How do i know i have the right pitch?

[drewmitch44]

I am using a 18' sea ray 1972 srv 180. I have a yamaha 115 eltj on the back. I thought that everything was ok but im thinking i might be able to get more preformance out of it. I took it out this afternoon and it acellerates so fast that i get to max rpm too quickly. I get to 5600 and it seems like there is still more to give her. I know that its not slipping because im going 37-42 mph (depending on waves) and the boat and motor still preforms at that speed. It neaver jumps up by a good bit or feels like a clutch slipping on a car. By preforming i mean that i can raise and lower the throttle and it responds well and verry quickly. I was happy with its preformance but i read a post that talks about if the prob is too little then you could be under propping your rig. I bought the motor from a guy that used to use it on a 16' speed/ski boat. It was a smaller boat but i thought that if the prop was wrong that it would not get my bigger boat up to WOT rpm. But i think that im getting there too easily. Does what im saying make any sence or should i just get over it? I bet i could over spin the motor if i wanted too. Now its fast as hell as far as for me but when im taking off it seems like its too easy for the motor to shoot it out of the hole. I can go from standing still to going the full speed in a second. i dont know how to tell my pitch on the prop im using.

 

[drewmitch44]

Re: How do i know i have the right pitch?

And i am told that with a better sized prop for the boat my crusing speed will save me some gas mileage. I get up on plane at around 2800 to 3100 rpm and am going about 22 mph. at about 3500 rpm is where i usually cruise and im generaly going about 26-29 mph. Does that seem correct. Thats about where i seem to get the best gas mileage also. Im just wondering if im running as eficent as possible? Thanks a lot guys!

 

[Cricket Too]

Re: How do i know i have the right pitch?

You said you can get to 5600 and feel like you can still go more, but have you pushed it wid eopen? What is the WOT RPM?

 

[Silvertip]

Re: How do i know i have the right pitch?

You have the optimum prop pitch when the engine runs at the top of the manufacturers recommended wide open throttle rpm band. Usually that rpm range is 5500 -6000 RPM. For test purposes, you need to put an average load in the boat (people and gear). Then run the boat wide open and note the engine RPM. It's OK to overrev the engine for a short burst f for this test but until you do that, you can't determine what prop is needed. Lets say the engine runs at 6000 RPM but the manufacturer recommends 5500 as the upper limit. For each inch of prop pitch up or down, RPM will change about 200 RPM. So to bring the RPM down 500 RPM you would increase pitch by two inches (say 17P to 19P). That would lower RPM by 400 so the theoretical WOT rpm would then be 5600 RPM. 100 RPM over the upper limit is OK.