15hp evinrude, need help choosing new prop.

I recently rebuilt a 1974 15hp evinrude model 15404s, now runs like a champ but may be pushing too much boat. I mounted it on a aluminum flat bottom that measures 15'10" x 50" with the factory prop I have good idle speeds and maneuvering and very nice take off that lifts the bow nicely but tops out at around 15mph witch leaves me just shy of getting on plane. the factory prop is a 3blade im not sure of the pitch. I was thinking of changing props to try and get that last bit of speed to get on plane. maybe 4 blade 10 pitch?

any tips or pointers on prop type or pitch to get that little more push would be appreciated.

Re: 15hp evinrude, need help choosing new prop.

Without knowing your current pitch or RPMs, any guess is as good as another...

If I were you I'd get an inductive tach or a tiny tach, and figure out what my RPM is right now at your top speed. Proper range for that motor is 5500 to 6500, and to get it to plane out you want to be closer to the higher #. If you RPMS are too low indeed you need less pitch. If you're already spinning over 6500 RPMS, you might need more pitch.

Also if you have a factory prop, it is clearly labeled with diameter and pitch. Remove it from the motor and look it over. Sometime #'s on the face of the hub are hidden when it is installed, but a true OEM Evinrude/Johnson prop I believe has diameter & pitch stamped right on the side of the barrel between blades somewhere. You should be able to find this info.

Also, have you tried different trim positions and/or motor heights? It is EXTREMELY common for people to mount these motors too low in the water creating extra drag...might try laying a board on top of the transom to raise the motor up a couple inches. Also pay close attention to the angle of the motor...set your A/V plate about even and level with the bottom of the boat. Small angle changes make a noticable difference...you have 4 holes to choose from for tilt position. Even if it looks about right, try all 4 different positions and a couple different motor heights anyway, and try to figure out which works best. And while you're out there, try a 1" thick board, 2" thick board, and 3" thick board on top of the transom to try your motor in 1" increments of height positioning. Again you just want to pick the one that performs best.

I have the same motor as a kicker on my 17 ft Landau (aluminum Bass Tracker type of boat with full floors, swivel seats, steering console, full livewell, 50 hp motor, 20 gallons of fuel, 2 adults, and a huge amount of gear...presumably much heavier than yours.) While it does not plane out, it comes really close. When I had that same motor as a short shaft on my 14 ft V-bottom rowboat with about 650 pounds of people/motor/gear not only planed out, but ran darn close to 30 mph...while turning about 6600 rpm. That motor loves to spin and scream...

I learned that a cheap inductive tach off Ebay (about $20 shipped) works real easy on that motor with a piece of reflective tape on the flywheel. You need to do it on a cloudy day though...direct sun screws up the reading.

Not sure how heavy your boat is, but I believe if not overly loaded, and motor positioned properly, your boat should plane relatively easily and run a solid 20 mph or better.

You absolutely certain you have a 15hp carb, and not a 9.9 carb? In 1974, the carb was the ONLY difference between those 2 motors, and if someone in history put a 9.9 carb on it (much much much more common) as part of a repair, you may actually have a 9.9 motor and not even realize it. See Leroy's Ramblings website (google it) for details on how to ID the difference...they look awful similar.

Re: 15hp evinrude, need help choosing new prop.

A 15 is good for about 25 on a light reasonably fast boat. If your boat is a bare 15 footer I think it should easily make about 20
with you alone.Your present prop size should be near the base of the blades or under the prop nut.
The factory prop was a 9 1/2 X 10 though the present prop may depend on the intended use.
A 10" 4 bld would depend on the present prop.
The motor was rated for a wot of up to 6,500 according the the brochure in 74. So it can really rev.

Re: 15hp evinrude, need help choosing new prop.

thanks, I will test the moter height, after the rebuild I had serious cavitation issues i ended up cutting the transom down 3 inches and it took care of the problem but i may still need to raise or lower an inch or so. and will find a tach and get my rpm ranges before any more prop shopping. total weight of myself, fuel and gear is around 450lbs. the boat itself is probly around another 500 or so.

