Prop for Johnson 70 hp

[Gerald Barnes]

I have read all the great FAQs and recent prop questions on the forum. Sure helps educate someone who never bought a prop.<br /><br />Here's what I've got:<br />1999 Crestliner 17'8" flat bottom 1030 lbs.<br />1999 J70PLEE<br />Prop 14X19<br />WOT 5600 rpm<br />Cruises about 35 mph<br /><br />Boat performance is adequate for what I want to do. No real complaints since I had carbs rebuilt and decarbed engine.<br /><br />Question: I keep seeing, "Prop for the top", everywhere on the forum. Engine manual "Specifications" gives range of 5000 to 6000 rpm. Next line says "Power 70 hp at 5500 rpm." Is Johnson recommending 5500? Local boat mechanic says 5600 rpm will not hurt the engine. He says just stay with a 14X19 if I am happy with performance. <br /><br />Question: Since I am buying a spare prop anyway, would it be best to go to a 14X17, as recommended in manual, and try to get rpm up to 6000, or should I just get another 14X19 and stay with 5600 rpm? If 2" reduction in pitch is an increase of 400 rpm that should put it at about 6000. What do you recommend?

 

[Dhadley]

Re: Prop for Johnson 70 hp

Rather than reducing pitch (reduce efficiency / lose bite) and getting rpms that way we prefer to try and get more with the set up (increase efficiency / lose drag). Is your motor mounted all the way down now? Or up 1 hole? Have you ever tried raising it and see what happens?

 

[Gerald Barnes]

Re: Prop for Johnson 70 hp

Dhadley, thanks for the quick reply!<br />I'm not sure what you mean about motor being mounted all the way down, or 1 hole up. Are you talking about the trim limiting rod? There are two rows of 3 holes. It is in top row, hole nearest transom. If we are talking about the motor itself, it is almost resting on top of transom. About a 10 thousandths clearance on right side of motor and about 100 thousandths clearance on left side. Top motor/transom bolts are in second hole down from top on motor.

 

[Dhadley]

Re: Prop for Johnson 70 hp

Hummm..... if the motor is (almost) down on the top of the transom and the bolts are not in the top holes, someone has drilled the transom incorrectly. The motor being crooked isnt good but its not off a lot.<br /><br />Look at the bottom bolts. If we can raise the motor without drilling new holes I'd try that first.<br /><br />Your rpms are decent but could be better.

 

[Gerald Barnes]

Re: Prop for Johnson 70 hp

Dhadley, how much do you think moving the motor up 1" will increase rpm? Is there a rule of thumb or does that change from boat to boat?