real quick rpm question

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mattttt25

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2002 yamaha f225. during recent sea trial, wot produced 6100 rpms. load was 3 adults, 3/4 fuel, minimal gear.<br /><br />that's good, right? manual calls for 5500-6000 max, but i've read so much about being a little on the high side is just fine. just hoping one of the old salts chimes in and tells me i'm right where i want to be.
 
JB

JB

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Re: real quick rpm question

I think you are right where you want to be for that load, mattttt.<br /><br />Would you call that a heavy load, normal or light?<br /><br />If that is typically your heavy load I think you are under propped a bit. If it is a light load you are spot on.<br /><br />Good luck. :)
 
JB

JB

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Re: real quick rpm question

In that case, there may be a better prop for you, Mattttt.<br /><br />Post the make and specs of your prop and I'll bet DHadley can put you onto somethng just a wee bit better.<br /><br />Good luck. :)
 
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mattttt25

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Re: real quick rpm question

jb, prop reads 17M. it's the standard black ss prop that yamaha uses. i think it's actually a 15 1/4 x 17, if that sounds right.<br /><br />while i'll take all the advice i can get, i've always questioned why so many of the educated, trusted guys on the site recommend getting the rpms above what the manual says. i would guess that since that's my average load, and i'm already a tad higher than the listed max, i'm fine. first thing we always tell people to do is read and follow the manual. but this always seems to be the exception. what's up?
 
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steelespike

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Re: real quick rpm question

It is my opinion you dont want to exceed the WOT rpm reccomendation neither too high nor too low.With your present setup you are certainly<br />close and could surely run as equipped as long as you run in the accepted range by merely running the throttle so as not to exceed the rpms.This would give you a little extra for heavier loads or skiing.And if you are careful with light loads the motor should be happy.
 
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imported_Curmudgeon

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Re: real quick rpm question

.. i've always questioned why so many of the educated, trusted guys on the site recommend getting the rpms above what the manual says.<br /><br />Educated and trustworthy doesn't guarantee they have a clue about which they speak. Follow your manual and you'll likely have mostly pleasant boating experiences. ;)
 
Dunaruna

Dunaruna

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Re: real quick rpm question

Your manual calls for between 5500 and 6000 WOT RPM. "On the high side" would be 6000, not 6100.
 
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Dhadley

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Re: real quick rpm question

Mattttt25, thats a good point. The answer gets very long and involved but heres a short version. <br /><br />After the fuel had the lead removed we found that the existing 2 stroke outboard motors actually lived longer when set up at a higher rpm. Lots of in-water and dyno testing was involved. Lots. Its all related to todays fuel and combustion temps.<br /><br />Remember with a 2 stroke theres no valves to hit the pistons, no lifters to float, no push rods to bend, no timing chain/belt and so on. <br /><br />As for your rig, I would -- if I were propping it out -- try the same model prop in a 19. However, the holeshot is going to suffer unless you change the X dimension. 4 strokes have to have that low gear ratio and big diameter prop to get it as good as it is now.<br /><br />I'd guess that whoever rigged your boat was used to rigging that boat with a 225 2 stroke which has a much different gear ratio. And a 17 would be about right. <br /><br />That being said, I might try a large diameter Turbo but they dont have anything, today, in that diameter. Itd be interesting to compare though.<br /><br />If youll post your top speed, with an average load, we'll run the numbers and see where your slip is.
 
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mattttt25

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Re: real quick rpm question

dhadley- thank you for the response. i follow you on what you have seen, but don't the engine manufacturer's see the same thing? wouldn't they recommend the proper rpm range?<br /><br />i ran on flat water last night for the first time. just me in the boat, but i was still at 3/4 tank of fuel and moderate gear. i hit 6100 rpms and 43 mph. may have been able to squeak out a little more if messed with trim and tabs (new boat, still exploring it).<br /><br />boat lists at 3500 lbs dry. but this one is optioned out with factory hardtop, aux fuel tank (150 gal total), marine head, etc. i also had my curtains down, which may have reduced speed by a little.<br /><br />engine is mounted on a factory bracket. anti-vent plate is higher than the keel as it should be, but can't give you exact measurements.<br /><br />you mentioned big diameter prop. isn't my 15 1/4 small in comparison? when walking around the grady floor room, all the big yamahas seemed to have small diameter props, in my opinion.
 
