Stainless Steel Prop for a 90V4 Evinrude ?

StingRay_90V4

Petty Officer 2nd Class
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Aug 26, 2014
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155
I am currently running a 13.25dx17p aluminum prop on my 16' bowrider with a 90V4 Evinrude. It tops out at about 40-41mph at 5100rpm. I would like to get a ss prop that would increase my rpms a few hundred while also giving the same top speed or just a tad MORE top speed. But from what I have been reading there are a few things to take into account. What size of ss prop would give the same or slightly better top end for a 1994 ( 90 HP ) V4 Evinrude ?

Thanks for the help !
 

steelespike

Supreme Mariner
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Apr 26, 2002
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19,069
Your numbers don't add up. You can't go 41 at 5100 rpm.It shows zero slip.typical slip numbers are double digits when necessary I use 13%
at 13% slip @ 41 mph = about 5800 rpm.If we gat a gps speed it will eliminate one suspect number.
The Speedometer and tach are always suspect when doing test runs. Speed by gps is always best.
You may be able to borrow a shop tach to verify your tach numbers.Or buy an induction tach.
we know you have a 17" prop,2.00 ratio,speed and rpm are suspect.accurate numbers will bring the slip numbers into line.
What year and model is the motor?Do you know the rpm range numbers?
 
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StingRay_90V4

Petty Officer 2nd Class
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Aug 26, 2014
Messages
155
The speed was tested with a gps a few days ago. Granted at that piont the boat had two extra people in it and the speedo said 39mph and the gps said 38mph. So i figured the speedo is about 1 mph off. The boat is a 1996 stingray 501zp. It has a v plane hull. What ever that means. Lol... The motor is a 1994 ( I beleive ) Evinrude 90 HP V4. The recomended rpm range from evinrude's site AND my local dealer both said for that motor to be from 5000-5500 rpm. My tach has said 5000-5100 ish rpm. So I figured I am within the realm that they recomend. The boat dealer told me at 5000rpm I would be fine for that at wot. But NO LESS !!! So i was thinking my prop and rpm are okay ?? But my speed has been gps confimed at 38 AND 39 mph. However when I hit 41 the other morning I did not have the gps with me to confirm it. What do you think the speed actually is ? Because the gps said 38mph constantly and occasionally saying 39 mph for a few minutes and then back down to 38. Then back up to 39 on the gps.
 

steelespike

Supreme Mariner
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Apr 26, 2002
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Speedometer error is not necessarily lineal. But The gps should be accurate so if we use 13% slip at 38 mph we get about 5426 rpm
Look for a selector switch on the back of the tach note the position and operate it a few times to clean the contacts.
You do have to keep in mind wind,waves and current. when using a gps.
 

Cap'nHandy

Seaman
Joined
Aug 27, 2014
Messages
64
The 13 3/8 X 17 SS OMC prop is a good one, well matched to that engine. Used ones in good condition can be frequently had for a wallet sized portrait of Benjamin Franklin.

Oh yeah, see if your running your engine too low - try raising it a bolt hole or two. NOT the same effect as trimming it out. Engine should have thrust line as close to parallel to hull as possible for max efficiency, and as high as you can go without ventilation issues. Then trim tabs - which will allow you to remove a whale tail if installed. Many people have reported great results from the automatic trim tabs. And if you DO have a whale tail - make sure it is not IN the water at planing speeds. Have someone look over the stern at WOT, and see if they can snap a photo of the engine where it meets the water. Post results here. ( and be careful please. )

You may squeeze a bit more out of it - but your approaching the limit for this setup. To go faster you gotta burn more gas with a bigger engine. Or keep the engine, and get a smaller faster boat. :becky:

Blaine
 
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StingRay_90V4

Petty Officer 2nd Class
Joined
Aug 26, 2014
Messages
155
Speedometer error is not necessarily lineal. But The gps should be accurate so if we use 13% slip at 38 mph we get about 5426 rpm Look for a selector switch on the back of the tach note the position and operate it a few times to clean the contacts. You do have to keep in mind wind,waves and current. when using a gps.
Thanks for the tip on the selector switch. Apparently my tach was on the wrong setting according to my local dealer. Although my rectifier is now shot and not working the tach at all. I have the new part. I just need to pull the flywheel of to remove the old one. And install the new one. So I know there is a math equation to use to kind of figure out rpms with a certain prop and an average slip ratio. Bit I don't know what it is. I took my boat out last night with my Garman GPS again. As well as a brand new prop. As the old one was really old and kwight dinged up. The new prop is an OMC stock prop that is the same diameter and pitch as my old one. ( 13.25D x 17p ) . And my GPS conferms a top speed of 40 mph right on the money. It hangs at 39mph for a bit. Then climbs to 40mph for a bit. So she is definitely hitting 39 and 40 mph. Can anyone tell me sense my tach isn't working roughly how many rpms that should be by using that math equation ? I am fairly confident to say my setup is pretty maximized now. And I think she is running at pretty neat 5500 rpm at top speed. If it is. I may even try a 19p and see is I can still maintain at least 5100rpm + and blow past 40mph and see if I can grab 41-42mph.
 

steelespike

Supreme Mariner
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Apr 26, 2002
Messages
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The problem with estimating rpm by gps speed is we are guessing what the slip is. at 123% the rpm calculates at 5712.
If we guess 10% slip it is a more reasonable 5522. The slip could be lower than 10 But doesn't happen too often
It appears your pretty closer to the rpm range
 
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