[Bushwacker]

Fr. Frank said it well! About all I'd add is to consider going up in diameter as well as down in pitch. The 13 3/4" dia. x 15" pitch 3B SST was the optimum prop for my boat with the 115 on it. A 3B will probably be fine with a light motor, but I never tried a 4B on my 115. After I added bracket w/30" setback and repowered with a much heavier motor however, it was obvious I needed a 4B! With those changes, which probably shifted the CG aft by about a foot, judging by how much I had to shift axle on trailer to get adequate tongue weight, the 4B prop made a huge difference!

I just ordered a 4B PowerTech stainless prop for my rig; I had my 14 x 20" pitch Michigan Vortex 4B aluminum prop repitched to 18" for use as power prop for heavy load. Max load rpm was perfect at 5500-5600, but it cost me about 1 mpg compared to my 4B SS 14.125" dia x 20" pitch Michigan Apollo. I'm keeping the Apollo for light load, but I wanted a SS 4B for heavy loads to get some of the mpg back. I filled out their extensive worksheet and they recommended a SS 4B 15" dia x 17" pitch. I like the idea of larger diameter and less pitch as it should have less slippage and be a little more efficient. Spoke to Marcus at PowerTech, who really seems to know his stuff. He said any 4B prop will have more stern lift than a 3B prop, and that may be why my Apollo works so well, even though it seems to have some amount of rake to it and is listed as a bow lifting prop on their web site. The prop he recommended has similar blade geometry to OMC's old SST except for the extra blade, which he says has turned out to be an excellent prop; it certainly was for my 115! He said it will have more stern lift than the Apollo, although he said it will probably not match it on mpg. I also tried the Merc website, to see what they recommended as far as engine revs etc. For max economy they recommended a prop that would turn about 5200 rpm, which is about what the Apollo does with light load, but for heavy load they recommended a 5500 rpm prop.

The prop I'm getting has the same plastic hub system as the Michigan, similar to the new Merc props, so I can use existing splined adapter. Marcus said those plastic hubs will break pretty easily if you hit something, and they have a new patented hub design that cures that problem, but it's 2X the price! In addition to keeping the Apollo as a spare, I'll also carry a spare plastic insert, as they're only about $15. A friend of mine who runs a Merc with that style hub says he's heard that if you do break one, you can just rev the motor to 3000 rpm for a minute or so to get the hub real hot, then just shut it down for a minute or so and it will "weld" itself back together well enough that you can probably make it back home on a plane if you're real gentle with the throttle and just run at min planning speed!

Go to PowerTech's web site, fill out their VERY comprehensive worksheet and see what they come back with. I'll bet it's 15" pitch and a little more diameter than you've been running! Denny