Re: 15hp evinrude, need help choosing new prop.

wolfman77 said:

thanks, I will test the moter height, after the rebuild I had serious cavitation issues i ended up cutting the transom down 3 inches and it took care of the problem but i may still need to raise or lower an inch or so. and will find a tach and get my rpm ranges before any more prop shopping. total weight of myself, fuel and gear is around 450lbs. the boat itself is probly around another 500 or so.

Click to expand...

That's interesting info. It sounds like you have a short-shaft motor, and you put it on a boat that needed a long shaft? But you fixed it by modding the boat's transom. Loosely speaking smaller boats have either 15" transoms, or 20" transoms, and motor shaft length likewise has a 5" variance in short vs long to fit these transoms. So if you cut only 3" off of a 20" transom, you now have a 17" transom. And if so, and you have a short shaft motor designed for a 15" transom, it's possible your motor is actually still too high, and possibly still ventilating when you get close to planing out? This could cause slight over-rev and loss of bite as it just gradually sucks in a few bubbles of air, and thus prevent planing and gaining any more speed?

I've made a whole lot of assumptions there, and whether I'm right or wrong it might give you some clues as to what to look for.

Rough rule of thumb for a starting point you want the A/V plate of the motor to be just above level with the bottom of the boat...maybe an inch higher....never below for a planing boat. And with the trim angle, you want that to be about level with the bottom of the boat. There are nice photos and explanation of motor height and trim angle concepts here:
http://media.channelblade.com/boat_...s/dts/dealerlibraries/8441/copetipstricks.pdf

Some photos of your setup might help us visualize what you have and what you're doing to offer different suggestions.

Re: 15hp evinrude, need help choosing new prop.

yes with the 3" notch the transom is now 17" tall. I didn't want to go lower because I wont be able to keep my 45deg sides due to the transom braces. without at least a 45 on the side cuts my steering will be blocked. if I do need to go lower when I fab up the new saddle to cover the transom I could cut my angles back then weld in plates for the braces to bolt to, then the steering would still be clear. here are some pictures

Re: 15hp evinrude, need help choosing new prop.

Height looks good to me and I think if you were to measure Most "15" motors they would measure around 17"
A picture of the inside of the boat If its bare the boat alone is probably about 260 lbs.
I have a 18 ft Crestliner bare utility with heavy wood seats.It weight 330 lbs dry weight.

Re: 15hp evinrude, need help choosing new prop.

Yeah it's definitely not too high, so no need to cut more from transom. But certainly the trim angle looks incorrect to me if that's how you run it...your prop is trying to lift the back of the boat up instead of using all thrust to push the boat exclusively forward. That A/V plate of the motor should be largely at the same angle as your straight-edge, and it's not. So you definitely need to tilt the motor out a hole or two to get that A/V plate leveled. And I still wonder if you'd gain something by lifting the motor an inch or so and see if you can get away with it without ventilating. To me it looks a little too low, but hard to tell for sure since it's not tilted properly. Get the tilt right, then cut yourself 3 strips of scrap 3/4" plywood/OSB scraps, and add them one at a time to the top of the transom during test drives and see what happens. If it starts to ventillate, take one out so the ventilation stops...that's your final height...now make something permanent of same thickness and attach it permanently to the top of the transom.

And any moveable weight from rear to front might help it plane also. I see you have trolling motor. Between driver/motor/trolling motor/battery/fuel tank all hanging out in the back, sometimes those boats run around with nose in the air and can't settle down. Maybe move your trolling motor battery up front along with an anchor or anything else heavy/mobile and see if that helps it plane. You might just have a weight distribution issue more than anything. I know when I fished alone from my 14 foot boat and I had everything in the back, I made longer fuel lines and longer trolling motor cables so I could set the fuel tank and trolling battery as ballast toward the front...which helped overall handling dramatically. But if you normally have a human sitting up front, likely no need.

That looks like a relatively heavy boat, don't expect miracles. But it SHOULD plane...

Re: 15hp evinrude, need help choosing new prop.

