Propellers - 90 Merc - performance

[dblandford]

Hello..I am new here, and new to the boating world. Hoping someone can help.

I have a 2009 Trophy Center Console 1703 boat with a 2009 90 Mercury Outboard 2-Stroke engine. I’ve had the boat out a few times now and noticed that when I am in wide open throttle the engine RPMs are well above the recommended RPM stamped on the engine. The recommended Max RPM is 5500, but when I open it up to full throttle the RPMs on the gauge hit around 7000. I did this for no more than 5 seconds and scaled the speed back to around 4800 RPMs and have not exceeded 5000 RPMS since. Top speed at 5000 RPM with only a couple people aboard is around 22 MPH. It also seems like it is difficult to get up on plane unless I hit and maintain around 4800 RPMs.

I have done some reading and have heard that my boat and engine are under propped...whatever that means! The prop I currently have on the boat is a 13.25 diameter prop with a pitch of 17. Anyone have any suggestions on how to improve performance?

 

[jimmbo]

Find out if the Tach is accurate and set correctly. Those numbers aren't adding up. I believe your engine has a 2.3:1 gear ratio, even at 5000, you would be going around 34mph with a 17" prop. Either the Tach is way off, or you are experiencing severe Ventilation/Blowout.

 

[dblandford]

What is ventilation/blowout and how can this be Resolved?

 

[flyingscott]

Are you running an optimax? If so i am pretty sure they have rev limiters that kick in well before 7000 rpm. Check your tach.

 

[QBhoy]

Hi. A 90hp 2 stroke from 2009 would be a rare thing indeed. At least where I live. It would need to have been bought commercially and would at least be an optimax. If it is, then it would have a limiter...way way before 7000 rpm.
If it’s genuinely pulling that (really doubt it) but you say it’s slow to accelerate...then it’s a real mystery to me. I think maybe you need to go try again and think what might not be true of your figures. Best of luck.

 

[jimmbo]

What is ventilation/blowout and how can this be Resolved?

Ventilation is where air/exhaust is introduced to the prop and it loses its grip, Blowout is a little different, usually not occurring until higher speeds, and is a result of severe Cavatation, That's where a Vacuum Pocket forms, due to poor hydrodynamics, mods/damage to gearcases, or exceeding the speed of a gearcase design. Again the result is loss of bite for the prop
Perhaps I shouldn't have used the word Blowout, as it likely doesn't apply in your case

 

[dblandford]

It is not an optimax...I’ll have to get my tach checked I guess. But it does show 7000RPM at WOT currently.

 

[jimmbo]

I doubt it it getting that. those 3 cylinder models tend to have a very short life when operated above 5500 rpm

 

[WesNewell]

Mercury prop selector says you should be running a 19P prop @ 5250rpm wot and 37 mph with 700# people and gear and 20 gal of gas. If you're reading 7000 rpm either the tach is set wrong, you have a hub slipping badly, or your prop is venting. Check it yourself,
https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one

 

[Texasmark]

The tach number is 5 on selector dial on the rear of Merc and Farnia to name 2 tachs for a 12 pole alternator which your engine has (6 pickup coils in the Stator but when the tach rectifies the sine waves, you have 2 DC peaks per coil which is what the tach counts).

On the slipping hub, remove the prop nut or just remove the whole prop....and grease the prop shaft and thrust washer/prop shaft interface while you are at it. Scribe a straight line from the 15 toothed brass center spline to the outer prop barrel. Put it back on, torque prop nut to 55 ft-lbs, and run it hard for a bit.

If the prop is slipping, regardless of what type of insert it uses (pressed in rubber...old style, or current popular brass/plastic removable), the line will no longer be straight.

That engine may or may not have a rev limiter module. Follow the grey wire from the rectifier/regulator module. If it goes to another module rather than go straight to the grey wire of the cable harness that goes to the remote control, you have one and it will not let the engine go over it's rated rpms.

Excessive engine height and or excessive trim (out) can cause excessive rpms (call it what you like) and loss of boat speed.

Besides Mercury's prop slip calculator I use go-fast.com and they are very accurate. 12-15% at WOT for a good planing hull, loaded and setup right, is a good number for me.

Instruments will lie to you for their own reasons, some due to internal problems and some due to input errors.

If your engine was running 7k rpm it would be screaming, not singing.....BTDT on a 2002 90 Merc 2 stroke.......had a pre ignition occur once, on muffs setting up/checking timing adjustment, and I pegged the 7k Merc tach! Was running on it's own and I had to unplug the fuel inlet to get it to shut down.....and the engine wasn't all carboned up, Piston crowns were clean....ran Sea Foam in every tank of gas.

 

[Texasmark]

That engine may or may not have a rev limiter module. Follow the grey wire from the rectifier/regulator module. If it goes to another module rather than go straight to the grey wire of the cable harness that goes to the remote control, you have one and it will not let the engine go over it's rated rpms.

I was out mowing grass just now and realized that I offered some bogus information. The grey wire doesn't feed the RPM limiter if it had one. That's only used to feed the tach.

In consulting the engine wiring harness for the 2015 service manual revision, for the 1998 and newer 75 thru 125 2 stroke carbed engines, the 75 and 90 ELPT engines have no RPM limiter. The 100 up may/does have one.

Sorry about the disinformation but I think I caught it before it went viral!