ORIGINAL: gplarry
The OS advertisement for the 120AX says 3.1 hp at 9,000 rpm but it recommends a 15X6 prop.
3.1 hp at 9,000 rpms have to be more like an APC 17X6 prop I expect.
My saito 100 can turn an APC 15X6 well over 9,000 rpms.
I wonder what gives with that.
Larry
What gives is that
1) you are confusing the HP rating and a recommened prop. They have absolutely no correlation. HP rating is always published along with a relavent rpm, since they are directly related. And, as documented by many, published HP is nearly a meaningless number since all engine manufacturers do not use the same method or standards to derive that number. At best, its a reference.
2) the mfg documentation on several OS engines is sorta goofy based on real-world experience and operation.
3) They are trying to illustrate that this is a low-rpm/big prop/torque engine, not intended for high rpm.
The 15x6 recommended prop will turn around 10,500 rpm on the engine. The engine runs well with that. The 16x6 16x8 are good choices too, and some are running 17x6 props as well. Any prop load resulting in the engine turning between 9000 and 11,000 rpm is fair game for the engine. You select the prop based on your application and what the engine is actually capable of.