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Author Topic:   Speed increase of 1mph for 600 rpms ?
Fishcop posted 02-13-2004 07:22 PM ET (US)   Profile for Fishcop   Send Email to Fishcop  
Here are the facts:

1987 25' Outrage w/T-top, 140 gal fuel, three adults and lots of gear, 225 Merc Optimax on 30" setback bracket, SS Mirage Plus/3 blade/15p turning 4900 rpms = 40.9 mph on gps. Increase the rpms to 5500 rpms and get 42 mph. At 5600 rpms, the overspeed sensor kicks in and the warning bell starts. When I checked the GPS at 5600 rpms, 42.6/42.9 mph.
Why so little gain for 600 rpms?

Any help appreciated.


Sal DiMercurio posted 02-13-2004 09:19 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Cop, something dosen't sound right about the over rev.
If i'm not mistaken that 225 maxes out at either 5,800 or 6,000 rpms, theres no reason at all for any hi rev signal to come on at 5,600 rpms.
As far as the 2 mph gain, thats a very big heavy boat with 1 prop., but I can't tell you if it's right or not.
Try the prop calculator & see what it says.
Fishcop posted 02-13-2004 09:44 PM ET (US)     Profile for Fishcop  Send Email to Fishcop     

Checked the prop calculator and it shows a 17p putting out the same numbers as my 15p. Did not show a 15p for my application and the max rpms were higher.
You are right, the max rpms on the motor are 5750, but I have never been able to reach that number. I have the smart-craft guages on the boat and they have some type of over speed function. I was just curioius why increasing 600rpms at top speed would only produce 1mph gain.

Thanks for the help.


bsmotril posted 02-13-2004 11:52 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Does trimming down slightly have any effect? Could be that the prop is slipping more at that higher RPM, or sucking air. Just for grins, try trimming down a bot and run your test again. The speeds won't be as high, but I am curious to see what the speed difference will be for those extra 600 rpms with the motor tucked in more. Also, the relationship of horsepower to speed is not linear, nor is the relationship of drag to speed. But those effects are minor for the range we're talking about here. BillS
lhg posted 02-19-2004 03:52 PM ET (US)     Profile for lhg    
Andy - You may be under-propped. On your engine, my Merc prop charts show it maxed out at 41/42 mph with a 15" pitch prop, carrying 6000-8000# loading. So this matches your report of prop "floating". I doubt if you're that heavy. For a 17" pitch prop, they are showing speeds of up to 47 MPH with a load of range of 4300-6300. My guess is that you should get about 45 mph out of the Opti 225 on your rig. You might want to look into a 17" pitch 4-bladed Revolution 4 prop as an alternative to the Mirage. The 4 blade will, however, eat up about 100 rpm at the top. Too hard to call without testing as to which series prop would be best for you.

When are you going to be upgrading to a 275 Verado?

jimh posted 02-19-2004 08:14 PM ET (US)     Profile for jimh  Send Email to jimh     
This may be a proper question for a propeller designer, but let me give you my seat-of-the-pants opinion, if you will.

My supposition is that when propellers are designed there are certain target speeds at which they are made to perform best. We know for a fact that at lower speeds there is a bad angle of attack on the blades of a propeller, and as a result there is a great deal of "slip." As the boat speed increases, the forward motion of the propeller through the water affects the angle of attack of the propeller blades, and the propeller moves into a region where it is more efficient. As the boat speed increases, generally the slip on the propeller decreases, with the result that the propeller becomes more efficient. This has the effect of making incremental changes in engine RPM have more effect at high speeds than at low speeds.

Now fishcop is reporting the opposite effect; the last 600 RPM of engine speed is not producing much boat speed increase at all. Perhaps the explanation for this is that the propeller being used has reached the boat speed for which it was designed, and as the boat goes faster the angle of attack of the propeller blades is moving into a region where their efficiency is going down, not up.

The only other explanation that seems reasonable to me would be that there is some mechanical slipping going on between the propeller shaft and the propeller itself, perhaps due to a rubberized hub that has shrunk from aging.

Fishcop posted 02-19-2004 10:50 PM ET (US)     Profile for Fishcop  Send Email to Fishcop     
Thanks to all.

The latest is that the performance remains the same with a lighter load and less fuel. Fresh or saltwater, the boat maxes at 42mph at approx 5500rpms. Around 41mph at 4900rpms. As I recall, with the additional weight of the forward cuddy, I had the same performance of 42mph. As I look at the prop calc, the performance should increase with the 17P. The max rpms for my motor are 5750 which I can not reach now. With the 17p, as LHG stated, I can probably increase mph by 2-3mph. Well, it is only money...rebuild the hub and get a new 17p. I will keep you posted with the results.


The Verado 275 looks sweet. My only concern is that for that $$$'s, I would rather get a new Armstrong and a second CR 225 Optimax!
Man, that sounds good. 450hp on the transom and no concerns over a simple 1-2mph gain in 600rpms. Might even make that my Spring project.

Thanks again,


quattro20vt posted 02-20-2004 04:24 AM ET (US)     Profile for quattro20vt  Send Email to quattro20vt     
You mentioned you have the Mirage Plus--that uses the Flo-Torq II hub which really doesn't get rebuilt, but replaced, for $40, and doesn't likely slip as much as break (tho' Dick P says he's never seen one break).