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Dhadley

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Re: real quick rpm question

Very good observation about what the factory sees. This is where it gets real complicated to us in the "real world". You have to try and understand the corporate mentality.<br /><br />The factories will only address issues on what they call "current" engines. To them, "non-currents" don't exist. A "current" engine is a model that should still be under factory warranty. If there is an issue with a "current" model motor it has to reach a certain level or percentage of that model motor before its addressed. At the time this coking and stuck rings showed up (everybody knew it was a by product of the fuel-of-the-day) OMC's answer was with "low octane" head gaskets, reduced timing and bigger main jets. It really didn't help the stuck ring situation. <br /><br />Back in the late 70's when we discovered that a bit higher rpm helped, OMC did not / would not change their recommendations. If they did they felt they would be admitting they were "wrong" in the past. They stuck with rating the motor at the same low (loaded) rpm. And thats what it is -- an rpm at which a motor is rated. It doesnt mean the motor cant be ran at a different rpm and live a long happy life. Motor ratings have to do with where hp and torque cross in the rpm range. Usually at about 5250.<br /><br />With big companies and government, go by what they do -- not by what they say. If they didnt want the motors to turn more rpm, or they thought it was harmful, they wouldnt have set the limiters as high as 6200 and some at 6700.
 
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Dhadley

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Re: real quick rpm question

At 43 mph youre just under 15% slip. Not too bad, could be better. It might be worthwhile to try a 19, just to see.<br /><br />The "standard" diameter for a V6 for years has been 14 1/4" for a 3 blade. The 4 strokes with the lower gear ratio all seem to be running larger diameters. A V6 Suzuki uses a 16" diameter prop. <br /><br />Personaly I'm a fan of larger diameter. It means you can increase the X dimension and lose drag.
 
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mattttt25

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Re: real quick rpm question

thank you. good explanation.<br /><br />do you agree that i probably have a 15 1/4 x 17? haven't busted out the tape measure, but i know the prop is the standard black ss yamaha uses, and is marked with 17M.<br /><br />if i try a 19, do i keep the diameter the same? what if i go with a polished ss prop? no correction needed?<br /><br />and if i decide to stick with what i have, do you feel the 6100 rpms will be fine?
 
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moderator1

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Re: real quick rpm question

I could answer the last question, but Dhadley is the "spert" on this so I'll stay out....<br />No I won't...6100 is fine!!!!<br />You may gain a bit of speed with the 19 though, and I think Dhadley wants to improve your slip #'s, so listen to him...He knows his *****....
 
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dajohnson53

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Re: real quick rpm question

Originally posted by moderator1:<br /> ... so listen to him...He knows his *****....
Click to expand...
Hey, you're allowed to spell out "stuff" (*****).<br /><br />Now if you intended to write ****, that's a different story! <br /><br />Maybe you intended to write "he knows his ********* **************" (technical specifications)? <br /><br />D. (AKA **** for brains) ;)
 
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Dhadley

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Re: real quick rpm question

Lets say that with the 19, of the same brand and style, your rpms dropped to (using the standard of 200 per inch) to 5700. And assuming the slip % stayed the same at 15% you'd be at 44-45 mph. <br /><br />But lets say if the slip dropped to 12-13% you'd be at 45-46 mph. Lets further say that the 4 stroke experts would like to see that motor at a bit more than 5700, then we could raise the motor a little and pick things back up to -- say perhaps 5800-5900. Assuming 12-14% slip youre now at 47 to 48 mph.<br /><br />So assuming the hull and motor responds as hoped, you have the potiental to pick up 4-5 mph.
 
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