1957 18hp Johnson prop selection

[BamaBuckshot]

Need a little info on which prop to get. The prop that is on the motor now is a 9X10. The blade tips are chipped pretty good and has alot of vibration when in use. I'm pretty sure it's out of balance.<br /><br />I have done the prop selector. It lists about five props but two are recommended. I called the i-boats hotline to help me select the right one. They were unable to help me due to it being an older motor. They gave me a number to Johnson, but they are closed due to the holidays.<br /><br />The motor will be on a 14' vee bottom boat. The two props they recommend is a 9X9 and a 9X10. What are the difference between the two?

 

[Paul Moir]

Re: 1957 18hp Johnson prop selection

The 9X9 is for heavier boats, where the 9X10 is for ligher ones. On the 9X10, 9 is the diameter in inches, and 10 is it's pitch in inches. If the propeller's stream did not slip at all (ideal and no drag) the 9x10 prop would move your boat 10" forward every time it turned. Of course, the 10" propeller is harder for the engine to turn than the 9".<br />When choosing a propeller, it's important to know what rpm your achiving with your current set-up. The most reasonable way to measure that is with a Tiny Tach. The RPM range for your engine is 4000-5000RPM at wide open throttle. If you're below that range, the 9" propeller may get you into it.<br />My guess though, is that unless your 14' boat is very heavy for it's length, the 9x10 is the prop you want. I hope others will be along with more informed comments.

 

[BamaBuckshot]

Re: 1957 18hp Johnson prop selection

Thanks Paul. Thats the info I was looking for.<br /><br />Even thought the boat is pretty light, should I get the 9X9 for the times I have a little heavier load?

 

[Paul Moir]

Re: 1957 18hp Johnson prop selection

I don't think so. First, you should prop for your average load, not your worst-case. Second, I think the 9x9 prop is aimed for really heavy loads like displacement hulls and such. The standard prop that came with the 18hp in '57 was 9x11 afterall.<br />If you went to the 9x9, you would take off faster like running in low gear. But you might loose top speed since you'll have to turn more RPMS to go the same distance in the same time. But if your into tuning the boat for speed and such, you should be beginning with experimenting with your boat setup and only switch props after you've got that the way you want it.

 

[BamaBuckshot]

Re: 1957 18hp Johnson prop selection

Paul,<br /><br />Once again thanks. You just made selecting a prop so much easier. <br />I'm not really concerned about speed right now. I'm just gonna use the boat and motor for hunting and fishing.