Speed of a 20 foot Pontoon boat with 70 horsepower Johnson 1983 model year?

Big Bubba · #1 May 29th, 2008, 01:31 PM

Hello everyone,
How are you guys doing today? I am fine but I have to work today and the weather is nice out there compared to last weekend. Anyway, I was wondering how fast a 1991 Lowes Pontoon boat, 20 foot long with the 2 logs large sized under her that has a 1983 year model# J70ELCTD 70 horsepower Johnson on the back of the pontoon boat should go? I am able to go 6000 rpms at 16 miles per hour in choppy water with a 13.75 X 13 pitch aluminum 3 blade prop and can go up to 20 miles per hour in choppy water with the rpms at 6700 rpms. Let me know I am curious that is all.

tashasdaddy · #2 May 29th, 2008, 02:09 PM

at 6700 rpms that motor is not going to last very long. 6000 is max for that motor. it is not the speed that counts, it is being propped correctly. your wide open rpms should be around 5800. you need to prop correctly, or you will be replacing the motor.

ShellBack101 · #3 May 29th, 2008, 02:21 PM

Bub,

I have a 22' crest pontoon with a 50hp Johnson 1996 J50tleds, and I get about 18mph @ WOT turning 4800 RPM

flipjoe · #4 May 29th, 2008, 11:39 PM

seems a litttle slow, i have a 20' harris flote boat sunliner with a 2004 40hp johnson, getting 5500rpms making about 17mph.

Big Bubba · #5 May 30th, 2008, 12:07 AM

Thanks for the reply. What do you think I should do? I just bought a new Aluminum 13.75 X 13 pitch 3 blade and it is brand new but with how the weather was is that I did ding it up very oh so very litely and I don't think Cope Marine will take it back for an exchange. It seems like my motor is running right though but I guess my prop setup isn't quite there yet. I don't know and I need your guys help here. What do you suggest and does anyone out there have a prop that they would sale to me really cheap, if so PM me and let me know. Thanks again, Bob

Big Bubba · #6 May 30th, 2008, 12:36 AM

O.K. guys. I found a 13.75 X 15 aluminum genuine Johnson/Evinrude 3 balde prop on EBay see item number 180247083546 and tell me if this will work for me or not. Thanks again Bob

The-Machinist · #7 May 30th, 2008, 04:55 AM

Look for a local prop shop and tell them your problem. Most aluminum props can have the pitch increased a few inches. Probably cheaper than a new prop.

flipjoe · #8 May 30th, 2008, 05:41 PM

the key is to get wot rpms lower than 5800 increasing the pitch will do it, for you i would probably go a little bigger than 15

Effect of Prop Pitch on RPM
A pitch change can increase or decrease the RPM's and bring RPM's into the recommended range. A 2" increase in pitch (for example, from 21" to 23") typically results in a decrease of approximately 300-400 RPM.