FWIW, on my Revenge 22 WT WD with a relatively new Opti 225 and the Mirage Plus 17p, I see 44+ mph @ 5500 rpm, and about 39 mph @ 4900 rpm.

Your 42mph @ 5500 gives about 6%
My 44mph @ 5500 gives 12% slip (must be the ugly bottom paint I have)

Your 41mph @ 4900 gives NEGATIVE 3% slip...HUH?
Can you verify your tach's accuracy?

lhg posted 02-20-2004 02:33 PM ET (US)     Profile for lhg    
Mercury performance props that have the replaceable plastic shear sleeve all would have the round vent plug system. If your Mirage prop deos not have vents at all, it has the rubber hub. Either way, I would first do the economical thing, and check out the prop. A new rubber hub is about a $60 job, and a new replaceable drive sleeve is only about $10. I think Jimh's suggestion that the hub is slipping could be a strong possibility.

Regarding the so-called negative slip that Andy is reporting, this happens all the time with the Merc performance props. Actually, depending on the height they are run, the EFFECTIVE pitch is often higher than advertized. That is exactly the situation Andy is experiencing with the 15. I am guessing the prop is being run fully submerged, which will increase it's effective pitch by 2". Running an elevated 17" pitch would be a better alternative. Either way, it appears this boat should do 45 with the Optimax 225.

Sal DiMercurio posted 02-20-2004 06:23 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Andy, are you certain the alarm that comes on at 5,600 rpms is an over rev alarm?
The my knowledge the engine will start shutting down cylinders not buzz an alarm at over rev for which I think your over rev is 6,200 rpms, thats still 700 rpms away from over reving.
Are you sure it's not a bad thermostat thats kicking in the alarm ?
Are you trimming that engine back enough to raise the bow & break free of the whole bottom being wet?
I don't think anything is slipping because theres a whole lot of power being pored to that lower unit & once it starts to slip, it normally lets go right now unless your at idle.
I really doubt it's the hub.
Do you have a big "V" spray coming up on both sides of your lower unit ?
VMG posted 02-20-2004 06:53 PM ET (US)     Profile for VMG  Send Email to VMG     
I've had two boats with Mirage plus props (one single engine, one twin). IMHO, Jim's observation about the top end performance increment is consistent with my observations. The M+ delivers wonderful hole shot and mid-range performance, but I think all that diameter works against you toward the top end.
Fishcop posted 02-20-2004 09:50 PM ET (US)     Profile for Fishcop  Send Email to Fishcop     
Thanks again to all!

I went out today and ran the boat in freshwater, against a current and with the current.
4900rpms gets 39.9 to 40.9 trim down both upstream and downstream.

Trim up, the rpms increase to around 5600+/-rpms and the smartcraft gauges read "over speed" and an alarm signals.

Looked in the manual (I hate doing that!) and the "over speed" alarm is installed as a protection via the ECM and routed to the gauges. The Optimax motors also shut down functions to protect the motor against failure (AKA fault codes).

Now, as far as prop slip...I do not know. The hub seems intact and the local shop will do the work for $50.

Can you get 5750rpms from your Optimax? Do you have the smart craft gauges? I have spoken to some bass boat owners and they say they get 6000rpms and 70mph from their Optimax motors. Far in excess of what I can accomplish.

The motor is max rated for 5750rpms. This is on the motor and in the manual. There is no V-spray off the lower end and I believe that the addition of the 30" bracket adds to the limit of available trim.
By the way, caught and released a 76" sturgeon at Rio Vista today. Kind of a tricky feat with 17lb mono and lots of floating trees and debris.

I am still leaning towards the twin 225's and a good life insurance policy.

Thanks again to all.


Sal DiMercurio posted 02-20-2004 10:38 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Andy, good going on the sturgeon, heck of a battle on 17# test, fish must have been 150 lbs.
Darn, I wish i'd have known you were out today, I was only about 6 miles down stream from you, [ my island ] in Montezuma slough.
My best is 327 lbs [ 109 " long ]on 25 lb test & about 4 hours of give & take before the over size limit [ 72" ] took effect
lhg posted 02-20-2004 10:55 PM ET (US)     Profile for lhg    
Andy - when I repowered mine in 1997, I looked into twin 225 EFI's. Prop charts were showing 23" pitch and 67 mph. For 225 Optimax's, prop charts show 65 mph, as across the board, the Opti's are about 2-3mph slower than the EFI's. So if you do it, hang on! You'll also be carrying max weight, leveraged back 30", so be sure the bracket and built up transom will handle it. I use a 26" setback bracket.
quattro20vt posted 02-20-2004 11:27 PM ET (US)     Profile for quattro20vt  Send Email to quattro20vt     
I see 5490 rpm on the Smartcraft system monitor. This was after trimming out some and letting the boat settle in a bit...if I tried trimming out any more, she'd start to porpoise. I suspect I'd get to 5750 if I raised the engine one hole (it's about 1/2" lower than it ought to be due to the way the WD was notched), which I might do, but I'm probably gonna leave it alone this year to learn more about it as it is.
Fishcop posted 02-21-2004 09:17 PM ET (US)     Profile for Fishcop  Send Email to Fishcop     
One more thing.

If I stick another 225 Opti on a new bracket, is CR really going to make a difference?

Your thoughts are appreciated.



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