Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

[AgnotGt]

Hi

Next year I am taking the 16ft Glastron up to Lake Tahoe; 6,000ft. Right now, @sea level, I am turning approx. 4500rpm @ 42mph with a 14 x 19 prop. While I won't be pulling a skier I would like to have the right prop for getting around the lake.

I'm thinking maybe a 14x17? But I am only guessing....

Thanks for the suggestions!

 

[alldodge]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

Hi

Next year I am taking the 16ft Glastron up to Lake Tahoe; 6,000ft. Right now, @sea level, I am turning approx. 4500rpm @ 42mph with a 14 x 19 prop. While I won't be pulling a skier I would like to have the right prop for getting around the lake.

I'm thinking maybe a 14x17? But I am only guessing....

Thanks for the suggestions!

I take it you don't plan to re-jet the carb. I would lean more to the 15 pitch then the 17. You loose 4% of your power for every 1000 feet of elevation. You could look at the link below and put in your info to get a rough idea. It only goes back as far as 2000 and would have to use a 3.0 but will give you an idea.

Prop Selector | Mercury Marine

 

[AgnotGt]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

I take it you don't plan to re-jet the carb.

Originally this boat was bought in Tahoe but I don't know the history beyond that. It is possible that is still jetted for higher altitude.... She seems to run good down here, been here for a few years, but the timing is way too advanced at the moment. I had to turn my timing light 20* advanced to see the mark. I verified the pulley and timing mark are correct today. Since it is winterized now I can't fire her up to re-adjust, to the correct timing, till next spring.... (Receipts showing a tune-up so I had left it alone)

I just found out that the former owner has a new prop but it is the same specs. Black Max, p/n#48-832830 A45 Value? I had hoped it was the old prop from Tahoe.... But no such luck.

.

 

[alldodge]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

Originally this boat was bought in Tahoe but I don't know the history beyond that. It is possible that is still jetted for higher altitude.... She seems to run good down here, been here for a few years, but the timing is way too advanced at the moment. I had to turn my timing light 20* advanced to see the mark. I verified the pulley and timing mark are correct today. Since it is winterized now I can't fire her up to re-adjust, to the correct timing, till next spring.... (Receipts showing a tune-up so I had left it alone)

I just found out that the former owner has a new prop but it is the same specs. Black Max, p/n#48-832830 A45 Value? I had hoped it was the old prop from Tahoe.... But no such luck.

.

It is probably not currently jetted for high altitude. High altitude requires more fuel during operation to preform but more fuel at lower levels will damage the engine. Basically you can go from low to high and not hurt anything, but you cannot go the other way without damage (way to rich). Advancing the timing can be used at higher levels to help, and since things don't burn very well when up high it can be done with less possibilities of damage.

If your not going to be there long you could get away with what you have it would just be slower. Stated you don't plan to ski or tube, could be WOT would be 30mph instead of 42. But if not, at least 17 but think 15 would be better, but use the calculator and see what it comes back with

 

[fishrdan]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

Seems like that 2.5L runs pretty well in the Glastron, 4500RPM at 42MPH with a 19P prop, is that GPS speed or off the speedo? What type of load are you going to carry while up in Tahoe,,, and how does that compare to the load while at sealevel?

The boat seems to run strong so you could try a 17P prop, but I bet a 15P would work much better. I'm running a 140HP Mercruiser in an 18'er and a 17P is a dog up at that type of altitude, but a 15P works fine. Different boat, with a different load, so your prop selection could need to be,,, different....

High altitude requires more fuel during operation to preform but more fuel at lower levels will damage the engine. Basically you can go from low to high and not hurt anything, but you cannot go the other way without damage (way to rich).

Ummm, it's the other way around... High altitude requires less fuel, so the carb could be leaned out to help compensate for high altitude, but this would need to be reversed when dropping in altitude,,, so the engine doesn't burn up due to too lean of a fuel mixture.




When I go up in altitude 6000-8800' I re-jet to lean the fuel mixture, bump the timing a few degrees and swap props, but it's kind of a hassle since those changed need to be made before and after a high altitude trip. Mercruiser recommendation is to only swap props, unless the boat is going to live at high altitude.

