71 Johnson 50 hp Initial Prop?

[Maclin]

I just sold a 16 foot fiberglass bowrider with a 90HP engine, had a 17p on it. It maybe could have swung a 19p, but it felt so good out of the hole and planed almost instantly, could hit almost 40 with just me in it. A 50hp on that same boat would probably needed to be in the 13 range, that is what I keep coming back to on this. Good luck.

 

[StingRay_90V4]

I just sold a 16 foot fiberglass bowrider with a 90HP engine, had a 17p on it. It maybe could have swung a 19p, but it felt so good out of the hole and planed almost instantly, could hit almost 40 with just me in it. A 50hp on that same boat would probably needed to be in the 13 range, that is what I keep coming back to on this. Good luck.

I agree with this. I have a 1992 Evinrude 90 Hp crossflow. On a 1992 StingRay 501zp. It has a 17p on it. Incredible hole shot and does 45mph right on the mark at 5650 ish rpm. It will spin a 19p but it doesn't have the same hole shot at all. And the rpms drop to about 5100 rpm. And it doesnt really gain any extra speed. Maybe a mph or so. So if my perfect working 90 hp spins a 17p perfectly. At 45mph. I cant see a 50 hp doing it . the exact size is 13 1/4 x 17p. We also have a 1988 Evinrude 60 hp. It has a 13 1/4 x 15 on it. If that helps any.

 

[jbuote]

Yeah... That helps a lot actually! lol...
​I MAY have a line on a cheap local used 14 x 17 (<$30)... If that can happen, I'll probably grab that for initial tach testing to get some real numbers, with my setup, to work with.
​If that fails, I'll probably head more towards the 15 New...
My WOT range per factory is 4500-5500 with peak hp at 5000 rpm..
​Going to load the boat with 2 people, fuel, fishing gear and cooler (as it will be loaded 99% of the time) and test with that load and aim for 5000 rpm...

​I know some say to tach test to the high end of range, but that's because a lot of people test with an empty boat, then load it and wonder why it's not getting to rpm...
​Figure I'll do all my testing with it actually loaded the way it will be normally, and aim for the peak hp rpm..

Sound like a plan?

 

[Maclin]

For getting an initial where-am-I-at, ok. Just be aware that sometimes if you start out too far over-propped it gets more difficult to judge how far to go down. Other factors come into play in regards to actual engine performance while lugging just below its' powerband at full throttle.

 

[jimmbo]

Actually your 50hp is rated @5500rpm and a WOT range of 5000 - 6000. and comparison to different HP is often comparing Apples to Oranges. Your 50 has a very high gear ratio. 32:12, so it is able to swing props like the 19, 17, 15 pitches
Propping with a light load has it merits. It ensures the engine won't be over revving if you lend it to someone and they run it empty.

 

[Maclin]

Thanks jimmbo, I had thought to ask about the ratio but was just totally stuck at thinking it would not be quite that torque-muliplying of a factor, my bad. That certainly puts the pitch numbers of 17 and 19 into a better perspective. Seems like the trial with that 17p will be valuable.

 

[StingRay_90V4]

Yeah... I agree to.. I didnt even think of the gearing in the lower leg. My 90 4 cyl must have a totally different gearing then our 60 hp in rhe leg unit. I had never thought of that. It makes sense though. Because both boats are 15 spline. And both use the exact same 13 1/4 x 17 p prop. But the 60 will only hit 32 to 35 mph . and the 90 hits 45mph with a much better hole shot on the exact same prop. I giess based on the 50 having a more aggressive gearong in the leg. You should use the 17p as a starting point.

 

[jbuote]

OK!! Sounds like a plan then! LOL
​
​@jimmbo : I totally forgot to mention the gearing.. I actually looked up the parts (Fwd gear, and pinion) counted the teeth (32:12) and then factored down to a 2.66:1 gear ratio... That's what I plugged into my spreadsheet to find my "On Paper" numbers in the first place..
​
​The RPM though... Interesting! I knew the manual I have says WOT RPM is 5500. I couldn't find a power curve chart for a 71, but a 75 showed the range of 4500 - 5500.. with 5000 being the sweet spot.. Guess I made the mistake of presuming the 71 was about the same, and 5500 was the MAX it should get to...