I have not felt any cavitation since lowering the transom and at full throttle the motor runs very smoothly I don't know what rpm its running at full throttle but from the sound of the engine I seriously doubt its anywhere near 6500. I ordered a cheap tachometer so I can get a better idea of engine speed. as far as boat weight the boat itself measures 15'10" length and 50" wide interior bottom 19" sides 68" across the top. it has 2 built in benches with a live well in the middle witch I have just put plugs in and used as storage. I used one sheet of 3/4" floor decking sealed with krylon rubber sealant "cheaper version of flexseal" cut into 3 pieces to put in a deck and keep myself from having to walk on the braces, also I just wasn't comfortable with how much flex the boat had since I run mostly on big water that like to turn rough once im a few miles from the ramp.

Re: 15hp evinrude, need help choosing new prop.

I have had someone up front my last few times out. i will try moving some weight forward when i go out alone, the 6gal fuel cell usually sits directly below the motor between the transom braces, i do have a very heavy battery for the trolling motor that usually sits beside me on the bench so i can easily unhook the power to trolling motor when not in use. and i keep a small lawnmower battery behind me that runs the nav and flood lights. cooler, tackle bags, trot line buckets etc... usually sit forward of the live well so they are out of the way until use. im still wondering about the motor depth with 3 people in the boat i trimmed the motor out 1 notch and it started cavitating at top speed.. i need to go run by myself and check the hight and trim. everybody is giving great tips. this is not my first boat but its the first boat that i started with a hull and a junk motor and rebuilt everything myself. i was a helicopter mechanic in the army before being medically retired last year, so im catching on pretty quick to whats going on with boat trim and prop pitch and rake, force to weight etc etc.. you guys keep the tips coming and ill end up with the fastest 15hp on the river. shouldn't be hard since im probably the only person around running one on Kentucky lake

Re: 15hp evinrude, need help choosing new prop.

What you are experiencing when the prop loses grip and the motor revs is actually ventilation.
For many many years called cavitation even in some factory manuals.
You don't feel cavitation it occurs on the under side of the blades. It is a low pressure area and the water boils there.
Over time it erodes the surface of the blade leaving scars.
It can be an indication that a prop is working really well.
Your more likely to replace a prop from age and other wear and tear than cavitation.
There are new props that should resist venting better than your present prop especially if its from 74.

Re: 15hp evinrude, need help choosing new prop.

ok so the tach came in and I ran the boat about 20 miles yesterday. the prop that is currently running is really beat up so before leaving I used a file to smooth the blade edges back out witch seemed to help a lot. running upriver with the wind I still had a lot of ventilation issues I was able to run around 3900rpm when I would run higher than 4000rpm the prop would begin to slip. on the return trip running against the wind on pretty rough water the ventilation stopped and I was able to consistently run wide open throttle at 4320rpm still around 15 mph and just not enough to plane. the current prop does not have any markings on the outside, I plan to pull it off and hope the pitch is stamped into the hub. if not I guess il just assume that it is the 9 1/2x10 factory prop. I plan to stay with a 3 blade prop and drop pitch to try and get up to around 5500/6000 rpm so that I still have a little play room before the engine over revs in case the new prop vents. the lowest pitch prop I have found so far is a 5" so I may not even be able to get enough engine rpm. but I have the choice of 9 1/2 or 10" props if I measure and the 10" will fit should I switch? and how will I account for the larger prop when trying to figure pitch for my desired rpm range?

Re: 15hp evinrude, need help choosing new prop.

You need to do a test run with as much weight removed as possible.You have to plane for the numbers to have any value.
If your not planed then the the results have no wot value except the obvious result of not planing.
Your boat is just what the 15 is intended for.The flat bottom should get it up on the water easily.
How is the hole shot?
I do suspect there is a motor issue Is the throttle opening all the way?
Have you checked the lync and sync?
Check that both cylinders are firing. You wouldn't be the first to not realize you are on one cylinder except for the lack of power.