 

[AgnotGt]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

The speed was assessed using the dash gauges; no gps. I had full fuel tank but no other weight. No gear or passengers. Tahoe will be plus one and a cooler.

At some point I will check out the jets so I can record their number but for now she's running great. I will only be up at Tahoe for a few days so no reason to make changes to anything except the prop. You both agree that a 15p is probably where I need to be so I will be looking to see what is available.

I was thinking that higher in altitude needs leaner mixture.... My old 53 cad would "just" get around up there but get back down to the flat land and look-out..... I tried the calculator but unless I was using it wrong it doesn't give a separate recommendation for "you're here / going there"

.

 

[jestor68]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

Given that your motor is turning almost to the max of it's WOT rpm now (4200-4600); I would think that a 17 pitch is all the change you would need to make.

The motor will tend to run a little rich at 6000 ft, but since you're only going to be there for a short time, it won't matter; except for a little higher fuel consumption.

The problem arises when you take a motor jetted for high altitude to a low altitude( runs lean....boom).

Insure the timing is set correctly before going up there.

The general rule of thumb is 3-4% power reduction per thousand ft of altitude increase for normally aspirated engines.

 

[steelespike]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

Your numbers don't add up. Assuming a 2.00 gear ratio a 19" at 4500 can't go 42.
Even a slightly different gear ratio isn't likely to make the numbers balance.
It's likely both the tach and speedometer aren't quite right.
Does it have a strong hole shot? Does it handle a load well?

 

[alldodge]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

The speed was assessed using the dash gauges; no gps. I had full fuel tank but no other weight. No gear or passengers. Tahoe will be plus one and a cooler.

At some point I will check out the jets so I can record their number but for now she's running great. I will only be up at Tahoe for a few days so no reason to make changes to anything except the prop. You both agree that a 15p is probably where I need to be so I will be looking to see what is available.

I was thinking that higher in altitude needs leaner mixture.... My old 53 cad would "just" get around up there but get back down to the flat land and look-out..... I tried the calculator but unless I was using it wrong it doesn't give a separate recommendation for "you're here / going there"

.

On the second page "USAGE" to the right of selecting how you use the boat is the High Altitude selection

 

[AgnotGt]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

On the second page "USAGE" to the right of selecting how you use the boat is the High Altitude selection

Not knowing the boats performance @6,000ft you are left to guess. I just put in the same info as last night but changed the WOT to 3,800rpm and 30MPH. That resulted in needing a 15p to reach ideal rpm @ 34mph.

I will wait till I re-time this engine and get more accurate rpm & speed info. In the Monterey Bay with chop and swell, relying on dash gauges and no trim adjustments the numbers were just reference info anyway!

I had put the outdrive all the way down and never touched it.....

So I'm learning and reading! Thanks!!!

 

[buckhorn7]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

No need to change anything. I boat from Tahoe to the Delta and surounding lakes at all different elevations. The boat just goes a little slower at higher elevation no matter what you do.

 

[Rick Stephens]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

No need to change anything. I boat from Tahoe to the Delta and surounding lakes at all different elevations. The boat just goes a little slower at higher elevation no matter what you do.

Agreed. Unless yer staying for quite a while and wanting to pull skiers, just not all that big a deal to run it a little bit slower on the stock stuff. It'll run rich, but that won't hurt things any more than it does your car.


I spent years in that part of the country - launch at Moss Landing, head around the point at Monterey on down to Point Sur for the fishing. And hauling the boat up to Huntington or Tahoe on occasion. You'll have no problem as is with your setup. And after trying it out up at Tahoe, you'll know exactly what you want to do to it the next time, if any adjustment is necessary.

Have fun!

 

[fishrdan]

Re: Sea level to 6,000ft: which pitch would be good? '87 Mercruser 2.5l Aphha1

^^^ Agree and disagree...

I've have taken several boats up to high elevation and all of them have suffered huge performance losses, they still ran, but ran like carp if nothing was done to compensate for the elevation change. Going from boats that snap up on plane at 1000' elevation to dogs that were barely able to climb on plane at elevation, or wouldn't plane. Making prop and carb changes made a big difference in performance.

If I'm just going up for a day trip I don't mess with anything, but if going up for a couple of days I change it over for high elevation. On the day trips it's not a huge deal as I usually use the electric trolling motor 90% of the time anyway...