1978 70 horse johnson prop selection

[boarder290]

I?m looking at buying a new prop this season as the old one is starting to slip occasionally when taking a tight turn pulling a tube. Currently the prop is the original 13 1/4 X 17 and runs at 5100 rpm at 33 mph (gps). The boat is a 1960-1970 mirrocraft 19 ft.

I?m looking at going to a 13 3/4 X 15 cupped michigan wheel prop (011002). I?m hoping this will give me another 100-200 rpms along with a little more hole shot but comparable top speed.

Just wondering if my logic makes sense or if anyone has any other input (as I am new to boating) before I spend 150 on a new prop

 

[SparkieBoat]

Re: 1978 70 horse johnson prop selection

the slipping is because of air cavitation, nothing wrong with the prop other than it is the wrong pitch for your boat. you defiantly need to go to a 15 or possibly even a 13 or 14. that motor loves to run at around 5800 RPMs. the 15 will be a good choice and put you right around 5500 RPM.
yes you will have better hole shot and probably same top speed +- 1-2 MPH.

 

[boarder290]

Re: 1978 70 horse johnson prop selection

I also forgot to mention that the current prop is un-cupped. I have read that cupping a propeller is the same as adding 2 to the pitch. So essentially a 15 cupped is a 17 un-cupped. Is there any noticeable advantage between cupped or un-cupped at my slower speeds? (Compared to a 70 mph boat where the speed vs. rpm range is much broader) The prices are essentially the same.

Based on what you said I would think that a 13 cupped would be optimum for hole shot and top end.

 

[SparkieBoat]

Re: 1978 70 horse johnson prop selection

you should check with a local shop that will allow you to test several different props before a final purchase. there are some prop shops that will be willing to do this for you, some charge a minimal fee for each prop tested. this is the best way to go. If you cannot find a shop that will do this a 13 cupped should be about right. but just the slightest change in prop pitch or cupping can make a big difference sometimes, the 200 RPM per pitch is just a rule of thumb, it can vary widely in real life.

 

[steelespike]

Re: 1978 70 horse johnson prop selection

Interesting calculator numbers 5100 @ 33 comes up less than 3% slip. using 2.42 gear ratio.
A 15" prop at 5500 same slip runs about 31.Its hard to believe you can improve on 3 % slip even with a cupped prop.
I do wonder if your tach is reading low.5500 rpm would be about 9%slip probably reasonable for a 19' boat.

 

[SparkieBoat]

Re: 1978 70 horse johnson prop selection

I think he said he was using a 17 pitch prop not a 15. he is considering going to a 15 or a 13 cupped.

 

[steelespike]

Re: 1978 70 horse johnson prop selection

I think he said he was using a 17 pitch prop not a 15. he is considering going to a 15 or a 13 cupped.

Perhaps I worded it awkwardly. I was refering to the possible results if he was to go to the 15 he was considering.
Assuming a 400 rpm increase going to a 15.
Assuming the ratio is correct at 2.42(I checked a number of years)
a 17" prop 5100 rpm and 33 mph is just under 3% slip.
With an uncupped prop 3% seems unlikely it would seem the tach could be reading slightly low.
Not a deal breaker as far as prop selection but perhaps needs to be considered.
For a motor said to like higher rpm it has a way to go even if true rpm is 5300 or 5400.