Propeller size

[flyingfrizzle]

Originally Posted by rich10

Hello, I have a 20' sea craft c.c. that I'm looking to repower with a150h.p. Four stroke engine can any one tell me what size and pitch propeller to use to get 40-45m.p.h.

The main thing you need to do is get your RPM's in the right range. You mainly prop for rpm, stability and running conditions. A heavy motor like that you will want something cupped to lift the back of the boat to counter the weight (stern lifting prop). Far as MPH you just get what you get once you get the rest of the boat happy and your rpms in the right range. You can look for features to give you more mph but its a give and take. You may gain mph but loose ride quality or low end. I like to go fast and run larger hp motors than necessary sometimes but its a give and take when trying different props. I can run faster in the calm with one prop but it may not plane out well at low speeds on a choppy snotty day making the ride bad. It all depends on many factors like weight in the boat, balance of the weight, how loaded you run the boat during different trips, conditions of the water where you run the boat, what you are willing to live with or looking for in ride quality and even things like bottom paint creating drag can vary prop selection. If you are just trying to get more speed out of your hull and willing to give on the other areas you can get a smaller diameter prop with thinner blades. You can get the lower unit up higher and out of the water to loose drag. You can try semi surfacing props but make sure what ever you do you keep it in the right rpm range. You can deal with bad handling and ride to gain a few mph but your motor will not deal with being lugged with a over pitched prop dragging it down. Most look for the best ride and over all performance threw out the full range while keeping the motor in the right rpm range. If you get all that right you should have a fair mph with it but at the same time you can do things to gain 4-5 mph and have a boat that rides like crap. There are so many different props out there and each will change the ride of the boat. Even two of the same type props like mercury mirage props. The newer model year A45 is a heavy thicker bladed where the older A40s are lighter and have thinner blades. I have seen a 3 mph difference between the two even tho they are the same prop size and pitch. Over the years the newer ones have gotten heaver and the slightest design changes effect the boat differently.