1959 johnson 35hp

[coolguy147]

what is the gear ratio for a 35hp johnson? found this neat thing on fiberglassics

Hector Gutierrez wrote on Wed Aug 1, 2001 3:39:19 PST in response to a posting on props:

Here is a handy little formula to calculate your theoretical speed for a given RPM. Us Mechanical Engineers have nothing better to do than to come up with this stuff. How close is your actual speed to the calculated? For my boat it was off by 1 MPH!

(Prop Pitch x RPM)
------------------- = Theoretical Speed in MPH
(Gear Ratio x 1056)

Example:

Prop size = 15" x 17"
Gear Ratio 1.68
WOT = 4,000 RPM


(17 x 4,000)
------------- = 38.33 MPH
(1.68 x 1056)

My actual was about 37 MPH

 

[coolguy147]

Re: 1959 johnson 35hp

^

 

[samo_ott]

Re: 1959 johnson 35hp

The ratio looks like 12:21. How'd you get 1.68?

 

[coolguy147]

Re: 1959 johnson 35hp

no that just the example i found online

 

[coolguy147]

Re: 1959 johnson 35hp

that cant be right i get 4.5 mph

 

[Sea18Horse]

Re: 1959 johnson 35hp

I think a gear ratio of 12:21 expressed another way is 1.75/1. So using 1.75 and guessing 4,500 rpm with a prop pitch of 13 1/4 I get 32.26mph. Does that sound reasonable?

Do you have a tach on that motor yet?

Using that formula on my boat with my 18 just for giggles and assuming 4,500rpm I get 26.78mph. I don't have a tach for mine so I have no idea what it's turning but on the gps I can get 19mph. I think the formula assumes a fairly standard planing hull. Mine is a wide transom drift boat with a fin for use with an outboard. I don't feel too bad. I think 19mph is pretty darn good for a drift boat.

.....................Todd