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  If max RPM is too low, is that bad for engine? (prop question)

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Author Topic:   If max RPM is too low, is that bad for engine? (prop question)
David Jenkins posted 06-12-2003 11:22 PM ET (US)   Profile for David Jenkins   Send Email to David Jenkins  
When my 15' is not loaded with gear and fuel, my 4-stroke Evinrude 70 turns a maximum of 5800 RPM. But fully loaded, I get only 4600 RPM.

I am about to take a long trip with the boat fully loaded. Is it bad for the engine to run it this way or is the only downside that I will not be able to go as fast as I could go if I were to use a smaller prop?

Sal DiMercurio posted 06-12-2003 11:50 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Your over propped.
I believe that engine is rated for 6,000 rpms.
If i'm correct, you should prop the engine to turn 6,000 with a "normal load", not with a light load, prop it for the way you normally run the boat on an everyday trip.
Yes you will damage that engine by going on a long trip with the boat propped for a no load situation.
I don't know what prop your now running but if I were you, i'd get a prop with at least 5" "LESS" pitch just for this trip.
This will allow the engine to get close to it's maximum rpm range, but wont put any load or strain on it.
Yes it will knock your top end speed down, but with the current prop, i really feel you may not even finish the trip.
The way it's propped right now is the same as driving your truck in 4th gear all the time even in the mountains.
It sounds like your running either a 19 or 21 pitch & need to drop back to a 15 or 16.
What size prop are you running.
You really should take this post seriously because your going to ruin that engine.
JBCornwell posted 06-13-2003 12:18 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Redline on that engine is 5800, Sal.

I agree with everything else you said.

Red sky at night. . .

David Jenkins posted 06-13-2003 01:12 AM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
Thank you very much, Sal and JB. I will change props before the trip. My current prop is stainless, three blades, and it has 13 1/4 x 19 on it.
lhg posted 06-13-2003 03:27 PM ET (US)     Profile for lhg    
Running full throttle at low RPMS's causes detonation, which WILL ruin the engine, as stated.
Sal DiMercurio posted 06-13-2003 10:38 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
You should be looking for a 13.25 x 14 or 15 & possibly raising the engine "up" one set of holes.
A 14p would put you right at 5,800, but a 15p & raising the engine should also put you very close to 57 - 5,800
David Jenkins posted 06-13-2003 11:21 PM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
Today I bought a 13.25 x 16 but I can return it for a prop with a smaller pitch if that is necessary. I took a test run with it but there was a good chop and a strong wind and I did not get a chance to open it up all the way. I did make it to 5000 RPM, though.

I rarely run the engine at full RPM. I understand that I could ruin the engine if I continually ran it at full throttle and it maxed out at less than 5000 RPM. But I rarely run it more than 80% full throttle.

I am glad that I got a new prop and I want to get it right and will return it for one that has even less pitch if that is necessary. But I'd like to think that it is not necessary.

Remember that under a light load, I got 5,800 RPM with my 13.25 x 19. So with thie 13.25 x 16 I am going to have to be careful when operating under a light load, especially if I let someone else drive the boat. For this reason, I am reluctant to get a prop with a pitch of less than 16.

I eliminated the jack plate today and have the engine mounted directly to the transom, raised 3 holes. The boat sits much better in the water without the jack plate (it is less back-heavy).

I will test it again tomorrow, now that I am back inland (I was on the NC coast today).

I tried to trade in my 13.25 x 19 but the dealer said that no one would want that prop because it had so much pitch. Can a prop shop change it to a smaller pitch? If not, would it bring anything on eBay?

Thank you!

Sal DiMercurio posted 06-14-2003 08:42 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Your main concern right now is getting a prop for your trip, I think that 16 pitch might not be the right one for your trip.
Your engine "MUST" be able to get to 5,800 rpms with a loaded boat, don't even use the low pitch prop when your not going on a heavily loaded boat.
Your not going to have 1 prop that is going to do both jobs, you either run it empty all the time & stay with the 19 pitch, or get a 14 or 15 pitch & use it only when your loaded heavy.
It takes about 60 seconds to change the prop.
I feel the 16 pitch was not a good choice, to much pitch.
Try it with the engine raised & alot of weight in the boat, maybe you will be lucky.
David Jenkins posted 06-14-2003 10:41 PM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     

Is it bad for my engine if I am over-propped but I never get near to pushing throttle to dead rack?

Is it bad for the engine if I am under-propped but I don't give it 100% throttle and I never go over 5800 RPM?

I had three young children with me on Friday and there was too much wind to give it a good test. But I will test it again tomorrow.

I have a 42 gallon tank on my 15'. So my top RPM will vary from when I am fully loaded with fuel to when I am near empty. We are planning a two-week trip on the ICW and I need a prop that will will be okay in a variety of situations. If I get 5200 RPM fully loaded and it will go over 6000 with minimal load, and if I plan to cruise at 3500 RPM, I am hoping that you will tell me this is okay. I'm guessing on the numbers at this time.

More data coming soon. Thanks again....

Sal DiMercurio posted 06-15-2003 01:23 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
When your over propped, your engine struggles right through the "WHOLE" rpm range, not just at wot.
For instance, if your propped right & put the throttle handle exactly 2-1/2 inches up from idle to get 3,500 rpms, you would have to put the throttle handle 3" inches up to get 3,500 rpms if your over propped,....yep, it struggles "ALL THEY WAY"
from 2,000 rpms to 5,000.
If your under propped the engine runs real easy, not struggling in any way, just don't over rev her & you will be fine.
Over propping is the one of the main cause's for most engines to die an early death.
How long do you think your tow vehicle would last pulling a boat in 3rd gear only?
Up hill, down hill, stop signs, you name it......same thing as over propping.
Another example, your propped right [ you, yourself, your body ], you take a deep breath, your lungs fill with air, you feel good, now lets over prop you, I'm 6' - 225 lbs & i'm gonna sit on your chest, now try & take a deep breath, .........whats the matter, you don't look to good ?.....thats being over propped.
David Jenkins posted 06-15-2003 11:52 AM ET (US)     Profile for David Jenkins  Send Email to David Jenkins     
Thanks, Sal. I just came in from a test run with the 13.25 x 16 prop, no jackplate, engine raised with bolt in third hole (from the top). Topped out at 5800 RPM with the boat fairly loaded (30 gallons of fuel and 2 adults). That was 36 mph. I was hoping for more speed but since I normally cruise at 18-22 mph I suppose it does not matter. Thanks again. David

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