High altitude prop

First, the general theory is that you lose about 3% of your horsepower for every 1,000 feet of altitude. There is about 2.9-3.0% less air in the same volume due to decreased density. 6000 feet is 18%. Your 250 is getting real close to 200hp, based on that rule of thumb. New EFI/DFI engines are pretty good at adjusting fuel flow to match the air density entering the motor so that you dont lose even more power by running too rich. Your task becomes trying to figure out what prop you would run on a motor that is down almost 20%. Going from 24 to 22″ is not quite 10%, so you are still too high.2008 Pantera Classic2014 Mercury Pro XS 200

Not being able to turn a 24 Trophy up to at least 5800 rpm, with no hole shot , I question being able to turn a Fury24 inch prop at higher rpm than the 5500 rpm Im getting now. It seems to me that I need to go down in size to get higher rpm and a better hole shot. I dont want to just go buy a bunch of props, and dealers here have nothing to really try. I think Id like a fury but Im missing something here. Remember Im running at fairly high altitude. Billy

Heres my thoughts. 1. hole shot. Im not a Fury expert, but have driven one boat using one and they do seem to be better than anything else around at the moment. So as BCB says, that is a starting point. 2. 5500 is not horribly low, but we are in April and I assume you are talking about current weather / RPM. In the Summer you are are going to fall off a cliff, in that as you drag rpm downward, you can fall right out of the torque band. Ive seen boats (even have a prop that will do this to mine) that run like a banshee in cold weather, and cant even get up on plane in August without a prop change. Do you have a hydraulic jack plate? If so the next question is moot, because it is so easy to adjust. But when you change props, or run a lot slower than normal, engine height might well become sub-optimal and need to be changed as well. Used to have a friend in Denver that mentioned this problem more than once. Never experienced it myself. Most of my fishing is around 500 above sea level here in Birmingham. 3. What about motor height? The trophy is a smaller diameter prop than a triple-ear. You may want/need to raise the motor a little to compensate when you try a fury or tempest. About all l can suggest is to go down in pitch, and experiment. This is a significant problem. Your motor is nothing but a big air pump, and you have 20% less air at that altitude. Just remember that this is a dual-whammy. Not only do you lose that top end RPM, you are dropping the RPM to the bottom end of the torque band in the middle throttle settings. If you cant twist a prop fast enough to let the motor get up to the torque band, you lose a second time. Sort of like a 21 speed bike. There is a gear setting where you simply cant climb a hill and have to get off and push. This is a little hard to do in a boat. Somehow a 22″ prop on a 250xs just seems wrong. But it may well be when you need depending on your load. The only way to determine proper prop size is to ask your motor what it likes. It tells you via the tach. This time of year I want to be right up on the limiter, because I know August is going to drop a couple of hundred RPM just due to air temp/density and humidity. As a last note, blade shape does make a difference. Good props (Fury as one example) make a big difference. Stainless is good, but not all stainless props are created equal. Probably the best solution for 6000 feet is a 300hp motor. 2008 Pantera Classic2014 Mercury Pro XS 200

only thing I dont get is your “right back at the numbers…” You are down over 15% in horsepower. You dropped 2″ in pitch. You cant possibly run as fast as you did at near sea level with 15% more power and 2″ more pitch. Did I misunderstand what you meant???2008 Pantera Classic2014 Mercury Pro XS 200