Prop fine-tuning 1990 Merc 60hp

[olyerickson]

Hi Folks!<br /><br />I'm getting 44 mph on my homebuilt Rascal (see link below) using a new 10R17 (Michigan Wheel) on a '90 Merc 60hp, WOT 5500 rpm. I'm pretty happy with this performance, except for the fact that the designer pushed the prototype (in 1989) to 52 mph with essentially the same engine (merc 60hp), same prop (10R17) but with a "super cup", running at slightly lower RPM (I think he came down to 5200rpm).<br /><br />Conventional wisdom says, since I'm at the upper range of RPM, I should try the next higher pitch. But this is not so easy to do, "10R19" is not an easy prop to find!<br /><br />I don't know what the rule of thumb is about jumping up in diameter, and where then to go with pitch. Thoughts?<br /><br />My boat: http://www.geocities.com/olyerickson/Boat

 

[LubeDude]

Re: Prop fine-tuning 1990 Merc 60hp

My suggestion would be to take your prop to a prop shop and have the cup increased, you could then probably raise your engine slightly.

 

[olyerickson]

Re: Prop fine-tuning 1990 Merc 60hp

The prop shop suggestion is one I've thought of. Is it an option for alum props? And what about painted props, such as the one I have from Michigan Wheel (a "Michigan Match")<br /><br />I'm assuming that the effect of increasing diameter is to increase thrust for the same RPM. So I guess one option is to up the diameter with same pitch, then back off the pitch if the RPMs go too far down.<br /><br />I have an older alum prop, larger diameter and unknown pitch, but heavily oxidized. It was on the motor when I got it, I replaced with a prop that was "known good" for the boat and the motor. Maybe I should give it a try...

 

[olyerickson]

Re: Prop fine-tuning 1990 Merc 60hp

Note that I was mistaken, if I wanted to "step up" from my 10R17 to a 10R19, it is possible with a Mercury prop (48-73146A40)<br /><br /> http://shorl.com/dabrakatraprofro (mercury parts express)<br /><br />The "old prop" that I have is a 48-85632A40, apparently a Black Max 10-7/8 X 11 RH Alum 3 BL <br /><br />This does NOT seem like a good prop for my application, and I'm glad I replaced it!

 

[olyerickson]

Re: Prop fine-tuning 1990 Merc 60hp

Sorry, this is a much better prop reference than my previous (mercury parts express doesn't have a simple "catalog" mode, only that darn shopping cart...):<br /><br /> http://shorl.com/dyhudroprasulu (outdrives.com)