prop selection for twin outboards

[galvic]

I have a prolem with propping a 15 footer. The boat is some 1200 pounds with twin mercury 500's on a stern bracket. If propeller selector says that for 16 foot, and some 1600-2000 pounds of gross weight single mercury 500 would require propeller 12" x 11", does that rule of thumb changes with twinson the stern, and should I use higher pitch, or a bigger diameter propellers. By the way, since one of these 500's is on a repair, i am using the boat with one, and with 11 1/2" x 13" propeller, and it, for lack of better words 2sucks". can't get it in a plane with more than 3 people onboard, and the rpm (by the sound of it) are lower that usualy (with 12" x 11" propeller). Thank You in advance, Niksa.

 

[Silvertip]

Re: prop selection for twin outboards

Without a tachometer and accurate speedometer prop selection will be a guess at best. There is no "standard" for selecting props. The best prop is one that allows the engine to operate right at its maximum wide open throttle RPM specified by the manufacturer. With one engine, a given prop will likely make the engine struggle. With both engines, that same prop may not be the best choice. You need to get both engines installed and properly tuned. Then make some wide open throttle runs and note maximum RPM and speed. If the engines run faster than the recommended RPM you need to increase pitch. If they are slower than recommended, you decrease pitch. Prop diameter has less to do with performance than pitch. Diameter is limited by clearance between nearby components. Engine RPMs change about 150 - 200 RPM for each inch of pitch change. So yes, the 13 pitch prop would cause the engine to run about 400 RPM slower than the 11 pitch and also struggle to get on plane. However, with 13 pitch props on both engines, that may be the best combination.