1977 johnson 140 prop question

[mike3469]

Hi guys i have a 1980 thundercraft 16' boat with a 1977 johnson 140hp outboard. the propeller on the engine was damaged and i never got a chance to try it out. it was 13x19. i just got a new prop today it is 12-1/4x21 and i was wondering if this was too high of a pitch for my boat?is there a recommneded prop for this engine/boat? i plan on pulling a tube. Thanks

 

[crb478]

Re: 1977 johnson 140 prop question

You may be able to use the 21 pitch, but you should be looking for 5600-5800 rpm and match the prop to get the rpm's. If you are pulling a tube or skiing the 19 may be a better fit as it will ginve you a better hole shot

 

[ezeke]

Re: 1977 johnson 140 prop question

As CRB478 indicated, if you cannot get in the range of 5,800 RPM at WOT with your average load, you will lug your motor and shorten its life.

 

[mike3469]

Re: 1977 johnson 140 prop question

only problem is that i don't have a tach, guy at the marine shop that sold me the prop told me to try it with my regular load and if i don't have a problem getting the boat to plane then its fine.

 

[rndn]

Re: 1977 johnson 140 prop question

only problem is that i don't have a tach, guy at the marine shop that sold me the prop told me to try it with my regular load and if i don't have a problem getting the boat to plane then its fine.

If I were you I'd find a different marina. The advice is so wrong. Many props will get the boat on plane and be able to over rev or bog the motor. Over revving is the same as taking off in first gear in a car and leaving it there while trying to do highway speeds. Bogging the motor is the same as trying to take off in 4th gear with a standard transmission in a car. You should always try to get a prop that puts the WOT within the manufacturers specifications. Anything else will only shorten your motor's life. Listen to the above advice and get a prop that puts it in the correct range.
You can do a search for a prop calculator and most will allow you to enter some information about your boat to get you close to the right size.