Prop help for 1981 classic

C

C-Val

New member
Hi Everyone

I am struggling with low rpm on my 75 etec 2012 on my classic
I am using a three blade aluminum 13.25 x 17” prop and am only getting 4200 wot with 2 people and normal gear.
At this rpm I am around 25 knots.

I was going to order a three blade aluminum prop
13.25 x 15” but I understand this will only put me up to 4600

Do you think a 4 blade prop will give me a bit more?

If I only get 4600 I am still under 5000
Will this be a bad thing?

If I go to a 15” how much speed will I loose?

Thanks
David and Val
 
More blades with the same diameter and pitch usually means more load on the engine, thus lower rpms. The lower pitch will increase rpm, but unless you run at wot, it won't increase speed.

In my motorcycle days leaking rings on a two stroke let blowby in the crankcase and diluted the incoming fuel/air charge and reduced power. That is my guess as to what is going on. The engine is just tired.

But replacing the prop with a lower pitch one is probably a good temporary solution.

David
 
David and Val,
I have been thru this exercise, so here are my opinions for what they are worth.
First, there is a lot of info online. One reference which helped me with a step wise approach was: http://www.veradoclub.com/index.php?topic=137.0
We have a CD 22 with a 2007 90hp Etec. While a 15" prop put us in the acceptable rpm window with a light load [2 people, 20 gal fuel, no water, gear], it was on the low side. We usually carry [over half the time] a load more like 4-6 people and more gear [800-1100 lbs], so we decreased pitch to 11". Now we can turn about 5400 rpm lightly loaded and stay in the 45-5500 range loaded. Yes, speed dropped a few mph, but I think it is more important to work the engine comfortably than to milk a bit more speed out of it. We run at about 4000-4200 usually which yields about 16- 18 mph. This has been our experience, and comports with our priorities for our circumstances. Ours is only an example of the exercise and of our thinking.
What about you? To my mind, 4200 is low and you are lugging your motor, which will shorten its life even if run at less than full throttle. Your acceptable rpm range is 4500-5500. The idea of about 200 rpm per inch of pitch seemed to hold for me and is widely quoted. If you cited your usual load [2 people and gear] for 4200 rpm, you might consider a pitch of 13" which would probably get you around 5000 rpm WOT. A Solas prop for Etecs is about $100 US. If your rpms are then too high [doubt it] you can get the prop cupped to decrease rpm/increase load on prop. If your marina or friends can help with loaner props, that would be nice. If you like to run stainless, maybe try a cheapy aluminum and when you get it right, go stainless and keep the 17" and the cheapy for spares. You can figure all of that out!
DavidM had good observations on 2-stroke motorcycles. While my experience with 2-stroke motorcycles is limited to a 50-something Zundapp, my experience with true 2- stroke outboards is that failing rings resulting in increased blow by compromise idle first and foremost. If it won't idle, and it is tuned, check compression. Etecs are a different breed, tho, because they have direct injection, and do not depend on crankcase pressure for fuel delivery. They do depend on crankcase pressure for air intake. I doubt that your motor is worn out. If you do prop down in pitch and it does not help, checking compression is pretty straightforward - again, I doubt that you will need to go there.
I hope this helps!
andy
 
Thank you guys for sharing your experiences.

I have to redo my rpm test making sure the motor is not trimmed in too much. I often trim down to keep bow down.

That is a scary thought if getting gas in the oil if lugging too much!
The motor only has around 275 hours on it so I think it’s more of adjustment and prop than wore out. I have to work this weekend but the following weekend I will redo my test and let you know the results.

I would be happy with a 15” but I hope I don’t have to go down to 13” for reason of loosing speed.

Is there a formula for how much speed you loose per reduced inch of pitch ?
(Like the rpm formula for 1” of pitch)

Thanks
 
C-Val":3svfbp5w said:
Thank you guys for sharing your experiences.

That is a scary thought if getting gas in the oil if lugging too much!


Is there a formula for how much speed you loose per reduced inch of pitch ?
(Like the rpm formula for 1” of pitch)

Thanks
Click to expand...


Too many weight / hull design variables for a one size fits all formula.
Your engine has to make what ever RPM the book tells you it is supposed to be at WOT or you are killing it at all rpm ranges.
You will probably will need a 13 or even an 11 pitch prop.

Good luck
:mrgreen: :beer
 
When determining top RPM, you want to put the engine at full throttle, and then gradually experiment with engine trim--often to get the full WOT, you do trim up some. Just avoid cavitation/ventilation or porpoising.

Often you will gain speed as you bring the engine up to its rated RPM--since the engine is now developing full hp.

Retry the WOT, normal load, and experiment with the trim. If still low--go to the 15" aluminum prop. A 13" pitch is normally one you would use at a higher altitude. But I ended up with an 11" pitch being the best at Lake Powell, with a 25 and 130 Honda--actually had several mph more speed than the 13" and 15" props we had tried previously.
 
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