Thanks for clearing that up!

​So, I'll hopefully get that cheap 14 X17 and get some starting point numbers!
​I don't ever plan on lending it out, but..... Ok.. I'll go light load and prop to the high end!
​I'll let you know how it goes..

​Thanks a million to ALL of you!!

 

[jimmbo]

Here is the 72 Evinrude Brochure, the 71 brochures are poorer res but all the specs are the same

http://www.boatinfo.no/lib/evinrude/brochures/1972evinrude.html#/22

75
http://www.boatinfo.no/lib/evinrude/brochures/1975evinrude.html#/30
http://www.boatinfo.no/lib/johnson/brochures/1975johnson.html#/22

 

[jbuote]

Thanks for those jimmbo!

 

[jbuote]

Update...

​got a hold of the 14 X 17..
​Won't be able to lake test it for a couple weeks though.. Have a few things to take care of, but I'll keep you posted as I get things done... LOL

 

[jbuote]

Ok... Interesting issue...

​I had some other issues with the engine, but I think they may be solved. So I went out today to install the new 14 x 17 prop, but it's too big!
​No clearance at the top under the anti cavitation plate.. It would hit if I tried to use it.

​Can anyone shed any light on why if I pulled off a 14 x 17(aluminum), a new 14 x 17 (aluminum) would be to big???

 

[ahicks]

Have you confirmed you have the prop you think you have by checking the part number or size info located on the prop (not the box)?

 

[jbuote]

Yeah, I did.. The old prop is 14 x 17. Actually a pic of it and the size stamp earlier in this thread.
​The new one has the size stamp on the outside of the prop on the hub and it states 14 X 17..
Hence the confusion....

​I'm wondering now, since the old prop is so beat up, and PO seemed geared for a quick sell, maybe he did just throw a prop on it to "Look Complete"....
​Since the old prop is bent at the tips, it fits, but if it wasn't bent maybe it wouldn't fit either..

​I dunno.... lol

 

[jbuote]

Well,
​Broke down and bought a 13-1/4 X 17 here at iBoats. This size matches the "STD" listing in most parts breakdowns I've found..
Still interesting that a 14 x 17 is listed as a possible prop, but was too big...
​Oh well...

 

[jimmbo]

Where was the too big prop making contact? The AV plate, or the trim tab?

 

[jbuote]

Tips of prop blades would have hit the AV plate.. It's just baffling to me...

 

[jimmbo]

Both the old and new 14 X 17 were OMC/BRP props?

Would be nice to get my hands on them, but since your at least 2500 - 3000 miles away...

 

[jbuote]

Yeah... LOL

​I have to take something back though.. Seems it would hit the round base of the trim tab.. My mistake..
Took some pics so you can visualize it...

​Here is the 14 tips.. If I turned the prop any more, it'd scrape on the tip, and I certainly don't want to do that!! lol



​Then, the 13-1/4:


Lots' more clearance (by 3/4 inch.. lol )
Seems they are different designs too..
​
​I don't know what the old prop was (OMC/BRP, Michigan, Solas et.al. ) .. It was pretty banged up when I got it so I knew I'd need another one..
​The 14 is a Michigan prop, and the 13-1/4 is a Solas....

​I guess I have to wonder now, is the trim tab base protruding down too much?
Should it be more flush to the AV plate?

 

[jimmbo]

Might be the wrong trim tab, or something is keeping it from seating. paint? The Michigan prop looks like a carbon copy of one the later designs for the 70hp. There were some fitment issues if I recall.
I don't mean to dump on you here, but the transom is showing an awful lot of cracking. You may have a rotted transom