ADVICE ON PROPELLER PITCH vs CUP

[JOHN CLINTON]

I need some advice on propeller pitch vs. cup.
I own a 1959 Glasspar G-3 powered by a 1978 Johnson 75hp short
shaft “Stinger” motor. Nitro gear housing.

I had been running a reworked Michigan Wheel
Stainless Steel propeller, reworked from 15” pitch to 16” and a little
cup added. RPM’s max’d out at 6000rpms. I was happy with
hole shot and top end. About 44MPH.

I added a CMC PT130 for trim & tilt and raised the motor up
an additional 1”. My RPM’s jumped up to 6500 RPMs when fully
trimmed up. I also gained 2.5 to 3 MPH. Hole shot was even better
due to the trim & tilt. Problem now was too high RPM’s.

I had the propeller shop (same one that reworked the 15” to 16” change)
add an additional 1” pitch and add additional cup to the propeller. My logic
was 200 rpm for the additional 1” pitch and 200 rpm for the additional cup.
When I boat tested the adjusted propeller, the boat would not
plane off without laying over the front deck, motor trimmed down.
When on plane, RPM’s dropped to 5500 rpm at full throttle with motor fully trimmed up.
Speed went down 4 MPH. So it’s back to the propeller shop.

My questions are: What has more influence on hole shot, Pitch or Cup?
What has more influence on top end, Pitch or Cup?

Should I have the propeller shop remove pitch or cup or both?

I would like to have the performance near or better than with
the CMC unit installed and the 16” pitch prop. and get the
RPM’s near 6000rpm.

Thanks for your help and advice

John

[olmo40]

if im following right you had 2 inch pitch added ?? which is a lot to a stock prop . Id say your problem is Pitch .Id say if you took the cupping off you may get some speed back at the cost of grip, Take the 1 inch of pitch off and it should be happy days

It is a thin line to balance when you are nearing the limit of performance .You need a combination of both ,you then need to see how much cavitation you have (grip) vs top speed .

Here are some figure i ran on a job this week for a kids race boat .

11inch solas stock no cupping 41.5mph no grip sluggish out of hole and lots of cavitation
11inch with cup 37 mph better out of hole good grip .
10 1/4 cut 3/4 inch pitch out left the cupping ,good out of hole ,good grip 40mph. Game over

[ponyboy]

John,
Its JM from Indiana, I think your in the right place ,Now we need MORE info. Maybe we can find some X racers of this motor from the ol J class.

[Hottrucks]

I would leave the cup and take out the pitch
the pitch will restore your planing issue and a bit more cup that you added will give a tiny bit more loss of the rpm you are looking for....... you may also try to raise the motor a bit before trying any of this......


all that being said I would have just run the motor at 6500 with a little extra oil it shouldn't hurt it a bit........

[thelightning]

Arent those stinger engines built to run 7k rpms? they are not stock engines.

[minister]

The manual says 5800. I run mine at 6600, it doesn't seem to bother it much.
The problem with these motors is too little torque at low RPM.
Mine runs at standard hight (cav plate 1" above bottem) 50MPH with one on board. Put 3 people in it and it wont go on plane. So for usability keep pitch low and RPM high


My neighbour who used to race these motors in t-850 (in the Netherlands) claims running them at 7k doesn't pose a problem. But they didn't mind rebuilding em....

[JOHN CLINTON]

It sounds like Pitch is the answer to my question and solution
to my current performance problem. I’ve now got too much for
this boat and normal load.

I’ll get it back to the propeller shop and discuss it with them.

I’m glad to hear that a lot of 49.7 cu. in. 3 cylinder OMC
owners run them in the mid 6000 rpm range. Maybe I should have
left well enough alone.

Thanks for the replies guys!!