johnson 75 hp prop

diveplane

Petty Officer 2nd Class
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Feb 5, 2017
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guys need help what prop will i need for this outboard

1979 johnson stinger chequered flag
75ELR79
 

Sea Rider

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Need to start with something, check if possible which prop was factory delivered with that OB as a starter. Will need a tach to know wot rpm runs and go from there.

Happy Boating
 

phillnjack2

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Apr 30, 2011
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No such thing as a factory prop it came with, that would of been down to original dealer.
.
you need to tell us more about the boat type and the use you want from it.
if this is a long shaft motor (number suggest it is) then start with a 13x19 prop for a say 14-15 ft family speedboat top speed with decent pull away.
a 13.5x15 for pulling a skier or slightly heavier boat.
real heavy cabin cruiser a 13 x 13 prop, so until you tell us what type of boat and use etc we cannot say what is a good choice or not
 

jimmbo

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That engine used two different gearcases. The 15 in model had a smaller gearcase with a 15:28 gear ratio and was shipped with an 11 1/4 X 17 prop. The 20 in model had a larger gearcase with a 12:29 gear ratio and shipped with a 13 X 17 prop.

What prop you need will depend on the boat it is on, and the intended usage.

A word of warning, the 75hp is a little weak on low end torque, compared to the 70hp. Getting 1.5 hp/cubic inch of displacement does that. So propping will be more critical as the engine has a much narrower and peakier torque curve. It is rated 75hp @5500 rpm and the WOT range is 5200 - 5800. The milder 70 made its power at 5000 and had a wider rpm range of 4500 - 5500.
 
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David Young

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According to my 1977 Evinrude Factory repair manual, the 75 horsepower motor was shipped with a 17 inch pitch propeller from the factory. I would start out with a 17 inch pitch and see what your wide open throttle RPM is :)
 

phillnjack2

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Look at the model number the poster gave, its a longshaft so its 20 inch mid with 2.42 ratio.
the 75 hp motor can rev to around 7,000 rpm and still be making power , the 75hp is a higher ported motor than the 70hp (both at 49 cu inch.)
75 got different carbs to 70hp and different jet sizes too.
prop for that around 19 to 21 pitch, if boat is lightweight you can use the 13 3/4 x 23 prop that omc made for that and the v4 motors.
you might find that motor dont even have a rev limiter, lots of the stingers didn't have limiters unlike some of the others.
the evinrude hustler is a 75 hp identical motor in just different colours.. go tell the race boat people it is only for 5500 rpm and they will think your nuts.

johnson and evinrude 75hp did NOT come from factory with a standardised prop. the range was from 11 pitch to 19 in alloy and upto 23 pitch in stainless.
most people who opted for stainless used the 13x19 sst prop . a 17 prop is ideal for small sports cruiser and a 15 pitch for pulling up a mono..

David young
if you look at the sales brochure youl see the listed sizes of props is huge for the triples, there is no standard prop.
you tell the deal what you wanted and they put that size on for you, only time it cost more was the sst props.
 

flyingscott

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Wonder what the life expectancy was of that motor at 7000 rpm? The one thing I would say is stay within the recommended RPM range, jimmbo is exactly right on that. And find a dealer that has the service bulletin on porting the prop as that will help with holeshot.
 

jimmbo

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7000 rpm? It will be making about 55 hp at that speed, and not for very long.

BTW the brochures did list a prop installed and shipped with the engine. How do I know, I am holding the 1979 Evinrude Brochure in my hand

Porting/venting the prop is probably a good idea, being the engine is a little weak at the bottom end.
 
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phillnjack2

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Jimmbo I suggest you go and learn a bit more about outboards made by omc before making such comments.
porting is done on the engine sleeves, venting is done at the prop. 75 stingers were not a bottom end motor, thats was due to being a higher top end motor
also the cylinderhead was a higher compression head on the 75's to help compensate for the higher porting..
The johnson and evinrude 75hp stingers made over 80hp at the flywheel at 7200 rpm. (about 68hp at the propeller). mercury 70hp motors realy made about 50/55 at the prop. unless you found a 650xs, totally different to normal motor.
why do you think they were so successful in their racing boats ? I am just talking the fishing motors not the mod 50 that produced over 105hp like the 31m or the
triple that had the exhaust on the other side that produced 115hp !!!!

Most 75hp motors did not have a rev limiter and could infact climb to around 8,000 rpm with small props. this obviously will make the life expectancy drop[ if anything should go wrong.
on the 75 hp motors the 15inch models came with what was known as the nitro foot gearcase,
nitro foot is the small gearcase found on most 15 inch motors of 75hp running at 1.86 to 1 ratio and using props around 12x22 pitch...
The longshaft (20 inch mids) came in 2.42 ratio and used what became known as the clubfoot gearcase.
.
At 7,000 rpm the life expectancy is not hindered at all on the 75hp stingers, those that did have limiters were often 7200 rpm not 6700 like the later 56 cu in ignition.

Being as the OMC 75hp motor was very well designed it was the reason that mercury could not keep up with the omc triples .they became known as the mercury killers.
if you look at the much later 56 cu in omc SST60 race motors and see them race against the old 49cu in 75hp stingers you willl see there is hardly any difference in top end speeds attained. maybe 10mph unless the sst60 has been modified..
What you seem to be forgetting is that the stinger 75hp was a very high performance motor, it came with its different carbs, higher exhaust ports, and the ports were chamfered at the tops, unlike the previous 55,60,65 and 70hp variants.
maybe this will help you understand a bit about the 75's.
these boats below are run with total stock motors, NOT the omc race motors that are over 100hp, these are stock motors you could just walk into a dealer and buy.
engine pictured is a stock Evinrude Hustler 75 short shaft, identical apart from colours to the johnson 75 stinger.
 

