Propeller on 1979 16ft StarCraft with 70 HP Johnson

[marcusknight]

I finally hooked my tach up to my 70 Johnson because I was eager to learn what the RPMs are with a 14 diameter 17 pitch propeller that is on my engine. It came with it and I am surprised it fits given I hear lots about people using 13.75 props for this engine.

Anyway, the idle speed in neutral seems to hover around 1000 rpms and WOT is about 5200 with just one person in the boat. How do those numbers sound? Other thoughts?

 

[Texasmark]

Sounds great to me. If you are talking about the difference in performance with only a diameter change of 1/4 inch, don't expect to see anything.

Mark

 

[jestor68]

The "standard" size prop would be a 13-1/4 X 17.

If you're running a 14" diameter prop, switching to the 13-1/4" diameter will allowing better acceleration and higher top rpm (+200-300 rpm) and higher speed.

Your rpm range being 5000-6000, you need to get at least 5500 rpm to develop full power; I.e.; 70 hp @ 5500 rpm.

 

[steelespike]

While the rpm seems reasonable without the gps speed at wot we can't tell if your tach is actually accurate or if your setup seems to be working.
Diameter is a secondary consideration.Doesn't usually enter into the equation except for problematic applications and to be sure it is a proper fit.
Diameter generally increases as as pitch decreases and of course as pitch increases diameter may decrease.
Diameter may decrease as blades are added.

 

[phillnjack2]

14x 17 is more for a cruiser, you would be far better off with having a 12 3/4 or 13 by 19 pitch for top speed and pull away.
your top rpm is way too low, you need to get it at max revs for just one person in boat to know the engine is breathing correctly.
a 13x 17 pitch is ok for light stuff when ski-ing etc.
now you don't say the year of the engine !! if its before 1992 then 5500 is your top end, if its later then 6,000 is the max throttle.

above someone stated the standard prop size !!! there is no such thing as standard prop size for any outboard engine.
just because one shop sells an engine with a certain size it don't mean a different shop will be the same.
the propeller is your final drive ratio, and with wrong final ratio the engine will not perform and use too much fuel.
props are an experiment, just because an identical boat engine set up use prop A don't mean that will be good for you neither.
extra's in the boat, type of water you intend on main use, altitude it all plays a huge part in selecting the right prop.
BUT a 14x 17 is not the right prop, as that is for a cruiser not a speedboat.

phill

 

[marcusknight]

Engine is a 1979 Starcraft 70 HP Sea Horse, as someone asked.

 

[marcusknight]

No Title

 

[jestor68]

above someone stated the standard prop size !!! there is no such thing as standard prop size for any outboard engine.


phill

If you visit the Evinrude site and look up a 17 pitch prop, you will see the the diameter is 13-1/4". That's what I meant by the "standard" size 17 pitch prop, since that's the size BRP offers in a 17 pitch.

And yes, there is such a thing as a "standard" size prop for a outboard motor. Look at most any owner manual and you'll see a "standard" prop listed.

A good example is(google it) is the Force 85/125 owner manual, which states on the specs page the "standard" prop size of 13 X 19.

 

[steelespike]

If you visit the Evinrude site and look up a 17 pitch prop, you will see the the diameter is 13-1/4". That's what I meant by the "standard" size 17 pitch prop, since that's the size BRP offers in a 17 pitch.

And yes, there is such a thing as a "standard" size prop for a outboard motor. Look at most any owner manual and you'll see a "standard" prop listed.

A good example is(google it) is the Force 85/125 owner manual, which states on the specs page the "standard" prop size of 13 X 19.

I think that the reference to no standard prop refers to the actual prop selection. Many times prop selection depends on the boat size ,design and weight.
Then the typical load and activities. The "standard prop" should be considered a starting point and may not be selected.

 

[walleyehed]

This engine likes 5500-5700 wot normal load and I agree, 14X17 sounds more like a load pusher. "Usually" this 3 banger uses a prop in the 12-1/2 to 12-3/4" dia., although prop design dictates this, I would say something in the 15P range is going to give best over-all use. The early OMC props in the 4.25" gearcase size where 13.25 for 17P, the later (pre BRP) was 14X17 for the 4.25. Most 3 cyls. used 12.75 dia. props, but 13 is not that different unless you have a larger blade surface.