Aluminum props typically have no cupping, and the blades reportedly flex some under load and are thicker. Props designed for higher speed boats typically have the most cupping. The more a prop is cupped, the more it will reduce rpm at a given pitch. Stainless props often have some degree of cupping at the prop edge to decrease slippage, which improves efficiency.

Aluminum vs stainless also makes a difference. The same pitch and diameter in a particular brand will not turn the same rpm as that of another brand in the same size, due to difference in design, surface area of blades etc. Trying to find the best prop for your application can be a pita and expensive. The cheapest source I found for it was Dan's Discount Props on the internet at $224.95. The solas prop appears to be very good quality. You might pick up a little better top speed and rpm, but I think that cavitation would become excessive. I don't, think that a higher mount would have a good result.

It appears that it would be in hole #2 from top to bottom. With regards to mounting height of engine, I cannot see the holes, as they are full of caulk. My brother also has an R180 with the 115 yamaha, and using this prop with good result. Therefore, I have concluded that the 15p is the best for me. This extra weight has dropped my wot rpm to about 6200. I have since added a second battery, and often run with the live well full, since I mostly fish offshore. With the same load as above, I was able to turn only 5700 rpm, with no gain in top speed, which was a disappointment. I thought that I might to be able to turn a 17 pitch solas and gain some top speed, so I purchased a 17p solas stainless, model #3431-133-17, which is 13 1/4 x 17.

The rpm range for this engine is 5300-6300. This achieved 38-39 mph per gps and speedometer. When I lake tested the boat after break-in, the 15p solas turned 6300 rpm with 2 adults and full of fuel. The exchange occurred prior to taking delivery of the boat, so I did not run it with the 14p prop. This was recommended over the yamaha stainless, which is 2x more in cost. I don't recall what the diameter was, as I exchanged it for a Solas stainless 13 1/2 x 15 pitch, model# 3431-135-15, new saturn model, that the dealer carried. Mine came factory equipped with the 14 pitch aluminum prop.

Yamaha prop calculator