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phillnjack2

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you need to watch a few things like this, where peope all get together and race their boats.. all stock engines.
In these races their is mainly 3 motors. the johnso/evinrude 7 hp 49 cu in, the later 56 cu in bridgeport and the SST60. you get the odd mercury but he is just having fun and dont mind coming last... and recently they are using oval port 56 on the older 15 inch mids.
go to youtube and type in six mile lake t-party 2013... you might see why the stingers and hustlers were so popular on lightweight boats..
 

flyingscott

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People telling you to over rev will not warranty their advice so I would be careful taking that. When a rod goes through your block and you come back here looking for advice why destroyed your motor and you tell what you did, They will tell you it's because of over revving. A motor that is raced or over revved consistently IS NOT going to be reliable anymore just a natural fact of abusing a motor and racing and over revving abuses a motor. Things wear out and break, if the manufacturers who spent millions of dollars on designing your motor wanted your motor to rev to 7000 rpm. They would have put that in the owners manual and warrantied it but they say if you race the motor there is no warranty anymore. They put the rpm recommendation in there because they want that motor to be reliable and last along time. Yours has lasted almost 40 yrs sounds like the designers knew what they were doing. Get a tach and prop your motor for the recommended RPM and enjoy it for a good long time.
 
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phillnjack2

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flyingscott to come in like that you must obviously be a OMC 2 stroke GOD, fantastic to have such a wizzard on here.


Can you now tell the people exactly what happens when you lug a highly ported 2 stroke motor by over propping it to keep the rpm down ?
maybe explain just how stressful lugging is compared to letting a 2 stroke motor breathe.
tell them what the pitfalls realy are and just how common this problem is !!!
oh and of course the price of a rebuild kit that will soon be required by doing so.
.
Tell them what happens when they get all that nasty carbon build up that can get stuck in the rings and cause ring failure that leads to deep scores in the bores ?
and the little piston ring pins to fall out the gaps and run up and down the bores like demented pixies between the piston and sleeve.

tell them about the over heating of the two bridges in each exhaust port that get so hot they bend inwards causing ring failure, then they finally break and the
broken exhaust port bridges run around here there and everywhere causing nice little custom gouges in the block, in the head and everywhere possible.
piston and bore scoring and destruction of the cylinder head due to shrapnel flying about in the combustion chamber ?

and please tell the people what else over propping realy does to a motor like this. .

Oh I am sure the readers will want to know why lugging can cause all this plus the big ends and small ends to wear premature crack up and send the rods through the side of the block. maybe the main bearings and end seals will also suffer from chunks of metal doing the rounds ?
surely this cannot be true just by causing the engine to be lugging is it ?

Now mind how you go when getting down off that High chair.


p.s how many con rods have you seen go through a OMC 2 stroke block due to over revving ?
 

diveplane

Petty Officer 2nd Class
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Feb 5, 2017
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the engine is mounted on my 15 footer glastron futura 162
the boat empty weighs 720 pounds, the outboard weighs 230
..

so any recommended props guys>?

prop that won't strain the motor.
 

diveplane

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Feb 5, 2017
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so say calculate passengers gear and gas as well, x2 passenger weight 250 me my wife 190
say 12 gallons gas one spare tank and another for engine.
cooler and some gear.
 

diveplane

Petty Officer 2nd Class
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Feb 5, 2017
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if i was to add another 2 passengers 200 pounds each.

max my boat i will allow is 4 adults.
 

diveplane

Petty Officer 2nd Class
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Feb 5, 2017
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best prop for this mass weight? same time not knocking the crap out the engine, i wont run the motor beyond its spec.. or speed.
 

ondarvr

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I think you already had a few suggestions of where to start, and a starting point is all it is, you need to dial it from there.

​Pick an RPM range you're comfortable with and choose a prop, maybe borrow one from the shop to get some numbers so you know which way to go from there. It's very unlikely you'll buy the correct prop the first time, so if you get a used one first it can always be used as a spare to get you back to the dock if needed. Or if the dealer will let you try a couple so you get the right one it's even better.
 

jimmbo

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Well without an existing prop as a baseline... Normally a 3 blade 17" is a good starting point for that size of boat. However, as said before propping in a little more critical with that engine... I think somewhere between 15" and a 17" would be right. Speak nicely to a dealer to let you take a few props for a test, with the understanding you WILL buy one from him, and if you damage any of them, you will buy them.
 

flyingscott

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I would start with a 15" pitch. I ran a similar motor on a 16' sylvan that was a little lighter than yours and that kept my RPMs between 5500 and 5800 rpm depending on load. Venting the prop made a world of difference on the motor to.
 

Sea Rider

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No one will tell you eyes closed to buy an XXX diam/pitch prop which is best performer for that combo. If you dial a prop that makes combo to run near max wot revs as usually loaded, you're a extreme lucky boater. Usually best prop is dialed at second attempt is lucky enough, if not surely at third attempt. N? of passengers, max load is an inportant factor to have in mind when dialing best top prop..

Happy Boating
 
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