1976 Evinrude 15604 prop

[alumistar]

I have been playing with this motor for about 4 years. It is jetted for 4000-8000 ft. elevation, 9.25x9 4 blade Solas prop and runs like a boss. It planes a14ft Acecraft with me in the back and my wife up one row with my 12 year old and my 6 year old in the bow. I have just purchased a Solas 9.25x11 ss prop. What do you guys suspect it will turn out? I have added forward controls and will be sitting in the middle, 12 year old in the back and 6 year old in the front. Should be good weight distribution.

 

[steelespike]

Re: 1976 Evinrude 15604 prop

I think its going to struggle unless you are by yourself.
You've added a little more weight and increased prop size 2" sure to knock
about 400 off the rpm.
Might be fun to see what you can get out of it with as much as possible stripped out of the boat.
If it does plane with your load it will probably be outside the range on the low end.
The 15 likes to rev according to my info rpm range is 5500-7000.
I saw a newer one on You Tube turning 6960 rpm at 28 mph.It was on an empty 10 foot boat.
What is the actual length and widest part of your boat?

 

[alumistar]

Re: 1976 Evinrude 15604 prop

It is 13ft6in and a 63 inch beam. IIRC, red line is 5400.

 

[steelespike]

Re: 1976 Evinrude 15604 prop

Do you mean your present wot rpm is 5400? Is that with your family aboard?
"Redline" is actually the maximum wot rpm rating; 7000 for your motor.
Unfortunately your allready below the minimum rpm at 5400.
I will be very interested to see the results with the new prop just to see the actual numbers from a 2" pitch change.
I have a feeling you will have to test lightly loaded with each prop in order to get within the rated rpm range.

 

[steelespike]

Re: 1976 Evinrude 15604 prop

It is 13ft6in and a 63 inch beam. IIRC, red line is 5400.

Your boat is what I call a small 14.What I call a big 14 might be 14'6" and 6' wide.It has much more capacity and will plane easier with the family aboard.This is assuming a bare bones tiller steering boat not a full blown rig with floors live well steering console etc.
The bigger boat will be easier for your 15 to get up on the water.

 

[alumistar]

Re: 1976 Evinrude 15604 prop

ok, i just had a chance to look at my owner's manual and here is what's written:

"Horsepower.........15 O.B.C. Certified Brake Horsepower at 4500 RPM
"Full Throttle Operating Range............4000 to 5000 RPM."

that's right out of the OEM 1976 Owner's Manual.
I have a "TinyTach" that read 5400 rpm on Chatfield Reservoir, here in Denver. that was last september, with the boat loaded as described above.

a couple things i should note here:
1)reservoir elevation - 5620ft above sea level
2)jetted for altitude
3)new oil and compression rings
4)new crank bearings
5)not a full rebuild, but close to it as of march 2012
6)fully loaded boat with ~600 pounds including people, poles, tackle, lunch and gas, which displaces 6 to 8inches of water, so it rides really shallow while trolling. on plane, it appears to displace about 3 or 4 inches.
7)while considered a V hull, it is very flat, more of a U hull really.
8)i weigh 250. my wife and 12 year old weigh 125, my 6 year old weighs 50. with me and the 5 gallon gas tank at the tiller and the rest up a row we can plane with the 9.25x9 4blade solas........approximate speed is about 15-20mph-ish, TinyTach reading ~5400rpm, so by technicality, it looks like i am 400rpm above what it should run at. although i have to say, it sounded like it was dead on for rpm in real time conditions.

i hope this helps some.

 

[alumistar]

Re: 1976 Evinrude 15604 prop

ok. so if i am reading the theory correctly, i have essentially increased pitch by 1", not 2". my current 4 blade prop is 9" pitch, which ought to be equivalent to a 3 blade 10" prop. so, if my ordered prop is a 3 blade 11", it would be equivalent to a 4 blade 10".....essentially, a 1" increase in pitch. at that, i should see only a 150-250rpm decrease. that being said, if my TinyTach is correct at ~5400rpm with my current setup, i should still be able to achieve overreving with a redline of 5000rpm and actually spin up to 5150rpm. someone tell me if i am "theoretically correct" on this? obviously, the true knowledge will be gained once i test this in real life, in the same spot. i have always run aluminum props. the new ordered prop is SS, so hopefully it'll help with overall aesthetics of the boat and a little performance bump. i've been rebuilding over the winter. maybe i'll make a video or something, now that BettyRoxx is starting to look more camera friendly

p.s.- does a twin cylinder, 15 hp, 2 stroke "marine" engine have a "power band" like an equivelant 2 stroke two cylinder "motorcycle" engine? if it does:
1-has anyone ever operated within the power band?
2-specifics inluding, rpm range, pro's and con's, feel of operation
3-how did you get into the power band?
4-would you suggest doing it? why or why not?
5-any observations i haven't expressed.
6-potential damage is obvious.

thanks,
james

 

[steelespike]

Re: 1976 Evinrude 15604 prop

Here is the info I have OMC original sales brochures for 75 and 77 15 hp rpm range 5500-6500 No 76 catalog but I think the service manual covers them all.
69-89 after market service manual all 15s are listed at 5500-7000.
I think you would call the rpm range the "power band". Ideal performance is produced within that range.
The 77 9.9 (a choked down 15) is rated @ 5,000 with a range of 4500-5500 Ser. manual says prior to 89 9.9 rpm range is 5000-6000.
Gear ratio is 2.42 On both 9.9 and 15 using the same "standard" prop for both of, course might be bigger or smaller depending on what boat or load. If 5000 is rated rpm your max speed would only be 15.8 with a 9" prop. A 50 year old 10 hp Johnson will do about 21 at about 5000. Your rpm is obviously higher than 5,000 With the stated hp rating at 6,000 in several sources.
At 5600 ft your power is down about 15%
The 11" prop is the highest listed here at iboats it's hard to believe it will do well at 5600 feet.
I hope it does well for you.

 

[alumistar]

Re: 1976 Evinrude 15604 prop

weird. i have a clymer for evinrude/johnson covering 1973-1990. it says the redline is 6200. but my owner's manual for the motor says 4000-5000. which is correct is beyond me. i know it sounds about right, in the sweet zone, on the lake. hopefully i will know more once the prop arrives and i can test in real time. i will keep this updated once i know for sure. who knows, maybe my TinyTach is off a bit.

 

[steelespike]

Re: 1976 Evinrude 15604 prop

From your owners manual specs I wonder if there is a miss print with specs from the 54-56 15 hp motors.
These are 15 hp at 4500 rpm and 4000-5000 rpm range. Displacement at 19.94 where your motors displacement is
13.2. I think you can see that for a 13 cu in motor to develop 15 hp it would have to turn more than 4500 rpm.