High Altitude Ratio - 1.49 : 1

[sam60]

My drive has this high-altitude plate on it - 1.49 : 1 . I figured out that the engine turns 1.49 times to turn the prop once, but will this gear setup perform poorly at 1,500' elevation ?

King Cobra Cone Clutch

 

[Howard Sterndrive]

Re: High Altitude Ratio - 1.49 : 1

with the right prop it should run just fine. prop it to run 4200-4600 rpm at WOT

 

[sam60]

Re: High Altitude Ratio - 1.49 : 1

So I want the prop to turn at 2,818 to 3087 RPM.....the math doesn't seem right. My engine is 4200-4600 WOT. What prop do I need? Not looking for speed, just want to make it home.

 

[Howard Sterndrive]

Re: High Altitude Ratio - 1.49 : 1

you want the prop that makes your (properly tuned) engine turn between 4200 and 4600 rpm at WOT.
Doesn't matter what you're looking for, correct props will fit that spec. If your engine is lugging below 4200 @WOT, then select a lower pitch prop to bring the revs up.
If your engine is revving over 4600, choose a higher pitch prop to bring the revs down

3,4,5 blade... alum or steel... there are many props to choose from that will provide different characteristics, but WOT rpm is the critical thing

if you just want to get home, a 3 blade aluminum can be had here on iboats for under $100

 

[HT32BSX115]

Re: High Altitude Ratio - 1.49 : 1

So I want the prop to turn at 2,818 to 3087 RPM.....the math doesn't seem right. My engine is 4200-4600 WOT. What prop do I need? Not looking for speed, just want to make it home.

It really doesn't matter what RPM your prop turns (to a point) you only want to select a prop that will give you around 4600 at WOT.

Using a prop calculator and estimating that you can use a 19p prop you should go about 45-50 MPH at 4600.

http://www.go-fast.com/Prop_Slip_Calculator.htm


Assume about 15% or so slip

The earlier 460 Cobra was a 1.41:1 drive I think, so 1.49:1 isn't all that far off..... I still have a 19p (cupped to 20) that ran 60mph at 5000 RPM! and a nearly new 17p prop that was way too low! I think you'll be fine with a 19 though. 21p might be a little "tall"


Cheers,


Rick

 

[fishrdan]

Re: High Altitude Ratio - 1.49 : 1

You need to know what RPM the engine is turning at WOT for anyone to make suggestions on what prop you need, maybe it's propped correctly right now? If the engine overshoots the WOT RPM, you know you need a higher pitched prop than the one that's on there right now.

Is this boat new to you, was it brought down from high altitude, was it tuned for high altitude? If it was tuned for high altitude, I would be verifying the correct sealevel jets are in the carb and that the timing is set correctly.

I'm no expert on gear ratios, but I think your drive will work at low elevation, how well would depend on what pitch prop you need to use . It's not ideal and you will most likely need to use a high pitch prop, but it should work. I go up to high elevation with a sea level geared drive and have to drop pitch for the engine to perform. From a 19P to a 15P when up at 7000-9000', not ideal, but it works.

 

[sam60]

Re: High Altitude Ratio - 1.49 : 1

The boat is down and out and in the middle of an engine rebuild and I have never had it in the water. The prop that is on it has an M logo in a crooked parallelogram and "15 x 17", the previous owner has only used it at Lake Mead, but the boat is originally from Salt Lake City. He also said that it porpoised with this prop. There is a new spare prop up in the bow storage that I'll look at next time cover is off.

Thanks

 

[HT32BSX115]

Re: High Altitude Ratio - 1.49 : 1

He also said that it porpoised with this prop.

Props don't cause porpoising.....

I think you'll find that 17p prop to be close to what you need. You might also want ensure that you actually have a 1.49 (1.5) to 1 drive.

turn the input shaft (place it in gear while you're turning the shaft) 3 turns and see if the prop turn 2.

3/2=1.5

It's worth KNOWING what your drive ratio is. After so many years, (even though unlikely) someone could have swapped the drive or rebuilt it with different gears.

 

[sam60]

Re: High Altitude Ratio - 1.49 : 1

The curiosity got the best of me so I went in after the new prop. It's a 14.5 x 19 - 15 spline part # 391201. It appears from a search that this is for smaller boats. If so, it's ebay bound.

The 1.5' cubed center-low bow storage compartment that it came out of will be a great place for sand bags to help with porpoising.

 

[HT32BSX115]

Re: High Altitude Ratio - 1.49 : 1

The curiosity got the best of me so I went in after the new prop. It's a 14.5 x 19 - 15 spline part # 391201. It appears from a search that this is for smaller boats. If so, it's ebay bound.

The 1.5' cubed center-low bow storage compartment that it came out of will be a great place for sand bags to help with porpoising.

It's probably worth trying it...