High altitude prop for good hole shot

[larry1167]

I have an '87 Sanger TX with OMC Cobra drive and Chevy 350 carbureted(Quadrajet) engine. Boat weighs about 2,600lbs. My local lake is at 500' and I have never operated the boat above 3,000ft.

Gear ratio on the Cobra is 1.41:1. WOT operating range is 4200-4600 RPM. Currently running a SS 14.5x19 three blade prop . At 500', I hit the WOT range and boat runs well with the 14.5x19 prop. Hole shot is good. Pulls 200+ lb. guys out of the water. Top speed is 50+mph.

Planning to take the boat to Donner Lake this summer. Elevation is 5,900'. I know I need to reduce the pitch, but I am wondering how much. I need to maintain a good hole shot because I use the boat to slalom ski and wakeboard. Not too concerned with losing top end speed because I only need to get to 31-33mph for skiing. My main concern is bogging down when pulling a skier from a deep water start on a slalom ski.

I don't want to spend the money on a stainless steel prop, therefore, I am going aluminum. What size prop would you guys recommend for what I'm looking for? I was thinking 14.5x15 three blade, but would also consider a 4 blade.

 

[fishrdan]

Re: High altitude prop for good hole shot

I run a 19P prop for low elevation, then swap to 17P for lakes up to 5000'. Above 5000' I go to a 15P prop since I'm lake hopping at that point and can be at lakes anywhere from 6000' to 9000'. This seems to work out well for my 140HP Mercruiser. BTW, I bought my 15P prop off a guy who used it for 1 trip up to Lake Tahoe...

If spending a few days up at 9000' I'll swap to smaller jets in the carb too. It's a hassle (takes 15-20 minutes with my carb) but it makes the engine run much better.

 

[larry1167]

Re: High altitude prop for good hole shot

Thanks Fishrdan. Anyone else care to chime in? I was hoping to accomplish acceptable performance without adjusting my carb and just swapping my prop. Anyone out there with a 260HP carburetated I/O operating at high altitude? What prop are you using?

 

[Tahorover]

Re: High altitude prop for good hole shot

I live on the delta and boat on Tahoe during the summer. My delta prop is 2" larger in pitch then my Tahoe prop. You will be running rich with out carb adjustment. I turn my mixture screws in 3/4 of a turn when at the lake.

 

[larry1167]

Re: High altitude prop for good hole shot

Thanks Tahorover. Are you running a 3 blade prop on an I/O? I was thinking of going town 4" on pitch, but maybe this is too much. I read a few old posts and some people suggested 1" for every 1,500' of elevation. Sounds like you have a lot of experience doing this though and I would lean toward your suggestion if we have similar boats.

I guess I will have to adjust the mixture. I have never done it on my carb before. When you say you are turning your screws "in", is that clockwise or counter clockwise? Thanks in advance for your help.

 

[larry1167]

Re: High altitude prop for good hole shot

Tahorover- I just took a look at my carb. I see what you mean. Looks like I need to go clockwise, aka "in". Also looks like you're a classic woody guy so I assume you have inboards. Are you running 4 blade props?

When I fire the boat up for the first time at altitude, will the engine run really rough because it's running rich? Just wondering how evident it will be when I adjust the mixture screws to the correct location. Will it be pretty easy to tell when I've got the mixture correct?

Now I have to track down the OMC socket to adjust the mixture screws.

 

[craze1cars]

Re: High altitude prop for good hole shot

Generic "how to set mixture screw" for pretty much every carburated engine on planet:

1. Set idle speed with idle screw to spec idle speed WITH ENGINE FULLY WARMED UP. 2. Turn mixture screw in 'till engine stumbles due to idle speed decreasing. 3. Then turn it out 'till engine stumbles due to idle speed decreasing. 4. Set mixture screw at max RPM & smoothness, usually approx 1/2 way in-between those 2 stumble points. 5. Reset idle speed with idle screw.

Keep in mind you probably have two screws on your Qjet. And they should generally be set the same...so turn them in or out "equally and alternately" while doing this.

Before making changes it is usually smart to first note your starting point...assuming it runs good now. Most people do this by turning the screw all the way in until you feel it just touches bottom (do NOT tighten this screw!!) WHILE COUNTING the # of turns it takes (be aware it won't take many turns...usually you'll find it to be somewhere between 3/4 and 2.5 full turns). Whatever it is write this number down. Now with that note you can always return to your baseline/start point if you get lost, or when you return to home altitude, by simply bottoming the screw and turning it back out exactly that many turns.

Bear in mind this screw only effectively impacts your idle quality. It will still be running rich at most speeds if you don't change to smaller main jets...but might be good enough for your temporary use purposes, and won't be too rich to hurt anything unless you run into stumbing/flooding/plug fouling problems, which I doubt you'll experience at 5,900 ft. But, I would suspect your engine mfr recommends dropping a jet size or two for anything above 5,000 ft...that's prettty typical. So if lake is at 5,900 you're possibly pushing the envelope just a hair...but that entirely depends on how you're jetted now, maybe it's been "tuned" a hair different from spec already at some time during its life, being that your boat is now pushing 25 years old? Do you even know what jets are in there now? And what it calls for? Pull a plug...if it's soot black here at low altitude it might indicate you're a bit rich already, and moving that boat up to 5,900 feet may give you VERY poor running. But if the plug looks clean/mean/lean down here, you might be good enough to go as-is...

In winter I often take my snowmobiles from my normal 1,500 ft altitude riding area up to 10,000+ feet in the Rockies. For such a change they take substantial adjustments in carb/gearing/clutching, and I have found dealers in the low lying areas and other people who don't do this everyday often have only theoretical advice, and no personal experience. So usually their advice is a bit "off."

So now I ALWAYS call a dealer near the riding area I am going to and ask THEM what I should do for adjustments. And I get much better advice, and they're always happy to help in an effort to earn your business if you need them while you're in town. Heck...they might even have a selection of high altitude jets and props to rent to you for reasonable cost so you can avoid making a permanent investment...might be cheaper for you than buying another prop.

So the best thing you can do of all IMO, is chat briefly on the phone with a marina or two that services that lake you're visiting. They probably answer this question every single day...both regarding prop and carb...and have more personal experience with visitors from the lowlands than anyone on this forum does. Even if you decide to try it as-is, in which you will probably be fine, you'll at least have someplace to turn to if you get out on the lake on day one and can't get your boat to plane out for some reason....

 

[larry1167]

Re: High altitude prop for good hole shot

Craze1cars- thanks so much for all the information. This really helps.

I don't know what jets are in there now. I am the second owner. The first owner only operated at sea level, therefore, I assume the original jets are in the carb. I just changed my plugs and they look good. Insulators look perfectly clean with no deposits.