90hp Johnson Speed/RPMs

[Clint37]

If I want to have better top end speed, should I increase or decrease prop pitch? What will this do to my RPMs? My 2000 Johnson 90hp V4 is turning 5300 rpms at WOT with a lightly loaded 17 footer.(40mph) This is with the standard aluminum 14X19 prop. It has good hole shot. The transom ID plate doesn't show me the max RPM's but I think its 5500. I can't raise the motor on the transom because the prop already ventilates in turns when the motor is trimmed up. Admiral UU provided some information on prop design a few weeks ago and I've decide that a new prop would solve many of my problems but I need more info to narrow the choices before I start trial and error with props. Thank you for all of your help!

 

[Hooty]

Re: 90hp Johnson Speed/RPMs

If it was mine, I'd turn the engine about 5800rpm so you need to go down in pitch. One inch will change the engine speed approx. 200rpm. If you went down to a 17" pitch, you're engine would turn in the neighborhood of 5700rpm. Changing to a stainless prop would probably pick another hundred or so rpm. <br /><br />c/6<br />Hooty

 

[Dhadley]

Re: 90hp Johnson Speed/RPMs

Going to a ss prop will certianly help. The built in technology will allow a higher X dimension which in itself will increase efficiency. Choose a good ss prop that has an extended cup length (not to be confused with cup angle) like Stiletto. <br /><br />I agree that 5800 (average load) is the target. What you need is to hook up with a good performance center and test several props and set ups. A good case can be made for either the 17" or the 19" pitch. If you are going for strictly top speed -- I would lean toward the 19" and really fine tune the set up. Perhaps adding a jackplate (with set back) and a good cav plate fin in the process. <br /><br />If overall performance is what you desire and dont plan on a jackplate, perhaps a 17" would be better. Once you do the set up and then if you need more bite, its easily added by extending the cup length. Again a good performance center can be helpful unless you want to do the set up yourself.<br /><br />Be aware that not all ss props are created equal. Decide ahead of time if you are spending money for performance or you are going to buy strictly based on price.<br /><br />Good luck!

 

[Clint37]

Re: 90hp Johnson Speed/RPMs

Thanks very much to both of you. I'll call the local marina to see if they will allow me to test a few prop setups. <br />Dhadley, I've read several of your post about the X-dimension and the jackplate really does make sense but I doubt that I can raise the engine any more because the prop already ventilates so easily. I'll let you know what happens. Thank you again!

 

[Dhadley]

Re: 90hp Johnson Speed/RPMs

Your current prop cannot be raised higher. That doesnt mean a different one cannot be raised.

 

[TRITONRULES]

Re: 90hp Johnson Speed/RPMs

hey clint37<br />I am thinking of buying a 04 johnson 90hp V4, how do you like yours?? would you recomend it? Had any problems?I am trying to decide between the johnson or the yamaha. I was wondering what kind of a 17 footer you are pushing. I am pushing a 18 foot Triton aluminum, and would HOPE I can get more than 40 at WOT. Do you know what youre boat weighs? Thanks for the help. Bill

 

[Clint37]

Re: 90hp Johnson Speed/RPMs

Tritonrules,<br />In July I traded 1992 Mercury 115 with a broken crank shaft (and some cash) for my current 2000 90hp Johnson V4. The boat is a 1992 17ft fiberglass Astro Fish/ski (17FSX). I don't know how much the boat weighs but the maximum weight capacity is 1250lbs if that helps. The Merc 115 would push it 51mph with a new SS prop. The 90hp pushes it 39-40mph with a slightly worn aluminum prop. I plan on taking Dhadley 's advice to try to gain better top speed. As far as reliabilty, I am very pleased so far with the Johnson. With good plugs/gas/oil/battery it fires right up and runs smooth and strong though it is kinda loud at WOT in my opinion. Slow it down from WOT to about 80% throttle and its very quiet. Much quieter than the Mercury at that speed.<br />As far as my preference compared to Yamaha....I know a guy who has a Yamaha 150 on a 18.5ft Cajun and he is very happy with it (60+mph)though I have no first hand knowledge of Yamaha outboards' performance or reliability. Being Japanese they may be more expensive to repair but I don't know for sure. I've only had this motor for a couple of months but so far, I've been pleased with it and would recommend it.