Prop diameter for a Mercury 90

[dandreye]

Hello All,

Recently bought a used boat with a 2-stroke Mercury 90hp (I think 90 ELPTO) made around 2002, w/o any documentation. Looking to buy a SS prop for it. Have seen a general recommendation on this forum to start with a 19" pitch SS prop - but which diameter? I've come across some posts saying that it's fixed for each outboard model.

This 20-feet boat (pictures attached) will often carry 4 - 6 persons + typical camping belongings, which I guess would be at least the "normal" load. I seek optimal fuel consumption.

A hint at a few good SS prop brands and models to choose from would be really appreciated.


Many thanks in advance,

Dmitriy

 

[Silvertip]

Re: Prop diameter for a Mercury 90

Prop diameter for that motor is typically 13-1/4 inches but it may vary by up to 1/2 inch depending on the prop design and manufacturer. Small diameter diameter differences are normal and are the result of design characteristics. You will be best by starting with a 19P.

 

[dandreye]

Re: Prop diameter for a Mercury 90

Thanks for that - I can see that among 13-1/4" x 19P SS props iBoats recommends the following (3-blade):

TurningPoint - Express
Stiletto - Advantage I
Solas - Titan
Michigan Wheel - Apollo, Avenger, Ballistic

Any particular ones from this list or some other ones that are known to be of very high quality, durable etc.? Price isn't an issue but I won't have a chance to return it once I buy.

 

[Silvertip]

Re: Prop diameter for a Mercury 90

Go here and read this. If it has blades and spins Walleyhed has probably tested it although he tends to favor the Stiletto.

http://forums.iboats.com/showthread.php?t=218877

 

[Texasmark]

Re: Prop diameter for a Mercury 90

First order of business is to clean the hull. If I see what I think I see you ain't-a-gonna get anywhere with that mess. Need a slick hull if you expect the boat to perform as it was designed...big drag factor on mph.

The engine you mention has a 2.3:1 gear ratio which is 15% higher or so than a lot of engines in the mid hp range. That means that you will run a higher pitched prop than a comparable engine with the lower ratio that others may be used to.

So, 19 is not a bad place to start. As far as diameter, for that gearbox housing, you can run up to a 14" with no problems (BTDT).

I have that same engine and run a 21P or 24P. For what you said about your plans, I think a 19P in 13 + diameter is a good starting point. The engine is rated for 5500 max rpm and I would prop it to be there on your normal load, the one where you do the most running....and the point I am referring to is you and your other occupant are on the way to the fishing hole (boat is the heaviest) in relatively smooth water throttle firewalled, trimed out to max speed which is usually just below where it wants to porpose, unless you get on the other side, and on that rig I doubt you have enough power to blow past that point and smooth back out.

It will however overrev (no overrev limiter on that engine) if you let it when out alone lightly loaded, but just yesterday I was out with my 21P and with the cool weather it easily punched out to 6300. I didn't stay there very long, backing off to 6000 which it handled nicely, which is what you can do.....just cut her back to where you feel comfortable.

Also, do a decarb (archives) and keep some Sea Foam in the tank. She just runs better. If it has been sitting up, or you know that the impeller hasn't been changed recently, a change of that and drain and refill the lower unit gearbox with Merc high perf oil. What you drain out should be black; the new will be dark blue. If your drain is not black you have a seal leaking which needs to be fixed.

While at your Merc dealer getting your oil, pick up a couple of blue washers to seal the screws that you undo to drain and refill the oil.

Need more ask, PM or on here.

Mark

 

[dandreye]

Re: Prop diameter for a Mercury 90

Thanks a lot to Silvertip and Texasmark for the valuable feedback.