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What should max RPM be

G

gone_postal

Regular Contributor
"J90ELECM - '98 - 90hp

"J90ELECM - '98 - 90hp

I finally have my tach working now and now know that my top end is 5500RPM. I thought the range for this motor was 6200-6300, but might be wrong. I just don't want to do any harm by putting too much strain on it. I do not know my prop size/pitch - guess I have to pull it to find that out. The motor is the original from the factory, so I'm hoping that it's at the correct height for this boat."
 
"Hmm. Kinda a bummer then. I h

"Hmm. Kinda a bummer then. I had this motor rebuilt a few months ago and before the rebuild, the boat did 43mph w/ just me & a little over 1/4 gas (30g tank). Compression at that time was 100,120,120,120.

Now though, the highest I've seen is 35mph w/ just me and 1/2 tank (with wind & current).

Prop, motor position, etc all the same, so I was kinda hoping the RPM's were low and it was a simple fix - something w/ timing, jetting, etc. I guess it is what it is. I just expect it to be faster - 16' boat w/ 90hp.

So, what is cruising range? I'm guessing 4000-4500 which puts me at 20-26mph."
 
"The nominal max range is 4500

"The nominal max range is 4500-5500.
Is Your speed measured by log or GPS?
If You are after top speed, it might pay off going up on the pitch 2"."
 
"GPS

Wow, 4500-5500? So wha

"GPS

Wow, 4500-5500? So what is cruising? 3500-4000? If so, this thing is a dog - I'm looking at 20mph at 4000. Don't get me wrong - it's not a raceboat - it's a flats boat, but only weighs 1400lbs w/ motor. I would just expect it to be faster. I'll be cruising w/ pontoon boats at this pace
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Funny, everyone I talk to says "90hp on a 16 footer?!? That must haul!!!"

I have a 3 blade stainless prop on there right now. How do I go about finding the pitch, etc? I heard it's stamped on the other side - that I have to remove the prop to find out. I'm just not sure if that's true or not.

Honestly though, how much can be gained from a prop if I'm already in the right range? SS props aren't cheap...."
 
"Well, just left Evinrude'

"Well, just left Evinrude's website. I looked up the performance tests of their 90hp ETEC on various boats and 35-40 is the norm. Guess I'm not doing too bad afterall.

Any ideas on why the boat USE to do 40+ though?"
 
My old 17ft with a 115 Yamaha

My old 17ft with a 115 Yamaha 2 stroke topped out at just over 40mph. 17" ss prop So I'm thinking your speed is about right for a 90.
 
"I've seen a lot of 'f

"I've seen a lot of 'funny' GPS readings and speed readings. Quite often it has come out to the GPS Setting.
Km/Hour
Mph
Knots
To get 'what You want', You may have to do some testing with different props, both types and pitches, different engine mounting height and perhaps even moving weight around.
The std BRP/OMC props (Alu and SST ll) have their optimum performance at about 5000rpm engine rev which is where You have to max HP.
It is a lot more to props than just Alu/SST and Dia and Pitch."
 
"Morten - yeah, I might be ope

"Morten - yeah, I might be opening myself up to a lot of frustration if I start messing w/ props.

I forgot to mention that the previous owner installed a hydrofoil (whale tail or whatever you like to call it) on the motor. What is the general consensus on these things? Obviously marketing leads one to believe you get a better holeshot AND higher top end, but I don't really see how. The holeshot makes sense and even keeping the boat on plane at a lower speed, but top end? I would think more drag would reduce speed."
 
"I had about 20K (USD)

"I had about 20K (USD) worth of props at all times in my stock. Alu, SST/Vipers, Rakers You name it. I did some rigging of boats/engines for magazine testing for the OMC/BRP distributor back in Norway. Some of my choices were not quite out of the 'book' even though they were always OMC original. I used the 4-blade alu on a test of the 2-cyl 25, for the first time 'we' beat Yamaha and Mercury on the most. I put an 'old' type Viper on a 150 Ficht, came out with 'best handling and overall performance'. (Viper was originally intended as a Cobra prop!)
Try to find a decent prop shop/dealer than may borrow You some alternatives and I'm sure You may find a reasonable solution without loosing Your mind and money."
 
"Your propeller is the only so

"Your propeller is the only source for getting available horsepower to the water. Without the correct prop and pitch you could have a 500 horsepower motor but go nowhere. Pitch in a prop is a lot like gears in a car or truck. Higher pitch equals higher top end but terrible low end. Pitch is the theoretical travel a prop makes in one revolution. Example: A 20 pitch prop will travel 20 inches per revolution if there is no slip.

Props come in a variety of configurations known as flat pitch, regressive pitch and progressive pitch. Props with a flat or straight pitch have the same pitch on the leading edge as on the trailing edge and are usually considered the fastest. Props with regressive pitch are somewhat rarer because they have a higher pitch on the leading edge than on the trailing edge, this configuration utilizes the higher pitch to create drag to enhance the hole shot. Props with a progressive pitch are the most common and are the best for overall performance, these have a lower pitch on the leading edge for hole shot purposes and a higher pitch on the trailing edge for speed. These props are generally stamped with a median number, an example being a prop stamped 25 pitch will more likely read 27 to 28 pitch under the cup when measured on a pitch gauge.

Rake of the blades is the next consideration, a low rake prop has the most power and a high rake has the most speed and lift. Rake is the angle of the blade in relation to the hub, 0 rake being 90 degrees. The better running props are usually in the 20 through 30 degree rake angles.

Cup added to a prop blade controls what the prop does for the boat. Cup on the trailing edge controls stern lift and aids to some degree hole shot. Cup on the blade tips controls bow lift and speed. Cup is not a generic item, it has to be designed to the application and what works for boat and motor X will not necessarily work for boat and motor Y.

Porting a prop aids hole shot, these are the holes found in most props behind the leading edge of the blades on the barrel of through hub exhaust and over and through applications. In the through hub configuration the porting allows extra exhaust into the prop blades so the prop can spin up faster to help the hole shot. On over and through hub configurations, the porting actually creates a vacuum pulling water back into the prop blades which aids hole shot.

Selecting which combinations of the above that will work the best for your application is usually best left up to someone who has a good working knowledge and a lot of experience in the field."
 
"'u fellers...have toooo m

"'u fellers...have toooo much time on your hands! :>)'

Not quite, but I had a lot of customers that wanted to get it right in the first place and even payed for it!
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"
 
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