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Thread: Yamaha 25 4-stroke...

  1. #1

    Default Yamaha 25 4-stroke...

    I saw the other thread talking about prop pitch, RPM's, etc. and it got me thinking about our own rig. If you're running a small outboard (no gauges, etc.), how can you possibly tell if you're in the right RPM range and getting the most out of the outboard?
    We have a 16' jonboat for the Kenai w/25 Yamaha, prop. Does anyone know what the standard pitch/diam prop is? We run the boat pretty loaded; 3 guys over 200 lbs, food, cooler, drinks, fishing gear and usually 6-10 gals of gas for long days on the river.
    Thanks!
    Jim

  2. #2
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    You can't tell without a tach.
    Tennessee

  3. #3

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    Is there a way to hook up a tach temporarily? How do the dealers rig small boats without knowing the right prop, etc.?
    Jim

  4. #4

    Default tiny tach

    I have two 25's, one jet one prop. These little things work good. Alot of folks sell them, I ran quick search and Cabellas came up. If you run small power plants like us every little bit helps.

    http://www.cabelas.com/prod-1/0005943520319a.shtml

  5. #5
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    Those Tiny Tachs are supposed to work pretty slick.

    The easiest way is just to listen with your ear. Certainly not exact like a tach would be, but it can get you in the ball park if you pay attention.

    For example I have a two stroke Yamaha 25 with a 9 7/8" X 14" prop and a 9 7/8" X 12" prop. If it's just me and a buddy in the boat, the 14" prop gets us up on plane quickly and I can feel a bit of acceleration all the way to about 7/8's open throttle. Now if you toss in another person or two, it's takes a fair amount more time to get on plane and you just don't hear the engine reving up to the rpm it should be...it sounds a little bogged down even at full throttle.

    Now toss on the 12" pitch prop. With me and one passenger it jumps right up on plane and has great acceleration, but from 3/4 throttle and up there isn't any noticeable gain in speed even though you hear the rpms increasing...obviously in this situation I need more prop pitch as I can over rev the motor. With three or more folks in the boat the 12" prop is perfect, gets me up on plane quickly, I have all the speed I could want and usually cruise about 3/4 throttle.

    Not sure if that explanation helps at all, but maybe it'll give you some things to look out/listen for. It's pretty easy to tell when you're at one extreme or the other.

  6. #6
    Member garnede's Avatar
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    I have a 5hp sea king on my scanoe. When the engine is at full throtle the engine still sounds bogged down. If I could get another 100-200 RPM's it would be in the "comfort zone". I have no real experience with boat motors, so how do I get the RPM's up?

    Quote Originally Posted by akfishinguy View Post
    Those Tiny Tachs are supposed to work pretty slick.

    The easiest way is just to listen with your ear. Certainly not exact like a tach would be, but it can get you in the ball park if you pay attention.

    For example I have a two stroke Yamaha 25 with a 9 7/8" X 14" prop and a 9 7/8" X 12" prop. If it's just me and a buddy in the boat, the 14" prop gets us up on plane quickly and I can feel a bit of acceleration all the way to about 7/8's open throttle. Now if you toss in another person or two, it's takes a fair amount more time to get on plane and you just don't hear the engine reving up to the rpm it should be...it sounds a little bogged down even at full throttle.

    Now toss on the 12" pitch prop. With me and one passenger it jumps right up on plane and has great acceleration, but from 3/4 throttle and up there isn't any noticeable gain in speed even though you hear the rpms increasing...obviously in this situation I need more prop pitch as I can over rev the motor. With three or more folks in the boat the 12" prop is perfect, gets me up on plane quickly, I have all the speed I could want and usually cruise about 3/4 throttle.

    Not sure if that explanation helps at all, but maybe it'll give you some things to look out/listen for. It's pretty easy to tell when you're at one extreme or the other.
    It ain't about the # of pounds of meat we bring back, nor about how much we spent to go do it. Its about seeing what no one else sees.

    http://wouldieatitagainfoodblog.blogspot.com/

  7. #7
    Member SusitnaAk's Avatar
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    Garnede Sea King Wow haven,t heard that one in long time! All Different Companys made engines for them, Simply put the heaver the load the less pitch prop you want, More speed higher No# pitch, Prop. All things being in the same diameter, Then there,s 4blades you could throw into the mix,Taller blades, shorter blades,Weedless, goes on and on. some for lake boats, pulling skiers, pantoons ,sailboat ect... Find the No# on your Prop, One is size blades the other is pitch for you drop down about -two from there so if it is say 7x10 try 7x8 so forth, hope this helps! all in a nut shell, finding one for your motor well? Good prop shop could re pitch your old one to as option. Not alot you can do with 5hp. Also as stated above be able to pull the weight but you will lose some of the top end speed.

  8. #8

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    Thanks for the tips guys. I'll be getting one of those Tiny Tachs and play around with the prop a little (I've got two).
    Let you know how things work out (not there until July).
    Jim

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