Re: 15hp evinrude, need help choosing new prop.

both cylinders are firing. originally the head gasket was blown and the bottom cylinder was flooding. after replacement it ran great until the spark plug that was in that cylinder went dead (my guess was factory defect there were tiny pin holes in the ceramic allowing discharge). the stator plate rotates to its limit and opens the carb linkage fully. im going to pull the carb today and double check to make sure that it is actually the 15hp carb. if it has been replaced with a 9.9 carb wouldn't that be the reason my wot rpm are so low? if it is a 9.9 carb and the only difference is the diameter of the carb throat is it possible for me to machine the throat out to the 15hp diameter?

Re: 15hp evinrude, need help choosing new prop.

Even at 9.9 its my opinion that it should plane unless the prop is totally wrong.
A 9.9 can make 20 or 21mph on a light reasonably fast boat.

Re: 15hp evinrude, need help choosing new prop.

Even at 9.9 its my opinion that it should plane unless the prop is totally wrong.
A 9.9 can make 20 or 21mph on a light reasonably fast boat.
Check out the 9.9 and 15 on the water tests on youtube.
These motors are pretty impressive.

Re: 15hp evinrude, need help choosing new prop.

ok.. its the 15hp carb... so what other motor problems could be keeping my wot rpm 1200rpm low.... I cant even drop enough prop pitch to gain that much engine speed.. this is getting frustrating. just pulled the prop too its the factory 91/2x10 so if the motor was set up from the factory to run 5500-6500 rpm with the 91/2x10 what is holding my rpm so low..

Re: 15hp evinrude, need help choosing new prop.

Hi Wolfman 77,

Check : http://forums.iboats.com/inflatables-ribs/abc-sib-rib-installation-guidelines-628902.html

Related Topics 3-4-5

Happy Boating

Re: 15hp evinrude, need help choosing new prop.

thanks I read over the info. im still not sure to drop my motor another inch or raise it. the attached pictures show the the trim set to the 3rd hole the plate sits about 3/4 inch above the bottom of the boat but I have such terrible prop ventilation at this setting that I cant get anywhere near wot. the problem is the higher I raise the trim the worse the ventilation is. also as the foot moves through the water it displaces water at such an angle that the exhaust port is out of the water.. so should I raise my motor and drop trim or lower the motor and raise trim?

Re: 15hp evinrude, need help choosing new prop.

wolfman77 said:

im still not sure to drop my motor another inch or raise it.View attachment 213781View attachment 213782View attachment 213783

Click to expand...

That's why I recommended you carry a bunch of 3/4" sticks out on the water and try both. Takes about 2 minutes to raise and lower a motor one stick at a time on the water.

If it's ventillating in all trim positions, it's too high. Drop it one 3/4" stick and see if it stops. If it doesn't, drop it again.

From those latest pics, it looks like it may be a bit too high to me. Plus the A/V plate appears to be angling upward toward the back. So to answer your latest quesion I think both of your suggestions might be off...I'd be inclined to drop the motor another 3/4" AND drop the trim/tilt one more hole.

For tilt, you simply want the A/V plate as close as possible to being on the same plane as the hull of the boat. If it ventilates in this position, it's because the motor is mounted too high on the transom.

If it still can't get on plane with motor in that position you are either 1. Too heavy for motor, 2. Down on HP due to unrealized problem in the engine, or 3. Overpropped.

Steelspike earlier suggested dropping as much weight as possible and trying to get it on plane to get a true RPM test. Have you done this? No fishing gear, no battery, no anchor, no trolling motor...just you, life vest, motor, and a 1/2 full fuel tank. Does it plane, and does it reach near max RPM in this condition?

Alternately, do you have access to another smaller rowboat you can beg/borrow and slap this motor on and go for a rip and do the same? Because if it doesn't plane effortlessly/instantly and scream right up to proper RPM on a small V-hull bare aluminum boat, you have confirmed you have a problem with the motor itself and your rebuild wasn't done quite properly or completely.

You indicate you "rebuilt" the motor. Later you indicate you just replaced a head gasket. What exactly was done in the rebuild? Have you done a compression test since to confirm balanced?