Johnson 50 - Revs over spec - just want your thoughts

lakelover · Apr 17, 2012

I had my Jet Star out for a test run last fall, 50hp motor & 12-1/4 x 15 prop.

I figured the total weight including my son and myself, all gear etc. was about 905 lb. The motor topped out at about 6200 rpm, well over the recommended 4500-5500 rpm. It didn't sound like it was really screaming or anything at WOT, in fact it sounded pretty good.

I anticipate most outings the weight will vary from about 240 lb. less to 100 lb. more.

Should I go to a higher pitch to slow it down a little, or just take it easy when light & see how it goes with the extra weight?

lakelover · Apr 22, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

^^^

steelespike · Apr 22, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

I think your tach may be in error.There may be a switch on the back You could try operating it to clean the contacts.Are you saying boat,motor and gear is 900 lbs.
If thats the case It may be the prop,What is the gps speed at 6200?

lakelover · Apr 22, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

The tach is brand new, the switch is set at 6 per the instructions & info in the service manual.

I estimated the approx. 900 to be the total weight including 2 people, but now after rethinking it, I think it was more like 1120; sorry, don't know the speed, I don't have a gps.

Hull=415
Motor=190
Fuel, battery etc.=110
2 people= 405

Total=1120

steelespike · Apr 22, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

Speed and rpm is a good way to evaluate the performance.
Some cells have a gps app.
I can tell you I had a 18' bare bones Crestliner utility of about the same weight with a 88 50 hp Evinrude. Unfortunately I never had a tach or gp[s at the time. But I can say it had/has(son has the boat) a 19" prop And it performs very well.I hate to use this term
"it doesn't seem to over rev"It seems to fly,can get up a slalom skier on one ski.Hardly notices 1000 lbs of additional weight. Has been setup like this since 88.
Your little boat might struggle a little more when loaded at the max weight but should do well with a average load.
It does seem that a 19" prop would knock about 800 off your rpm.

lakelover · Apr 22, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

Thanks, steelespike. Do you see any problem running at those rpm's with a light load? When I get the boat out again, I hope to borrow a gps to check the speed. (I'm one of those holdouts that doesn't even own a cellphone, I like to be unreachable.)

steelespike · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

I forget the year of your motor but::
My info for 89 says 4500 to 5500 and my aftermarket service manual says 50 hp is developed at 5000.
I wouldn't run over 5500 unless you see it in an Evinrude publication.
A limited test run to check your speed at wot is fine but not routinely.

lakelover · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

That's what I was sort of thinking. I'll try it out again and probably look into a different prop to get the rpm's down some.

lakelover · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

After a little research, I'm going to have trouble finding a 19 pitch that will fit my 50.

Johnson does not recommend a larger than 12-1/2" diameter for that motor, and to get a 19 pitch without paying $230+, I'd have to go larger. Not gonna invest that in a prop.

There are lots of 17 pitch available with 11-3/4-12-1/2 dia., I may have to settle for that & hold back at 5500 when (if) the rpm's start maxxing out. I've found several 17 pitch available for $80-123. That's more along my budget.

According to what I'm reading, a 17 should knock off 300-400 rpm. I guess I could not trim out so far too.

steelespike · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

I don't understand?What year is your motor and is it a 2 cylinder?
what prop in a 19" Is 12" The higher the pitch usually the lower the diameter.
As near add I can tell 75 -05 are the same design.? The list 19" here at boats is just one and its 113/4"

lakelover · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

The motor is a 2 cylinder 2003 Johnson 50, J50PLSTD. I think that might be the difference in availability.

I found this new on ebay:

Size: 13 x 19 Pitch RH Replaces Part Numbers:

382765
390896
176215
177139
763464
765184

I contacted the dealer & asked if it would fit my motor and they responded "Johnson does not recommend a larger than 12-1/2" diameter for that motor".

With the 12-1/4 dia. on there now, there is just about 3/4" clearance between the prop and the cavitation plate.

steelespike · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

I do wonder if the 13 is intended for a large gearecase I think yours is a small geaqrcase.
I've seen plenty of motors on here running about 1/4" At 13" it appears you would be at 3/8"as I'm sure you realize your only adding
3/8" to the radius.Of course you may want to double check your present dimension.
Are you buying only a certain brand? There must be dozens of 19" props out there less than 13" I would think the 13 is the exception.Typically a 19 would be the same or smaller diameter than a 17.
Just found a 13 X 19 13 spline OMC prop and its for a OMC stern drive.
Small gear case is 3 3/8" I do have to agree that 19" E/J props are not that many especially
at bargain prices, and the 17 Is available in all shapes and sizes.

lakelover · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

From what I've read, I have the small gearcase.

There's no particular brand I favor, I just don't want to spend my life savings on one.

You're right, I'd only be adding 3/8 to the diameter, I confess I didn't think of that. I'd like to find a diameter on the smaller end, I don't know, just feels like a little more clearance would be good. I never had to buy a prop before.

But, I'm having trouble finding the small diameters at 19p that don't cost a ton.

steelespike · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

I say give up on a 19 and take a close look at a 17" Hustler. This is an aggressive prop that should lower slip and control the rpm
better.Probably get that last little bit of speed. all for under $100. Usually described as ss performance at aluminum prices.

lakelover · Apr 23, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

Thanks, I'll definitely look into that.

lakelover · Jun 15, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

OK, I ended up buying a Solas 12x17 prop and that has solved my problems. With a light load (just me), it tops out around 5300 rpm and I can trim it out a bit for 5500; more trim is possible for when I have a heavier load. I still don't know my top speed, I have to borrow a gps to check that out. Hole shot is fantastic.

Thanks for the help!

steelespike · Jun 16, 2012

Re: Revs over spec - just want your thoughts

Re: Revs over spec - just want your thoughts

Well done.It will be interesting to see how the speed is.

lakelover · Jun 24, 2012

Re: Revs over spec - just want your thoughts **additional info & update

Re: Revs over spec - just want your thoughts **additional info & update

I finally was able to borrow a gps.

The prop I have gives me 36 mph @ 5500 rpm, in a light chop and a full tank of gas, just me in the boat. If I leave it trimmed in or mostly in, the rpm's top out at 5500, but if I trim out, it will still scream up to 6200 without "significant" speed gain. My thought is that the prop is just getting too close to the surface of the water?

I also had it out with a total of 4 people in it and it still performed well with good speed at WOT (didn't have the gps with me that time), just took a little longer to plane out when I sped up from a slow cruise. With a load of 4, if I trim it out, it will still exceed 5500, but not by much.

Johnson 50 - Revs over spec - just want your thoughts

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