New Yamaha 70hp correct prop size

[tlcausey]

Hello, I have a 1964 Glastron 17', had a new Yamaha f70 installed. Trying to figure out the best prop size. Dealer installed a 17" prop to try. WOT I get 5400 rpm's, they suggest a range 5300 - 6300. I know I'm in that range but barely , wondering if should go to a 15" to get more in the middle or top of the rpm range. Tested it without a lot of extra weight, trimmed out in calm waters. Just trying to get feedback.
Thanks

 

[ahicks]

I think a lot of the "correct for you" prop is going to depend on your goals for the boat. The prop you have may be fine for a boat/owner that doesn't do a lot of pulling, where the emphasis is slanted more towards economical cruising.

However, if looking for max wide open performance (damn the gas mileage!), and pulling power, you would be looking for a prop that turns right at the max rpm rating for the engine with a lightly loaded boat - where the 15" prop might be closer.

 

[Sea Rider]

Hoew loaded do you like go boating. Lightly, medium or heavily loaded, with a fixed or variable load. Whichever is the answer prop it right for OB to rev towards max 6.3 K wot range as currently loaded. That way combo will have an oustanding hole shot, nice fuel consumption at cruising speed. Once there throttle to whatever rpm you like boating most of the time.

Before take off, set trim one position downward from neutral, go full throttle, once on plane trim OB to neutral for combo to ride parallel to water level, for that to happen need to have deck load well distributed. Check if combo planes faster.

If OB sits at the sweet transom height spot, combo should plane as a breeze when going full hammer down provided that OB is in excellent running shape.

Happy Boating