Max Rpm on 90 hp! 2 stroke Yamaha.

1stoutboard

Seaman Apprentice
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Aug 21, 2003
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38
04 model on a 17ft Glastron. I have been reading a lot about the max rmp between 4500-5500.My owner manual states that as well. I have 9 hours on the motor so far. Prop is 13 1/4X 17 pitch. I have not been able to get the Rpm past 5100 at 39MPH. Water was semi calm , not as bad as it usually is at this time of the year in sw florida, and all the rain we get. I raised the prop as much as i can , it was almost out of the water. I did not get it to vent. Would I be able to reach the 5500 rmp on a WOT? Is that what I am looking at ?Or maybe the engine is so new. Can the prop be a factor here? I did not ask the dealer yet. I want some of your opinions . Thank you.
 

roscoe

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Oct 30, 2002
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21,657
Re: Max Rpm on 90 hp! 2 stroke Yamaha.

You are well within the recommended rpm range, this is good.<br />What are you trying to achieve?<br />I see no reason to change anything.<br /><br />If you go to a 16"pitch prop, your rpm will be a little higher, but don't expect to see much increase in speed.
 

Dhadley

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Feb 4, 2001
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16,978
Re: Max Rpm on 90 hp! 2 stroke Yamaha.

Describe what you mean when you say you raised the prop as much as you can. Do you mean with the trim or did you raise the motor on the transom (increase the X dimension)?<br /><br />Good luck!
 

Forktail

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Feb 11, 2002
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Re: Max Rpm on 90 hp! 2 stroke Yamaha.

"Would I be able to reach the 5500 rmp on a WOT?"<br /><br />RPM range is always based on WOT, and this outboard makes its maximum power at 5500, not 5100. So open her up and then see where it tops out. <br /><br />If you're still only at 5100, then a 16 pitch would be perfect.
 

1stoutboard

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Aug 21, 2003
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Re: Max Rpm on 90 hp! 2 stroke Yamaha.

Here what we have gentlemen: I am not trying to achieve any higher speed Roscoe, if thats what you thought. I thought there could be something wrong with the fact that I did not see the rpm hit 5500 .I had the throttle all the way down, it would not go any further. I highly doubt it if the speed will go any higher on this motor. Forktail: I actually put her on the water today and opened up the throttle , I was able to get 5100 rpm and 40mph,(of course water conditions are never as smooth as you would want them to be) , which I think is the top speed for this little guy of an engine. I am not a speed lover , I just love to enjoy the ride of a new boat. What I meant by raising the prop was , I trimmed it up as much as I can. Sorry for not using the right lingo. I have 13 hrs total on this motor as of today.So if I put a 16 pitch, then the rpm would go highr , and consequently , the speed would be more. Am I right? Is that safe for that 90 hp . I don't wanna kill it by going over 40mph as top speed. <br /> Thank you for your valuable input guys.
 

Solittle

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Apr 28, 2002
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7,518
Re: Max Rpm on 90 hp! 2 stroke Yamaha.

I kinda agree with roscoe that the best thing for now is to do nothing. You are not going to hurt the motor at 5100 rpm or 40 mph.<br /><br />When you get 25 or so hours on it you might want to go to a 16" prop (if your selling dealer will swap now, do so). Speed is not the important thing. RPM is for both performance and durability of the motor. You want to achieve max recommended rpm at WOT. Anything less (ie. 5100) results in your motor lugging.
 

1stoutboard

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Aug 21, 2003
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Re: Max Rpm on 90 hp! 2 stroke Yamaha.

Solittle. What does lugging mean.?I don't wanna pretend that I know . I have seen the term fly around in this forum before. Does that mean the motor is not giving its best at WOT?
 

rickdb1boat

Supreme Mariner
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Jan 23, 2002
Messages
11,195
Re: Max Rpm on 90 hp! 2 stroke Yamaha.

Lugging is when your motor should be turning a higher RPM for given conditions. If you underprop your motor, it will always be working harder at ALL speeds. Kinda like trying to climb a steep hill in 4th gear of a manual transmission car. Best to prop for the highest RPM range allowed for your motor. There is a given range(Example:4500-5500 RPM's) that is acceptable, but toward the top end is always better.
 

roscoe

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Oct 30, 2002
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21,657
Re: Max Rpm on 90 hp! 2 stroke Yamaha.

Sorry I wasn't too clear in my question--what are you trying to accomplish?<br /><br />I should have said, Are you looking for more speed, better hole shot, of trying to set up to run at recommended rpm?<br /><br />Yes, you are right, a smaller pitch prop will give you higher rpm, but since the prop covers less distance thru the water(per revolution), the speed is not affected a great deal.<br /><br />Dropping down to a 16" pitch should give you about 200 more rpm.<br /><br />If you are interested in tweaking your rig for the absolute best performance, then you may have to experiment with raising or lowering the engine on the transom, as Dhadley mentioned when referencing the "x" dimension.<br /><br />If your engine is reaching the recommended rpm, then you know that the engine and prop are matched to the load(boat), and that means that there is not too much resistance on the inner workings of the engine.<br /><br /> You may want to read this post:<br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=034516 <br /><br />Have fun.
 

1stoutboard

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Aug 21, 2003
Messages
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Re: Max Rpm on 90 hp! 2 stroke Yamaha.

Thank you all for your great ideas. This is a great forum with so many experienced members who offer great suggestions. I am glad I found this forum.
 
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