Correct RPMs

[Rudy Brown]

What are the correct rpm range for my motor? I've heard everything from 4500-6000. Its a 84 16' fibreglass bass boat w/ 90hp rude. Right now it has a oem 13x19 SS prop and gets 5100 rpms @ 47 mph(gps checked). Just tryin to set it up properly. Thanks for the help, Rudy.

 

[mikeyzx2]

Re: Correct RPMs

Well I don't know the exacts, but it should be in the neighborhood of 5500-5800, my book for '77 85hp is ideally around 5800, and like you say, you've heard it all. I've been told 5800-6000 for mine.

 

[Rudy Brown]

Re: Correct RPMs

Thanks for the responce. I think my tach has given up the ghost. When I punch the numbers in a prop calc, I get less than 1% slip. I know I aint that effecent! I'm gonna get a new tach from BRP before I change props. I know when I drilled vent holes in the prop I was amazed at my holeshot. I think I have the motor at the right height. At 90% of the trim level and WOT the boat runs fine but the prop vents in turns and I have to trim back to about 40%. Also seems like I gain about 300rpms from no trim to full. I'll have to play with it some more this season.

 

[emdsapmgr]

Re: Correct RPMs

The engine was orig rated to run between 4500 and 5500, with the rated hp made at 5000. You will find the engine HP falls off significantly over 5500. Ideally, you want to get from 5100 rpm up to the 5500-5600 range at top end. If the prop ventilates in turns, the engine is either mounted too high for the hull, or the prop is not cupped enough. Try a raker stainless-they are cupped more than the SS props. When you gain 300 rpm's does the speed go up, or just rpm? Could be cavitating.

 

[Silvertip]

Re: Correct RPMs

The tach may be perfectly ok but the speedometer is very likely very inaccurate. It's the most inaccurate gauge on the boat. Use a GPS to check speed before investing in a new tach.

 

[Rudy Brown]

Re: Correct RPMs

Originally posted by Upinsmoke:<br /> The tach may be perfectly ok but the speedometer is very likely very inaccurate. It's the most inaccurate gauge on the boat. Use a GPS to check speed before investing in a new tach.

I took the boat out yesterday. The tach is shot. Its sticky. You have to knock on it to get it to move. The only time the prop ventalates is when its trimmed up in turns, if its not trimmed you could turn very sharp and not loose bite. I had the transom rebuilt last year and the motor is where the rebuilder placed it. I do not know if its set lower or higher than before. I believe its higher. I know it runs 40.9 mph with Me, wife and 2 kids.(tach said 5200) and 47/48 with just me and 1 kid.(tach said 5200 again). Both speeds vere verified with my Garmin Legend C.

 

[Dhadley]

Re: Correct RPMs

By any chance is the tach a Teleflex?

 

[Rudy Brown]

Re: Correct RPMs

Originally posted by Dhadley:<br /> By any chance is the tach a Teleflex?

Ha Ha, nope its a Faria. And yes I'm gonna get an OMC or BRP next. Its sticky, you have to bang on it for it to settle down and give you a reading. Thanks for everybodys help. I have learned so much from this forum.

 

[Rudy Brown]

Re: Correct RPMs

Originally posted by emdsapmgr:<br /> The engine was orig rated to run between 4500 and 5500, with the rated hp made at 5000. You will find the engine HP falls off significantly over 5500. Ideally, you want to get from 5100 rpm up to the 5500-5600 range at top end. If the prop ventilates in turns, the engine is either mounted too high for the hull, or the prop is not cupped enough. Try a raker stainless-they are cupped more than the SS props. When you gain 300 rpm's does the speed go up, or just rpm? Could be cavitating.

On the rpm rise, the boat does get faster. Cant remember how much, but it does. I'm gonna try a new prop this summer, but I think the first thing is get a good tach. I really dont want to over rev this motor. Of course everyones looking for speed, but what I really want is a great holeshot, the motor in the right rpm range and if possible getting close to 50mph would be nice. Its not a race boat, I just fish out of it. I just want the best of all the worlds.

 

[Dhadley]

Re: Correct RPMs

Its not that we're looking for speed. We want the motor to be capable of 5800 rpm so it will live a long and stress free life. As you reduce the slip % your holeshot will indeed become better. So will your cruise rpm and your top speed. And the mpg. Its all just in the numbers.<br /><br />With a 2 stroke there really isnt such a thing as over rev. As long as theres a load on the prop, youll be fine.

 

[emdsapmgr]

Re: Correct RPMs

On the 300 rpm rise question, if the speed goes up, then the prop is not cavitating. Sounds like you have the engine mounted high on the transon. It is probably OK the way it is. Your setup should result in max speed. High as it is, you might want to add a water pressure guage to be sure you are maintaining good pressure when in tight turns.

 

[walleyehed]

Re: Correct RPMs

If it can be trimmed to 90% of max trim with the prop that's on it, it's nowhere near high enough to worry about water pressure yet.