Need higher rpm

[joewilshire]

1973 Johnson 115 hp.
Recommended rpm's at wot...4500-5500.
I have a aluminum propeller marked K922.
I only get rpm of around 3800.
I tried to measure the prop and I think the diameter is just a bit bigger than 13".
I also tried to measure the length of one fin at 6".
Does anyone know what size the stamped numbers mean and what you would recommend to get recommended rpm's.
It's a 24 ft pontoon boat.
Thank you.

 

[steelespike]

There may be numbers under the prop nut.
​What is the gps speed at 3800?
Assuming a 2.00 gear ratio an 11 or 13" pitch would be a good starting point
Diameter generally increases as pitch goes down.
Today an 11" pitch will likely be about 14"
Your 13 1/4"? diameter may be an indication of a higher pitch.
I think a 73 may call for an 11" pitch as it may be a little tired.
Your better off erring on the low side than to have too much prop.

 

[joewilshire]

The gear ratio is 13:26 according to service manual. Does that in turn mean something like a 1 to 2 ratio? In other words...each revolution of the motor equals 2 on the prop or is it 2 revs of the motor equals 1 on the prop?
To answer your question...gps speed at 3800 rpm is 20 mph.
I will see if there are any numbers under the prop nut this weekend and let you know.
Thanks steelespike.

 

[joewilshire]

pic 1

 

[joewilshire]

pic 2...may be stainless but it doesn't seem that heavy.

 

[steelespike]

13/26 is 2 to 1, motor 2 revs prop one rev.If it is a ss prop it will be magnetic, aluminum will not attract a magnet.
The angle of the blades seems extreme, top picture. Perhaps it is a 22" pitch.
I lean toward an 11" pitch as I've said before, better to err on the light side.

 

[joewilshire]

Thanks again steelespike. I forgot about the magnet trick.
I will be ordering a propeller with 11" pitch. Probably one not too expensive at first. Would you, or could you recommend a brand?

 

[steelespike]

The Solas Amita and Rubex have a good reputation. Michigan Wheel has been around for close to 100 years.

 

[joewilshire]

Done. Ordered the Solas Anita. Should be here Thursday. Hope to install it and check out this weekend. I'll try and get picture of GPS and tachometer at WOT.
Talk at you later.

 

[walleyehed]

This may sound crazy but I recognize that prop by blade shape and profile...It's a 13X21 by MWC.. Steelespike has ya headed it the right direction...

 

[joewilshire]

Got the old propeller off. Hub was stamped 12 3/4 x 21.
Put new propeller on and went to the lake. Great hole shot take off. Got 5100 rpm at 23 mpH at WOT.
I may bring trim up one more hole to see how it works but don't want to get cavitation. Is it really noticeable if the trim gets too high?
Thanks greatly steelespike. You did me good.

 

[joewilshire]

This may sound crazy but I recognize that prop by blade shape and profile...It's a 13X21 by MWC.

Do you think it's stainless steel? It is not magnetic but some stainless isn't either. How much would you think it's worth? It's not in too bad of condition.
Thanks