I have a propeller question

67-69johnman

Petty Officer 3rd Class
Joined
Feb 17, 2012
Messages
75
Hi, I have a propeller question, obviously. The pride and joy of my collection is a 69 Johnson 20. I'm wanting to put an 11 pitch prop on as I've run it with a borrowed one in the past and I am pretty pleased with the performance. I'd like to purchase one for my very own but I'm not understanding the prop terms used nor do I really understand really anything about my own prop.
What is thru hub exhaust.
Is 11 pitch appropriate for my boat?(16 foot jumboV )
http://boatpropellers.iboats.com/Jo..._11.00-Pitch_Propellers/29/?*******=772689138
Is this really what this propeller looks like or is that just a generic photo
Is this a prop you personally would choose?
Thanks Alex.
 

BonairII

Commander
Joined
Jun 7, 2011
Messages
2,727
Re: I have a propeller question

There's a separate section for prop questions. A mod will probably move this thread to that section.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: I have a propeller question

If you read "read this first" at the top of the page it will help you understand what is involved in prop selection.
The prop image is generic.
Thru hub exhaust the exhaust comes out the center of the hub.
Your motor has an exhaust snout which you have probably observed.
Also you may have noted the your prop has no provision for exhaust exit.
Some motors you can use a thru hub on when they are not through hub.
Not the case for your motor.
Prop selection is not as simple as picking a number and sticking it on.
Before making a change you should know your present prop size
and the wide open throttle(wot) rpm and speed of your present setup.
Its important that your motor be able to rev freely within its rated rpm.
69 is rated @ 4000-5000 rpm You would normally prop toward the higher number.
A Tinytach is reasonable and easy to install to get your rpm and a gps is excellent for speed.
11" probably isn't ideal for your setup unless the boat is light and you carry light loads.
If you can get us your prop size near the base of the blades. and a gps speed we may be able to
loosely estimate your rpm but will have to use too many guesses of other parameters
to be very accurate.
 

67-69johnman

Petty Officer 3rd Class
Joined
Feb 17, 2012
Messages
75
Re: I have a propeller question

ok thats a lot of info at once. most of which i have
if i turn the dummy light in my head off.
i would notice that i do have an exhaust port above the gearcase assm.
the dimension is a 9x10
and with just me and the dog in and a cooler full of ice i clocked us at 23mph
i just so happened to install a tach for a brief minute and at full rev we are betweet 2300 and 2400 rpm thats on one plug so im assuming i just multiply by 2 and get between 4600 and 4800 rpms for the crank.

well the prop i use seems to push the boat fine with two men and about 5 bags of floating decoys and all our gear (probably an overloaded boat) when the 9 by 11 was spun off the hub and consequently donated to the lake gods i went back to the 9x10 that came with the motor.

im still not clear on how that happened. i've always been told that shear pins should be made of a somewhat soft metal to prevent damage to the gearcase and ultimately the powerhead. But the shear pins that are sold by the local marine shops here are stainless steel. i just dont understand how a prop can spin completely off of its hub break through the plastic hub-nut and still not break the shear pin.

i can only assume the rubber hub was either damaged or inferior.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: I have a propeller question

Using 10% slip, high end of normal, I get an rpm of 4720
A reasonable number your figures seem to confirm that.
I'd like to try for more speed but I don't think an 11" prop is the right solution.
Is the prop in excellent condition?Motor in fine tune?
Does the throttle open all the way?
Is the boat a bare bones utility with a smooth bottom?
Is the prop an older style with somewhat thick blades?
 

67-69johnman

Petty Officer 3rd Class
Joined
Feb 17, 2012
Messages
75
Re: I have a propeller question

the boat: an '83 lowe 16' jumbo-V bottom boat we are about 48" at the floor with a 68" beam. the bottom has runners not smooth. rated for 40hp motor, 835lb load or 4 persons and gear.
The prop is usual condition for its age its not a new prop but does have a few nicks but the blades have not been excessively worn.

the motor has new points set to 18 thousandths as per instruction from the factory. plugs are set a 30 thousandths. and the carb has been rebuilt and i would have to say we are winding all the way up. im not experiencing a loss in power by any means. i basically just think that other prop pushed the boat a little faster than this one does.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: I have a propeller question

I'd like to see how it operates with a new 10" prop.
I feel that an 11" will lower rpm by about 200 maybe more.
Will slow hole shot and may struggle more with a load.
The motor will be better off reving freely closer to is top rpm.
Your antivent (cavitation)plate just above the prop needs
to be at least even with the bottom of the boat. Adjust the trim carefully.
Most setups you would trim out 2 to 3 notches and attempt to balance the
boat so neither to much fore or aft.Though usually with a stern bias.
The boat could run about 27 mph on a light reasonably fast boat.
To raise the motor just use a stick on the top of the transom
to hold the height till you tighten the clamps.
 

67-69johnman

Petty Officer 3rd Class
Joined
Feb 17, 2012
Messages
75
Re: I have a propeller question

I too would like to see that and 27mph is really cookin in a little boat like that.
I mean I guess I theres not really any time I would NEED to do that but I guess I just want the best performance I can get out of that little gem.
But I do have a new prospect though guy on Craigslist has a 78 evinrud 35 horse short shaft tiller that runs but hasn't been used in a few years he wants 75 kno for it I told him I WANT THAT motor.
Would it be wise to leave the stick in to adjust the height. I can visualize the motor slowly making its way back down in travel.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: I have a propeller question

You can leave it if you want.If it works its way down its not tight enough.
Just tack the stick in place.It doesn't add any real support for the motor.
be sure the clamp has a good grip.
That 35 will sure solve any speed/load issues.35 is rated for about 33 mph.
Be sure its the right shaft size for your boat.
Keep in mind the 35 probably weighs about 128 lbs assuming manual start.
Your 20 is probably about 77 lbs.Probably no big deal unless you want to carry it
a lot.
The 20 would use about 2 gph(gallons per hour) at wot. The 35 about 3.5 gph.
If you throttle back to about the speed of the 20 the 35 will be just loafing along
saving a little wear and tear and gas.
 

67-69johnman

Petty Officer 3rd Class
Joined
Feb 17, 2012
Messages
75
Re: I have a propeller question

From the pictures it appears to be a fifteen inch short shaft and I happen to have a fifteen inch transom.
But I consider myself a student.
Do I need a 35hp probably not.
But it is an affordable project. I would probably do it just to see how it works.
It's pretty new to me but I looooove working on these things

But it would be pretty cool to take a 75 dollar motor and turn it into a lake monster like that thing could be
Anyway though of my three late 60's Johnson 20's I have. I took the best looking prop I have (another 9x10)
And slapped it on there and tomorrow we will be going for a test run.
 

67-69johnman

Petty Officer 3rd Class
Joined
Feb 17, 2012
Messages
75
Re: I have a propeller question

Question. Is there a place I can purchase the hub alone? I have a 25 ton press at work
Do you know of a source?
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: I have a propeller question

There is another factor to consider when buying props. a 9x11 prop from manufacturer "A" may not and generally does not perform identically to a 9x11 from manufacturer "B". One manufacturer may optiminze for hole shot while the other may optimize for top end speed. You don't get both with the same prop. This is why prop selection is such a crap shoot. But the prop you selected from the iBoats store is proper for your engine. By proper however, that means it fits and was designed for that engine. that does not mean it is the best prop for your specific combination of boat, load, etc. You really need to get accurate WOT rpm readings with what you consider an "average" load. Only then can you determine if what you have actually needs to be changed.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: I have a propeller question

If you go to check out the 35 do a compression check.Your shooting for at least 100 psi and within 10%
between cylinders.The 78 has the CD2 capaciter discharge ignition.A spark check wouldn't hurt as well.
The 35 should have the new style thru hub gearcase, probably have a wider choice of props.
As you probably know the 35 is the younger broher of your 20 with a proven basic design that dates back to the early 1950s.
You could probably get a hub insert from a prop shop.You might try a search useing words like prop hub replacement etc.
I wouldn't expect much difference with your prop change but nothing ventured nothing gained.
I do believe the new props arew more sofisticated than props of the era of your motors.
 

67-69johnman

Petty Officer 3rd Class
Joined
Feb 17, 2012
Messages
75
Re: I have a propeller question

TEST RESULTS
Okay went out sunday with two motors the current user (69 johnson 20hp) and the recent rebuild (67johnson 20hp)
both had 9x10 but the prop on the 69 is in the best shape i have. bot motors tach out at 4800-4900 rpm
the prop in bad shape pushed me and one other person to 18 mph.
the 69's prop however pushed us along at full rev at a blistering 21 mph

my conclusion is in agreeance with yours in that a 10 pitch prop in tip top shape will probably push me the desired mph's
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: I have a propeller question

Just for laughs I used google to go to youtube I put in youtube 20 hp johnson.
Several runs came up of 68 ish 20 hp on various boats they seem to .
get out of the hole quick and scoot pretty good. One was jacked up pretty good.
You can see the stick and you can here it growl and ventilate a little. Probably could go pretty good if not so rough.
Only one had a gps that was a 83 or 93. showed 24 I think with only one aboard.
 
Top