Re: Need Prop 101 for 87 3.0L OMC Cobra
Prop diameter has little effect on performance. Your outdrive is designed to work with props of a specific diameter. Too big and the blades will hit things. Pitch and prop design (shape of the blades), cupped or not (a lip on the edge of the prop), and other design characteristics determine how a prop performs on a given boat. Two identical props may perform very differently on different boats. Props are identified for example as 14-1/4 x 21. 14-1/4 being the diameter and 21 being the pitch. Pitch refers to the theoretical distance the boat would move with one revolution of the prop (21 inches in this case). Since the prop operates in a liquid there is some slippage so if the boat did indeed move 21 inches the prop would be 100% efficient which is simply not possible. When looking for a prop, you need to look at the catalogs carefully and make sure you pick one for your specific drive. In your case its a 3.0 Cobra. Before you buy, determine what diameter and pitch you have now. Numbers are usually provided on the prop hub. Make some wide open throttle runs with normal load noting speed and maximum RPM. If RPM is at the low end of the engine manufacturers recommended RPM range, you'll want a prop with slightly less pitch. If the engine is reving higher than recommended you want a prop with more pitch. If you at the top end of the range you are propped correctly. Do a search on "Prop Theory". You will have reading material to last you the rest of your life.
