First Run with the New Props
 

Went out in Boston Harbor this afternoon - beautiful out there today.

Seas calm - some swells and wakes.
75 gallons fuel (3/4 tank)
2 adults

4400 RPM (200RPM more)
54 MPH on the non-GPS bouncing speedo (2-4 MPH more than before)

Not a wild improvement, but definite improvement. :drink:

Have the props thinned and balanced.. you should pick up a couple more mph.

Chris

open up to thru-hull exhaust and better flowing manifolds and it will B go time, then some 4-barrel carbs and intakes, well then ya can try to hold on to ya channungass :eek:

I'm sure that's what's holding me back. Staying in Boston for a couple nights now. Dropped her in Medford today with the family. Got a slip at Boston Yacht Haven - great place! Our huge room overlooks the slip. After arrival, got the family settled and went out for a quick spin alone. 4500rpm, 54mph, 50g fuel. 2' chop and high winds. A lot of banging. Broke the cabin door latch bouncing, rear seat back fell off! I had a blast!!!!!

Looking forward to tomorrow. Could be the last weekend. I was going to head to the CT River run next weekend (Mohegan Sun the night before), but we couldn't get a babysitter....

I think ya Still need more RPM's.keep playing with it:drink: