RPMs Too Low!

KilroyJC · Sep 14, 2005

Hello all - I have a thread in Boat Topics & Questions (http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic&f=1&t=012914#000013) that evolved into a prop question, so here goes: 1976 Starcraft Holiday 18 Aluminum 1975 Evinrude 70HP (model 70573) With a 14X17 aluminum prop, and just me in the boat, I get 4100RPM WOT at 25.2MPH (on GPS) With a 13.75X15 aluminum with a couple chunks out of two blade edges, and just me in the boat, I get 4500RPM WOT at 27.3MPH (GPS) Do I stay at max diameter & go down in pitch, like a 14x13, or do I start reducing my diameter & staying with a -15 pitch? Someone suggested I go to a 14X11, but I don't think that'd do anything for my top end. . . Any recommendations? thanks, Matt M

walleyehed · Sep 15, 2005

Re: RPMs Too Low!

I have a Starcraft supersport 18...I had a 115 johnny on it, and due to my altitude, my best prop was the 13-1/4X17, but that put me at around 5700RPM with 2 in the boat. Your 15 is still way low on RPM, so do we know the tach is working??? And, do we know the engine is healthy over-all?? Don't get hung-up on diameter yet, it serves a function but we don't need to worry about that right now. I'm going to assume the engine is mounted in the lowest position on the transom...is this correct???

KilroyJC · Sep 15, 2005

Re: RPMs Too Low!

Kenny - The tach is brand new, but I have another & if I get out this weekend I will connect that & compare numbers to verify. As to overall engine health, compression is 115/110/110, and it usually starts up on the first or second revolution of the crank! Don't know what else to evaluate engine health with. Shifts are solid, and other than beat-up paint and a slight divot in the skeg (half the size of a dime), it's in good shape. As to lowest position on the transom, I'll have to get back to you - I keep my boat 65 miles from my house. . . One of thie things that has to be a factor is that the boat is rated mor a max of 140HP, and I'm only running 50% of that. I'm just fighting the law of diminishing returns, I guess. I mean, I could go to a 12x9 and max out my rpms quite easily, but would I even get on plane??? I looked into those adjustable props, but they don't make them in the range of adjustability that I need. If they did, I'd geet one just to experiment, and then once I found the right size, I'd invest in a good prop & keep the composite as a spare. ONE OTHER THOUGHT - Will running in freshwater and saltwater give noticeably different result if nothing else chenges, due to the different densities? Thanks again, Matt

KilroyJC · Sep 16, 2005

Re: RPMs Too Low!

Follow up post - Yes - the motor is mounted at the lowest position. Using a straitedge, with the trim pin in the second holeout from the transom, the top of the cav plate is about 1/2" above the bottom of the keel, and prett close to parallel to it, too. I can't use my other tach, because some wonderful out-of-state driver (SUMMER"S OVER! WHAT THE *&%^&% ARE YOU DOING HEADING TO THE JERSEY SHORE NOW FOR???!?) cut me off, and when I slammed on the brakes it left my car seat & met my firewall, and the pointer came off the pin. . . Tomorrow I'm going to hit the local Evinrude dealer & see if I can't find a cheap used -13 prop & get it out on the water so I can get more numbers up. Thanks again, Matt

ivar · Sep 18, 2005

Re: RPMs Too Low!

I guess with the 15" prop, the dinged up blades cost you a couple hundred rpms. Would be interesting to see how it would work with a good 15" prop with the motor raised on the transom 1 or 2 holes.

walleyehed · Sep 18, 2005

Re: RPMs Too Low!

If you get a 13 for a trial run, give us the numbers and we'll go from there. The larger diameter is going to give you a few more options in the long run. 50% of the max recommended is tough to deal with, but I think we can get it so you can use all that engine has to give.

KilroyJC · Sep 22, 2005

Re: RPMs Too Low!

W - You'd think that at this point in the season there'd be used props out there - not at the Jersey Shore! Went to 3 different marinas, and a flea market, & some garage sales that looked promising, and nothing in the size/pitch that I need... I'll keep looking because I'd still like to get one more run out on the water to generate numbers, but after Columbus weekend the boat is not in the water again until next year, & I'll start on my winter project of repairing the transom, while the weather is still decent. Once again, thanks for all the advice everyone!

KilroyJC · Sep 23, 2005

Re: RPMs Too Low!

OK - I decided to start working on replacing my transom, so no numbers until next year. Info: I degreased the serial number plate, and found out that the WOT operating range for this motor (according to Evinrude) is 4500 to 5500, with the motor having certified output of 70HP @ 5000 as per BIA. So the damaged prop is juuuust getting me into my power band.

KilroyJC · Oct 9, 2005

Re: RPMs Too Low!

Just made a score on eBay! Picked up a complete lower unit from a '76 70HP that had a blown powerhead (a rod bolt worked loose & went through the bloce - ouch!), with a 14X9 prop for $75.00! New Water pump assembly on it, too. Guestimate about 5600RPM@WOT, but will I be moving enough water to not lose top end?

walleyehed · Oct 10, 2005

Re: RPMs Too Low!

When that engine was built in 1975, yea, 4500-5500 was ok with fuel of that era. With todays fuel, we have PROVEN that much below 5500 for a WOT causes coking, leading to ring problems, temp problems and eventually engine failure. 5700-5800 is BEST for that engine on todays fuel. If you are thinking of using the 9P prop, don't...you'll lose alot of speed (alot) and be way over 6000RPM. We need a known 13 or 15 in good condition and get some numbers...I think the 13 will be close, but we can adjust the engine heigth and maybe get the 15 up in R's...don't know until we get a good base-line.

KilroyJC · Oct 11, 2005

Re: RPMs Too Low!

W - Thanks for the heads-up on the 9! I guess there's a curve involved, not just the flatline 200RPM/inch, huh? If nothing else, the 9P is swappin' ammunition for something else I may need - I just couldn't resist a complete LU for the $$$...expecially since it was within convenient (sort of) pick-up distance. The seller didn't know the pitch, & from the small photo I could only guestimate - I thought it was an 11P, because I didn't know that there was something as low as a 9P. Anyone wanna swap? The only baseline I have with a good prop is from the 17P, which other than a couple of paint scratches and a little grease is in excellent shape. This coming weekend I winterize the motor & pull it from the transom, so that I can rebuild my transom, which is another thread I started under boat repair/restoration...

walleyehed · Oct 11, 2005

Re: RPMs Too Low!

The 9P will be good sell/trade material... The 200 per inch of pitch is pretty close if you are within 500-600 either way of the middle of the RPM range..I still think a 13 will be close.

RPMs Too Low!

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