prop

[rc.chris]

I am thinking of putting an older YS 91 4 stroke engine on an air boat, but the cage will only allow an eleven inch prop to swing. Does anyone have any suggestions on prop pitch for an 11" or maybe a 3 bladed prop?

[Mr Rossi]

Easy, more pitch equals more speed less pull (5th gear in a car) less pitch equals less speed more push (1st gear) try an 11x6 as a start point. three blade props are hard to come by (in the UK anyway) and are less efficient.

[invertmast]

Originally Posted by Mr Rossi

Easy, more pitch equals more speed less pull (5th gear in a car) less pitch equals less speed more push (1st gear) try an 11x6 as a start point. three blade props are hard to come by (in the UK anyway) and are less efficient.

Wow... thats somewhat right, but the prop sizing is WAAYYY off. an 11x6 is 1" smaller in diameter than what a 60 size 2 stroke would run (12x6 typically).

This being a 90 size, its going to need a larger prop or a smaller prop with ALOT of pitch. And the fact its a 4stroke means it can run an even larger prop than a 90 2 stroke.

Meaning you'd need between a 13x6 to 15x8, since this is a 4-stroke you should really use the larger 15x8 prop. So if you have to have a 11" prop, that means you should be runnin an 11x12. which APC make an11x7,11x8 11x9,11x10,11x11 and 11x12

Now that u have a recommended prop, its best to purchase one and see where it falls in your RPM range recommendation for your particular engine. I believe my YS 1.70DZ which spins a 15.5x12.5 4-blade prop spins this at a max 7800 rpm for comparison.