Rude 70 hp, New Prop, Loss of RPM?

[btmoyers]

I have an E-Rude, 1979, 70 HP. On 15' Blue Fin. I <br />broke a prop last week (rough driveway). It is an OMC 13" x 19" (#382465). I replaced it with a Mich. Wheel "Vortex" 13" x 19". When I went out today with the new prop, I lost my hole shot, lost about 6 mph on top end and could tell the motor did not get up to normal RPM at WOT. Not sure what is happening . Still the same dia. and pitch. The old prop was in rough shape and I had to file it down a few times. I expected to gain a little muscle, not lose it. Any input is greatly appreciated.

 

[wilde1j]

Re: Rude 70 hp, New Prop, Loss of RPM?

I would guess the new prop is cupped, which causes loss of maybe 150 RPM @ WOT. What was and are the WOT RPM's? You may have been at the low end of the acceptable range before the new prop. Generally, you want to be propped at the top of the operating range with a normal load.

 

[btmoyers]

Re: Rude 70 hp, New Prop, Loss of RPM?

Hi Wildeone, Thanks for the reply. I don"t have a tach. Just going by ear on the RPM idea. The new prop was indeed cupped. Which brings me to the next question. Mich Wheel suggest going down 2" in pitch if going from a straight prop to cupped. But what does the plastic trim ring at the outermost end of my original prop do? I was wondering if that acted in some way similar to cupping. Nothing like answering a question with a question Anyhow hope to get to the bottom of it all. agn tnx for the quick response.<br /><br />Bill

 

[btmoyers]

Re: Rude 70 hp, New Prop, Loss of RPM?

Just a note. I took the new prop back to the dealer and now am not entirely positive the "Vortex" is cupped. Darn it now I'm just not sure.....Anyhow they suggest I call in the AM Monday to talk to the resident Guru...Hope you guys/gals out there have some more ideas that way I can sound educated!! When I make that call <br /><br />Bill

 

[DJ]

Re: Rude 70 hp, New Prop, Loss of RPM?

boat202,<br /><br />Unless your boat is extremely light, a 70 hp. John/Rude would have a hard time pulling a 19" wheel.<br /><br />I would think a 17" or even a 15" would be a better choice.<br /><br />The three cylinder loopers cannot stand lugging. They need rpms.

 

[wilde1j]

Re: Rude 70 hp, New Prop, Loss of RPM?

It's easy to tell if the prop is cupped. The trailing edge of each blade has a little hook in it, rather than flat like the oEM and cheap props.<br /><br />The trim ring on your old prop has nothing to do with pitch or prop loading, just a kind of flow straightener.<br /><br />I think djohns19 is right, you're overpropped, but w/o a tach, there's no way we can be of more help.

 

[btmoyers]

Re: Rude 70 hp, New Prop, Loss of RPM?

Thanks, Djohns19.<br />This prop is what came on the motor when I bought it. My top end is/was apx. 31 to 32 mph/GPS which didn't seem too bad. I just load the normal fishing gear, tool box, usually 2 people on board. I have a built in gas tank/ 18 gals. I carry duel starting/accessory batts. in the aft area. A trolling batt and trolling motor in front. I never really thought about the load other than I have a fairly shallow draft. I'm not sure if that helps give an idea as to load/versus what the 70 HP is capable of pushing. I know all this is slightly academic sinse past performance hasn't been the question. Just wanted to include this info. so I can make proper prop decisions. I am keen to going to 17" but still wonder why I'm not getting the same performance with the new 19" which I thought would "run" better than that old beater.<br />Sorry for the long post........<br /><br />Bill.

 

[btmoyers]

Re: Rude 70 hp, New Prop, Loss of RPM?

Thanks Wildeone, You replied as I was typing in respone to Djohns reply. <br /><br />I realize now that that was an improper impulse about the trim ring(I love learning on the net!!!). Anywho...I will be leaning to the over propped idea. But still wondering about the loss of power on the new prop. Now that it is not in my possesion, I don't know that it was cupped but that certainly would be one culprit. Thanks for spending some of your time on my problem. Will forge on......<br /><br />Bill.

 

[btmoyers]

Re: Rude 70 hp, New Prop, Loss of RPM?

just a little bump...

 

[sloopy]

Re: Rude 70 hp, New Prop, Loss of RPM?

The three cylinder loopers cannot stand lugging. They need rpms.<br />

Music to my ears...... <br /><br />lower the pitch a bit

 

[DJ]

Re: Rude 70 hp, New Prop, Loss of RPM?

boat202,<br /><br />After reading the description of your load, I definetely think a 19" is too steep. <br /><br />As an example, my father runs a 17' BW Montauk with a John. 70. He switched from a 17" to a 15" and did not lose any speed-but the engine is alot happier and the holeshot is drastically improved.<br /><br />By the way, you can buy a tach. for less than $50.00. Also, I am NOT a fan of Michigan wheel props. I have never had one that I thought worked correctly-in my opinion.

 

[ob]

Re: Rude 70 hp, New Prop, Loss of RPM?

michigan props are un balanced hecho en mexico junk.dealer tried to pawn one on me when i bought my rig new. 15 ft. boston whaler with 70hp evinrude.michigan props are not designed to match the torque curves of your motor.I use a 17 x 131/4 omc stainless.Michigan props shake like a dog passing razor blades.yours should turn 5800 to 6000 top end.If you don't have a tach I highly recommend getting one hooked up.This will help you determine the optimal prop application and aid in monitoring engine performance.